WO2015172802A1 - Scheduling in wireless local area networks - Google Patents

Abstract

There is provided time scheduled multiple access in a wireless local area network. A method performed by an access point comprises acquiring signal strength values for stations associated with the access point. The method comprises scheduling the stations such that stations having determined signal strength values within the same signal strength intervals are scheduled to transmit concurrently.

Description

SCHEDULING IN WIRELESS LOCAL AREA NETWORKS

TECHNICAL FIELD

Embodiments presented herein relate to wireless local area networks, and particularly to methods, an access point, a station, computer programs, and a computer program product for time scheduled multiple access in wireless local area networks.

BACKGROUND

In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.

For example, in wireless communications networks, and in particular in the uplink (i.e., the transmission path from a wireless device (e.g. a station or user equipment; UE) to a network node (e.g., a radio base station such as an access point (AP) in a cell site)) in such wireless communications networks, signals from different stations may be received at the access point at the same time. The term AP is here used to denote the network node to which the station or UE connects wirelessly. This is the term used in the IEEE 802.11 standard, whereas in other standards the terms base station (BS) or Node B, or evolved Node B may be used instead. This is for instance the case for

Wideband Code Division Multiple Access (WCDMA) based communications networks and Long-Term Evolution (LTE) based communications networks, both developed by the 3rd Generation Partnership Project (3GPP).

To ensure that the signals from the different stations are received by the AP at similar power levels, transmit power control (TPC) may be used. This essentially means that the AP sends control information to the individual stations and informs the stations if the transmitted power of the stations needs to be adjusted. In WCDMA, where the signals from the stations are not perfectly orthogonal to one another, very fast power control may be needed. The stations are here controlled on a millisecond scale. For LTE, the signals from the different stations are essentially orthogonal and the requirement on the power control may be less stringent.

WO2007/126385 A2 addresses issues with largely different power spectrum densities. Two different types (denoted Type I and Type II, respectively) of interference that could be experienced are considered. Type I corresponds to interference that predominantly is problematic for adjacent channels, and where a typical source for such interference is phase noise. Type II

interference corresponds to interference generated at mirror frequencies, where a typical source is IQ-imbalance. US2011/0243073 Ai discusses potential issue with very different received powers if IEEE 802.11 is used. IEEE 802.11 relates to communications standards for wireless local area networks. In particular, when two or more narrow channels are combined to provide a wide channel, in

US2011/0243073 Ai it is suggested that if different stations are allocated to different narrow channels, their signals should preferably be received by a common access point with similar powers. US2011/0243073 Ai is concerned with that the acknowledgements can be received simultaneously or concurrently from two or more stations using the same spatial channel, but different narrow frequency channels in order to reduce the time for receiving the acknowledgement from all stations.

In general terms, when signals from two or more transmitters are received by a receiver it may be advantageous that the signal powers for the different signals are as similar as possible in order to minimize the amount of interference between the different signals. This may be caused by that the signals are not perfectly orthogonal. Then a strong signal may severely interfere with a weak signal. This is often referred to as the near-far problem. This issue is addressed in W 2007/126385 A2. In WO 2007/126385 A2 the approach taken to address the near-far problem is essentially to ensure that stations that are scheduled at the same time have similar signal power densities, which if found necessary is accomplished by changing the power and/or the bandwidth of the signal. However, also when the received signals are orthogonal there may, for example because of non-linear effects, be practical issues with receiving signals with very different powers, even if the power density is the same. Alternatively, it may be so that different power densities are preferable in order to, for example, obtain similar powers in situations where the used bandwidth is very different for the different received signals. A typical example of this is the analog-to-digital converter (ADC) in the receiver. The ADC should be able to handle the largest possible signal amplitude in order not to cause clipping, since the latter would severely degrade performance of the ADC and hence of the receiver. However, for a fixed number of bits in the ADC, the larger the signal that needs to be handled, the larger the

quantization step must be. Therefore, if a signal of low power is received at the same time as a signal of high power, the large quantization step enforced by the strongest (high power) signal may cause significant quantization noise for the weak (low power) signal.

Hence, there is still a need for an improved handling of stations in wireless local area networks.

SUMMARY

An object of embodiments herein is to provide improved handling of stations in wireless local area networks.

A particular object is to provide efficient scheduling of stations in wireless local area networks.

According to a first aspect there is presented a method for time scheduled multiple access in a wireless local area network. The method is performed by an access point (AP). The method comprises acquiring signal strength values for stations (STA) associated with the access point. The method further comprises scheduling the stations such that stations having acquired signal strength values within same signal strength intervals are scheduled to transmit concurrently. Advantageously this provides efficient handling of STAs in wireless local area networks.

Advantageously this provides efficient scheduling of STAs in wireless local area networks. Advantageously this allows for two or more STAs to transmit simultaneously in the UL without the need for transmission power control (TPC) of the STAs.

Advantageously this alleviates the need for an excessive number of bits being required in the ADC used in the AP.

Advantageously, also in case of TPC this allows for improved performance of the aggregate throughput in the UL.

Advantageously this enables high spectrum efficiency in wireless local area networks.

Advantageously, by reducing the total transmission time for the STAs, it also reduces the power consumption of the STAs. According to a second aspect there is presented an access point for time scheduled multiple access in a wireless local area network. The access point comprises a processing unit. The processing unit is configured to acquire signal strength values for stations, STAs, associated with the AP. The processing unit is configured to schedule the STAs such that STAs having acquired signal strength values within same signal strength intervals are scheduled to transmit concurrently.

According to a third aspect there is presented a computer program for time scheduled multiple access in a wireless local area network, the computer program comprising computer program code which, when run on a processing unit, causes the processing unit to perform a method according to the first aspect. According to a fourth aspect there is presented a method for time scheduled multiple access in a wireless local area network The method is performed by a station (STA). The method comprises receiving a list of stations from an access point (AP), the list comprising information about to which one of at least two signal strength intervals the stations belong. The method comprises determining a need to access a bandwidth portion of a communications channel of the wireless local area network. The method comprises, in response thereto, determining whether or not another STA is transmitting on the channel. The method comprises initiating transmission on the

communications channel if the bandwidth portion is not occupied, and if no STA belonging to at least one signal strength interval outside that of the STA is transmitting.

According to a fifth aspect there is presented a station for time scheduled multiple access in a wireless local area network. The STA comprises a processing unit. The processing unit is configured to receive a list of STAs from an access point, the list comprising information about to which one of at least two signal strength intervals the STAs belong. The processing unit is configured to determine a need to access a bandwidth portion of a

communications channel of the wireless local area network. The processing unit is configured to, in response thereto, determine whether or not another STA is transmitting on the channel. The processing unit is configured to initiate transmission on the communications channel if the bandwidth portion is not occupied, and if no STA belonging to at least one signal strength interval outside that of the STA is transmitting. According to a sixth aspect there is presented a computer program for time scheduled multiple access in a wireless local area network, the computer program comprising computer program code which, when run on a processing unit, causes the processing unit to perform a method according to the fourth aspect. According to a seventh aspect there is presented a computer program product comprising a computer program according to at least one of the third aspect and the sixth aspect, and a computer readable means on which the at least one computer program is stored. The computer readable means may be non- transitory computer readable means.

It is to be noted that any feature of the first, second, third, fourth, fifth, sixth and seventh aspects may be applied to any other aspect, wherever

appropriate. Likewise, any advantage of the first aspect may equally apply to the second, third, fourth, fifth, sixth, and/or seventh aspect, respectively, and vice versa. Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

The inventive concept is now described, by way of example, with reference to the accompanying drawings, in which:

Figs la, lb, and 11a are schematic diagrams illustrating a communication network according to embodiments;

Fig 2a is a schematic diagram showing functional units of an access point according to an embodiment; Fig 2b is a schematic diagram showing functional modules of an access point according to an embodiment;

Fig 3a is a schematic diagram showing functional units of a station according to an embodiment; Fig 3b is a schematic diagram showing functional modules of a station according to an embodiment;

Fig 4 shows one example of a computer program product comprising computer readable means according to an embodiment; Figs 5, 6, 7, and 8 are flowcharts of methods according to embodiments; and

Fig 9 schematically illustrates pathloss as a function of distance according to an embodiment;

Fig 10 schematically illustrates received power as a function of frequency according to an embodiment; and Figs 11b and 11c schematically illustrates reception of packets according to embodiments.

DETAILED DESCRIPTION

The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the inventive concept are shown. This inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout the description. Any step or feature illustrated by dashed lines in the drawings should be regarded as optional.

Fig la is a schematic diagram illustrating a communications network 10a where embodiments presented herein can be applied. The communications network 10a comprises network nodes in the form of access points (APs) 11a, 11b. The APs na-b are configured to provide network coverage to wireless devices or stations (STAs) 12a, 12b, 12c, I2d. The STAs I2a-d may be any combination of hand-held wireless transceiver devices, such as mobile phones, smartphones, tablet computer, or laptop computers or the like, or other types of user equipment (UE). The APs na-b thus act as radio base stations for the STAs i2a-d. Each STA i2a-d is configured to be operatively connected to at least one AP na-b via a wireless link 15a, 15b, 15c, isd. The communications network 10a further comprises a core network 13. The APs na-b are operatively connected to the core network 13. The core network 13 is in turn operatively connected to an Internet Protocol (IP) based service network 14. The STAs i2a-d are thereby enabled to access content and services as provided by the IP based service network 14. The communications network 10a may be a wireless local area network

(WLAN). In WLANs basic access to the communications channel is based on carrier sense multiple access with collision avoidance (CSMA/CA). When the access to the communications channel is handled in a completely distributed fashion, this is referred to as a distributed coordination function (DCF). This means that all STAs, as well as the AP, contend for the communications channel. There are also possibilities for a more centralized channel access scheme where the AP polls the different STAs, and in addition it is possible to combine the DCF with more centralized scheme in a hybrid manner, referred to as hybrid coordination function, HCF. With the evolvement of the IEEE 802.11 standard, progress has been made in achieving high data rates, most notably because of an increase in the bandwidth and the introduction of multiple-input multiple-output (MIMO) compliant communications technologies. However, just increasing the data rate on the physical channel (PHY) layer may not be enough; also

modification to the medium access control (MAC) layer may be needed. In particular, the need to contend for the channel may introduce overhead. That is, since the channel will be unused during the contention, it is beneficial if the channel can be used for longer times between the contentions. Therefore, mechanisms such as frame aggregation and block acknowledgement (BA) have been introduced in the IEEE 802.11 standard. Essentially, the increased data rate on the MAC layer may be achieved by ensuring that once the STA gets access to the communications channel the STA can use it to transmit a relatively large amount of data such that any overhead caused by the channel contention is reduced or even minimized.

Assume that a large number of s STAs are associated with one AP. In such scenarios the total time for transmission between the STAs and the AP may be shared between the different STAs. With a large number of associated STAs, not all individual STAs may sustain a high average data rate. In order to achieve a high aggregated throughput, the communications channel should be divided among the STAs such that once a particular STA gets access to the communications channel the particular STA should have a sufficiently large amount of data to transmit.

One issue with requiring that each particular STA should transmit a large amount of data each time each particular STA gets access to the

communications channel may be that the duration between two time instants when a particular STA gets access to the data channel will be large, and therefore there will be an inherent delay which may not be acceptable for some application programs run by the STAs. If for instance the application program would involve a two-way voice or video service, thus requiring network access by the STA running the application program, this would put rather stringent restriction on the delay and the delay jitter that can be tolerated when the STA accesses the network.

To solve the issue with supporting many STAs but without introducing a large delay, one approach is to schedule two or more STAs in parallel. This approach is commonly used in cellular systems where there is a strict centralized control, and where transmission power control (TPC) is used to ensure that the signals in the uplink reach the network node at approximately the same power level. Some of the herein disclosed embodiments are concerned with achieving the same benefits but without the need for TPC and optionally without the need for centralized scheduling.

As an illustrative, non-limiting, example, assume that four STAs I2a-d are associated with one AP 11a, as illustrated in Fig lb. Fig lb is a schematic diagram illustrating a communications network 10b where embodiments presented herein can be applied. The communications network 10b comprises an AP na. The AP na has a coverage area 16a within which the AP na may provide network coverage by transmitting and receiving signals to/from STAs. In the schematic diagram of Fig lb four STAs 12a, 12b, 12c, I2d are located within the coverage area 16a. STAs i2a-d may thus be provided network coverage by the AP 11a by means of wireless communications links 15a, 15b, 15c, lsd being established between the AP 11a and the STAs i2a-d.

In Fig lb STA 12a and STA 12b are considered to be closer to the AP 11a than STA 12c and STA i2d. To give some numerical values to more easily describe the situation, it is within the present illustrative, non-limiting, example assumed that the distances from the AP to STA 12a, STA 12b, STA 12c, and STA I2d are 5m, 10m, 50m, and loom, respectively. Moreover, it is within the present illustrative, non-limiting, example assumed that all STAs i2a-d are using a transmit power of 20 dBm. Finally, it is within the present illustrative, non-limiting, example assumed that that the pathloss (PL) as a function of distance can be expressed as PL(d) = 40 + 30 · log_io(d), where d is the distance in meters. Fig 9 schematically illustrates pathloss in decibels (dB) as a function of distance. In Fig 9 the '*'-symbols indicate the pathloss at 5, 10, 5, and 100 m, respectively. Thus, each '*' corresponds to the distance and pathloss for one of the STAS i2a-d. As can be seen in Fig 9, if STA ii2a-d would transmit concurrently to the AP 11a, the signal from STA 12a would be received at a power of 20 dBm - 61 dB = -41 dBm, whereas the signal from STA i2d would be received at the AP at a power of 20 dBm - 100 dB = -80 dBm, which is 39 dB less than the power received from STA 12a.

In general terms, the receivers used in WLAN devices or products (such as APs na-b and STAs i2a-d) typically have an analog-to-digital converter (ADC) with 10 bits resolution. Each bit in the ADC reduces the quantization noise with about 6 dB (depending for example on the distribution of the information source to be quantized by the ADC), so as a reasonable estimate the dynamic range of a 10 bit ADC is about 60 dB. WLANs today typically use orthogonal frequency division multiplexing (OFDM), which has a peak-to- average ratio of 6-7 dB. Since the ADC should be able to handle also the peak amplitudes an extra headroom in the ADC may be needed owing to the automatic gain control not being ideal. It can therefore be assumed that the average power of the signal is about 10 dB below a full scale ADC, i.e., 50 dB above the quantization noise floor. In case of several signals, automatic gain control (AGC) will be based on the total power, which in case of largely different power levels implies that it will be based on the power of the strongest signal.

Returning to the situation that STA 12a, 12b, 12c, and I2d would all transmit concurrently or simultaneously, it is readily understood that the signal from STA 12a would determine the AGC setting. Concurrently or simultaneously here and elsewhere in the present disclosure means that the transmissions from two or more STAs I2a-d at least partly overlap in time domain and/or that the transmissions from an AP 11a and at least one of its STAs I2a-d at least partly overlap in time domain. This also includes a perfect overlap, meaning that, for example, two or more STAs I2a-d start transmitting and stop transmitting at the same time. As the signal from STA I2d is 39 dB weaker it is concluded from the calculations above that the signal power from STA I2d will only be 11 dB above the quantization noise of the ADC in the AP 11a. That is, the highest possible signal-to-noise ratio (SNR) for the signal transmitted from STA I2d to the AP 11a is limited to 11 dB. This may effectively limit the transmission from STA I2d to the AP 11a to very robust modulation and coding schemes (MCSs). Further, MIMO may not be considered for the signal transmitted from STA I2d to the AP 11a. The received signal power of STA I2d is -80 dBm, which may be reasonably good. The thermal noise floor can here be assumed to be around -94 dBm

(assuming a 20 MHz bandwidth and a noise value of 7 dB), which means that with no quantization noise the SNR would be 14 dB.

In WO 2007/126385 A2, which is concerned with Orthogonal Frequency- Division Multiple Access (OFDMA) in the UL (i.e., corresponding to transmission from STA I2a-d to AP na-b), focus is on the interference which occur at the mirror frequency. That is, it is envisioned that the main cause of degradation is due to imbalance between the in-phase and quadrature components of the signals (i.e., so-called IQ-imbalance), and primarily in the transmitter. However, in case the issue is due entirely to properties of the receiver (such as due to a limited dynamic range in the ADC in the receiver), WO 2007/126385 A2 does not provide any suggestions regarding how to handle the interference issues.

In US2011/0243073 Ai, it is assumed that the bandwidth of the UL is wide enough to consist of two or more narrow channels. Frequency Division Multiple Access (FDMA) is assumed in the UL and filters are used to distinguish the different UL signals before being the signals are further processed. Different ADCs can be used for the different narrow channels to relax the requirements on the ADC, for instance in term of sampling rate. In addition, only acknowledgement (ACK) responses from the stations to the access point are considered, which means that a fixed modulation and coding scheme is employed. In the present disclosure, OFDMA is considered in the UL. That is, the actual user data is multiplexed on different sub-carriers in an OFDM system, and the composite UL signal from all the UL users is processed in the same ADC. Further, different from FDMA, OFDMA is associated with a strict requirement on time alignment. Timing alignment is not treated in US2011/0243073 Ai.

US 20120063406 Ai relates to multi-user MIMO (MU-MIMO) in UL, i.e., where two or more STAs are simultaneously transmitting in the UL using the same frequency resources. MU-MIMO requires multiple antennas at the AP na-b. (One is enough at the STAs I2a-d.) OFDMA, which is the main focus of the herein disclosed embodiments, works fine with only one antenna at the AP. Thus, US 20120063406 Ai does not teach a solution for OFDMA as it relies on multiple antennas at the AP na-b. Further, US 20120063406 Ai is based on the AP transmitting a response message (typically a clear to send (CTS) frame), which includes criteria information, see [0010]. The criteria information may be received power, see [0019]. That is, according to US 20120063406 Ai a STA needs to determine the received power of the DL signal, compare it with the criteria information, and based on this

comparison determine whether it is allowed to access the channel.

According to the IEEE 802.11 standards, e.g. 8o2.ng, 802.1m, 8o2.nac, there is currently never more than one signal in the uplink at the same time; at least not on the same spatial channel. The medium access control (MAC) protocol does not allow for more than one station transmitting concurrently. Moreover, although TPC is enabled in these standards, it may typically not be used in practical implementations, which means that signals from some stations in the uplink are received at high power whereas signals from some other stations are received at low power. Thus, changing the MAC to also allow for several stations to transmit in the uplink at the same time implies that also changes should be made such that TPC is introduced. However, in order to take full advantage of OFDMA in the UL, such TPC commands may be introduced. The herein disclosed embodiments are based on allowing several stations to concurrently or simultaneously transmit in the UL, without the need for TPC. This may be achieved by proper scheduling of the different transmitting stations such that the power levels at the receiving access point are similar. In case of TPC, means for improved performance in the UL are disclosed by some of the herein disclosed embodiments where the desired received powers are taken into account in the UL scheduling. As some of the herein disclosed embodiments relate to OFDMA, time alignment is, in addition to suitable alignment of the received powers, also addressed.

The embodiments disclosed herein particularly relate to time scheduled multiple access in wireless local area networks. In order to obtain such scheduling there is provided an access point 11a, 11b, a method performed by the access point 11a, 11b, a station 12a, 12b, 12c, i2d, a method performed by the station 12a, 12b, 12c, I2d, at least one computer program comprising code, for example in the form of a computer program product, that when run on a processing unit, causes the processing unit to perform at least one of the method of the access point 11a, 11b and the station 12a, 12b, 12c, i2d. Fig 2a schematically illustrates, in terms of a number of functional units, the components of an access point (AP) 11a, lib according to an embodiment. A processing unit 21 is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), application specific integrated circuit (ASIC), field programmable gate arrays (FPGA) etc., capable of executing software instructions stored in a computer program product 41a (as in Fig 4), e.g. in the form of a storage medium 23. Thus the processing unit 21 is thereby configured to execute methods as disclosed herein. The a storage medium 23 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory. The AP 11a, lib may further comprise a communications interface 22 for communications with another AP 11a, 11b, the core network 13, and at least one station 12a, 12b, 12c, i2d. As such the communications interface 22 may comprise one or more

transmitters and receivers, comprising analogue and digital components such as a digital-to-analogue converter and an analogue-to-digital converter, a suitable number of antennas for radio communications, and a suitable number of ports for wired communications. The processing unit 21 controls the general operation of the AP 11a, 11b e.g. by sending data and control signals to the communications interface 22 and the storage medium 23, by receiving data and reports from the communications interface 22, and by retrieving data and instructions from the storage medium 23. Other components, as well as the related functionality, of the AP 11a, 11b are omitted in order not to obscure the concepts presented herein.

Fig 2b schematically illustrates, in terms of a number of functional modules, the components of an AP 11a, lib according to an embodiment. The AP 11a, 11b of Fig 2b comprises a number of functional modules; an acquire module 21a, a schedule module 21b. The AP 11a, lib of Fig 2b may further comprise a number of optional functional modules, such as any of a send and/or receive module 21c, a generate module 2id, and a determine module 2ie. The functionality of each functional module 2ia-e will be further disclosed below in the context of which the functional modules 2ia-e may be used. In general terms, each functional module 2ia-e may be implemented in hardware and/or in software. The processing unit 21 may thus be configured to from the storage medium 23 fetch instructions as provided by a functional module 2ia-e and to execute these instructions, thereby performing any steps as will be disclosed hereinafter.

Fig 3a schematically illustrates, in terms of a number of functional units, the components of a station (STA) 12a, 12b, 12c, I2d according to an

embodiment. A processing unit 31 is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), application specific

integrated circuit (ASIC), field programmable gate arrays (FPGA) etc., capable of executing software instructions stored in a computer program product 41b (as in Fig 4), e.g. in the form of a storage medium 33. Thus the processing unit 31 is thereby configured to execute methods as herein disclosed. The a storage medium 33 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory. The STA I2a-d may further comprise a communications interface 32 for communications with an AP 11a, lib or another STA I2a-d. As such the communications interface 32 may comprise one or more transmitters and receivers, comprising analogue and digital components, such as a digital-to- analogue converter and an analogue-to-digital converter, and a suitable number of antennas for radio communications, components for infrared communications, etc. The processing unit 31 controls the general operation of the STA I2a-d e.g. by sending data and control signals to the communications interface 32 and the storage medium 33, by receiving data and reports from the communications interface 32, and by retrieving data and instructions from the storage medium 33. Other components, as well as the related functionality, of the STA I2a-d are omitted in order not to obscure the concepts presented herein. Fig 3b schematically illustrates, in terms of a number of functional modules, the components of a STA I2a-d according to an embodiment. The STA I2a-d of Fig 3b comprises a number of functional modules; a send and/or receive module 31a, a determine module 31b, and an initiate module 31c. The STA I2a-d of Fig 3b may further comprises a number of optional functional modules, such as a read module 3id. The functionality of each functional module 3ia-d will be further disclosed below in the context of which the functional modules 3ia-d may be used. In general terms, each functional module 3ia-d may be implemented in hardware and/or in software. The processing unit 31 may thus be configured to from the storage medium 33 fetch instructions as provided by a functional module 3ia-d and to execute these instructions, thereby performing any steps as will be disclosed hereinafter.

Figs 5 and 6 are flow charts illustrating embodiments of methods for time scheduled multiple access in wireless local area networks as performed by an AP 11a, 11b. Figs 7 and 8 are flow charts illustrating embodiments of methods for time scheduled multiple access in wireless local area networks as performed by a STA i2a-d. The methods are advantageously provided as computer programs 42a, 42b. Fig 4 shows one example of a computer program product 41a, 41b comprising computer readable means 43. On this computer readable means 43, at least one computer program 42a, 42b can be stored, which computer program 42a can cause the processing unit 21 and thereto operatively coupled entities and devices, such as the communications interface 22 and the storage medium 23, to execute methods according to embodiments described herein, and which computer program 42b can cause the processing unit 31 and thereto operatively coupled entities and devices, such as the communications interface 32 and the storage medium 33, to execute methods according to embodiments described herein. The at least one computer program 42a, 42b and/or computer program product 41a, 41b may thus provide means for performing any steps as herein disclosed.

In the example of Fig 4, the computer program product 41a, 41b is illustrated as an optical disc, such as a CD (compact disc) or a DVD (digital versatile disc) or a Blu-Ray disc. The computer program product 41a, 41b could also be embodied as a memory, such as a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM), or an electrically erasable programmable read-only memory (EEPROM) and more particularly as a non-volatile storage medium of a device in an external memory such as a USB (Universal Serial Bus) memory. Thus, while each computer program 42a, 42b is here schematically shown as a track on the depicted optical disk, each computer program 42a, 42b can be stored in any way which is suitable for the computer program product 41a, 41b. Reference is now made to Fig 5 illustrating a method for time scheduled multiple access in wireless local area according to an embodiment. The method is performed by the access point (AP) na-b.

The time scheduled multiple access is based on signal strength values of signals from stations i2a-d transmitting to an access point na-b. The processing unit 21 of the AP na-b is therefore configured to, in a step S102 acquire signal strength values for stations (STAs) 12a, 12b, 12c, i2d associated with the AP na-b. The acquiring in step S102 may be implemented by executing functionality of the acquire unit 21a.

Scheduling of a particular STA I2a-d is then based on which signal strength interval the particular STA I2a-d belongs to. The processing unit 21 of the AP na-b is therefore configured to, in a step S104 schedule the STAs i2a-d such that STAs I2a-d having determined signal strength values within the same signal strength intervals are scheduled to transmit concurrently. The scheduling in step S104 may be implemented by executing functionality of the schedule unit 21b. Examples of how to schedule the STAs i2a-d in S104 will be further disclosed below. Examples of signal strength intervals, how the signal strength intervals may be determined, and how to the STAs I2a-d may be associated with signal strength intervals, will be disclosed below.

General embodiments relating to further details of time scheduled multiple access in wireless local area will now be disclosed. l8

The time scheduled multiple access may be time scheduled orthogonal frequency division multiple access (OFDMA).

The AP na-b may in step S102 acquire already determined signal strength values for the STAs I2a-d. Alternatively, the acquiring in step S102 involves the AP na-b to determine the signal strength values for the STAs i2a-d.

The STAs I2a-d may be STAs in idle mode or in connected mode within a coverage area 16a of the AP na-b.

Widths or sizes of the signal strength intervals (e.g. in dB) may be

determined based on analogue-to-digital converter (ADC) requirements of the AP. Further, partitioning of the signal strength intervals may be determined based on at least one of the number of STAs i2a-d in each signal strength interval and the current load of the STAs I2a-d.

In general terms, the signal strength intervals may be determined to meet limiting requirements of the AP na-b, for example determined by the dynamic range in the ADC of the AP na-b. For example, the difference in received power for two STAs belonging to the same signal strength interval may not exceed a given value, say in the order of 10 dB. How the intervals are partitioned, e.g. if the boundaries of the signal strength intervals are at -90 dBm, -80 dBm, -70 dBm, etc. or at -85 dBm , -75 dBm, -65 dBm, etc. may be fixed or may be selected and/or adjusted based on what particular power levels are received from the different STAs. The absolute positions of the signal strength intervals may be determined such that the STAs are divided among the different available signal strength intervals according to a predetermined criterion, for example such that each signal strength interval comprises the same number of STAs, or such that each signal strength interval has a predetermined width or size.

In general terms, the scheduling of the STAs i2a-d is achieved by the AP 11a- b transmitting messages comprising packets to the STAs I2a-d. All messages sent between the AP na-b and the STAs i2a-d may be IEEE 802.11 messages. Reference is made to Fig 6 illustrating methods for time scheduled multiple access in wireless local area according to further embodiments.

There may be different ways to perform the scheduling in step S104. Different general embodiments relating thereto will now be described in turn. The scheduling may relate to transmission grants. Particularly, the

scheduling may further comprise an optional step Si04a of sending transmission grants to the scheduled STAs I2a-d. According to an

embodiment the processing unit 21 of the AP na-b is configured to perform step Si04a. The sending in step Si04a may be implemented by executing functionality of the send/receive unit 21c.

The scheduling may relate to ACK/NACK reports. Particularly, the scheduling may further comprise an optional step Si04b of allowing scheduled STAs i2a-d to transmit at least one of an acknowledgement (ACK) report and a negative acknowledgement (NACK) report to the AP na-b. According to an embodiment the processing unit 21 of the AP na-b is configured to perform step Si04b. The allowing in step Si04b may be implemented by executing functionality of the send/receive unit 21c.

The scheduling may involve generating a list of determined signal strength values of the STAs i2a-d. Such a list may be distributed to the STAs i2a-d. Particularly, the scheduling may further comprise an optional step S104C of generating a list of STAs I2a-d having acquired signal strength values within the same signal strength intervals. The list may then, in an optional step Si04d, be sent to the STAs i2a-d associated with the AP na-b. According to an embodiment the processing unit 21 of the AP na-b is configured to perform steps S104C and Si04d. The generating in step S104C may be implemented by executing functionality of the generate unit 2id. The sending in step Si04d may be implemented by executing functionality of the send/receive unit 21c. The list may further comprise information about to which one of the signal strength intervals the STAs I2a-d belongs. How such lists may be used by the STAs i2a-d will be further disclosed below. The scheduling may involve the use of a transmission power control approach. Particularly, the scheduling may further comprise an optional step Si04e of sending transmission power control (TPC) commands for the STAs i2a-d. The sending in step Si04e may be implemented by executing functionality of the send/receive unit 2ic. According to an embodiment the processing unit 21 of the AP na-b is configured to perform step Si04e.

Individual TPC commands may be sent during the scheduling of the STAs i2a-d. Hence the TPC commands in step Si04e may be sent to at least one STA in each signal strength interval. i04ei04ei04eThe TPC command may comprise instructions for at least one STA I2a-d in each signal strength interval to reduce its transmission power. Examples of how such TPC commands may be used will be disclosed below.

The scheduling may consider the modulation and coding schemes (MSCs) used by the STAs. Particularly, the scheduling may further comprise an optional step Si04f of determining the average throughput for STAs I2a-d in at least one of the signal strength intervals for at least two different MCSs used by the STAs i2a-d. The determining in step Si04f may be implemented by executing functionality of the determine unit 2ie. The MCSs for the STAs i2a-d in the at least one of the signal strength intervals may be determined, in an optional step Si04g, such that the average throughput for the STAs in the at least one of the signal strength intervals is maximized. The determining in step Si04g may be implemented by executing functionality of the determine unit 2ie. Information about the determined MCSs may then, in an optional step S104I1, be sent to the STAs in the at least one of the signal strength intervals. The sending in step S104I1 may be implemented by executing functionality of the send/receive unit 21c. According to an embodiment the processing unit 21 of the AP na-b is configured to perform steps Si04f,

From step S104 follows that two STAs I2a-d may be scheduled concurrently and from optional step Si04e follows that one STA i2a-d may reduce its transmission power. Scheduling based on steps Si04e, Si04f, Si04g and S104I1 may thus accomplish the following. Firstly, two STAs may be scheduled concurrently, and in this way the communications network would benefit from OFDMA, but with the possible drawback that the STA being closest to the AP would have to reduce its transmission power to guarantee similar received powers of the two STAs. Secondly, two STAs may be scheduled at different times, hence no OFDMA is applied. However, in the latter case it would be possible for the STA being close to the AP to use the MCS giving the highest throughput, which in turn would mean that this STA would need less total transmission time and thus would give more total transmission time to the other STA. Packets transmitted from STAs i2a-d at different distances from the AP na-b may be received by the AP na-b at different time instances, thus resulting in different packet delivery delays. The scheduling may consider such packet delivery delays. Particularly, the scheduling may further comprise an optional step Si04j of acquiring respective packet delivery delay requirement values for the STAs i2a-d. The STAs i2a-d may then, in an optional step Si04k, be scheduled such that STAs with packet delivery delay requirement values within a packet delivery delay interval are scheduled to transmit

concurrently. According to an embodiment the processing unit 21 of the AP na-b is configured to perform steps Si04j and Si04k. The acquiring in step Si04j may be implemented by executing functionality of the acquire unit 21a. The scheduling in step Si04k may be implemented by executing functionality of the schedule unit 21b.

Timing offset values may be used in order to mitigate any impact the STAs i2a-d being located at different distances from the AP na-b. Particularly, the scheduling may further comprise an optional step S104I of determining individual timing offset values for the STAs I2a-d. The individual timing offset values may then be used by some of the STAs i2a-d to delay their transmission of data packets or messages to the AP na-b. Which STAs i2a-d that should delay their transmission generally depends on the length of the transmission paths between the STAs I2a-d and the AP 11a. In general terms, STAs I2a-d with shorter transmission paths should delay their transmission more than STAs I2a-d with longer transmission paths; STAs I2a-d with longest transmission paths may not delay their transmissions at all. This may result data packets or messages transmitted by different STAs i2a-d located at different distances from the AP na-b are received within a predetermined time interval. According to an embodiment the processing unit 21 of the AP na-b is configured to perform step S104I. The determining in step S104I may be implemented by executing functionality of the determine unit 2ie.

The timing offset values may be related to timing advance (TA) commands. Particularly, the scheduling may further comprise an optional step Si04m of sending a TA command to STAs having a timing offset value outside a time interval so as to adjust for the timing offset. According to an embodiment the processing unit 21 of the AP na-b is configured to perform step Si04m. The sending in step Si04m may be implemented by executing functionality of the send/receive unit 21c. Examples of how such TA commands may be used will be disclosed below. Alternatively, the scheduling may further comprise an optional step Si04n of scheduling the STAs I2a-d such that STAs with timing offset values within a time interval are scheduled to transmit concurrently. According to an embodiment the processing unit 21 of the AP na-b is configured to perform step Si04n. The scheduling in step Si04n may be implemented by executing functionality of the schedule unit 21b Examples of situations where TA commands are not available will be disclosed below.

Reference is now made to Fig 7 illustrating a method for time scheduled multiple access in wireless local area according to an embodiment. The method is performed by a station (STA) I2a-d.

The processing unit 31 of the STA I2a-d is configured to, in a step S202, receive a list of STAs I2a-d from an access point (AP) na-b. The receiving in step S202 may be implemented by executing functionality of the send/receive unit 31a. The list comprises information about to which one of at least two signal strength intervals the STAs i2a-d belong. The list may have been generated by the AP as in step S104C. The STA i2a-d may, for some reason, want to access the communications channel of the wireless local area network. The processing unit 31 of the STA i2a-d is therefore configured to, in a step S204, determine a need to access a bandwidth portion of a communications channel of the wireless local area network. The determining in step S204 may be implemented by executing functionality of the determine unit 31b. The reason for accessing the bandwidth portion may be to transmit end-user data, to transmit control data, or to send an ACK or a NACK report to the AP na-b.

The STA I2a-d then checks the channel in order to determine whether or not it is possible for the STA I2a-d to access the bandwidth portion. Particularly, the processing unit 31 of the STA I2a-d is configured to in response thereto, in a step S206, determine whether or not another STA is transmitting on the channel. The determining in step S206 may be implemented by executing functionality of the determine unit 31b. Depending on which (if any) other STAs I2a-d are currently accessing the communications channel the STA may be allowed to access the

communications channel or be prevented from accessing the

communications channel. Particularly, the processing unit 31 of the STA 12a- d is configured to, in a step S208, initiate transmission on the

communications channel if the bandwidth portion is not occupied, and if no STA belonging to at least one signal strength interval outside that of the STA is transmitting. The initiating in step S208 may be implemented by executing functionality of the initiate unit 31c.

Reference is now made to Fig 8 illustrating methods for time scheduled multiple access in wireless local area according to further embodiments.

There may be many ways to determine whether or not another STA is transmitting on the channel. For example, the method may comprise an optional step S2o6a of reading the source addresses of packets transmitted by other STAs so as to determine whether or not other STAs belong to a signal strength interval outside that of the STA are (currently) transmitting. According to an embodiment the processing unit 31 of the STA I2a-d is configured to perform the optional step S2o6a. The reading in step S2o6a may be implemented by executing functionality of the read unit 3id.

Some particular embodiments based on the above disclosed general embodiments will now be described in detail. However, although each particular embodiment is described in isolation, features from at least two different particular embodiments may be combined.

To ease the description of the particular embodiments, but without limiting the general scope of the herein disclosed general embodiments, the particular embodiments are described by means of specific examples.

According to a first particular embodiment, channel access is accomplished in a centralized fashion such that the AP 11a has complete control of exactly when different STAs I2a-d will (be allowed to) transmit. The AP 11a is configured to, based on the received signal strength of the associated STAs I2a-d, determine which STAs I2a-d can transmit concurrently. The AP 11a is configured to use this determination when scheduling the different STAs 12a- d. One way to implement this is to perform step S102 and step S104.

As an illustrative, non-limiting, example, if there are four STAs I2a-d associated with AP 11a, as illustrated in Fig lb, the AP 11a may for instance schedule STA 12a and STA 12b to transmit concurrently and STA 12c and STA I2d to transmit concurrently. The difference in pathloss (PL) for STA 12a and STA 12b is less than 10 dB. Also for STA 12c and STA I2d the difference in PL is less than 10 dB, so scheduling the four STAs I2a-d in such pairs (i.e., where STA 12a and STA 12b form one pair and where STA 12c and STA I2d form another pair) ensures that the UL signal is received with a power difference that on average is less than 10 dB. According to the first particular

embodiment, the received signal strength is assumed to directly be a consequence of the experienced pathloss. This could be the case when no TPC is used, and is therefore particularly relevant for WLAN based

communications systems. However, as will be disclosed in some of the particular embodiments below, in case TPC is available this may also be taken into account during the scheduling of the STAs I2a-d such that it is received power rather than the pathloss that is considered as the metric for grouping the different STAs i2a-d to be scheduled. In general terms, the first particular embodiment covers at least two scenarios. A first scenario relates to when the AP 11a schedules the suitable STAs I2a-d by sending transmission grants, allowing the STAs I2a-d to send UL data. One way to implement this is to perform step Si04a. A first scenario relates to when the AP 11a schedules downlink (DL) data to suitable STAs I2a-d, allowing the STAs i2a-to concurrently send ACK/NACK reports to these respective data packets. The scenario where the AP sends transmission grants is somewhat related to WO2007/126385 A2. One difference is that the first particular embodiment is not concerned with the power density but by the power itself. Also, as at least some of the embodiments disclosed herein are related to the dynamic range of the ADC, particular issues with the mirror (image) frequency or adjacent frequencies do not occur and are therefore not treated explicitly. The scenario where the AP 11a schedules data to particular users in the DL is not at all mentioned in WO2007/126385 A2.

One situation may assume that the ACK/NACK report from the STA is not scheduled explicitly, but instead that the ACK/NACK report is sent a predetermined time after the data packet to which the ACK/NACK report is a response. One way to implement this is to perform step Si04b.

In general terms, in the situation where the AP 11a sends transmission grants for the UL transmission this implies that some signalling may be needed from the AP 11a to the STAs I2a-d. However, there is no need for the different STAs i2a-d to have knowledge of what other STAs i2a-d are scheduled, and how much bandwidth is allocated for other STAs i2a-d.

In general terms, in the situation where the scheduling is for the DL, and where the scheduling of particular STAs I2a-d are primarily performed in order to ensure that the ACK/NACK reports will be received at similar powers, there may be no need for dedicated signalling.

A second particular embodiment is applicable to scenarios where the channel access is distributed. Such scenarios are not covered by WO2007/126385 A2. According to the second particular embodiment the different STAs I2a-d associated with one AP 11a are made aware of which of the other STAs i2a-d it is allowed to concurrently transmit with. In more detail, according to the second particular embodiment each particular STA i2a-d has a list of STAs that the particular STA is not allowed to concurrently transmit with. One way to implement this is to perform step S104C, step Si04d, and step S202.When determining if a STA i2a-d may access the channel, the STA i2a-d therefore determines if another STA I2a-d is transmitting, and if so, what STA(s) is/are transmitting and whether the transmitting STA(s) is part of the list or not. According to the second particular embodiment the STA i2a-d also determines what frequencies are used for the transmission. In case it is found that at least part of the bandwidth is not occupied and that no STA belonging to the list is transmitting, the particular STA may initiate a transmission. One way to implement this is to perform step S204, step S206, and step S208.

Returning to the example with four STAs I2a-d as illustrated in Fig 1 and assuming that STA 12a may want to access the communications channel. It is for illustrative, non-limiting, purposes assumed that the bandwidth of the communications channel is 80 MHz, and that a STA I2a-d may use 20, 40, or 80 MHz of the communications channel when accessing the communications channel. Assume that STA 12a senses the communications channel to determine if at least part of the total bandwidth of 80 MHz is not used. In addition, if at least part of the bandwidth is used, it is assumed that STA 12a determines which other STA(s) is/are transmitting on the communications channel. If STA 12a determines that at least one of STA 12c, STA I2d, or the AP 11a is transmitting, STA 12a defers from transmitting. However, if STA 12a determines that the communications channel is idle or that the only STA transmitting is STA 12b, then STA 12a may transmit on the part of the bandwidth of the communications channel that is not used. For instance, if no transmission is detected, the full 80 MHz bandwidth may be used by STA 12a. However, if, say, a bandwidth of 40 MHz of the communications channel is found to be occupied by STA 12b, then STA 12a may use the 40 MHz of bandwidth not used by STA 12b. STA 12a may use all of the unused 40 MHz of bandwidth or it may use 20 MHz of the unused 40 MHz of bandwidth.

Fig 10 illustrates received power in dB as a function of frequency in MHz. Particularly, Fig 10 schematically illustrates an example where the

communications channel has a (double sided) bandwidth 103 of 20 MHz and where each STA i2a-d may access one or several bandwidth portions 102 of 5 MHz. In the illustrative example of Fig 10, a first portion of the bandwidth 101a used by STA 12a corresponds to a first received power level and a second portion 101b of the bandwidth used by STA 12b corresponds to a second received power level. According to the illustrative example of Fig 10, STA 12b may access the communications channel only if STA 12b belongs to the same group as STA 12a. Two or more STAs I2a-d belong to the same group if their signal strength values are within the same signal strength interval.

In the second particular embodiment, the list, which may be unique for each STA I2a-d, is determined by the AP 11a and communicated from the AP 11a to the different STAs I2a-d. How often this list is communicated may vary. The list may remain the same for the duration of a session or it may be updated due to that one or more of the STAs i2a-d have moved. In order for a particular STA to determine which other STAs are transmitting, the particular STA may read the source address available in packets transmitted on the communications channel. If the source address matches one of the STAs in the list of the particular STA, no transmission is allowed from the particular STA. One way to implement this is to perform step S2o6a.

In the first and second particular embodiments described above, the scheduling has been made based on that no TPC is used. However, also in case TPC is used, it may be beneficial to take the PL and/or received signal strength into consideration. Assume, for example and without limitation, that STA 12a and STA I2d would be scheduled for concurrent transmission. Moreover, assume further, for example, and without limitation, that the received power from STA i2d suggests that a robust modulation and coding scheme (MCS) should be used, i.e., a MCS that can be used at worse channel conditions still allowing for successful reception. To ensure that the signals from STA 12a and STA I2d are received at the AP 11a at approximately the same power levels, STA 12a would, according to the present non-limiting example of Fig. 10, need to reduce its output power. Referring to Fig 9, STA 12a would have to reduce the power level by 39 dB for the powers from STA 12a and STA i2d to be the same. Although this is feasible, it means that STA 1 would no longer be able to transmit at the highest data rate (the MCS resulting in the highest data rate), but would have to reduce the data transmission rate to the degree its transmit power is reduced.

In a third particular embodiment, TPC is also taken into consideration in the scheduling of the different STAs. Specifically, the different STAs i2a-d are scheduled such that as high transmit power as possible can be used for all

STAs I2a-d scheduled to transmit concurrently, and still resulting in the same or similar received powers at the AP 11a. One way to implement this is to perform step Si04e.

Assume, for example, and without limitation, that STA 12a and STA 12b are scheduled together as the PL and/or received signal strength for these two STAs 12a, 12b are relatively similar. In order to obtain similar received powers, the transmit power for STA 12a is reduced, according to the present non-limiting example, by 9 dB, whereas STA 12b is allowed to transmit at maximum power. In this way the signal quality from STA 12b is improved as the dynamic range of the ADC at the AP 11a is better utilized for STA 12b than would have been the case if the signal from STA 12a would have been 9 dB stronger. That is, the dynamic range of the ADC may be adapted to fit the particular signal strength interval used, instead of having to cover the complete possible signal strength interval if any STA i2a-d is allowed to transmit. A fourth particular embodiment relates to scheduling of STAs I2a-d further based on the trade-off between being able to support several STAs 12a transmitting simultaneously in the UL and being able to apply the most suitable MCS. One way to implement this is to perform step Si04f, step

To illustrate the fourth particular embodiment in a non-limiting manner, assume that only STA 12a and STA I2d have data to send. Two possibilities exist. Firstly, STA 12a and STA I2d may transmit sequentially, whereby STA 12a can transmit at higher power than otherwise would have been the case, and where STA 12a therefore may use an MCS that allows for a higher data rate. Secondly, STA 12a and STA 12b may be scheduled to transmit concurrently, but then STA 12a would need to reduce its transmission power and therefore also reduce its data transmit rate.

To further illustrate the fourth particular embodiment, assume that if STA 12a is scheduled to transmit on its own, STA 12a can transmit at a rate of 20 Mb/s. Associated with this kind of transmission is assumed an overhead of 100 % if assuming that every packet includes a fixed, trailing preamble that is sent using the most robust MCS. This corresponds to an effective data rate of 10 Mb/s. To further illustrate the fourth particular embodiment, assume that if STA I2d is scheduled to transmit on its own, STA I2d can transmit at a rate of 2 Mb/s. Associated with this kind of transmission is an overhead (transmission that does not contain user data) of 20%. The relative overhead is smaller since the fixed duration of the preamble is less compared to the total packet sent at a more robust MCS resulting in lower data rate. This corresponds to an effective data rate of 5/3 Mb/s.

To further illustrate the fourth particular embodiment, assume that if STA 12a and STA I2d are scheduled to transmit concurrently, STA 12a and STA 12b will be allocated half the bandwidth each and can each transmit at a rate of 1 Mb/s. Associated with this kind of transmission is an overhead of 20%. The relative overhead is smaller as the duration of the actual data is less because of the lower data rate.

The AP 11a may be configured to determine the best approach for scheduling STA 12a and STA i2d, for example, based on the average throughput, For the current non-limiting example, assume that each one of STA 12a and STA I2d, has 100 Mb (12.5 MB) of data to send. If the AP 11a first schedules STA 12a and then STA I2d (i.e., STA 12a and STA I2d are scheduled to transmit sequentially), the total transmission duration will be 10 s + 60 s = 70 s. If instead STA 12a and STA I2d are scheduled to transmit concurrently, the total time will be 120 s. Thus, for the present non-limiting example it is preferable to schedule the STAs sequentially.

In a fifth particular embodiment, the packet delivery delay is taken into account during the scheduling of the STAs I2a-d. One way to implement this is to perform step Si04j and step Si04k.This could be regarded as an additional optimization target besides, for example, maximizing the average cell throughput, as considered in the fourth particular embodiment.

Alternatively, packet delivery delay can form the only metric to be considered during the scheduling. If there is a strict requirement on the packet delivery delay, it may, according to the present example, be preferable to transmit to STA 12a and STA I2d concurrently rather than to switch the transmission resources between the two.

In all of the above particular embodiments, it has been assumed that the OFDMA signals arrive in a time aligned fashion. In communications networks based on OFDMA it is important the time alignment are such that the different sub-carriers are orthogonal after the cyclic prefix has been removed. This is as such a well-known property of OFDMA based

communications networks and therefore not discussed further herein. To achieve this alignment, timing advance (TA) may be used. This allows the AP 11a to inform the different STAs I2a-d if they need to adjust their

transmission time somewhat. As an example, when a STA I2a-d is moving farther away from the AP 11a it may have to transmit its data somewhat earlier in order for the signal not to arrive too late at the AP 11a.

In two additional particular embodiments, the above particular embodiments are therefore complemented with information about the received timing. In a sixth particular embodiment, where there is no TA command available (implying that there is no possibility for the AP 11a to time align reception of packets from the different STAs i2a-d), the scheduling takes both received power and received timing into account. One way to implement this is to perform step S104I and Si04n. Since there may be a relation between distance between the AP 11a and the STA i2a-d, and received power at the AP 11a, respectively, it may in some cases be so that by ensuring that the received powers are similar, the time delay of the different signals may also be similar. However, it could also be so that a STA i2a-d that is very close to the AP 11a has a severely obstructed path, in which case the pathloss may be high although the path length (and by that the delay) is small. In the sixth particular embodiment, STAs i2a-d are scheduled together only if both received power and delay for the signals received at the AP from the STAs are sufficiently similar. Here sufficiently similar may for instance mean that the received powers differ at most 10 dB at the AP 11a and that the delay is within 200 ns.

Fig 11a schematically illustrates a communications system 10c comprising one AP 11a. Two STAs 12a and 12b are located at respective distances di and d2 from AP 11a. Fig 11b schematically illustrates reception of packets 110a, 110b as sent from STA 12a, and STA 12b, respectively, at the AP 11a. Each packet 110a, 110b has a cyclic prefix portion 112 and a payload portion 114. The cyclic prefix portion 112 corresponds to a time duration of ti, which for example may be 800 ns. The payload portion 114 corresponds to a time duration of t2, which for example may be 3.2 μβ. Since according to the illustrated example d2 > di, the packet 110a transmitted from STA 12a will by the AP 11a be received earlier than the packet 110b transmitted from STA 12b. This time difference is in Fig 11b denoted t3 and may be determined as (d2- di)/c, where c is the speed of light. Thus, for di = 25 m and d2 = 100 m, t3 = 250 ns.

In a seventh particular embodiment, it is assumed that TA commands are available. In the seventh particular embodiment, the different STAs i2a-d are first grouped based on received power at the AP and then TA commands are used by the AP 11a to ensure also that the STAs i2a-d in the respective groups are time-aligned. One way to implement this is to perform step S104I and Si04m.

Fig 11c schematically illustrates reception of data packets 110a, 110b as sent from STA 12a, and STA 12b, respectively, at the AP 11a, and where a TA command corresponding to t3 (i.e., 250 ns according to the present example) has been issued so as to align the reception of packets 110a and 110b, respectively.

The inventive concept has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended patent claims.

For example, some embodiments have been described for being used for IEEE 802.11 systems, and therefore the notation Access Point (AP) and

Station (STA) has bee used to denote the network node (base station) and the wireless devices or mobile stations (user equipment; UE), respectively.

However, as is obvious for the person skilled in the art, the herein disclosed embodiments are not limited to this standard, but may be applied also to other standards, mutatis mutandis.

For example, some embodiments have been described in scenarios where the physical layer (PHY) is based on orthogonal frequency division multiplexing (OFDM). The principles of the herein disclosed embodiments are applicable also to other PHYs, for instance a PHY based on direct sequence spread spectrum (DSSS). For example, some embodiments are concerned with improving the receiver conditions in the UL, i.e., at the AP. In general terms, the scheduling, coordination etc. is performed by the AP. Signalling needed for ensuring that the STAs do transmit at suitable time is transmitted from the AP to the STAs. The signalling may be provided in each packet, e.g. the MAC header of the packet, but it may also be transmitted less frequent, e.g. by the AP sending dedicated control packets, either to STAs individually or using e.g. multi-cast. The limited dynamic range in the ADC refers to the ADC in the AP.

Claims

CLAIMS l. A method for time scheduled multiple access in a wireless local area network (10a, 10b), the method being performed by an access point, AP, (na, lib) and comprising the steps of:

acquiring (S102) signal strength values for stations, STAs, (12a, 12b, 12c, i2d) associated with the AP; and

scheduling (S104) the STAs such that STAs having determined signal strength values within same signal strength intervals are scheduled to transmit concurrently.

2. The method according to claim 1, wherein the STAs are STAs in idle mode or in connected mode and being within a coverage area (16a) of the AP.

3. The method according to any one of the preceding claims, wherein size or widths of the signal strength intervals are determined based on analogue- to-digital converter requirements of the AP.

4. The method according to any one of the preceding claims, wherein partitioning of the signal strength intervals is determined based on at least one of number of STAs in each signal strength interval and current load of the STAs.

5. The method according to any one of the preceding claims, wherein said scheduling further comprises:

sending (Si04a) transmission grants to the scheduled STAs.

6. The method according to any one of the preceding claims, wherein said scheduling further comprises:

allowing (Si04b) scheduled STAs to transmit at least one of an acknowledgement, ACK, report and a negative acknowledgement, NACK, report to the AP.

7. The method according to any one of the preceding claims, wherein said scheduling further comprises:

generating (S104C) a list of STAs having acquired signal strength values within said same signal strength intervals; and

sending (Si04d) the list to the STAs associated with the AP.

8. The method according to claim 7, wherein the list further comprises information about to which one of said signal strength intervals said STAs belongs.

9. The method according to any one of the preceding claims, wherein said scheduling further comprises:

sending (Si04e) a transmission power control, TPC, command to at least one STA in each signal strength interval.

10. The method according to claim 10, wherein the TPC command comprises instructions for at least one STA in said each signal strength interval to reduce its transmission power.

11. The method according to any one of the preceding claims, wherein said scheduling further comprises:

determining (Si04f) average throughput for STAs in at least one of the signal strength intervals for at least two different modulation and coding schemes, MCSs;

determining (Si04g) MCSs for the STAs in said at least one of the signal strength intervals that maximizes the average throughput for said STAs in said at least one of the signal strength intervals; and

sending (S104I1) information about the determined MCSs to the STAs in said at least one of the signal strength intervals.

12. The method according to any one of the preceding claims, wherein said scheduling further comprises:

acquiring (Si04j) respective packet delivery delay requirement values for the STAs; and

scheduling (Si04k) the STAs such that STAs with packet delivery delay requirement values within a packet delivery delay interval are scheduled to transmit concurrently.

13. The method according to any one of the preceding claims, wherein said scheduling further comprises:

determining (S104I) individual timing offset values for the STAs to be used by the STAs to delay their transmission of messages to the AP.

14. The method according to claim 13, wherein said scheduling further comprises:

sending (Si04m) a timing advance, TA, command to STAs having a timing offset value outside a time interval to adjust for the timing offset.

15. The method according to claim 13, wherein said scheduling further comprises:

scheduling (Si04n) the STAs such that STAs with timing offset values within a time interval are scheduled to transmit concurrently.

16. The method according to any one of the preceding claims, wherein all messages sent between the AP and the STA are IEEE 802.11 messages.

17. A method for time scheduled multiple access in a wireless local area network (10a, 10b), the method being performed by a station, STA, (12a, 12b, 12c, I2d) and comprising the steps of:

receiving (S202) a list of STAs from an access point, AP, (11a, lib) the list comprising information about to which one of at least two signal strength intervals said STAs belong;

determining (S204) a need to access a bandwidth portion of a communications channel of the wireless local area network; and in response thereto:

determining (S206) whether or not another STA is transmitting on the channel; and:

initiating (S208) transmission on the communications channel if the bandwidth portion is not occupied, and if no STA belonging to at least one signal strength interval outside that of the STA is transmitting.

18. The method according to claim 18, wherein determining whether or not another STA is transmitting on the channel further comprises: reading (S2o6a) source addresses of packets transmitted by other STAs so as to determine whether or not said other STAs belong to a signal strength interval outside that of the STA are transmitting.

19. An access point, AP, (11a, 11b) for time scheduled multiple access in a wireless local area network (10a, 10b), the AP comprising a processing unit

(21) configured to:

acquire signal strength values for stations, STAs, (12a, 12b, 12c, I2d) associated with the AP; and

schedule the STAs such that STAs having determined signal strength values within same signal strength intervals are scheduled to transmit concurrently.

20. A station, STA, (12a, 12b, 12c, i2d) for time scheduled multiple access in a wireless local area network (10a, 10b), the STA comprising a processing unit (31) configured to:

receive a list of STAs from an access point, AP, (11a, 11b) the list comprising information about to which one of at least two signal strength intervals said STAs belong;

determine a need to access a bandwidth portion of a communications channel of the wireless local area network; and in response thereto:

determine whether or not another STA is transmitting on the channel; and:

initiate transmission on the communications channel if the bandwidth portion is not occupied, and if no STA belonging to at least one signal strength interval outside that of the STA is transmitting.

21. A computer program (42a) for time scheduled multiple access in a wireless local area network (10a, 10b), the computer program comprising computer code which, when run on a processing unit (21), causes the processing unit to:

acquire (S102) signal strength values for stations, STAs, (12a, 12b, 12c, I2d) associated with the AP; and

schedule (S104) the STAs such that STAs having determined signal strength values within same signal strength intervals are scheduled to transmit concurrently.

22. A computer program (42b) for time scheduled multiple access in a wireless local area network (10a, 10b), the computer program comprising computer code which, when run on a processing unit (31), causes the processing unit to:

receive (S202) a list of STAs from an access point, AP, (11a, 11b) the list comprising information about to which one of at least two signal strength intervals said STAs belong;

determine (S204) a need to access a bandwidth portion of a

communications channel of the wireless local area network; and in response thereto:

determine (S206) whether or not another STA is transmitting on the channel; and:

initiate (S208) transmission on the communications channel if the bandwidth portion is not occupied, and if no STA belonging to at least one signal strength interval outside that of the STA is transmitting.

23. A computer program product (41a, 41b) comprising a computer program (42a, 42b) according to at least one of claims 21 and 22 and a computer readable means (43) on which the computer program is stored.