This application claims priority from U.S. provisional application No.61/025,645 entitled "METHOD AND APPARATUS FOR SERVER SELECTION IN A COMMUNICATION NETWORK", filed on 2/1.2008, which is assigned to the assignee of the present application and is hereby expressly incorporated herein by reference.
Detailed Description
The techniques described herein may be used for various wireless communication networks such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA and other networks. The terms "network" and "system" are often used interchangeably. A CDMA network may implement a radio technology such as Universal Terrestrial Radio Access (UTRA), CDMA2000, etc. UTRA includes wideband CDMA (wcdma) and other variants of CDMA. cdma2000 covers IS-2000, IS-95 and IS-856 standards. TDMA networks may implement radio technologies such as global system for mobile communications (GSM). OFDMA networks may implement methods such as evolved UTRA (E-UTRA), Ultra Mobile Broadband (UMB), IEEE 802.11(Wi-Fi), IEEE 802.16(WiMAX), IEEE 802.20, and,Etc. radio technologies. UTRA and E-UTRA are part of the Universal Mobile Telecommunications System (UMTS). 3GPP Long Term Evolution (LTE) is a future release of UMTS that uses E-UTRA, which uses OFDMA on the downlink and SC-FDMA on the uplink. UTRA, E-UTRA, UMTS, LTE, and GSM are described in documents of the organization entitled "third Generation partnership project" (3 GPP). Cdma2000 and UMB are described in documents of the organization entitled "third generation partnership project 2" (3GPP 2).
Fig. 1 illustrates a wireless communication network 100, which may include a plurality of base stations and other network entities. For simplicity, fig. 1 shows only two base stations 120 and 122 and one network controller 150. A base station may be a fixed station that communicates with the terminals and may also be referred to as an access point, a node B, an evolved node B (enb), etc. A base station may provide communication coverage for a particular geographic area. The overall coverage area of a base station may be partitioned into smaller areas, and each smaller area may be served by a respective base station subsystem. The term "cell" can refer to a coverage area of a base station and/or a base station subsystem serving the coverage area, depending on the context in which it is used.
A base station may provide communication coverage for a macrocell, picocell, femtocell, or some other type of cell. A macro cell may cover a relatively large geographic area (e.g., tens of kilometers in radius) and may support communication for all terminals with service subscriptions in a wireless network. A pico cell may cover a relatively small geographic area and may support communication for all terminals with service subscriptions. A femto cell may cover a relatively small geographic area (e.g., a home) and may support communication for a set of terminals associated with the femto cell (e.g., terminals belonging to a home household). The base station used for the macro cell may be referred to as a macro base station. The base station for the pico cell may be referred to as a pico base station. A base station for a femto cell may be referred to as a femto base station or a home base station.
Network controller 150 may be coupled to a set of base stations and provide coordination and control for these base stations. Network controller 150 may communicate with base stations 120 and 122 via a backhaul. Base stations 120 and 122 may also communicate with each other, e.g., directly or indirectly via a wireless or wired interface.
Terminal 110 may be one of many terminals supported by wireless network 100. Terminal 110 may be stationary or mobile and may also be referred to as an Access Terminal (AT), a Mobile Station (MS), a User Equipment (UE), a subscriber unit, a station, etc. Terminal 110 may be a cellular telephone, a Personal Digital Assistant (PDA), a wireless modem, a wireless communication device, a handheld device, a laptop computer, a cordless telephone, a Wireless Local Loop (WLL) station, and so forth. Terminal 110 may communicate with a base station via the downlink and uplink. The downlink (or forward link) is the communication link from the base stations to the terminals, and the uplink (or reverse link) is the communication link from the terminals to the base stations.
Wireless network 100 may support HARQ to improve the reliability of data transmission. For HARQ, the transmitter may send a data transmission and, if necessary, one or more additional transmissions until the data is decoded correctly by the receiver, or a maximum number of transmissions has been sent, or some other termination condition is met.
Fig. 2 shows an exemplary data transmission on the downlink with HARQ. The transmission timeline may be partitioned into framing units. Each frame may cover a predetermined duration, e.g., 1 millisecond (ms). A frame may also be referred to as a subframe, slot, etc.
Base station 120 may have data to send to terminal 110. Base station 120 may process the data packets and send transmissions of the packets on the downlink. Terminal 110 may receive the downlink transmission and decode the received transmission. Terminal 110 may send an Acknowledgement (ACK) if the packet is decoded correctly; the terminal 110 may send a Negative Acknowledgement (NAK) if the packet is decoded in error. The base station 120 may receive the ACK/NAK feedback, send another transmission of the packet if a NAK is received, send a transmission of a new packet or terminate if an ACK is received. The transmission of packets and ACK/NAK feedback may continue in a similar manner.
M HARQ interlaces (interlaces) with indices of 0 to M-1 may be defined for each of the downlink and uplink, where M may be equal to 4, 6, 8, or some other value. Each HARQ interlace may include frames spatially separated by M frames. A packet may be sent on one HARQ interlace and all transmissions of the packet may be sent in different frames of the same HARQ interlace. Each transmission of a packet may be referred to as a HARQ transmission.
Wireless network 100 may be a heterogeneous network with different types of base stations (e.g., macro base stations, pico base stations, home base stations, etc.). These different types of base stations may transmit at different power levels, have different coverage areas, and have different effects on interference in the wireless network. Wireless network 100 may also support relay stations. A relay station is a station that receives a data transmission for a terminal from an upstream station and transmits the data transmission to a downstream station.
Terminal 110 may be within the coverage of multiple base stations. One of the plurality of base stations may be selected to serve terminal 110. The selection of the serving base station may be referred to as serving station selection. The base station with the best received signal quality may be selected as the serving base station. Received signal quality may be quantified by signal-to-noise-and-interference ratio (SINR), signal-to-noise ratio (SNR), carrier-to-interference ratio (C/I), and so on. In much of the description below, SINR and C/I are used to represent received signal quality. Selecting the base station with the best downlink SINR as the serving base station may have the following disadvantages:
● are not effective enough when a macro, pico, and/or home base station is present in a mixture;
● if the selected base station is a home base station with restricted association and the terminal 110 is not a member of the restricted set; and
● are not effective enough with relay stations.
In an aspect, a serving base station may be selected based on one or more metrics. In general, a metric may be defined in terms of one or more parameters, where the parameters may be measured or specified. Some of the metrics may be used as constraints, while others may be used as optimization variables. Constraints may be used to determine whether a given candidate base station may be selected as the serving base station. The constraint may be defined by requiring the metric to be above or below a predetermined threshold. The threshold may be set according to the base station capabilities or the threshold may be related to the minimum or maximum value in a group of base stations. The optimization variables may be used to determine the most suitable base station for selection. For example, the candidate base station with the best metric may be selected, where "best" may depend on how the metric is defined, or "best" may be the highest or lowest value. The selected candidate base station may have a lower SINR than another candidate base station. This serving base station selection scheme may provide advantages, such as reduced interference in the network.
The serving base station may also be selected based on one or more conditions. Conditions may be used to ensure that a suitable base station is selected. For example, the home base station is selected only when a condition that the terminal 110 can access the home base station is satisfied. As another example, a base station may be selected only if it can provide minimum quality of service (QoS) guarantees for QoS traffic of terminal 110.
In one design, the following metrics may be used to select the serving base station:
● emission energy metric-indicating emission energy;
● path loss-indicating the channel gain between the base station and the terminal;
● effective geometry-indicating received signal quality;
● predicted data rate-indicating the data rate that the terminal can support; and
● control channel reliability — indicating the reliability of the control channel.
Each metric is described in detail below. Other metrics may also be used for serving station selection.
Any combination of these metrics given above may be used to select the serving base station for the downlink and/or uplink. In one design, a single base station may be selected to serve terminal 110 on both the downlink and uplink. In this design, if the best base station for the downlink is different from the best base station for the uplink, a serving base station may be selected that is not far away from the best base stations for the downlink and uplink. In another design, one base station may be selected on the downlink to serve terminal 110 and another base station may be selected on the uplink to serve terminal 110. In this design, the serving base station for each link may be selected based on any metric.
For an Additive White Gaussian Noise (AWGN) channel and a 1 x 1 antenna configuration with a single transmit antenna and a single receive antenna, the transmit energy metric may be determined as follows. The energy at the output of the transmit antenna and the energy at the output of the receive antenna may be expressed as:
equation (1)
Where h is the channel gain from the transmit antenna output to the receive antenna output,
Eb，txis the energy per bit at the output of the transmit antenna,
Eb，rxis the energy per bit at the output of the receiving antenna,
Es，rxis the energy per symbol at the output of the receiving antenna,
r is the spectral efficiency in bits/second/hertz (bps/Hz),
c is the received signal power, an
I is the received interference power.
Equation (1) shows the transmit energy metric for the AWGN channel and the 1 x 1 antenna configuration. A transmit energy metric may also be determined for fading channels and different antenna configurations.
For the downlink, the transmit antenna outputs are at the base station and the receive antenna outputs are at the terminal 110. For the uplink, the transmit antenna outputs are at the terminal 110 and the receive antenna outputs are at the base station. C is the received power of the desired signal. I is the received power of the interference and thermal noise with respect to the desired signal. C and I may be the total received power PrxWherein P isrxCan be according to PrxGiven as C + I.
For linear regions, approximate log may be used2(1+ x) ≈ x/ln 2. Equation (1) can be expressed as:
equation (2)
Wherein E iss，rxS is the symbol rate and p 1/h is the path loss.
The emission energy metric E is shown in equation (2)b，txProportional to the interference I and the path loss p and inversely proportional to the channel gain h and the symbol rate S. Equation (2) may be used to calculate the transmit power metric E for the downlinkb，tx，DLAnd a transmit power metric E of the uplinkb，tx，UL. The path loss of the downlink may be estimated from the pilot transmitted by the base station. It can be assumed that the path loss of the uplink is equal to the path loss of the downlink. The interference of the uplink may be different from the interference of the downlink. The interference of the downlink may be measured by terminal 110 and used to calculate Eb，tx，DL. Interference on the uplink at each candidate base station may be used to calculate Eb，tx，UL. Each base station may broadcast the interference observed by that base station, which may be used to calculate Eb，tx，UL. For both downlink and uplink, the interference may be related to the base station that is calculating the transmit energy metric. Furthermore, the interference may be different for different HARQ interlaces. In this case, the transmit power metric may be estimated for each active HARQ interlace in which the candidate base station may schedule a data transmission for terminal 110.
In the example shown in fig. 1, one of the base stations 120 or 122 may be selected as the serving base station for the terminal 110. Base stations 120 and 122 are downThe uplink may interfere with each other. Eb，tx，DLThe following can be calculated:
● if interference mitigation is to be performed for the downlink between base stations 120 and 122, then E is calculated for base station 120 or 122b，tx，DLThe interference I used will be the sum of the ambient noise and the interference from other base stations. This condition often results in the selection of the base station with the lowest path loss.
● if interference mitigation is not performed for the downlink between base stations 120 and 122, then E is calculated for base station 120b，tx，DLThe interference I used will be the sum of the ambient noise and interference from base station 122 as well as other base stations. Similarly, E is calculated for base station 122b，tx，DLThe interference I used will include interference from the base station 120.
E may also be calculated by considering whether interference mitigation was performed on the uplink or notb，tx，UL。
In one design, the lowest E may be selectedb，tx，DLTo reduce interference on the downlink. May be selected to have the lowest Eb，tx，ULTo reduce interference on the uplink. As shown in equation (2), Eb，txProportional to the path loss. The base station with the lowest path loss may be selected to reduce interference and increase network capacity. A base station may be selected even though its downlink SINR may be weak, e.g., subject to no limited thermal noise on the downlink. Using Eb，txThe selection of a lower power base station with low path loss may be facilitated (instead of SINR or C/I), which may be more efficient when serving terminal 110.
The effective geometry can be determined as follows. The nominal geometry of the base station can be expressed as:
equation (3)
Wherein, Cavg，kIs the average received signal power of base station k;
Iavg，kis the average received interference power of base station k; and
Gnom，kis the nominal geometry of base station k.
The effective geometry of the downlink can be expressed as:
equation (4)
Wherein, Im，kIs the received interference power of base station k on HARQ interlace m.
FkIs a typical part of the resources allocated by base station k, and
GDL，eff，kis the effective downlink geometry for base station k.
FkIs the portion of the resources that may be allocated by base station k to a typical terminal. FkMay have a value between zero and one (or 0 ≦ F)k≦ 1) and may be broadcast by base station k or known by terminal 110. For example, for a home base station FkMay be equal to one, and F for macro base stationkMay be a value less than one. FkBut also on the number of terminals in the cell. FkMay be provided separately for each terminal and may be communicated to the terminals, e.g., via signaling.
Equation (4) uses the capacity function log (1+ C/I) to assign the geometry C corresponding to each HARQ interlaceavg，k/Im，kTo capacity. The capacities of all M HARQ interlaces are added and divided by M to obtain an average downlink capacity. The effective downlink geometry is then calculated from the average downlink capacity and the typical amount of resources that can be allocated. Equation (4) assumes that all M HARQ interlaces are available for terminal 110. The summing may also be performed on a subset of the M HARQ interlaces.
The effective geometry of the uplink may be expressed as:
equation (5)
Wherein IoTm，kIs the interference over thermal noise ratio of base station k on HARQ interlace m,
pCoTkis the carrier to thermal noise ratio corresponding to the uplink pilot at base station k,
d is the desired data Power Spectral Density (PSD) relative to the PSD, an
GUL，eff，kIs the effective uplink geometry for base station k.
IoTm，kMay be broadcast by base station k or may be estimated by terminal 110 based on downlink pilot measurements. pCoT corresponding to terminal 110 at base station kkAdjustments may be made using a power control mechanism to achieve the desired performance of the uplink. D may be determined based on the desired data PSD and the uplink pilot PSD for terminal 110 at base station k. D may also be assigned by base station k (e.g., via layer 1 or layer 3 signaling) or determined by terminal 110 running a distributed power control algorithm. D may also depend on the headroom of the Power Amplifier (PA) of the terminal 110, the interference mitigation scheme being used, etc. Carrier to thermal noise ratio of data CoTkCoT can be given according to the following formulak＝D·pCoTk。
Equation (5) converts the geometry corresponding to each HARQ interlace to capacity using a capacity function. Equation (5) then averages the capacities of all M HARQ interlaces and calculates the effective uplink geometry from the average uplink capacity.
Equations (4) and (5) provide effective downlink and uplink geometries for transmissions over the air interface on the downlink and uplink, respectively. The base station may transmit data to the network entity via a backhaul. The effective downlink and uplink geometries can be calculated taking into account the bandwidth of the backhaul, as follows:
and equation (6)
Equation (7)
Wherein B iskIs the normalized backhaul bandwidth for base station k and can be given in bps/Hz.
The expected data rate for each candidate base station may be determined based on the effective geometry as follows:
RDL，k＝Wk·log(1+GDL，eff，k) And equation (8)
RUL，k＝Wk·log(1+GUL，eff，k) Equation (9)
Wherein, WkIs the available bandwidth of the base station k,
RDL，kis the predicted data rate of the downlink of base station k, an
RUL，kIs the expected data rate for the uplink of base station k.
WkMay be the entire system bandwidth of base station k. Or, WkMay be part of the system bandwidth and may be broadcast by base station k. The expected data rate may also be determined in other ways, for example using parameters other than the effective geometry.
Terminal 110 may determine downlink and uplink transmit energy metrics E for each candidate base station according to equation (2)b，tx，DLAnd Eb，tx，UL. Terminal 110 may also determine an effective downlink and uplink geometry G for each candidate base stationDL，eff，kAnd GUL，eff，kAnd/or predicted data rates R for downlink and uplinkDL，kAnd RUL，k. Various parameters for determining the transmit energy metric, the effective geometry, and the expected data rate may be measured by the terminal 110, broadcast by the candidate base stations, or otherwise obtained.
The macro base station may reserve certain HARQ interlaces according to the information reported by the terminal to improve the effective downlink geometry of the pico or home base station. This may result in selection of a pico base station or a home base station and not a macro base station, for example, based on the transmit energy metric.
As described above, from the parameters of each candidate base station, the metric for that base station may be determined. This assumes a relay-less deployment in which base stations may communicate with other network entities via a backhaul. In the case of a relay deployment, data may be forwarded via one or more relay stations before reaching the backhaul. The metric may be determined by taking into account the capabilities of the relay station.
Fig. 3 shows a wireless communication network 102 with relays. For simplicity, fig. 3 shows only one base station 130 and one relay station 132. Terminal 110 may communicate directly with base station 130 via direct access link 140. The base station 130 may communicate with a network controller 150 via a wired backhaul 146. Alternatively, terminal 110 may communicate with relay station 132 via relay access link 142. Relay 132 may communicate with base station 130 via relay backhaul link 144.
Fig. 4 shows a frame structure 400 that may be used for network 102. Each frame may be divided into a plurality of slots 1 to S. In the example shown in fig. 4, slot 1 in each frame may be used for relay backhaul link 144. The remaining time slots 2 through S in each frame are available for the direct access link 140 and the relay access link 142. In general, any number of time slots may be used for each link.
Referring back to fig. 3, the terminal 110 may have an expected data rate R for the direct access link 140 to the base station 130dThe relay access link 142 may have an expected data rate R for relay station 132a. Relay 132 may have a data rate R for relay backhaul link 144 to base station 130b。RaAnd RbGiven by spectral efficiency, and R of the relay station 132rIt can be expressed as:
equation (10)
Equation (10) assumes that only one terminal is being served and the separation(s) between the relay access link 142 and the relay backhaul link 144plit) is done in an optimal way. If the separation between the relay access link 142 and the relay backhaul link 144 is predetermined (e.g., set by the base station 130 according to some criteria), the data rates (rather than spectral efficiencies) of the relay access link and the relay backhaul link may be calculated. Then, the data rate R is predictedrCan be given as Rr＝min(Ra，Rb) And may be associated with the data rate R of the direct access link 140dA comparison is made. When there are multiple relay stations, R may be scaledaTo address Spatial Division Multiple Access (SDMA) on the relay access link 142. For example, if there are N relays transmitting simultaneously, then N x R may be useda. In any case, both relay access link 142 and relay backhaul link 144 may be considered in calculating the expected data rate for relay 132, as shown in equation (10). Transmit energy metric E for relay 132b，txMay also be in accordance with E of the Relay Access Link 142b，txAnd E of relay backhaul link 144b，txIs calculated as the sum of.
Downlink and uplink energy metrics, effective downlink and uplink geometry, expected downlink and uplink data rates, and/or other metrics may be determined for each candidate base station. The metrics may be used for serving station selection in various ways. In some designs, the metrics may be used directly to select a serving base station. For example, in one design, the highest R may be selectedDL，kAnd/or maximum RUL，kTo obtain the highest data rate of the terminal 110. In another design, the lowest E may be selectedb，tx，DLAnd/or minimum Eb，tx，ULTo obtain minimum interference on the downlink and uplink, respectively.
In other designs, multiple metrics may be combined according to a function to obtain an overall metric. The base station with the best overall metric may then be selected. In one design, all E's with values below a predetermined threshold may be selectedb，tx，DLAnd/or Eb，tx，ULHas the highest R in the base stationDL，kAnd/or RUL，kThe base station of (1). This design may provide the highest data rate for terminal 110 while keeping the interference below a target level. For this design, one can rely on RDL，kAnd/or RUL，kDefine the total metric, and if Eb，tx，DLAnd/or Eb，tx，ULBeyond a predetermined threshold, the total metric may be set to zero.
One or more control channels may be used to support data transmission on the downlink and uplink. The serving base station may be selected such that a desired reliability may be achieved for all control channels, which may ensure reliable data service. The performance of a control channel may be determined by the signal quality it receives, where the signal quality may be given by SINR, SNR, C/I, CoT, etc. The received signal quality of each control channel may be measured and compared to an appropriate threshold to determine whether the control channel is sufficiently reliable. The reliability of the control channel may also be determined based on error rate and/or other metrics. If the control channel is deemed to be sufficiently reliable, a base station may be selected. In general, control channel reliability may be determined based on received signal quality (e.g., SINR, SNR, C/I, CoT, etc.), control channel performance (e.g., message error rate, erasure rate, etc.), and/or other information. The control channel may be deemed to satisfy control channel reliability if the received signal quality exceeds a predetermined quality threshold, the error rate or erasure rate is below a predetermined threshold, or the like.
The serving base station may also be selected based on terminal and/or network utility metrics. In one design, a network utility metric may be defined for each candidate base station according to one of the following equations:
equation (11)
Equation (12)
Equation (13)
Wherein, Tl，kIs the throughput of terminal i served by base station k,
l is the number of terminals served by base station k, an
UkIs the network utility metric for base station k.
Equation (11) provides the arithmetic average of the throughputs of all terminals served by base station k and can be used to maximize the overall throughput.Equation (12) provides a logarithmic average of the throughput of the terminal and can be used to achieve proportional fairness (proportional fairness). Equation (13) provides a harmonic average of the throughput of the terminal and can be used to achieve an equal grade of service (GoS). The average throughputs for a set of base stations may be summed to obtain an overall throughput or overall utility metric U for the base stations. May be based on utility metrics U of different candidate base stationskA total utility metric U, and/or a measure of energy such as E emittedb，tx，DlAnd/or Eb，tx，ULEffective geometric Condition GDL，eff，kAnd/or GUL，eff，kPredicted data rate RDL，kAnd/or RUL，kEtc. to select a serving base station.
In general, the serving station selection may be performed by the terminal 110 or a network entity such as a base station or a network controller. The base station may transmit information (e.g., via broadcast and/or unicast channels) to allow the terminal 110 to calculate the metrics. The terminal 110 may then select a serving base station based on the calculated metrics and the available information. Alternatively, terminal 110 may send the computed metrics and/or other information to a network entity. The network entity may then select a serving base station for terminal 110 based on the available information. The serving base station may be transmitted to the terminal 110 via a handover message or some other message.
The base station may transmit various types of information that may be used for serving station selection. In one design, the base station may send one or more of the following information, e.g., via a broadcast channel:
● the number and/or index of available HARQ interlaces on the downlink and uplink,
● partial resources F that can be allocated to the terminalk，
● interference level I for different HARQ interlaces and/or different frequency subbandsm，kThe values, for example, the actual value and the target value,
● intermediate or tail data rates on the downlink and uplink,
● QoS guarantees, e.g., whether a 50ms delay will be reached,
● backhaul bandwidth Bk，
● Effective Isotropic Radiated Power (EIRP),
● maximum Power Amplifier (PA) output power,
● receiver noise figure, an
● if battery powered, the power level of the base station.
As described above, such as M, Fk、Im，kAnd BkMay be used to calculate the metric. Other parameters such as QoS guarantees, intermediate or tail data rates, etc. may be used as constraints. The EIRP and maximum PA output power may be used to estimate the path loss, which may then be used to calculate a metric. The noise figure in combination with the IoT may be used to calculate the total interference power. If battery powered, the power level of the base station may be used to make handover decisions for terminals connected to the base station.
The base station may also send one or more of the following information, e.g., via a unicast channel:
● the expected experience of the user,
● bias of the handover boundary toward another base station, e.g., in terms of path loss difference, an
● result in expected changes in network utility due to terminals being switched in and out of the base station.
Terminal 110 may send one or more of the following information (e.g., in an extended pilot report) to the network entity performing the serving station selection:
● pilot strengths of the candidate and interfering base stations,
● path loss to candidate and interfering base stations,
●, e.g., a transmit energy metric, nominal geometry, effective geometry, and/or expected data rate for each candidate base station,
● broadcast information received from other base stations, an
● current capabilities of the terminal 110, e.g., data rate, delay, etc.
For initial access, terminal 110 may perform serving station selection based on broadcast information from candidate base stations and measurements obtained by terminal 110. Terminal 110 may also establish the initial connection using the best downlink SINR. For handover, terminal 110 may send an extended pilot report to the candidate base stations and may receive unicast and/or broadcast information that may be used for serving station selection.
In general, pilot measurements may be made based on any type of pilot transmitted by the base station and the relay station. For example, measurements can be made based on standard pilots transmitted by base stations and used by terminals for synchronization, acquisition, and so on. The measurements may also be made from low reuse pilots or preambles (LRPs), where LRP is a pilot that is sent by a small number of base stations and/or relay stations with low time and/or frequency reuse on a given time and/or frequency resource. Low reuse pilots may observe less interference and may thus result in more accurate pilot measurements.
Fig. 5 shows a design of a process 500 for selecting a serving base station for a terminal. Process 500 may be performed by a terminal or a network entity (e.g., a base station or a network controller).
A plurality of candidate base stations for the terminal may be identified, where each candidate base station is a candidate for selection as a serving base station for the terminal (block 512). The plurality of candidate base stations may belong to an open access communication system and may be accessed by any terminal having a service subscription. The multiple candidate base stations may include base stations with different transmit power levels and/or may support interference mitigation. In any case, one of the candidate base stations may be selected as the serving base station for the terminal (block 514). The selected candidate base station may have a SINR lower than a highest SINR of the plurality of candidate base stations. The difference between the highest SINR and the lower SINR of the selected candidate base station may be any value and may be greater than the hysteresis normally used for handover, e.g., at least 5 decibels (dB). The selected candidate base station may have a lower transmit power level than the highest transmit power level of the plurality of candidate base stations.
In one design, the serving base station may be selected based on at least one metric for each candidate base station. The at least one metric may include path loss, effective transmit power, effective geometry, expected data rate, etc. The serving base station may be further selected based on the utility metric for each candidate base station. For example, as shown in equations (11), (12), or (13), the utility metric may be determined as a function of the throughput of the terminal served by the candidate base station. The serving base station may also be selected based on control channel reliability and/or other metrics.
In one design of block 514, a transmit energy metric (e.g., E) may be determined for each candidate base station based on the path loss and possibly interference level of the candidate base station, e.g., as shown in equation (2)b，tx，DLOr Eb，tx，UL). The candidate base station with the lowest transmit energy metric or lowest path loss may be selected as the serving base station.
In another design of block 514, an effective geometry metric (e.g., G) may be determined for each candidate base station based on the received signal quality for the candidate base stationDL，eff，kOr GUL，eff，k). The received signal quality may be determined from C/I (e.g., as shown in equation (4)), from CoT and IoT (e.g., as shown in equation (5)), or from other parameters. The candidate base station with the largest effective geometry metric may be selected as the serving base station.
In another design of block 514, each candidate base station may be targeted based on its effective geometry and/or other parametersDetermining an expected data rate metric (e.g., R)DL，kOr RUL，k). The candidate base station with the largest expected data rate metric may be selected as the serving base station.
In one design, the capacity of each candidate base station corresponding to multiple resource sets may be determined based on received signal quality corresponding to the multiple resource sets. The multiple resource sets may correspond to multiple HARQ instances, multiple frequency subbands, multiple time intervals, etc. An effective geometry or an expected data rate may be determined for each candidate base station based on the capacity of the plurality of resource sets.
The plurality of candidate base stations may include a relay station. As shown in equation (10), a metric may be determined for a relay station based on (i) a first parameter value for a first link between the terminal and the relay station and (ii) a second parameter value for a second link between the relay station and the base station.
In one design, the serving station selection may be performed by the terminal. The terminal may determine at least one metric for each candidate base station based on measurements made by the terminal and information received from at least one candidate base station. The terminal may select the serving base station based on the at least one metric for each candidate base station. In another design, the serving station selection may be performed by a network entity (e.g., a designated base station). The terminal may send the measurement values, calculated metrics, identification of candidate base stations, and/or other information to the network entity to assist in serving station selection. For example, a handover message indicating the serving base station may be sent to the terminal via a previous or new serving base station.
The at least one metric for each candidate base station may include a first metric for the downlink (e.g., E)b，tx，DL、GDL，eff，kOr RDL，k) And a second metric for the uplink (e.g., E)b，tx，UL、GUL，eff，kOr RUL，k). A first candidate base station having a best first metric for downlink may be identified from among the plurality of candidate base stations. It is also possible to identify the uplinkThe second candidate base station for which the link has the best second metric. In one design, the first and second candidate base stations may be selected as serving base stations for the downlink and uplink, respectively. In another design, a single serving base station may be selected for both the downlink and uplink. The first or second candidate base station may be selected as the serving base station based on the first and second metrics. For example, a candidate base station with the best downlink and an uplink within a certain range of the best uplink may be selected. Alternatively, a candidate base station with the best uplink and a downlink within a certain range of the best downlink may be selected.
In one design, a terminal may communicate with a selected base station using interference mitigation to improve SINR. Interference mitigation may be used for system access by a terminal, for data transmission between a terminal and a selected base station, and so on. Interference mitigation may be achieved by sending interference mitigation request messages to interfering base stations and/or interfering terminals requesting them to reduce interference on certain specific resources. The message may be sent over the air from the serving base station to the interfering terminal or from the terminal to the interfering base station. The message may also be sent via a backhaul between base stations. The interfering base station or the interfering terminal may reduce interference on a specific resource by: (i) no transmissions are sent on these resources; (ii) sending transmissions on the resources at a low transmit power; (iii) utilizing beam steering to send transmissions on the resources to control power away from the terminal; and/or (iv) send transmissions in other manners to reduce interference on these resources. Interference mitigation is particularly applicable when the selected base station has a low SINR.
Fig. 6 shows a design of an apparatus 600 for selecting a serving base station for a terminal. The apparatus 600 comprises: a module 612 for identifying a plurality of candidate base stations for the terminal, wherein each candidate base station is a candidate for selecting as a serving base station for the terminal, and the plurality of candidate base stations comprises at least two candidate base stations with different transmit power levels; and a module 614 for selecting one of a plurality of candidate base stations as a serving base station for the terminal, wherein the selected candidate base station has a SINR lower than a highest SINR of the plurality of candidate base stations.
Fig. 7 shows a design of a process 700 for selecting a serving base station for a terminal based on different metrics. A plurality of candidate base stations for the terminal may be identified, where each candidate base station is a candidate for selection as a serving base station for the terminal (block 712). A first metric may be determined for each candidate base station and may be used as a constraint to determine whether the candidate base station may be selected as the serving base station (block 714). The first metric may be control channel reliability, etc. A second metric may be determined for each candidate base station and may be used as a variable to identify the most suitable candidate base station for selection as the serving base station (block 716). The second metric may be determined based on path loss, effective transmit power, effective geometry, expected data rate, and/or other parameters. One of the candidate base stations may be selected as the serving base station for the terminal based on the first and second metrics for each candidate base station (block 718). The selected candidate base station may have a SINR lower than a highest SINR of the plurality of candidate base stations.
Fig. 8 shows a design of an apparatus 800 for selecting a serving base station for a terminal. The apparatus 800 comprises: a module 812 for identifying a plurality of candidate base stations for the terminal, wherein each candidate base station is a candidate for selecting as a serving base station for the terminal; a module 814 for determining a first metric for each candidate base station, wherein the first metric is used as a constraint to determine whether the candidate base station can be selected as the serving base station; a module 816 for determining a second metric for each candidate base station, and the second metric is used as a variable to identify the most suitable candidate base station for selection as the serving base station; module 818 is configured to select one of the plurality of candidate base stations as the serving base station for the terminal based on the first and second metrics for each candidate base station, wherein the selected candidate base station has a lower SINR than a highest SINR of the plurality of candidate base stations.
The modules in fig. 6 and 8 may comprise processors, electronics devices, hardware devices, electronics components, logical circuits, memories, etc., or any combination thereof.
Fig. 9 shows a block diagram of a design of terminal 110 and base station 120. In this design, base station 120 is equipped with T antennas 934a through 934T and terminal 110 is equipped with R antennas 952a through 952R, where T ≧ 1 and R ≧ 1 in general.
At base station 120, a transmit processor 920 can receive data for one or more terminals from a data source 912, process (e.g., encode and modulate) the data for each terminal according to one or more modulation and coding schemes, and provide data symbols for all terminals. Transmit processor 920 may also receive broadcast and control information (e.g., information for serving station selection) from a controller/processor 940, process the information, and provide overhead symbols. A Transmit (TX) multiple-input multiple-output (MIMO) processor 930 may multiplex the data symbols, overhead symbols, and pilot symbols. A processor 930 can process (e.g., precode) the multiplexed symbols and provide T output symbol streams to T Modulators (MODs) 932a through 932T. Each modulator 932 may process a respective output symbol stream (e.g., for OFDM, CDMA, etc.) to obtain an output sample stream. Each modulator 932 may further process (e.g., convert to analog, amplify, filter, and upconvert) the output sample stream to obtain a downlink signal. T downlink signals from modulators 932a through 932T may be transmitted via T antennas 934a through 934T, respectively.
At terminal 110, R antennas 952a through 952R may receive the downlink signals from base station 120 and provide received signals to demodulators (DEMODs) 954a through 954R, respectively. Each demodulator 954 may condition (e.g., filter, amplify, downconvert, and digitize) a respective received signal to obtain received samples, and may further process the received samples (e.g., for OFDM, CDMA, etc.) to obtain received symbols. MIMO detector 960 may perform MIMO detection on the received symbols from all R demodulators 954a to 954R (if available) and provide detected symbols. A receive processor 970 can process (e.g., demodulate and decode) the detected symbols, provide decoded data for terminal 110 to a data sink 972, and provide decoded broadcast and control information to controller/processor 990. The channel processor 994 may measure parameters used for serving station selection (e.g., channel gain h, path loss p, signal power C, interference I, etc.).
On the uplink, at terminal 110, data from a data source 978 and control information (e.g., information for serving station selection or information identifying a selected serving base station) from controller/processor 990 may be processed by a transmit processor 980, precoded by a TX MIMO processor 982 (if available), conditioned by modulators 954a through 954r, and transmitted via antennas 952a through 952 r. At base station 120, the uplink signals from terminals 110 may be received by antennas 934, conditioned by demodulators 932, detected by a MIMO detector 936, and processed by a receive processor 938 to obtain the data and control information transmitted by terminals 110.
Controllers/processors 940 and 990 may direct the operation at base station 120 and terminal 110, respectively. Controller/processor 940 at base station 120 or controller/processor 990 at terminal 110 may implement or direct process 500 in fig. 5, process 700 in fig. 7, and/or other processes for the techniques described herein. Memories 942 and 992 may store data and program codes for base station 120 and terminal 110, respectively. A scheduler 944 may schedule terminals for transmission on the downlink and/or uplink and may assign resources to the scheduled terminals. A communication (Comm) unit 946 may support communication with other base stations and network controller 150 via a backhaul.
Those of skill in the art would understand that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof.
Those of skill would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the disclosure herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention.
The various illustrative logical blocks, modules, and circuits described in connection with the application may be implemented or performed with a general purpose processor, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.
The steps of a method or algorithm described in connection with the present application may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such the processor can read information from, and write information to, the storage medium. Of course, the storage medium may also be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal.
In one or more exemplary designs, the functions may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium. Computer-readable media includes both computer storage media and communication media including any medium that facilitates transfer of a computer program product from one place to another. A storage media may be any available media that can be accessed by a general purpose or special purpose computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a general-purpose or special-purpose computer, or a general-purpose or special-purpose processor. Also, any connection is properly termed a computer-readable medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, Digital Subscriber Line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, Digital Subscriber Line (DSL), or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. Disk and disc, as used herein, includes Compact Disc (CD), laser disc, optical disc, Digital Versatile Disc (DVD), floppy disk and blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media.
The previous description of the disclosure is provided to enable any person skilled in the art to make or use the present invention. Various modifications to the disclosure will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the disclosure. Thus, the present invention is not intended to be limited to the examples and designs described herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.