HK1091607A - Receiving apparatus with hybrid equalizer and rake receiver and corresponding method of receiving - Google Patents

Description

Receiving apparatus with hybrid equalizer and RAKE receiver and corresponding receiving method

This patent application claims delivery on 2/6/2003 and is entitled "COMMUNICATION RECEIVER WITH HYTHHYBIRD EQUALIZER" and is granted priority to provisional application No. 60/475,250 by the assignee.

Technical Field

The present invention relates generally to equalization in communication systems, and more particularly to a generalized receiver incorporating a RAKE receiver and a hybrid equalizer.

Background

Communication systems are used for the transmission of information from one device to another. Prior to transmission, the information is encoded into a format suitable for transmission over a communication channel. The transmitted signal is distorted as it passes through the communication channel; the signal is also subject to degradation of the vehicle caused by noise and interference generated during transmission. One example of interference typically encountered in band-limited channels is known as intersymbol interference (ISI). ISI is the result of the spreading of transmitted symbol pulses due to the dissipative nature of the channel, which results in overlapping of adjacent symbol pulses. The received signal is decoded and converted to the original pre-coded form. Both the transmitter and receiver are designed to minimize the effects of channel imperfections and interference. For purposes of illustration, interference or distortion due to channel imperfections, or any combination thereof, is referred to collectively as noise.

Various receiver designs may be used to compensate for noise caused by the transmitter and the channel. For example, an equalizer is a common choice to address ISI. The equalizer corrects for distortion and generates an estimate of the transmitted signal. In a wireless environment, equalizers are required to handle time-varying channel conditions. Ideally, the response of the equalizer can be adjusted according to changes in the channel characteristics.

Equalizers are often complex, tending to increase the power consumption of the communication device. Therefore, there is a need for an equalizer design that reduces power consumption. Furthermore, the equalizer needs to be controlled to operate during these channel conditions to get the optimum performance of the equalizer. It is also desirable to implement an equalizer in parallel with a RAKE receiver, wherein the equalizer operates only under specified operating conditions.

Drawings

FIG. 1 is a portion of a RAKE receiver in a communication system;

FIG. 2A is a model of a communication system;

FIG. 2B is a model of the transmission portion of the communication system, including modulation and analog receiver processing;

FIG. 3 is a receive data processor in a mobile station;

FIG. 4 is a receiver supporting data communications;

FIG. 5 is a state diagram illustrating the operation of a receiver employing a RAKE and hybrid equalizer; and

fig. 6 is a state diagram illustrating the operation of a receiver including a plurality of operating states.

Detailed Description

The term "exemplary" is used herein to mean "serving as an example, instance, or illustration. Any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.

Communication systems are used for the transmission of information from one device to another. Prior to transmission, the information is encoded into a format suitable for transmission over a communication channel. The communication channel may be a transmission line or free space between the transmitter and the receiver. As the signal propagates through the channel, the transmitted signal is distorted due to imperfections in the channel. In addition, the signal is subject to degradation caused by noise and interference generated at the time of transmission. One example of interference typically encountered in band-limited channels is known as intersymbol interference (ISI). ISI is the result of the spreading of transmitted symbol pulses due to the dissipative nature of the channel, which results in overlapping of adjacent symbol pulses. At the receiver, the received signal is processed and converted to the original pre-coded form. Both the transmitter and receiver are designed to minimize the effects of channel imperfections and interference. For purposes of illustration, interference or distortion due to channel imperfections, or any combination thereof, are collectively referred to as noise.

Various receiver designs may be used to compensate for noise caused by the transmitter and the channel. In one design, a RAKE receiver is used. In another design, an equalizer is used. In yet another design, both a RAKE receiver and an equalizer are used.

RAKE configuration

A communication system may use a RAKE receiver to process a modulated signal transmitted on the forward link or the reverse link. A RAKE receiver typically includes a searcher element and a plurality of finger processors. The searcher element retrieves strong instances in the received signal (or multipath). Finger processors are assigned to process the strongest multipaths to generate demodulated symbols for those multipaths. The RAKE receiver then combines the demodulated symbols from all assigned finger processors to generate recovered symbols, which are estimates of the transmitted data. The RAKE receiver effectively combines the energy received via multiple signal paths.

The RAKE receiver provides an acceptable level of performance for CDMA systems operating at low signal-to-noise ratios (S/N). For CDMA systems designed to transmit data at high data rates, such as HDR systems, a higher S/N is required. To achieve this higher S/N, the components that make up the noise term N need to be reduced. The noise term includes thermal noise (No), interference (Io) due to transmissions from other transmission sources and transmissions from other users, and intersymbol interference (ISI) resulting from multipath and distortion in the transmission channel. For CDMA systems designed to operate at low S/N, the ISI component is generally negligible compared to other noise components. However, for CDMA systems designed to operate at higher S/N, other noise components (e.g., interference from other transmission sources) are typically reduced, and ISI becomes a non-negligible component, possibly having a large impact on the overall S/N.

As described above, the RAKE receiver provides acceptable performance when the S/N of the received signal is low. A RAKE receiver can be used to combine the energy from each multipath, but typically does not remove the effects of ISI (e.g., from multipath and channel distortion). Thus, the RAKE receiver cannot achieve the higher S/N required by systems operating at higher data rates.

Fig. 1 is a block diagram of an embodiment of a RAKE receiver 100. Due to multipath and other phenomena, the transmitted signal may reach the receiver unit through multiple signal paths. To improve performance, RAKE receivers are designed with the ability to process multiple (and strongest) instances of the received signal (or multipath). For conventional RAKE receiver designs, each finger processor 110 comprises one finger of the RAKE receiver and can be assigned to handle a particular multipath.

In a spread spectrum communication system, such as a Code Division Multiple Access (CDMA) system, the received in-phase (I) signal from a particular pre-processor (not shown)_IN) And quadrature (Q)_IN) The samples are provided to a plurality of finger processors 110a through 1101. In each assigned finger processor 110, the received I_INAnd Q_INThe samples are provided to a PN despreader 120, and the PN despreader 120 also receives the complex PN sequences PNI and PNQ. The complex PN sequence is generated according to the particular design of the CDMA system being implemented, and for HDR systems, by multiplying the short IPN and QPN sequences by the long PN sequence by multipliers 138a and 138 b. Short IPN and QPN sequences are used to spread data at the transmitting base station, and long PN sequences are assigned to trusted receiver units and used to scramble the data. The PNI and PNQ sequences are generated using time shifts corresponding to the particular multipath being processed by the finger processor.

PN despreader 120 performs complex I_INAnd Q_INComplex multiplication of samples with complex PN sequences and despreading of the complex I_DESAnd Q_DESThe samples are provided to decover elements 122 and 132. Decover element 122 decovers the despread samples using one or more channelization codes (e.g., Walsh codes) used to cover the data and generates complex decovered samples. The decovered samples are then provided to a symbol accumulator 124, which symbol accumulator 124 accumulates the samples over the length of the channelization code to generate decovered symbols. The decovered symbols are then provided to a pilot demodulator126。

For a High Rate Packet Data (HRPD) system, such as the system defined by IS-856, a pilot reference IS sent during a portion of the forward link transmission. In this way, the decover element 132 decovers the despread samples using a specific channelization code (such as Walsh code 0 for HDR systems) used to cover the pilot reference at the base station. The decovered pilot samples are then provided to an accumulator 134 and accumulated over a particular time interval to generate pilot symbols. The accumulation time interval may be the duration of the pilot channelization code, the period of the entire pilot reference, or some other time interval. The pilot symbols are then provided to a pilot filter 136 and used to generate pilot estimates, which are provided to a pilot demodulator 126. The pilot estimates are pilot symbols estimated or predicted during the time period in which the data is present.

Pilot demodulator 126 performs coherent demodulation on the decovered symbols from symbol accumulator 124 using the pilot estimates from pilot filter 136 and provides demodulated symbols to symbol combiner 140. Coherent demodulation can be accomplished by performing point and cross multiplication on the decovered symbols using pilot estimates. The dot and cross multiplication effectively performs phase demodulation of the data and further scales the resulting output by the relative strengths of the recovered pilots. Scaling using pilots effectively measures the weights of different multipaths based on the quality of the multipaths for efficient combining. The point and cross multiplication then performs the dual functions of phase projection and signal weighting, which is characteristic of a coherent RAKE receiver.

A symbol combiner 140 receives and coherently combines the demodulated symbols from all assigned finger processors 110 to provide recovered symbols for the particular received signal being processed by the RAKE receiver. The recovered symbols for all received signals may then be combined (as described below) to generate overall recovered symbols, which may then be provided to subsequent processing elements.

Searcher element 112 may be designed to include a PN despreader, a PN generator, and a signal quality measurement element. PN (pseudo-noise)The generator generates complex PN sequences at various time offsets, possibly as directed by a controller (not shown) that is used to search for the strongest multipath. For each time offset to be searched, the PN despreader receives I at a particular time offset_INAnd Q_INSamples are sampled and despread with the complex PN sequence to provide despread samples. The signal quality of the despread samples is then estimated. This can be done by calculating the energy (i.e., I) of each despread sample_DES ²+Q_DES ²) And accumulate energy over a particular time period (e.g., a pilot reference period). The searcher element performs a search over a plurality of time offsets and selects the multipath with the highest signal quality measurement. Finger processor 110 may then be assigned to handle these multipaths.

The design and operation OF a RAKE receiver for a CDMA system is disclosed in greater detail in U.S. patent No.5,764,687 entitled "MOBILE DEMODULATOR ARCHITECTURE for a SPREAD SPECTRUM MULTIPLE ACCESS communication system," and U.S. patent No.5,490,165 entitled "DEMODULATOR ELEMENT ASSIGNMENTIN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALS" both assigned to the assignee OF the present invention.

In one embodiment, multiple forward link signals are received by K antennas and processed to generate a stream of samples x₁(n) to x_K(n) of (a). In this way, multiple RAKE receivers may be provided to process the K sample streams. A combiner may then be used to combine the recovered symbols from all of the received signals being processed. Alternatively, one or more RAKE receivers may process the K sample streams in time division multiplexing. In this Time Division Multiplexed (TDM) RAKE receiver architecture, samples from the K streams may be temporarily stored in a buffer for subsequent retrieval and processing by the RAKE receiver.

For each received signal, RAKE receiver 100 may process the highest L multipaths, where I denotes finger processingThe number of devices 110. Each of the I multipaths corresponds to a particular time offset determined with the aid of the searcher element 112. The controller or searcher element 112 may be designed to maintain the strongest multipath (α) for each of the K received signals being processed_Ji) And corresponding time offset (τ)_i) A list of (a).

In a combined receiver configuration with a RAKE and equalizer, these magnitude and time offsets may be used to initialize the equalizer coefficients and scaling factors, as described above. In one embodiment, the magnitude of each multipath of interest may be calculated as the square-of-the-accumulated-energy value divided by the number of samples (N) used in the accumulation process.

Equalizer configuration

The equalizer is the usual option to handle ISI. The equalizer may be implemented using a transversal filter (i.e., a delay line with T second taps, where T is the symbol duration). The capacity of the taps is amplified and summed to generate an estimate of the transmitted symbol. The tap coefficients are adjusted to reduce interference generated by those symbols that are temporally adjacent to the desired symbol. Typically, adaptive equalization techniques are used to continuously and automatically adjust the tap coefficients. The adaptive equalizer uses a prescribed algorithm, such as Least Mean Square (LMS) or Recursive Least Square (RLS), to determine the tap coefficients. The symbol estimates are coupled to a decision device such as a decoder or symbol slicer.

The ability of the receiver to detect a signal in the presence of noise is based on the ratio of the received signal power and the noise power. This ratio is commonly referred to as the signal-to-noise-power ratio (SNR), or carrier-to-interference ratio (C/I). These and similar terms and industry usage are often interchangeable. However, the meaning is the same. Thus, one of ordinary skill in the art will understand that any reference herein to C/I encompasses this broad meaning: measurement of noise effects at multiple points in a communication system.

In general, the C/I may be determined in the receiver by evaluating symbol estimates for a known sequence of transmitted symbols. This can be done in the receiver by calculating the C/I of the transmitted pilot signal. Because the pilot signal is known, the receiver can calculate the C/I based on the symbol estimates from the equalizer. The resulting C/I can be used for a variety of purposes. In a communication system using a variable rate data request scheme, a receiver may communicate with a transmitter at a maximum rate that it can support based on C/I. Furthermore, if the receiver includes a turbo decoder, the log-likelihood ratio (LLR) calculation requires an accurate estimate of the C/I, depending on the transmitted clusters.

Equalizers in wireless communication systems are designed to adapt to time-varying channel conditions. When the channel characteristics change, the equalizer adjusts its own response accordingly. These changes may include changes in the propagation medium or relative movement of the transmitter and receiver, among other conditions. As described above, adaptive filtering algorithms are typically used to modify the equalizer tap coefficients. Equalizers that use adaptive algorithms are commonly referred to as adaptive equalizers. The adaptive algorithms have common attributes: the adaptation speed decreases as the number of equalizer taps increases. A "long" equalizer (i.e., an equalizer with a large number of taps) is desirable because a long equalizer inverts the channel distortion more accurately, resulting in good steady-state performance. However, long equalizers react more slowly to channel variations, resulting in poor transient characteristics, i.e., poor performance when the channel changes rapidly. The optimum number of taps is a trade-off between good steady-state performance and good transient performance, balancing these conditions.

In practice, it is difficult to determine the optimum number of taps because the optimum value depends on a number of conditions and purposes, including, but not limited to, the instantaneous response of the channel, and the rate of change of the channel. Therefore, if the equalizer is used for a variety of channels, under a variety of time-varying conditions, it is difficult to determine the optimum number of taps in advance.

Fig. 2A illustrates a portion of the components of a communication system 200. In addition to the blocks shown, other blocks and modules may also be incorporated in the communication system. Bits generated by a source (not shown) are framed, encoded, and then mapped to symbols in a signaling constellation. The sequence of binary numbers generated by the source is called an information sequence. The information sequence is encoded by an encoder 202, and the encoder 202 outputs a bit sequence. The output of the encoder 202 is provided to a mapping unit 204, the mapping unit 204 acting as an interface to the communication channel. Mapping unit 104 maps the encoder output sequence to symbol y (n) in the complex-valued signaling cluster. Further transmit processing (including modulation blocks) and communication channel and analog receiver processing is illustrated by section 202.

Fig. 2B shows some of the details included in section 202 of fig. 2A. As shown in fig. 2B, the complex symbols y (n) are modulated onto the analog signal pulses, and the resulting complex baseband waveform is sinusoidally modulated onto the in-phase and quadrature-phase branches of the carrier signal. The resulting analog signal is transmitted by an RF antenna (not shown) over a communication channel. Multiple modulation schemes may be implemented, such as M-ary phase shift keying (M-PSK), 2^MQuadrature amplitude modulation (2)^MQAM), etc.

Each modulation scheme contains an associated "signaling cluster" that maps one or more bits to a unique complex symbol. For example, in 4-PSK modulation, two coded bits are mapped to one of four possible complex values {1, i, -1, -i }. Thus, there may be four possible values for each complex symbol y (n). Usually for M-PSK, log₂The M coded bits are mapped to one of M possible complex values located on a complex unit circle.

Continuing with fig. 2B, at the receiver, the analog waveform is downconverted, filtered, and sampled (such as at an appropriate multiple of the nyquist rate). The resulting samples are processed by equalizer 210 and equalizer 210 corrects for signal distortion and other noise and interference introduced by the channel, as illustrated by section 220. Equalizer 210 outputs an estimate of the transmitted symbol y (n). The symbol estimates are then processed by a decoder to determine the original information bits, i.e., the source bits, as input to the encoder 202.

As shown in FIGS. 2A and 2B, the pulse filter, I-Q modulator, and the receiver front-end,The combination of the channel and the analog processor has an impulse response of h_kThe z-to-h (z) -linear filter 206 is shown, where the channel-induced interference and noise is illustrated as Additive White Gaussian Noise (AWGN).

Fig. 2B details the processing portion 220 with a front-end processing unit 222 coupled to baseband filters 226 and 228 for processing the in-phase (I) and quadrature (Q) components, respectively. Each baseband filter 226,228 is then coupled to a multiplier for multiplication with a corresponding carrier. The resulting waveforms are then summed at summing node 234 and transmitted over a communication channel to a receiver. At the receiver, an analog pre-processing unit 242 receives the transmitted signal, which is processed and delivered to a matched filter 244. The output of the matched filter 244 is then provided to an analog-to-digital (a/D) converter 246. Note that other modules may be implemented depending on design and operating criteria. The components and elements in fig. 2A and 2B are provided for understanding the following discussion and are not intended as a complete description of a communication system.

RAKE and equalizer combination

In another design, the RAKE receiver operates in parallel with the equalizer. Such a design is described in detail in application No. 09/624,319 filed on 24/7/2000 by John Smee et al, entitled "METHOD AND APPARATUS FOR PROCESSING THE SIGNAL USE AN EQUALIZER AND A RAKERECEIVER (METHOD AND APPARATUS FOR PROCESSING A MODULATED SIGNAL USING AN EQUALIZER AND RECEIVER)". A selection is made between the RAKE receiver and the equalizer to determine the best estimate of the received signal. For example, the selection may correspond to a minimum Mean Square Error (MSE) between the transmitted pilot signal and the estimate, or a highest signal-to-interference-and-noise ratio (SINR) at each output, or some other criterion. The performance metric or estimate provides a means to compare the RAKE and equalizer. The selected receiver configuration is then used to process the received data signal.

A receiver is said to be "universal" if its performance is optimal over the "universe" of possible channel conditions and channel rates of change. A receiver with a RAKE and equalizer is "generic" if the receiver configuration selected based on the MSE estimate or C/I estimate is actually the best of the two configurations. Thus, an accurate MSE estimate or C/I estimate is required to make the receiver "universal".

Fig. 3 is a block diagram of a receive data processor 310 in a mobile station 300, according to an embodiment of the present invention. In this embodiment, the rx data processor 310 includes two signal processing channels that can operate in parallel to provide improved performance, especially at higher rates. The first channel processing path includes an equalizer 312 coupled to a post-processor 314 and the second signal processing path includes a RAKE 316.

In the receive data processor 310, a sample stream from a pre-processor (not shown) is provided to each of the equalizer 312 and the RAKE 316. Each sample stream is generated from a respective received signal. Equalizer 312 performs equalization on the received sample stream and provides symbol estimates to post-processor 314. Post-processor 314 may further process the symbol estimates based on processing performed at the time of transmission to provide recovered symbols. In particular, if PN spreading and covering are performed at the transmitter unit, the post-processor may perform despreading using the complex PN sequence and decovering using one or more channelization codes. After the filter coefficients are adopted, phase rotation (which is accomplished by pilot demodulation of the RAKE receiver) is implicitly achieved by the equalizer 312.

RAKE 316 may process one or more multipaths of each received signal to provide recovered symbols for the received signal. For each sample stream, the RAKE 316 may perform PN despreading, decovering, and coherent modulation for multiple multipaths. RAKE 316 then combines the demodulated symbols of all multipaths of the received signal to generate recovered symbols of the received signal. RAKE 316 may further combine the recovered symbols of all received signals to provide overall recovered symbols, which are provided by the RAKE receiver.

The recovered symbols from post-processor 314 and RAKE 316 may be provided to a Switch (SW)320, which switch 320 selects the recovered symbols from either post-processor 314 or RAKE 316 to provide to a deinterleaver 322. The selected recovered symbols are then rearranged by a deinterleaver 322 and then decoded by a decoder 324. A controller 318 is coupled to and manages the operation of the equalizer 312, the post-processor 314, the RAKE receiver 316, and the switch 320.

In accordance with the present invention, equalizer 312 may be used to provide equalization of the received signal to reduce the amount of ISI in the received signal. Each received signal is distorted by the characteristics of the transmitter unit, the transmission channel, and the receiver unit. The equalizer 312 may equalize the overall response of each received signal, which reduces the amount of ISI. Lower ISI increases S/N and supports higher data rates.

Continuing with fig. 3, rx data processor 310 includes two signal processing paths that may be used to process the received signal. The first signal processing path includes an equalizer 310 and a post-processor 314 and the second signal processing path includes a RAKE 316. In one embodiment, two signal processing channels may operate in parallel (e.g., in an adaptation cycle), and a signal quality estimate may be calculated for each signal processing channel. The signal processing channel that provides the better signal quality may then be selected to process the received signal.

For a conventional RAKE, the received signal quality can be estimated by calculating the signal-to-noise ratio (S/N). For a CDMA system that transmits a TDM pilot reference, the S/N may be calculated in the pilot reference period when the received signal is known. A signal quality estimate may be generated for each assigned finger processor. The estimates of all assigned finger processors may then be weighted and combined to generate an overall S/N, which may be calculated as:

equation 1

Where β is the weighting factor used by the RAKE receiver to combine demodulated symbols from the assigned finger processors to provide recovered symbols, which are an improved estimate of the transmitted data, Es is the energy per symbol of the desired signal (e.g., pilot), and Nt is the total noise of the received signal being processed by the finger processors. Nt typically includes thermal noise, interference from other transmitting base stations, interference from other multipaths of the same base station, and other components. The unit symbol energy can be calculated as:

equation 2

Wherein P is_IAnd P_QIs an in-phase and quadrature filtered pilot symbol, N_SYMIs the number of symbols over which energy is accumulated to provide the value of Es. The filtered pilot symbols may be formed by despreading the samplesAccumulated over the length of the channelization code used to cover the pilot reference. The total noise can be estimated as the change in the desired signal energy, which can be calculated as:

equation 3

Measurement of received signal quality is described IN further detail IN U.S. patent No.5,903,554 entitled "METHOD AND APPARATUS FOR MEASURING link quality IN a spread spectrum COMMUNICATION SYSTEM" AND U.S. patent No.5,799,005 entitled "SYSTEM AND METHOD FOR detecting receiving POWER AND PATH LOSS IN a CDMA COMMUNICATION SYSTEM" both assigned to the assignee of the present invention.

For signal processing channels that include the equalizer 312, the signal quality may be estimated using a variety of criteria, including mean square average (MSE). Again, for CDMA systems that transmit TDM pilot references, the MSE may be estimated in the pilot reference period and may be calculated as:

equation 4

Wherein N is_SYMIs the number of samples over which the error is accumulated to provide the MSE. Typically, the mean square error is averaged over multiple samples and averaged over one or more pilot references to obtain a desired level of confidence in the measurements. The mean square error can then be converted to an equivalent signal-to-noise ratio, which can be expressed as:

linearity

dB equation 5

S/N of signal processing path with equalizer 312_EQS/N compatible with signal processing channel with RAKE 316_EQA comparison is made. The signal processing channel that provides the better S/N may then be selected to process the received signal.

Alternatively, the MSE may be calculated for the signal processing channel with RAKE 316 and compared to the MSE calculated for the signal processing channel with equalizer 312. The signal processing channel with the better MSE may then be selected.

For an HRPD system, the S/N is estimated at the remote terminal and used to determine the maximum data rate that the remote terminal can receive at that operating condition. The maximum data rate is then sent back to the base station for which the S/N is estimated. The base station then transmits to the remote terminal at a data rate up to the identified maximum data rate.

According to the present invention, the data rate of a data transmission can be estimated using various methods. In one approach, the S/N may be estimated for the RAKE or for the equalizer based on the calculated MSE. The best S/N from all signal processing channels can then be used to determine the maximum data rate that can be supported. Alternatively, MSE may be used to directly determine the maximum data rate. The optimal S/N, MSE or maximum data rate may be sent to the base station.

Under certain operating conditions, a signal processing path with an equalizer may provide better performance than a signal processing path with a RAKE receiver. For example, a signal processing path with an equalizer works better at high S/N and for channels with ISI. RAKE may be used to handle multipath, which also generates ISI. In practice, the RAKE can be viewed as a filter with L taps (where L corresponds to the number of finger processors), where each tap corresponds to a time delay that can be adjusted. However, RAKE cannot effectively reduce ISI due to frequency distortion in the received signal.

The equalizer can more effectively reduce ISI due to frequency distortion. This is accomplished by providing a response that approximates the inverse of the frequency distortion when attempting to minimize the overall noise, including ISI. The equalizer thus "inverts" the channel and also attempts to eliminate the effects of multipath. In practice, each filter 410 is equivalent to a finger processor when the coefficients are initialized to { 0., 0,1, 0., 0 }. Then, once the zero-valued coefficients are adjusted, the filter frequency response is changed to equalize the signal distortion. In this way, the equalizer can be used to efficiently handle multipath-induced ISI as well as channel-induced ISI.

For simplicity, many aspects and embodiments of the present invention are described with respect to a spread spectrum communication system. However, many of the spirit of the present invention described herein can be applied to a non-spread spectrum communication system, as well as a communication system capable of selectively performing direct sequence spread spectrum, such as an HRPD system.

RAKE and hybrid equalizer configuration

According to one embodiment, equalizer 312 may be a hybrid equalizer, wherein equalizer 312 is enabled when operating conditions, including but not limited to channel conditions, cause equalizer 312 to be used. In other words, the equalizer 312 is activated when the equalizer is deemed to perform as well as, or better than, the RAKE 316. Otherwise, the equalizer does not operate. Thus, the system achieves power savings when the equalizer is considered to perform worse than the RAKE 316. Such an equalizer is referred to as a "hybrid" equalizer because the equalizer is responsive to operating conditions.

The hybrid equalizer and RAKE receiver architecture operates by comparing operating condition metrics, such as the potential demodulated SINR outputs of the RAKE and equalizer, and then selecting the mode that achieves the best performance. Modes may include, but are not limited to, RAKE only modes, RAKE and equalizer modes. One embodiment includes a test mode that periodically runs the equalizer and selects between a RAKE only mode and a RAKE and equalizer mode. Hybrid equalizers typically perform better in high geometry and slow fading conditions. Under these conditions, the equalizer provides performance gains over conventional RAKE-only designs. However, in the simplest implementation, the cost of running both methods is prohibitive, leading to increased power consumption even for the case where the equalizer does not provide any gain over the RAKE.

Ideally, the equalizer only works when a performance gain is achieved. Hybrid equalizers provide power reduction by using decision algorithms based on transient operating conditions, such as correlation statistics and/or receiver SINR. The hybrid equalizer only operates when channel conditions may produce performance gains.

Because the equalizer depends on slow fading channel conditions, one embodiment estimates the fading dynamics by estimating the intra-pilot impulse correlation statistics. The equalizer typically produces gain under higher geometry conditions (i.e., SINR), where another embodiment estimates SINR from pilot bursts. Both metrics can be used in the decision algorithm. If the correlation metric is greater than a given threshold and the SINR is also greater than another threshold, the equalizer is enabled, otherwise the equalizer is disabled. This reduces power consumption by avoiding the use of equalizers when benefits are not achieved.

Fig. 5 is a state diagram illustrating the operation 500 of a receiver according to one embodiment implementing a RAKE and hybrid equalizer. Two modes are implemented: RAKE-only mode 502; and RAKE and hybrid equalizer modes 504. Operation starts in the RAKE only mode. While in mode 502, operation remains in mode 502 until a change in operating conditions is sufficient to indicate that an increase in performance can be achieved after the equalizer is added. To determine if the operating conditions have changed sufficiently, (such as slowly varying channel conditions), a channel quality metric is evaluated. In this embodiment, the channel quality metric is the signal-to-interference-and-noise ratio (SINR) of the RAKE output (SINR)_RAKE) Measured and compared to a threshold (T) for triggering the equalizer_EQU) A comparison is made. Similarly, a correction metric (C) is determined for RAKE_RAKE) And to match it with the corresponding correction metric (C) of the equalizer_EQU) A comparison is made. When the SINR is_RAKEGreater than T_EQUAnd C is_RAKEGreater than C_EQURunning to RAKEAnd an equalizer mode 504. Thus, when operating conditions cause the equalizer to be used, mode 504 is entered and equalizer operation is initiated.

While in mode 504, the system continues to monitor the SINR of the RAKE output and the equalizer output (SINR)_EQU). When the SINR is_RAKEGreater than SINR_EQUOperation transitions to mode 502. The use of the equalizer is usually prompted by an increase in SINR, since SINR (according to the current geometry of the system) represents the channel condition. For low SINR, the equalizer is not running, so SINR acts as a good trigger to turn the equalizer on and off. The triggering of entering mode 504 (i.e., starting the equalizer) is effectively a two-part consideration. The first evaluation determines whether the channel conditions (e.g., SINR) are consistent with conditions under which the equalizer operates to improve performance. The second evaluation determines the channel speed or, in other words, how fast the mobile station is moving in the cellular network. According to an embodiment, the second evaluation determines a cross-correlation between the pilot bursts. Cross-correlation measures the degree of correlation between two levels. In this case, as the correlation of the signals increases, the delay between the two signals decreases. Similarly, the correlation decreases as the delay increases. Thus, as the distance between the mobile station and the receiver increases, or changes, the correlation between the received signals decreases. For low cross-correlation, the equalizer is enabled in mode 504, otherwise the RAKE only mode 502 is maintained. Cross-correlation can be measured on the pilot signal or pilot burst since this is a known signal that provides a confidence result.

As an example, the cross-correlation criterion is considered as follows. Given pilot symbol P_□The correlation between successive pilot symbols may be determined by calculating the correlation at N_SUMThe average over the pilot symbols is estimated as:

equation 6

The correlation metric ranges from 0 to 1. A correlation of 1 indicates a strong correlation, which results in good equalizer performance because the channel does not change between consecutive pilot symbols.

Other embodiments may define other parameters and metrics that can trigger the equalizer. A metric may be selected that provides the desired equalizer performance. Metrics may be selected that are specific to system design and performance goals.

Fig. 6 is a state diagram of operation 520 of an alternative embodiment having three modes of operation. In the RAKE-only mode 522, RAKE is used when the equalizer is not operating. The operation periodically enters a test mode 524 by measurement of the sampling period (either pre-specified or adaptive).

In test mode 524, equalizer operation is enabled. Test mode 524 enables the equalizer to determine whether the performance of the equalizer improves the performance of the receiver. The results of the RAKE and equalizer are compared to evaluate the performance of the equalizer. When the SINR is_RAKELess than SINR_EQUOperation transitions to RAKE and equalizer mode 526, where both RAKE and equalizer are enabled. In this case, the equalizer shows the ability to improve performance. If the results of test mode 524 indicate SINR_RAKEGreater than SINR_EQUOperation then transitions back to RAKE only mode 522. In this case, the equalizer does not improve performance and is not believed to provide an overall improvement in performance under the current conditions. Note that a margin value (δ) may be shown where SINR is biased according to system design and/or performance_RAKEOr SINR_EQU. The sampling period can be designed to be needed to run the equalizerA function of time, wherein the sampling period is sufficient to allow data to pass through the filter elements of the equalizer.

In mode 526, the system monitors the channel quality metric. When the SINR is_RAKEGreater than SINR_EQUAt this point, operation transitions to RAKE only mode 522. Note that the conversion uses a margin value (δ) to avoid the rotation between modes. Thus, a transition from mode 524 to mode 526 occurs when the equalizer generated estimated SINR exceeds the RAKE generated SINR by more than a margin value. Similarly, a transition from mode 524 to mode 522 occurs when the estimated SINR generated by the RAKE exceeds the SINR generated by the equalizer by more than a margin value. Further, a transition from mode 526 to mode 522 occurs when the estimated SINR generated by the RAKE exceeds the SINR generated by the equalizer by more than a margin value.

High rate packet data communication system

A specific high data rate system will be apparent from the following discussion. Other embodiments may be implemented that provide for the transmission of information at high data rates. For CDMA communication systems designed to transmit at higher data rates, such as High Rate Packet Data (HRPD) or High Data Rate (HDR) communication systems, a variable data rate request scheme may be used to communicate at the maximum data rate supported by the C/I. HDR communication systems are typically designed to comply with one or more standards, such as "cdma 2000 High Rate Packet Data Air Interface Specification", 3GPP2 c.s0024, release 2, year 2000, month 10, day 27, promulgated by the association "third generation partnership project".

In general, in AN HRPD system, AN Access Network (AN) is defined as a network device that provides data connectivity between a cellular network and a packet-switched data network (typically the internet) and AN AT. The AN in the HRPD system is equivalent to a base station in a cellular communication system. An Access Terminal (AT) is defined as a device that provides a data connection to a user. The AT in the HRPD system corresponds to a mobile station in a cellular communication system. The AT may be connected to a computing device, such as a laptop personal computer, or the AT may be a stand-alone data device, such as a Personal Digital Assistant (PDA). Note that the terms mobile station, remote terminal, and access terminal are used interchangeably.

Fig. 4 illustrates a receiver in an exemplary HDR communication system using a variable data request scheme. Receiver 400 is a subscriber station that communicates with a land-based data network by transmitting data on the reverse link to a base station (not shown). The base station receives the data and routes the data through a Base Station Controller (BSC) (also not shown) to the land-based network. Instead, communications to the subscriber station 400 may be routed from the land-based network through the BSC to the base station and transmitted on the forward link from the base station to the subscriber unit 150. The forward link refers to transmission from a base station to a subscriber station, and the reverse link refers to transmission from a subscriber station to a base station.

In an exemplary HDR communication system, forward link data transmission from the base station to the subscriber station 400 should be conducted using the maximum data rate or near the maximum data rate supported by the forward link. Initially, the subscriber station 400 establishes communication with the base station using a predetermined access procedure. In the connected state, the subscriber station 400 may receive data and control messages from the base station and may be able to transmit data and control messages to the base station. The subscriber station 400 then estimates the C/I of the forward link transmission from the base station 400. The C/I of the forward link transmission may be obtained by measuring pilot signals from the base stations. Based on the C/I estimate, the subscriber station 400 sends a data rate request message to the base station as a Data Rate Control (DRC) message on the allocated DRC channel. The DRC message may include the requested data rate or an indication of the quality of the forward link channel, such as the C/I measurement itself, the bit error rate or the packet error rate. The base station uses the DRC message from the subscriber station 400 to efficiently transmit forward link data at the highest possible rate.

A BSC (not shown) may interface with a packet network interface, the PSTN, and/or other base stations and be used to coordinate communications between subscriber stations and other users.

The forward link pilot channel provides a pilot signal that is used by the subscriber station 400 for initial acquisition, phase recovery, and timing recovery. In addition, the pilot signal may also be used by the subscriber station 400 to perform C/I measurements. In the exemplary embodiment depicted, each time slot on the forward link is 2048 chips long with two pilot bursts occurring at the end of one quarter and three quarters of the time slot. Each pilot burst is 96 chips in duration. Each slot contains two parts, where each half slot includes one pilot burst.

The forward link transmission is received through an antenna of the subscriber station 400. The received signal is routed by the antenna to the receiver, which has an analog pre-processing unit 402, a matched filter 404, and an analog-to-digital (a/D) converter 406. The receiver filters and amplifies the signal, downconverts the signal to baseband, quadrature demodulates the baseband signal, and digitizes the baseband signal. The digitized baseband signal is coupled to a demodulator. The demodulator includes a carrier and timing recovery circuit and also includes an equalizer 410. The equalizer 410 compensates for ISI and generates symbol estimates from the digitized baseband signal. The symbol estimates are coupled to a controller 416 by a communication bus 420. The controller then generates a DRC message. The output of the equalizer 410 is also provided to a decoder 412. The decoder 412, equalizer 410, and controller 416 are each coupled to a communication bus 420.

In addition to generating DRC messages, a controller 416 can be used to support data and message transmission on the reverse link. Controller 416 may be implemented as a microcontroller, microprocessor, Digital Signal Processing (DSP) chip, ASIC programmed to perform the functions described herein, or any other implementation known in the art. Timing unit 414 is also coupled to communication bus 420. The exemplary embodiment includes a sample memory unit 408 that is coupled to an equalizer 410 and a controller 416 by a communication bus 420.

RAKE 418 is also coupled to communication bus 420 and receives input for processing through a structure similar to that shown in fig. 1. The equalizer controller 422 receives estimates from the RAKE 418 and estimates from the hybrid equalizer 410 when in operation. The equalizer controller 422 then determines when the equalizer is in use and initiates operation. Similarly, the equalizer controller 422 determines when the equalizer is not being used and begins terminating operation. Various monitoring units may be implemented to check operational metrics such as channel quality and/or channel speed. The equalizer controller 422 uses this information to make equalizer decisions.

Performance measurement

As described herein, the equalizer configuration may be selected based on measurements of SINR, C/I, or other performance criteria. Other performance criteria may include, for example, the equalizer configuration mean square error measured on pilot samples. For example, if the equalizer output on the pilot samples is given asAnd the desired pilot samples are denoted as { { y { {_nN ═ 1.., K } }, the Mean Square Error (MSE) of the configuration is given as:

equation 7

One definition of SINR or C/I estimation is:

equation 8

Other representations or performance metrics are also possible.

The models, methods, and apparatus presented herein support different systems, channel conditions, and receiver designs, as examples of various embodiments. The application of the parallel equalizer described herein may be implemented as any of a variety of receivers adapted to operate in a variety of communication systems, including, but not limited to, high data rate systems.

Those of ordinary skill in the art will appreciate that the various logical blocks, modules, circuits, and algorithms 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 algorithms 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 the following: 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 functional processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may 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 methods or algorithms 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 processor and the storage medium are disposed 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.

The 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 half of the definitions herein may be applied to other embodiments without departing from the spirit and 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.

Claims (32)

1. A method of receiving data in a wireless communication system, the method comprising:

processing the received signal by a RAKE processing element to generate a RAKE processed signal;

measuring a first quality metric of the RAKE processed signal;

comparing a first quality metric of the RAKE processed signal to a first threshold; and

enabling an equalizer when the first quality metric exceeds the first threshold.

2. The method of claim 1, further comprising:

measuring a correction metric for the RAKE processed signal; and

comparing the correction metric to a second threshold,

wherein enabling the equalizer further comprises:

enabling an equalizer when the first quality metric exceeds the first threshold and the correction metric exceeds the second threshold.

3. The method of claim 2, wherein the first quality metric is a signal-to-noise ratio.

4. The method of claim 2, wherein the correction metric is a cross-correlation metric.

5. The method of claim 4, wherein the cross-correlation is measured between pilot bursts.

6. The method of claim 2, wherein after enabling the equalizer, the method further comprises:

measuring a first quality metric of the equalizer processed signal;

comparing a first quality metric of the equalizer processed signal to a first quality metric of the RAKE processed signal; and

disabling the equalizer when the first quality metric of the equalizer processed signal is less than the first quality metric of the RAKE processed signal.

7. A method of receiving data in a wireless communication system, the method comprising:

processing the received signal by a RAKE processing element to generate a RAKE processed signal; and

periodically testing the operating conditions, including:

processing the received signal by an equalizer to generate an equalizer-processed signal;

measuring a first quality metric of the RAKE processed signal;

measuring a first quality metric of the equalizer processed signal;

comparing a first quality metric of the RAKE processed signal to a first quality metric of the equalizer processed signal; and

determining whether to enable the equalizer based on the comparison.

8. The method of claim 7, wherein determining whether to enable the equalizer based on the comparison comprises determining to disable the equalizer if the first quality metric of the RAKE processed signal exceeds the first quality metric of the equalizer processed signal by a margin value.

9. The method of claim 8, wherein determining whether to enable the equalizer based on the comparison comprises determining to enable the equalizer if the first quality metric of the RAKE processed signal does not exceed the first quality metric of the equalizer processed signal by the margin value.

10. The method of claim 9, wherein the first quality metric is a signal-to-interference-and-noise ratio.

11. The method of claim 10, wherein when the equalizer is enabled, the method further comprises:

terminating the test;

processing, by the equalizer, a received signal to generate an equalizer-processed signal;

measuring a first quality metric of the RAKE processed signal;

measuring a first quality metric of the equalizer processed signal;

comparing a first quality metric of the RAKE processed signal to a first quality metric of the equalizer processed signal; and

determining whether to disable the equalizer based on the comparison.

12. The method of claim 11, wherein determining whether to disable the equalizer based on the comparison comprises determining to disable the equalizer if the first quality metric of the RAKE processed signal exceeds the first quality metric of the equalizer processed signal by a margin value.

13. The method of claim 12, wherein determining whether to disable the equalizer based on the comparison comprises determining to enable the equalizer if the first quality metric of the RAKE processed signal does not exceed the first quality metric of the equalizer processed signal by the margin value.

14. The method of claim 13, wherein periodically testing the operating conditions further comprises:

the test is initialized once in a sampling period, wherein the sampling period is a function of a time constant of an equalizer filter.

15. An apparatus for receiving data in a wireless communication system, the method comprising:

means for processing the received signal by a RAKE processing element to generate a RAKE processed signal;

means for measuring a first quality metric of the RAKE processed signal;

means for comparing a first quality metric of the RAKE processed signal to a first threshold; and

means for enabling an equalizer when the first quality metric exceeds the first threshold.

16. A receiver in a wireless communication system, the receiver comprising:

a processing element to process computer readable instructions; and

a memory device adapted to store computer-readable instructions, the instructions comprising:

a first set of computer-readable instructions for processing a received signal by a RAKE processing element to generate a RAKE processed signal;

a first set of computer-readable instructions for measuring a first quality metric of the RAKE processed signal;

a first set of computer-readable instructions for comparing a first quality metric of the RAKE processed signal to a first threshold; and

a first set of computer readable instructions for enabling an equalizer when the first quality metric exceeds the first threshold.

17. An apparatus for receiving data in a wireless communication system, the apparatus comprising:

means for processing the received signal by a RAKE processing element to generate a RAKE processed signal; and

apparatus for periodically testing operating conditions, comprising:

means for processing the received signal by the equalizer to generate an equalizer processed signal;

means for measuring a first quality metric of the RAKE processed signal;

means for measuring a first quality metric of the equalizer processed signal;

means for comparing a first quality metric of the RAKE processed signal to a first quality metric of the equalizer processed signal; and

means for determining whether to enable the equalizer based on the comparison.

18. A receiver in a wireless communication system, the receiver comprising:

a processing element to implement computer readable instructions; and

a memory device to store computer-readable instructions to:

processing the received signal by a RAKE processing element to generate a RAKE processed signal; and

periodically testing the operating conditions, including the operations of:

processing the received signal by an equalizer to generate an equalizer-processed signal;

measuring a first quality metric of the RAKE processed signal;

measuring a first quality metric of the equalizer processed signal;

comparing a first quality metric of the RAKE processed signal to a first quality metric of the equalizer processed signal; and

determining whether to enable the equalizer based on the comparison.

19. A wireless communication device, comprising:

a RAKE receiver adapted to receive a signal and generate an estimate of the received signal;

an equalizer; and

an equalization controller adapted to control operation of the equalizer in response to the estimate from the RAKE receiver.

20. The apparatus of claim 19, wherein the equalization controller enables the equalizer when the estimated channel quality metric exceeds a threshold.

21. The apparatus of claim 20, wherein the equalization controller enables the equalizer when the estimated channel quality metric is greater than the threshold and the estimated first correlation is greater than a second correlation of an equalized estimate generated by the equalizer.

22. The apparatus of claim 21, wherein the first correlation and the second correlation are based on received pilot signals.

23. The apparatus of claim 19, wherein the equalization controller disables the equalizer when a channel quality metric of the estimate from the RAKE receiver is greater than a channel quality metric of an equalized estimate generated by the equalizer.

24. The apparatus of claim 19, wherein the equalization controller periodically enables the equalizer to compare an equalized estimate generated by the equalizer to an estimate from the RAKE receiver.

25. The apparatus of claim 24, wherein the equalization controller compares a channel quality metric of an equalized estimate generated by the equalizer to a channel quality metric of an estimate from the RAKE receiver.

26. The apparatus of claim 24, wherein the equalization controller compares a channel rate of an equalized estimate generated by the equalizer to an estimated channel rate from the RAKE receiver.

27. The apparatus of claim 19, wherein the equalizer, when enabled, is adapted to operate in a first mode of operation and a second mode of testing.

28. The apparatus as in claim 27, wherein the equalizer transitions from the second test mode to the first operating mode when a channel quality metric of an equalized estimate generated by the equalizer is greater than a channel quality metric of an estimate from the RAKE receiver.

29. The apparatus of claim 28, wherein the equalization controller disables the equalizer when a signal-to-noise ratio of the estimate from the RAKE receiver is greater than an equalized estimate from the equalizer.

30. The apparatus of claim 19, wherein the apparatus comprises two modes of operation comprising:

a first mode in which the RAKE receiver is enabled and the equalizer is disabled;

a second mode in which the RAKE receiver and the equalizer are enabled.

31. The apparatus of claim 19, wherein the apparatus is adapted for two configurations, comprising:

a first configuration in which the RAKE receiver is enabled and the equalizer is disabled;

a second configuration, wherein the RAKE receiver and the equalizer are enabled.

32. The apparatus as recited in claim 30, wherein said apparatus has a third mode of operation comprising:

a test mode in which the equalizer is enabled for a sampling period and the equalized estimate is compared to an estimate from the RAKE receiver.