This application claims priority from U.S. provisional patent application No.60/470,724, filed on 14/5/2003, the entire contents of which are incorporated herein by reference.
Detailed Description
Fig. 2 shows a functional block diagram of a cellular OFDM wireless communication system 200 with a receiver integrated with subcarrier noise and interference detection. OFDM system 200 includes a plurality of base stations 210a-210g that provide communication for a plurality of terminals 220a-220 o. A base station, e.g., 210a, may be a fixed base station used for communicating with terminals, e.g., 220a, and may also be referred to as an access point, a node b, or some other terminology.
The various terminals 220a-220o may be dispersed throughout the OFDM system 200, and each terminal may be fixed, e.g., 220k, or mobile, e.g., 220 b. A terminal, such as 220a, may also be called a mobile station, a remote station, a User Equipment (UE), an access terminal, or some other terminology. Each terminal, e.g., 220a, may communicate with one or possibly multiple base stations on the downlink and/or uplink at any given moment. Each terminal, e.g., 220m, may include an OFDM transmitter 300m and an OFDM receiver 400m to enable communication with one or more base stations. Embodiments of OFDM transmitter 300m and OFDM receiver 400m will be described in further detail in fig. 3 and 4. In fig. 2, terminals 220a through 220o are capable of receiving pilot, signaling, and user-specific data transmissions, e.g., from base stations 210a through 210 g.
Each base station, e.g., 210a, in the OFDM system 200 provides coverage for a particular geographic area, e.g., 202 a. The coverage area of each base station is typically dependent on various factors (e.g., terrain, obstructions, etc.), but is typically represented by an ideal hexagon as shown in fig. 2 for simplicity. A base station and/or its coverage area are also commonly referred to as a "cell," depending on the context in which the term is used.
To increase capacity, the coverage area of each base station, e.g., 210a, may be partitioned into multiple sectors. If each cell is divided into three sectors, each sector of a sectorized cell is typically represented by an ideal 120 wedge representing one-third of the cell. Each sector may be served by a corresponding Base Transceiver Subsystem (BTS), e.g., 212 d. The BTS 212d includes an OFDM transmitter 300d and an OFDM receiver 400d, which are described in greater detail in fig. 3 and 4, respectively. For a sectorized cell, the base station for that cell typically includes all of the BTSs that serve the sectors of that cell. The term "sector," depending on the context in which the term is used, is also typically used to refer to a BTS and/or its coverage area.
As will be discussed in further detail below, each base station, e.g., 210a, typically has a transmitter configured to provide downlink communications, also referred to as the forward link, to terminals, e.g., 520 a. In addition, each base station, e.g., 210a, also has a receiver configured to receive uplink communications, also referred to as the reverse link, from terminals, e.g., 520 a.
In the downlink direction, the base station transmitter receives signals from a signal source, which may be the Public Switched Telephone Network (PSTN) or some other signal source. The base station transmitter then converts the signal to an OFDM signal to be transmitted to one or more terminals. The base station transmitter may digitize the signal, multiplex the signal into several parallel signals, and modulate a predetermined number of subcarriers corresponding to the multiple parallel signal paths. The number of subcarriers may be constant or may vary. In addition, the subcarriers may be adjacent to each other to define adjacent frequency bands, or may be separated from each other to occupy a plurality of independent frequency bands. The base station may allocate subcarriers in a constant manner, e.g., in the case of a fixed number of subcarriers, or the base station may allocate subcarriers in a pseudo-random manner, or in a random manner. The base station transmitter may also include an analog or Radio Frequency (RF) section to convert the OFDM baseband signal to a desired transmit frequency band.
In OFDM system 200, frequency reuse may occur in each cell. That is, within a first cell, e.g., 202d, uplink and downlink frequencies used by a first base station, e.g., 210d, may be used by base stations 210a-c and 210e-g in neighboring cells 202a-c and 202 e-g. As described above, each base station transmitter causes co-channel interference (CCI) experienced by neighboring receivers, in this example, neighboring terminal receivers. For example, the first base station 210f causes CCI to the terminals 220e and 220g in the neighboring cells 202c and 202d that do not communicate with the first base station 210 f. To facilitate minimizing the amount of CCI experienced by neighboring terminals, the base station transmitter may be part of a closed loop power control system.
To help minimize the amount of CCI experienced by terminals outside the cell, e.g., 202f, the base station transmitter may minimize the RF power transmitted to each terminal 220m and 2201 in communication with the base station 210 f. The base station transmitter may adjust the transmit power based in part on the determined noise level in each subcarrier band and a power control signal transmitted by the terminal and received by the base station receiver.
The base station, e.g., 210b, may attempt to maintain a predetermined SINR or C/I value for each subcarrier, e.g., to maintain a predetermined quality of service for terminals, e.g., 220 b-d. SINR or C/I greater than a predetermined value contributes little to the quality of service obtained from the terminal, e.g., 520b, but will result in an increase in CCI for all neighboring cells, e.g., 202a, 202d, and 202 e. Conversely, an SINR or C/I below a predetermined level may cause a significant degradation in the quality of service obtained by terminal 220 b.
The base station receiver may measure the noise and interference level in each sub-carrier band as part of a power control loop that sets the SINR or C/I of the transmitted signal. The base station receiver measures and stores the noise and interference levels in each sub-carrier band. When allocating subcarriers to a communication link, the base station transmitter checks the noise and interference levels in determining the power allocated to each subcarrier. Thus, the base station transmitter may maintain a predetermined SINR or C/I for each subcarrier such that the CCI experienced by terminals in other cells is minimized.
In another embodiment, for example, a 220I terminal, may attempt to maintain a minimum received SINR or C/I required to achieve a predetermined quality of service. When the received SINR or C/I is above a predetermined level, terminal 220I may transmit a signal to base station 210f requesting base station 210f to reduce the transmit signal power. Alternatively, if the received SINR or C/I is below the predetermined level, the terminal 220I may transmit a signal to the base station 210f to request the base station 210f to increase the transmit signal power. In this way, by minimizing the power transmitted to any given terminal, the amount of CCI experienced by terminals in neighboring cells is minimized.
Fig. 3 is a functional block diagram of an OFDM transmitter 300 that may be integrated, for example, in a base transceiver station or terminal. The functional block diagram of the OFDM transmitter 300 includes a baseband section that represents the baseband section of the transmitter in detail, however the figure does not show the signal processing, source interface or RF sections that may be included in the transmitter 300.
OFDM transmitter 300 includes one or more sources 302 corresponding to one or more data streams. When OFDM transmitter 300 is a base station transmitter, source 302 may include a data stream from an external network, such as a PSTN network. Each data stream may be intended for a separate terminal. The data provided by source 302 may be multiple parallel data streams, serial data streams, multiplexed data streams, or a combination of data streams. Source 302 provides data to modulator 310. Modulator 310 processes and modulates the input source. Modulator 310 may include functional blocks for interleaving, coding, grouping, and modulation as known in the art. The modulator 310 is not limited to performing a particular type of interleaving. For example, the modulator may independently block interleave the source data for each terminal.
The modulator 310 may also perform coding. Likewise, the transmitter 300 is not limited to a particular type of encoding. For example, modulator 310 may perform Reed Solomon (Reed-Solomon) coding or convolutional coding. The coding rate may be fixed or it may vary depending on the number of subcarriers allocated to the communication link of the terminal. For example, when a first number of subcarriers are allocated to the terminal, the modulator 310 may perform convolutional encoding using a rate 1/2 encoder, and when a second number of subcarriers are allocated to the terminal, the modulator 310 is controlled to perform convolutional encoding with a rate 1/3. In another example, the modulator may perform Reed Solomon coding, where the coding has a code rate that varies according to the number of subcarriers allocated to the terminal.
The modulator 310 may also be configured to modulate data using a predetermined format. For example, modulator 310 may perform Quadrature Amplitude Modulation (QAM), Quadrature Phase Shift Keying (QPSK), Binary Phase Shift Keying (BPSK), or some other modulation format. In another embodiment, modulator 310 processes the data into a format for modulating subcarriers.
Modulator 310 may also include an amplifier or gain stage to amplitude modulate the data symbols allocated to the subcarriers. Modulator 310 may also adjust the gain of the amplifier based on the subcarriers, and the gain of each subcarrier may depend at least in part on noise and interference in the subcarrier bandwidth.
The output of modulator 310 is coupled to the input of a 1: N multiplexer 320, where N represents the maximum number of subcarriers used in the transmit chain of the communication system. Multiplexer 320 may also be referred to as a "serial to parallel converter" because multiplexer 320 receives serial data from modulator 310 and converts it to a parallel format to interface with multiple subcarriers.
The subcarrier allocation module 312 controls the modulator 310 and the multiplexer 320. The number of subcarriers used to support the source data may be, and typically is, less than the maximum number of subcarriers used in the transmit chain of the communication system. The number of subcarriers allocated to a particular communication link may change over time. In addition, the identity (identity) of the sub-carriers may change over time even if the number of sub-carriers allocated to a particular communication link remains the same.
The subcarriers may be randomly or pseudo-randomly assigned to the communication link. Because the identity of the sub-carriers may vary, the frequency band occupied by the communication link may vary over time. The communication system may be a frequency hopping system using a predetermined frequency hopping scheme.
The subcarrier allocation module 312 may employ frequency hopping and may track the set of subcarriers used and the set of subcarriers allocated to the communication link. For example, in a base station with three forward link signals, subcarrier assignment module 312 may assign a first set of subcarriers to a first communication link, a second set of subcarriers to a second communication link, and a third set of subcarriers to a third communication link. The number of subcarriers in each subcarrier set may be the same or different. The subcarrier allocation module 312 tracks the number of subcarriers allocated to the communication link and the number of subcarriers that are idle and that may be allocated to the communication link.
The subcarrier assignment module 312 controls the modulator 310 to provide the desired coding and modulation needed to support the assigned subcarrier set. In addition, the subcarrier assignment module 312 controls the multiplexer 320 so that data from the modulator 310 is provided to the multiplexer channel corresponding to the assigned subcarrier. In this way, the subcarrier assignment module 312 controls the identity and number of subcarriers assigned to a particular communication link. The subcarrier assignment module 312 also tracks the identity of subcarriers that are idle and that may be assigned to a communication link.
The output of multiplexer 320 is coupled to an Inverse Fast Fourier Transform (IFFT) module 330. A parallel bus 322 having a width greater than or equal to the total number of subcarriers couples the parallel outputs from the multiplexer 320 to the IFFT module 330.
The fourier transform performs a mapping from the time domain to the frequency domain. Thus, the inverse fourier transform performs a mapping from the frequency domain to the time domain. The IFFT module 330 transforms the modulated subcarriers into time domain signals. The nature of the fourier transform ensures that the subcarrier signals are evenly spaced and mutually orthogonal.
The parallel outputs from the IFFT module 330 are coupled to a demultiplexer 340 using another parallel bus 332. The demultiplexer 340 converts the parallel modulated data stream into a serial stream. The output of the demultiplexer 340 may then be coupled to a guard band generator (not shown) and subsequently to a digital-to-analog converter (DAC) (not shown). The guard band generator inserts a time period between successive OFDM symbols to minimize the effects of inter-symbol interference caused by multipath in the communication link. The output of the DAC may then be coupled to an RF transmitter (not shown) for up-converting the OFDM signal to a desired transmit frequency band.
Fig. 4A-4B are functional block diagrams of an embodiment of an OFDM receiver 400. The OFDM receiver 400 may be used in a base station or a terminal such as a mobile terminal. The OFDM receiver 400 of fig. 4A performs noise estimation mainly in the digital domain, while the OFDM receiver 400 of fig. 4B performs noise estimation mainly in the analog domain.
The OFDM receiver 400 of fig. 4A receives RF signals transmitted by a complementary OFDM transmitter on an antenna 402. The output of the antenna 420 is connected to a receiver 410, and the receiver 410 may filter, amplify, and downconvert the received OFDM signal to baseband.
The baseband output from the receiver 410 is coupled to a guard removal module 420, which guard removal module 420 is used to remove guard intervals inserted between OFDM symbols at the transmitter. The output of the guard removal module 420 is coupled to an analog-to-digital converter (ADC)422 that converts the analog baseband signal to a digital representation. The output of the ADC 422 is coupled to a multiplexer 424 that converts the serial baseband signal to N parallel data paths. The number N indicates the total number of OFDM subcarriers. The symbols in each parallel data path represent gated time domain symbols of the OFDM signal.
The parallel data path is coupled to an input of a Fast Fourier Transform (FFT) module 430. The FFT module 430 transforms the gated time domain signal into a frequency domain signal. Each output of the FFT module 430 represents a modulated subcarrier.
The parallel outputs of the FFT module 430 are coupled to a demodulator 440 that demodulates the OFDM subcarriers. Demodulator 440 may be configured to demodulate only a subset of the subcarriers received by receiver 400 or may demodulate all of the outputs of FFT module 430 corresponding to all of the subcarriers. The output of demodulator 440 may be a single symbol or may be multiple symbols. For example, if the subcarriers are quadrature modulated, demodulator 440 may output either in-phase or quadrature signal components of the demodulated symbols.
The output of the demodulator 440 is coupled to a detector 450. The detector 450 is used to detect the received power in each sub-carrier band. The detector 450 may detect the received power by detecting or determining, for example, the power, amplitude, magnitude squared, magnitude, etc., or some other representation of the demodulated subcarrier signal that is related to the received power. For example, the amplitude square of the quadrature modulated signal may be determined by adding the square of the in-phase signal component and the square of the quadrature signal component. Detector 450 may include multiple detectors or may include a single detector that determines the detected value of the desired subcarrier signal before the next demodulated symbol occurs.
The processor 460 interfaces with a memory 470 that includes processor readable instructions. Memory 470 may also include a rewritable storage unit for storing and updating detected subcarrier noise values.
The sub-carriers allocated to a particular communication link may change at each symbol boundary. A frequency hopping sequence or frequency hopping information used to identify the subcarriers of the communication link assigned to the receiver 400 may also be stored in the memory 470. The processor 460 uses the frequency hopping information to optimize the performance of the FFT module 430, the demodulator 440, and the detector 450. In this way, the processor 460 can use the frequency hopping sequence or other frequency hopping information to identify which sub-carriers are allocated to the communication link and which sub-carriers are idle.
For example, when less than the total number of subcarriers are allocated to the communication link of receiver 400, processor 460 may control FFT module 430 to determine only those FFT output signals that correspond to the allocated subcarriers. In another embodiment, processor 460 controls FFT module 430 to determine the subcarriers corresponding to the communication links assigned to receiver 400 plus the outputs corresponding to subcarriers that are idle and not already assigned to any communication link. The processor 460 can relieve some of the load on the FFT module 430 by reducing the number of FFT output signals it needs to determine.
Processor 460 may also control modem 440 to demodulate only those signals for which FFT module 430 provides an output signal. In addition, processor 460 may control detector 450 to detect only those subcarrier signals corresponding to subcarriers that are idle and not yet allocated. Because detector 450 may be limited to detecting the noise level of the unassigned subcarriers, detector 450 may detect the signal prior to the demodulator. However, it may be advantageous to place the detector 450 after the demodulator 440 because the noise detected by the detector 450 will undergo the same signal processing as the symbols in the subcarrier. Thus, the statistical properties of the signal processing experienced by the demodulated noise will be similar to the statistical properties experienced by the demodulated symbols.
Processor 460 may track noise in the subcarriers by detecting the demodulated noise power in the subcarriers as long as the subcarriers are not allocated to the communication link. The detected power of the unassigned sub-carriers represents the interference and noise power within the sub-carrier frequency band. The processor may store the detected power in a memory location within memory 470 corresponding to the subcarrier. In frequency hopping OFDM systems, the identity of the unassigned sub-carriers changes over time and can change at any symbol boundary.
Processor 460 may store the plurality of detected power measurements for the first subcarrier in separate memory units. Processor 460 may then average a predetermined number of detected power measurements. Alternatively, the processor 460 may calculate a weighted average of noise and interference by weighting each stored detected power measurement with a factor that depends in part on the cumulative amount of detected power measurements (age). In another embodiment, the processor 460 may store the detected noise and interference power in corresponding units in the memory 470. Processor 460 then weights the stored values by a first amount and weights the newly detected power by a second amount, and thereafter stores the sum of the two in a memory location corresponding to the subcarrier, thereby updating the noise and interference value for the particular subcarrier. With this alternative updating method only N memory units are needed to store the N estimates of subcarrier noise and interference. It will be appreciated that other methods of storing and updating subcarrier noise and interference values may be used.
The detected power of the unassigned sub-carriers represents the total noise and interference for that sub-carrier band unless no interferer is broadcasting in that band. The detected power represents the power of the detected noise floor when no interferer is broadcasting in the sub-carrier band.
An OFDM system can ensure that no system source broadcasts interfering signals in the sub-carrier frequency band by synchronizing all transmitters and defining periods when all transmitters are not transmitting on a particular sub-carrier. That is, when noise estimation is performed in the receiver at the terminal, all base stations in the OFDM system may periodically cease transmission on one or more predetermined subcarriers during a predetermined symbol period. Communication in an OFDM system does not stop during the period in which a single subcarrier is unassigned, because all other subcarriers can continue to be allocated to a communication link. Thus, by synchronizing the transmitters and periodically not allocating each subcarrier to any communication link during one or more symbol periods, a noise level may be determined for each subcarrier band that is free of interference. Then, a noise power without an interferer may be determined for the subcarrier band during the unallocated period.
Fig. 4B is a functional block diagram of another embodiment of an OFDM receiver 400 in which noise and interference are detected using analog devices. Receiver 400 first receives an OFDM signal on antenna 402 and couples the output of antenna 402 to receiver 410. As in the previous embodiment, the receiver 410 filters, amplifies, and downconverts the received OFDM signal to baseband. An output of the receiver 410 is coupled to an input of a filter 480. The baseband output of the receiver 410 may also be coupled to other signal processing stages (not shown), such as a guard removal module, an FFT module, and a demodulator.
In one embodiment, filter 480 is a filter bank having a number of baseband filters equal to the number of subcarriers in the communication system. Each filter may have substantially the same bandwidth as the subcarrier signal bandwidth. In another embodiment, filter 480 is a filter bank having one or more tunable filters that can be tuned to any sub-carrier frequency band in the communication system. The tunable filters are tuned to the sub-carrier frequency bands of the communication link not allocated to the receiver 400. The bandwidth of the tunable filter may actually be the same as the bandwidth of the subcarrier band.
The output of filter 480 is coupled to detector 490. The output from filter 480 may be one or more filtered signals. The number of output signals from the filter 480 may be equal to the number of subcarriers in the communication system.
A detector 490 may be used to detect the power of each filtered signal. The detector 490 may include one or more power detectors. The power detector may correspond to the output of the filter 480. Alternatively, one or more power detectors may be used to detect the power output by each filter in succession.
The output of the detector 490 is coupled to the input of the ADC 494. The ADC 494 may include a plurality of converters, each converter corresponding to the output of one of the detectors 490. Alternatively, the ADC 494 may include a single ADC that sequentially converts the output of each detector 490.
A processor 460 interfaced with memory 470 may be coupled to the output of the ADC 494. The processor 460 may control the ADC 494 using processor-readable instructions stored in memory 470 to convert only those detected power levels in question. In addition, the processor 460 may track the frequency hopping sequence and update the detected noise and interference levels, as described in previous embodiments. The noise level may be detected independently of the interference level in a synchronous system that periodically stops transmission on predetermined subcarriers for a predetermined duration, such as one symbol period.
Fig. 5 is a schematic diagram of a spectrum of a portion of an OFDM frequency band 500 during a predetermined time period. The OFDM frequency band 500 includes a plurality of subcarriers, each occupying a predetermined frequency band, e.g., 502 a. Multiple communication links may occupy OFDM frequency band 500 simultaneously. The multiple communication links may use only a subset of all subcarriers available in the system.
For example, four subcarriers occupying four frequency bands 502a-d may be allocated to the first communication link. The subcarriers and corresponding frequency bands 502a-d are shown as being located in one contiguous frequency band. However, the subcarriers allocated to a particular communication link need not be contiguous, but may be any available subcarrier in an OFDM system. A second set of subcarriers, i.e., a second set of carrier bands 522a-d, may be allocated to a second communication link. Likewise, third and fourth subcarrier sets may be allocated to third and fourth communication links, respectively. The third set of subcarriers corresponds to a third set of frequency bands 542a-c and the fourth set of subcarriers corresponds to a fourth set of subcarrier frequency bands 562 a-c.
The number of sub-carriers allocated to a particular communication link may vary over time or may vary depending on the load placed on the communication link. Thus, a higher data rate communication link may be allocated a larger number of subcarriers. The number of subcarriers allocated to a communication link may vary at each symbol boundary. Therefore, the number and location of allocated subcarriers in an OFDM system may vary at each symbol boundary.
Because the total number of allocated subcarriers may not correspond to the total number of available subcarriers in the OFDM system, there may be one or more subcarriers that are not allocated to any communication link, i.e., idle subcarriers. For example, the three subcarrier bands 510a-c, 530a-c, and 550a-e shown in the OFDM band 500 are not allocated to any communication links. In addition, the unassigned sub-carriers, and corresponding sub-carrier bands, are not necessarily adjacent and do not necessarily exist between assigned sub-carriers. For example, some or all of the unassigned sub-carriers may exist at one end of the frequency band.
The receiver can estimate and update the estimate of noise and interference in a subcarrier by detecting the power in the subcarrier band when the subcarrier is unassigned. The unassigned sub-carriers may represent locally unassigned sub-carriers, such as in a cell or sector in which the receiver is located. Other cells or other sectors of a cell may allocate the subcarriers to communication links.
For example, a first receiver, e.g., a receiver in a terminal, may establish a communication link with a base station using a first set of subcarriers 502a-d in a first frequency band. The first receiver may estimate noise and interference in the unassigned frequency band by determining the power in the unassigned sub-carrier frequency band, e.g., 530 a. As described above, the receiver may update the estimate previously stored in memory by averaging using the most recently measured power level and the previously stored power level. Alternatively, the most recently determined power level corresponding to the most recent noise and interference estimate may be used in the determination of a weighted average of a predetermined number of recent noise and interference estimates.
In addition, in a synchronized system, one or more subcarriers may not be allocated to any transmitter for a predetermined duration, e.g., a symbol period. Thus, during the symbol period, the sub-carriers are not allocated in all cells in a particular OFDM system. Then, for a system-wide unassigned sub-carrier, the receiver can estimate the noise floor within that sub-carrier band (e.g., 550d) by determining the power of that sub-carrier band during periods in which all transmitters do not transmit on that sub-carrier band. The receiver may also update the noise estimate by averaging or weighted averaging the multiple estimates. The receiver may store the noise floor estimate for each subcarrier band separately. Thus, the receiver can periodically update the noise floor and noise and interference levels in each sub-carrier band.
Fig. 6 is a flow diagram of a method 600 of determining and updating noise and interference levels in an OFDM sub-carrier band. The method 600 may be used in a receiver in an OFDM system. The receiver may be, for example, a receiver in a terminal. Alternatively, or in addition, the receiver may be a receiver in a base transceiver station, for example.
The method 600 begins at block 602, where the receiver is synchronized in time with the transmitter. For example, the receiver may synchronize a time reference with a time reference in the transmitter. The receiver may need to synchronize with the transmitter for a number of reasons unrelated to noise estimation. For example, a receiver may need to synchronize with a transmitter in order to determine which subcarriers are allocated to its communication link in one or more symbol periods.
The receiver then proceeds to block 610 where the receiver determines unused or unassigned subcarriers in the next symbol period. The transmitter may send this information to the receiver in an overhead message. Thus, the message received by the receiver indicates which subcarriers are unassigned in a given symbol period. Alternatively, the allocation of the sub-carriers may be pseudo-random and the receiver may have synchronized a locally generated pseudo-random sequence with the transmitter in a previous synchronization step. In an alternative embodiment, the receiver determines the unassigned sub-carriers based on an internally generated sequence, such as a locally generated pseudo-random sequence or an internally generated frequency hopping sequence.
The receiver proceeds to block 620 where the transmitted OFDM information is received. The received symbols may include those assigned subcarriers that are assigned to the receiver communication link as well as those subcarriers that are not assigned to the receiver communication link.
The receiver proceeds to block 622 where the receiver converts the received signal to a baseband OFDM signal. The received signal is typically transmitted wirelessly as RF OFDM symbols to a receiver using an RF link. The receiver typically converts the received signal to a baseband signal for signal processing.
After converting the received signal to a baseband signal, the receiver proceeds to block 624, where the guard interval is removed from the received signal. As discussed previously in the discussion of OFDM transmitters, guard intervals are inserted to provide multipath immunity.
After removing the guard interval, the receiver proceeds to block 630 where the signal is digitized in an ADC. After digitizing the signal, the receiver proceeds to block 632 where the signal is converted from a serial signal to a plurality of parallel signals. The number of parallel signals may, in general, be as many as the number of subcarriers in an OFDM system.
After the serial signal is converted to a parallel signal, the receiver proceeds to block 640 where the receiver performs an FFT on the parallel data. The FFT converts the time domain OFDM signal to modulated subcarriers in the frequency domain.
The receiver proceeds to block 650 where at least some of the modulated subcarriers output from the FFT are demodulated. The receiver typically demodulates the sub-carriers assigned to the communication link of the receiver as well as the unassigned sub-carriers.
The receiver then proceeds to block 660 where the unassigned sub-carriers are detected to provide noise and interference estimates. If the subcarrier is a system wide unassigned subcarrier, the detected output represents a noise floor estimate for that subcarrier band.
The receiver then proceeds to block 670 and updates the noise and interference estimate and noise floor estimate stored in memory. As previously described, the receiver may store a predetermined number of most recently determined noise and interference estimates and perform an averaging process of the estimates. Likewise, the receiver may determine an average of a predetermined number of most recently determined noise floor estimates.
The receiver proceeds to block 680 where the noise estimate is communicated to the transmitter. For example, if the receiver is a terminal receiver, the terminal receiver may communicate the noise estimate to a transmitter in the base transceiver station. The terminal receiver may first communicate the noise estimate to the associated terminal transmitter. The terminal transmitter then transmits the noise estimate to the base station receiver. The base station receiver, in turn, communicates the noise estimate to the base station transmitter. The noise estimate may be used by a base station transmitter to adjust the transmit power level of the transmitter on the subcarriers corresponding to the noise estimate.
Similarly, the base station receiver may likewise communicate the received noise estimate to the terminal transmitter by first transmitting the noise estimate to the terminal receiver using the base station transmitter.
In block 690, the receiver determines a signal quality of a subsequently received symbol based in part on the noise estimate determined using the unassigned sub-carriers. For example, the receiver estimates the noise and interference for the unassigned sub-carriers. In the next symbol period, the receiver may receive symbols on the same previously unassigned sub-carriers. The receiver can then determine a signal quality, e.g., C/I or SINR, based in part on the previously determined noise estimate. Similarly, when the receiver determines the noise floor estimate, the receiver can determine the SNR of subsequent symbols received on the same subcarrier.
Because the number and location of unassigned sub-carriers typically vary randomly or pseudo-randomly, the receiver is able to periodically update estimates of noise and interference and noise floor for each sub-carrier band in an OFDM system. The receiver is thus able to generate and update estimates of noise and interference and noise floor that can be communicated to the transmitter stage in order to minimize CCI.
Electrical connections, couplings, and connections have been described with respect to various devices or elements. The connection or coupling may be direct or indirect. The connection between the first device and the second device may be a direct connection or may be an indirect connection. An indirect connection may include intervening elements that may process signals from a first device to a second device.
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 embodiments disclosed 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 embodiments disclosed herein 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 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 embodiments disclosed herein 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. In the alternative, the storage medium may be integral to the processor. The memory and storage medium may reside in an ASIC.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments 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 invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.