US 20030206577 A1 Abstract Methods and systems in a wireless receiver for enabling the reception of input signals at varied power levels in the presence of co-channel interference utilizing combinations of space-time adaptive processing (STAP), interference cancellation multi-user detection (MUD), and combined STAP/MUD techniques. In MUD, code, timing, and possibly channel information of multiple users are jointly used to better detect each individual user. The novel combination of adaptive signal reconstruction techniques with interference cancellation MUD techniques provides accurate temporal cancellation of interference with minimal interference residuals. Additional methods and systems extend adaptive signal reconstruction techniques to take Doppler spread into account. STAP techniques permit a wireless receiver to exploit multiple antenna elements to form beams in the direction of the desired signal and nulls in the direction of the interfering signals. The combined STAP-MUD methods and systems increase the probability of successful user detection by taking advantage of the benefits of each reception method. An additional method and system utilizes STAP techniques in the case where no pilot signal is available. This method compares the outputs of various hypothesized STAP solutions.
Claims(75) 1. In a wireless receiver for a CDMA system, a combination for enabling the receiver to receive input signals at varied power levels in the presence of interference, said combination comprising:
a control processor comprising a means for ordering user signals; and a plurality of SIC-ATRF processors combining successive interface cancellation (SIC) multi-user detection and adaptive temporal reconstruction filtering in a successive arrangement wherein the output of one of the said processors is the input to the next successive processor, each of said SIC-ATRF processors comprising:
a conventional detector;
a respread processor;
an adaptive temporal filter (ATRF); and
a complex mathematical operation processor for canceling the reconstructed signal for the user from the total received signal.
2. The combination of 3. The combination of 4. The combination of tap weights;
a tap delay line; and
a mathematical summing circuit.
5. The combination of 6. The combination of 7. The combination of 8. The combination of 9. The combination of 10. The combination of 11. The combination of 12. A method in a combination system for enabling the receiver to receive input signals at varied power levels in the presence of interference wherein said combination comprises a control processor and a plurality of SIC-ATRF processors in a successive arrangement, said method comprising:
ordering user signals according to a pre-defined methodology and assigning each user signal to one of the SIC-ATRF processors wherein each SIC-ATRF processor comprises a conventional detector, a respread processor, an adaptive temporal filter; and a complex mathematical operation processor for canceling the reconstructed signal for the user from the total received signal; communicating a separate user code associated with the user signal to each SIC-ATRF processor according to the ordering; in each successive SIC-ATRF processor, performing the steps of:
despreading, in the conventional detector, the received signal and estimating a symbol transmitted for the desired user signal;
communicating the symbol estimate to the respread processor;
spreading, in the respread processor, the symbol estimate;
estimating a channel for the user associated with the SIC-ATRF processor and reconstructing the signal interference associated with the user signal; and
canceling, in a mathematical operations processor, the reconstructed signal for the user associated with the SIC-ATRF processor from the total received signal.
if a plurality of SIC-ATRF processors remain, inputting the output of the current SIC-ATRF processor to the next successive SIC-ATRF processor. 13. The method of 14. The method of 15. The method of determining the adaptive filter tap weights that minimize a pre-determined cost function between the received signal and an output of the adaptive filter; and
updating, in the ATRF, the filter tap weights.
16. The method of 17. The method of determining the adaptive filter tap weights by jointly minimizing the cost function between the received signal and the sum of the outputs of the respread processors of previous SIC-ATRF processors; and
updating, in the ATRF, the filter tap weights.
18. The method of 19. A method in a combination system for enabling the receiver to receive input signals at varied power levels in the presence of interference wherein said combination comprises a control processor and a plurality of SIC-ATRF processors in a successive arrangement, said method comprising:
ordering user signals according to a pre-defined methodology and assigning each user signal to one of the SIC-ATRF processors wherein each SIC-ATRF processor comprises a conventional detector, a respread processor, a frequency shift processor, an adaptive temporal filter; and a complex mathematical operation processor for canceling the reconstructed signal for the user from the total received signal.; communicating a separate user code associated with the user signal to each SIC-ATRF processor according to the ordering; in each successive SIC-ATRF processor, performing the steps of:
despreading, in the conventional detector, the received signal and estimating a symbol transmitted for the desired user signal;
communicating the symbol estimate to the respread processor;
spreading, in the respread processor, the symbol estimate;
shifting, in the frequency shift processor, the symbol estimate generated by the respread processor for the user associated with the SIC-ATRF processor;
estimating a channel for the user associated with the SIC-ATRF processor and reconstructing the signal interference associated with the user signal; and
canceling, in a mathematical operations processor, the reconstructed signal for the user associated with the SIC-ATRF processor from the total received signal.
If a plurality of SIC-ATRF processors remain, inputting the output of the current SIC-ATRF processor to the next successive SIC-ATRF processor. 20. The method of determining the adaptive filter tap weights that minimize a pre-determined cost function between the received signal and an output of the adaptive filter; and
updating, in the ATRF, the filter tap weights.
21. The method of determining the adaptive filter tap weights by jointly minimizing the cost function between the received frequency shift estimate and the sum of the outputs of the frequency shift processors of previous SIC-ATRF processors; and
updating, in the ATRF, the filter tap weights.
22. In a wireless receiver for a CDMA system, a combination for enabling the receiver to receive input signals at varied power levels in the presence of interference, said combination comprising:
a plurality of processors in a parallel arrangement wherein the input to every processor is a received signal, each of said parallel processors comprising:
a conventional detector;
a respread processor; and
an adaptive temporal filter (ATRF); and
means for summing signals to form an interference estimate and subtracting the estimate from the received signal. 23. The combination of 24. The combination of 25. The system of 26. The combination of tap weights;
a tap delay line; and
a mathematical summing circuit.
27. The combination of 28. The combination of 29. The combination of 30. The combination of 31. The combination of 32. The combination of 33. In a wireless receiver for a CDMA system, a combination for enabling the receiver to receive input signals at varied power levels in the presence of interference, said combination comprising:
a plurality of processors in a parallel arrangement wherein the input to every processor is a received signal, each of said parallel processors comprising:
a conventional detector; and
a respread processor;
an adaptive temporal filter (ATRF); and means for summing signals to form an interference estimate and subtracting the estimate from the received signal. 34. The combination of 35. The combination of 36. The combination of 37. A method in a combination system for enabling a receiver to receive input signals at varied power levels in the presence of interference wherein said combination comprises a plurality of parallel processors and means for summing signals to form an interference estimate and subtracting the estimate from the received signal:
communicating a received signal to the plurality of parallel processors wherein each of the processors comprises a conventional detector, a respread processor, and an adaptive temporal filter (ATRF); generating a reconstructed signal in each of the parallel processors for a user associated with the parallel processor; said generation step further comprising:
despreading, in the conventional detector, the received signal and estimating a symbol transmitted for the desired user signal;
communicating the symbol estimate to the respread processor;
spreading, in the respread processor, the symbol estimate; and
estimating a channel for the signal associated with the parallel processor and reconstructing the signal interference;
communicating the output of each parallel processor to a means for mathematically summing signals and subtracting signal estimates from the received signal; generating, in the mathematical means, an interference estimate for each user; and subtracting, in the mathematical means, the interference estimate from the received signal. 38. The method of ordering user signals according to a pre-defined methodology; and
communicating a separate user code to a conventional detector in each parallel processor.
39. The method of determining the adaptive filter tap weights that minimize a pre-determined cost function between the received signal and an output of the adaptive filter; and
updating, in the ATRF, the filter tap weights.
40. The method of determining the adaptive filter tap weights by jointly minimizing the cost function between the received signal and the sum of the outputs of the respread processors of all the other parallel processors; and
updating, in the ATRF, the filter tap weights.
41. A method in a combination system for enabling a receiver to receive input signals at varied power levels in the presence of interference wherein said combination comprises a plurality of parallel processors, an adaptive temporal filter (ATRF), and means for summing signals to form an interference estimate and subtracting the estimate from the received signal, the method comprising the steps of:
communicating a received signal to the plurality of parallel processors wherein each of the processors comprises a conventional detector and a respread processor; generating a reconstructed signal in each of the parallel processors for a user associated with the parallel processor; said generation step further comprising:
communicating the symbol estimate to the respread processor; and
spreading, in the respread processor, the symbol estimate;
estimating a channel for the signal associated with each parallel processor and reconstructing the signal interference; communicating the output of the ATRF to a means for mathematically summing signals and subtracting signal estimates from the received signal; generating, in the mathematical means, an interference estimate for each user; and subtracting, in the mathematical means, the interference estimate from the received signal. 42. The method of updating, in the ATRF, the filter tap weights.
43. The method of determining the adaptive filter tap weights by jointly minimizing the cost function between the received signal and the sum of the outputs of the respread processors of all the other parallel processors; and
updating, in the ATRF, the filter tap weights.
44. A method in a combination system for enabling a receiver to receive input signals at varied power levels in the presence of interference wherein said combination comprises a plurality of parallel processors and means for summing signals to form an interference estimate and subtracting the estimate from the received signal, the method comprising the steps of:
communicating a received signal to the plurality of parallel processors wherein each of the processors comprises a conventional detector, a respread processor, a frequency shift processor and an adaptive temporal filter (ATRF); generating a reconstructed signal in each of the parallel processors for a user associated with the parallel processor; said generation step further comprising:
communicating the symbol estimate to the respread processor;
spreading, in the respread processor, the symbol estimate;
shifting, in the frequency shift processor, the symbol estimate generated by the respread processor for the user association with the parallel processor; and
estimating a channel for the signal associated with the parallel processor and reconstructing the signal interference;
communicating the output of each parallel processor to a means for mathematically summing signals and subtracting signal estimates from the received signal; generating, in the mathematical means, an interference estimate for each user; and subtracting, in the mathematical means, the interference estimate from the received signal. 45. The method of updating, in the ATRF, the filter tap weights.
46. The method of determining the adaptive filter tap weights by jointly minimizing the cost function between the received signal and the sum of the outputs of the respread processors of all the other parallel processors; and
updating, in the ATRF, the filter tap weights.
47. In a wireless receiver for a CDMA system, a combination for enabling the receiver to receive input signals at varied power levels in the presence of interference wherein said input signal is a vector comprised of one signal from each antenna in an antenna array of the receiver, said combination comprising:
a space-time adaptive processing (STAP) processor; means for hypothesizing possible symbols transmitted during a symbol period; a respread processor; means for weight computation wherein the hypothesized symbol and the vector input symbol are used to form a set of STAP weights which filter the input data spatially and temporally; a matched filter bank; means for determining a metric to measure the quality of the matched filter bank; and means for comparing generated metrics. 48. The combination of 49. The combination of 50. The combination of 51. The combination of 52. A method in a combination system for enabling a receiver to receive input signals at varied power levels in the presence of interference wherein said input signal is a vector comprised of one signal from each antenna in an antenna array of the receiver; the method comprising the steps of:
receiving the input signal vector; determining a metric, said determining step comprising:
hypothesizing which symbol was transmitted;
respreading the hypothesized symbol;
determining STAP weight update based on respread hypothesized symbol and input signal vector;
applying a tap delay line to each signal vector component;
applying STAP weights to each signal vector component, one per antenna;
summing the weighted results in a mathematical summation circuit;
despreading the output of the summation circuit and inputting the despread signal to a matched filter bank; and
generating a metric associated with the hypothesized signal;
repeating the metric determining step for each of the possible 64 Walsh symbols; comparing each of the 64 metrics; and determining, based on the comparison, the transmitted symbol. 53. A method in a combination system for enabling a receiver to receive input signals at varied power levels in the presence of interference wherein said input signal is a vector comprised of one signal from each antenna in an antenna array of the receiver; the method comprising the steps of:
receiving the input signal vector; determining a metric, said determining step comprising:
hypothesizing which symbol was transmitted;
respreading the hypothesized symbol;
determining STAP weight update based on respread hypothesized symbol and input signal vector;
despreading the delayed signal vector components, one per antenna;
applying the STAP weights to each of the despread signal vector components;
summing the weighted results in a mathematical summation circuit; and
generating a metric associated with the hypothesized signal;
repeating the metric determining step for each of the possible 64 Walsh symbols; comparing each of the 64 metrics; and determining, based on the comparison, the transmitted symbol. 54. In a wireless receiver for a CDMA system, a combination for enabling the receiver to receive input signals at varied power levels in the presence of interference wherein said input signal is a vector comprised of one signal from each antenna in an antenna array of the receiver, said combination comprising:
a control processor comprising a means for ordering user signals; and a plurality of STAP/VSIC-ATRF processors combining successive interface cancellation (SIC) multi-user detection, space-time adaptive processing and adaptive temporal reconstruction filtering in a successive arrangement wherein the output of one of the said processors is the vector input to the next successive processor, each of said STAP/VSIC-ATRF processors comprising:
a space-time adaptive processing (STAP) processor;
a plurality of adaptive temporal filters (ATRFs), one per antenna; and
a plurality of complex mathematical operation processors, one per ATRF, for canceling the reconstructed signal for the user from the total received signal.
55. The combination of a plurality of filters, one per antenna;
a mathematical summation processor for combining the outputs of all the filters;
a conventional detector; and
a minimum cost channel weight update processor.
56. The combination of 57. The combination of 58. The combination of 59. The combination of tap weights;
a tap delay line; and
a mathematical summing circuit.
60. The combination of 61. The combination of 62. The combination of 63. The combination of 64. In a wireless receiver for a CDMA system, a combination for enabling the receiver to receive input signals at varied power levels in the presence of interference, said combination comprising:
a plurality of processors in a parallel arrangement wherein the input to every processor is a received signal, each of said parallel processors comprising:
a space-time adaptive processing (STAP) processor; and
a plurality of adaptive temporal filters (ATRFs); and
means for summing signals to form an interference estimate and subtracting the estimate from the received signal. 65. The combination of a plurality of filters, one per antenna;
a mathematical summation processor for combining the outputs of all the filters; and
a conventional detector.
66. The combination of 67. The combination of 68. The combination of 69. The combination of 70. The combination of 71. The combination of 72. The combination of tap weights;
a tap delay line; and
a mathematical summing circuit.
73. The combination of 74. The combination of 75. The combination of Description [0001] This application claims the benefit of U.S. Provisional Application No. 60/190,803 filed Mar. 21, 2000. [0002] This invention relates to wireless communication networks and more specifically to CDMA wireless systems subject to co-channel interference. [0003] Code Division Multiple Access (CDMA) networks are widely deployed throughout the world. The current implementations of CDMA typically follow the IS-95 industry standards and are referred to as IS-95 wireless systems. With the advent of enhancements to CDMA technology such as third generation CDMA, CDMA2000 and W-CDMA, the deployment of CDMA is expected to increase dramatically. [0004] A typical CDMA system [0005] On the customer side, users connect to the wireless network through wireless mobile nodes [0006] One advantage of CDMA over other wireless access systems is that all users share the same spectrum at the same time. However, the fact that multiple users occupy the same bandwidth limits performance and capacity. Because the conventional matched filter receiver [0007] In CDMA wireless systems, power control is used to control the level of MAI at the base station. By adjusting every user's power so that all user transmissions arrive at the base station at approximately the same level, the base station receiver for each user sees the same amount of MAI, and the link quality is roughly the same for each user. If power control was not implemented, then a single user close to the base station could prevent the conventional CDMA receiver for other users from receiving a usable signal, resulting in the so-called near-far problem. [0008] Power control works reasonably well for currently deployed CDMA wireless systems although limitations in the speed of power control are a constant engineering concern and limit capacity and link quality. However, there are frequently situations where it is desirable to deploy auxiliary receivers that are not the target of mobile station power control. Auxiliary receivers can be used to monitor the health of a CDMA wireless system or assist in geolocation. These auxiliary receivers may even be used by law enforcement and military operators for non-cooperative monitoring of a CDMA system for drug-interdiction, counter-terrorism and international intelligence gathering. In these cases, the auxiliary receiver must contend with a wide range of received power levels. Often the auxiliary receiver may need to receive a signal from a mobile station whose received power level is far below (30 dB or more) below the strongest arriving signal. [0009] A need therefore exists for enabling a user in a CDMA system to receive user signals in the presence of interference from other users when the power level of all co-channel signals is not adjusted to be substantially the same. [0010] In accordance with an aspect of our invention, we combine concepts from space-time adaptive processing (STAP), interference cancellation, and multi-user detection (MUD) in multiple embodiments that are able to extract low-level CDMA signals in dense multi-user environments. The performance of these embodiments depends on the accuracy of the signal reconstruction and cancellation. This is particularly crucial if there is a wide range in received power (e.g., from lack of power control). For example, if there is an interfering signal that is 30 dB stronger than the signal we wish to receive and this signal is cancelled with 90% accuracy (meaning that 90% of the interfering signal power is canceled), then the residual portion is still 20 dB above the desired signal. Thus, in addition to symbol detection accuracy, channel estimation accuracy becomes very important in reducing the cancellation residuals. [0011] Our invention utilizes adaptive temporal reconstruction filter (ATRF) techniques for reconstructing the signal interference. This novel approach permits very accurate channel estimation and signal cancellation. Through our novel use of ATRF, individual multipath components do not need to be tracked and separately estimated. The ATRF recreates the multipath channel structure with accurate amplitude and phase estimates for each component. The use of cost estimation techniques within the ATRF further minimizes cancellation residuals. In addition, cancellation timing errors are mitigated because the filter weights do not need to be exactly centered around the main multipath peak in order to solve for them accurately. [0012] There has been extensive work on combined successive interference cancellation and multi-user detection systems. Much of this work is focused on simple channel estimation techniques, such as averaging the outputs of the conventional detector's correlators in order to estimate the amplitude and phase of signals to cancel. The reasons for this are that this approach is simple to describe, simulate and implement and the focus is most often on applications where power control is available to the receiver. Thus, small inaccuracies in cancellation do not significantly affect the performance. Also, there are only a limited number of multipath components which are strong enough to be worth tracking and canceling. [0013] There has also been some work on channel estimation for MUD with the more theoretical motivation of determining the limits of estimation accuracy. These works have often focused on complex maximum likelihood approaches. Because our invention applies successive interference cancellation to complex, non-discrete multipath channels encountered in the real world, our invention takes transmit filtering into account and compensates for timing errors. Our approach minimizes residuals and estimates all multipath components without the need to track them individually. [0014] Through the addition of STAP, the receiver is able to spatially separate the signals using array (smart antenna) receiver technology. This allows the STAP receiver to place spatial beam pattern nulls on strong interferers. In addition, the STAP receiver combines multipath energy, including both the resolvable multipath that is captured by the rake receiver, as well as unresolved multipath that the rake receiver cannot effectively exploit. We combine these techniques with MUD approaches, where the receiver jointly operates on the received waveform to extract signals for all users simultaneously. By carefully estimating higher level signals and canceling them from the array data for the STAP receivers for lower-level signals, the combined STAP-MUD approach is much more effective than either approach implemented individually. [0015] In multi-user detection (MUD), code, timing and possibly channel information associated with multiple users are jointly used to better detect each individual user. Thus, at the outputs of a conventional MUD detector, each user sees less multiple access interference and enjoys improved performance. One form of multi-user detection known as interference cancellation estimates, reconstructs and subtracts interfering signals out of the received signal. Unlike the traditional CDMA detectors, interference cancellation MUD utilizes information about other users when detecting a single user. One aspect of our invention is the novel combination of these interference cancellation MUD techniques and adaptive minimum cost channel estimation in the reconstruction of signals. This combination improves performance of signal reconstruction including symbol detection accuracy and channel estimation fidelity. [0016] Using this combination, we have demonstrated that the STAP-MUD receiver can operate independently of power control, extracting waveforms that are over 35 dB below the strongest arriving CDMA signals. [0017]FIG. 1 is a network diagram illustrating a typical wireless CDMA network. [0018]FIG. 2 is a network diagram of an illustrative embodiment of a SIC-MCCE combination system in accordance with our invention. [0019]FIG. 3 depicts an illustrative conventional detector for the combination of FIG. 2. [0020]FIG. 4 depicts an illustrative respread processor for the combination of FIG. 2. [0021]FIG. 5 depicts an illustrative adaptive temporal filter (ATRF) for the combination of FIG. 2. [0022]FIG. 6 is a flow diagram illustrating a method of operation for the SIC-MCCE combination system of FIG. 2. [0023]FIG. 7 is a network diagram of an illustrative embodiment of a SIC-JMCCE combination system in accordance with our invention. [0024]FIG. 8 is a network diagram of an illustrative embodiment of a SIC-MF-MCCE combination system in accordance with our invention. [0025]FIG. 8 [0026]FIG. 9 is a network diagram of an illustrative embodiment of a PIC-MCCE combination system in accordance with our invention. [0027]FIG. 10 is a flow diagram illustrating a method of operation for the PIC-MCCE combination of FIG. 9. [0028]FIG. 11 [0029]FIG. 11 [0030]FIG. 12 is a network diagram of an illustrative embodiment of a STAP receiver in accordance with our invention. [0031]FIG. 13 is a network diagram of an illustrative embodiment of a stage in a STAP/VSIC-MCCE combination system in accordance with our invention. [0032]FIG. 14 is a network diagram of an illustrative embodiment of a J-STAPSIC combination system in accordance with our invention. [0033]FIG. 15 depicts an illustrative J-STAPSIC stage for the combination of FIG. 14. [0034] I. Interference Cancellation MUD Combined with Adaptive Temporal Channel Estimation [0035] Interference cancellation can take the form of either successive interference cancellation or parallel interference cancellation. FIG. 2 depicts one illustrative embodiment of our invention comprising a system [0036] The illustrative system of FIG. 2 comprises a control processor [0037] Each SIC-ATRF processor [0038] The exact format of the conventional detector and respread processor will differ based on the modulation, coding, and spreading schemes of the particular CDMA system utilized in the wireless receiver system. Although the conventional detector and respread processor can be designed based on third generation CDMA, CDMA2000, or W-CDMA technology, FIGS. 3 and 4 are block diagrams of the conventional detector [0039] The IS-95 conventional detector [0040] The IS-95 conventional rake detector, a standard technique employed in practice, embodies several instantiations of the IS-95 conventional detector. Each detector uses the same long and short code, however a different delay is applied to each constituent IS-95 conventional detection. The delays correspond to different multipath components, so that a different IS-95 conventional detector tracks each significant multipath component. The outputs from the 64-ary matched filter banks of each of the IS-95 conventional detectors are combined in the IS-95 rake conventional detector using a non-coherent combining technique. Several non-coherent combining techniques are available; however, a simple example is the equal-gain combiner, in which the power from the corresponding ports from each of the 64 matched filter bank outputs in the constituent IS-95 conventional detectors are added, resulting in 64 new variables. These variables are compared, and the one with the largest power is selected as the receiver's estimate of the transmitted symbol from a 64-ary alphabet. [0041] The respread processor [0042]FIG. 5 is a block diagram of the ATRF [0043]FIG. 6 shows a flow diagram of the operation of the system [0044] An illustrative methodology ranks signals in descending order of received powers. An advantage of this methodology is that by canceling the strongest users first, the remaining users receive the largest benefit from MAI reduction. In alternative methodology, the control processor identifies signals above a certain threshold without performing a hard ranking of each signal. [0045] Based on the ordering, the control processor [0046] In step [0047] A more detailed description of the basic SIC-MCCE channel estimation and reconstruction performed in the ATRF [0048] In basic SIC-MCCE channel estimation (step [0049] this is expressed in vector form as:
[0050] where r [0051] Different weight vectors can be obtained by using minimizing different cost functions, each of which represents the quality of the performance of the SIC stage in some manner. One implementation of the minimum cost channel estimate solution is the minimum mean square error solution. The minimum mean square error solution for the weight vector w is the solution that minimizes the following cost function: [0052] which simultaneously minimizes both the residual at the output of the j [0053] Since this solution minimizes the mean square error between the ATRF filter output and the received data at this input to the stage, this is called the minimum mean square error (MMSE) solution. In an alternate illustrative embodiment of our invention, the channel is estimated jointly over multiple users. We will refer to this combination of a jointly optimized ATRF and SIC multi-user detection as the SIC-JMCCE system. An illustrative multi-stage SIC-JMCCE system is shown in FIG. 7. The SIC-JMCCE system [0054] The mode of operation in accordance with the SIC-JMCCE system is as described above for the SIC-MCCE system, FIG. 6. However, the channel estimation step [0055] The above approaches to channel estimation in accordance with our invention reconstruct the temporal structure of the signals. However, these approaches do not take into account the frequency content of the signals. In another illustrative embodiment, the ATRF is extended to take into account Doppler spread. We refer to this combination of SIC multi-user detection and multiple frequency adaptive reconstruction as a SIC-MF-MCCE system. The MF-MCCE ATRF can be implemented either in an independent or joint arrangement. An independent MF-MCCE ATRF is shown in FIG. 8. In this arrangement, a frequency shift processor [0056] The mode of operation in accordance with the independent SIC-MF-MCCE system is shown in FIG. 8 [0057] r
[0058] [0059] Using these equations, the MF-MCCE ATRF [0060] which gives: [0061] The MF-MCCE ATRF [0062] The SIC detection approach is particularly attractive where there is a wide range in received powers (e.g., due to lack of power control). The SIC approach exploits the power distribution by canceling based on signal strength ordering. For applications where signals are received at about the same power (e.g., through power control), the PIC approach is often preferable. [0063] The combination of interference cancellation and ATRF channel estimation can also be extended to parallel interference cancellation techniques. FIG. 9 depicts one stage of a system [0064] The PIC-MCCE system [0065] The conventional detector [0066]FIG. 10 shows a flow diagram of the operation of each processor [0067] The outputs from the ATRF [0068] A first approach to channel estimation in the PIC structure is the same as described above for basic SIC-MCCE channel estimation. A joint MCCE channel estimation approach, described above for the SIC-JMCCE system, can also be applied to the parallel structure. We refer to this system as PIC-JMCCE. [0069] A partial PIC-JMCCE system is shown in FIG. 11A according to an illustrative embodiment of our invention. In this embodiment, each processor [0070] An alternative embodiment of the PIC-JMCCE system is shown in FIG. 11B, having a single ATRF [0071] A third approach to channel estimation, PIC-MF-MCCE, extends the ATRF to account for Doppler spread. This approach is identical to the approach described above for SIC-MF-MCCE. In the PIC-MF-MCCE arrangement, a frequency shift processor [0072] The above embodiment assumes that all signals are used in the PIC-MCCE system at each stage. This condition can be relaxed to include groups of signals at each stage. For example, a control processor could be used to order the received signals in groups of similar power and successively detect groups of users in parallel. Similarly, the PIC-JMCCE system need not include all previously detected signals at each stage, but possibly, some subset of them. [0073] II. Application of STAP to Systems without a Pilot Reference Signal [0074] Through the use of space time adaptive processing (STAP), a receiver is able to spatially separate user signals using array (smart antenna) receiver technology. This feature allows a STAP receiver to place spatial beam pattern nulls on strong interferers. In addition, the STAP receiver combines multipath energy, including both the resolvable multipath that is captured by a rake receiver, as well as unresolved multipath that the rake receiver cannot effectively exploit. [0075] A single user space time adaptive processing (STAP) receiver is depicted in FIG. 12 in accordance with an illustrative embodiment of our invention. The STAP receiver [0076] An illustrative embodiment of our invention comprises a space time adaptive processing (STAP) processor, means for hypothesizing possible symbols transmitted during a symbol period, a respread processor, means for weight computation wherein the hypothesized symbol and the vector input symbol are used to form a set of STAP weights which filter the input data spatially and temporally, a matched filter bank, means for determining a metric to measure the quality of the matched filter bank, and means for comparing generated metrics. The STAP processor includes a plurality of filters, each comprising a set of STAP weights, and a plurality of mathematical summation circuits. In addition, each filter may also include a tapped delay line. In a preferred IS-95 implementation, the matched filter bank is a bank of 64 matched filters that correspond to the 64 possible Walsh symbols. [0077] When a user signal is received by the antenna array, the user signal from each antenna in the array is first downconverted to baseband in a processor (not shown) and sampled. Downconversion and sampling are performed by an external processor. After the resulting signal r [0078] A more detailed description of the metric determination step is described below. After the input signal vector is received, the hypothesizing means hypothesizes which symbol was transmitted. The hypothesized symbol is communicated to the respread processor and spread to create a replica of the transmitted waveform. The replica of the transmitted waveform and the input signal vector are input to the weight computation means. The weight computation means uses these inputs to determine the appropriate STAP weights for the STAP filters. After the determination is made, these STAP weights are communicated to the filters and applied to each signal vector component, r [0079] In an alternate embodiment, the STAP processor may despread the delayed signals from each antenna element and then apply the STAP weights. After the STAP weights are applied, the results are summed and used as input to the matched filter bank. [0080] For example in IS-95, the sharpness factor is computed by taking the ratio of the peak output (i.e., for the most likely transmitted symbol) to the sum of the outputs for all the other 63 hypothesized Walsh symbols. The sharpness factor can also be based on the distance between the peak output and the average of all other outputs. In either case the STAP solution with the largest sharpness factor is chosen to determine the correctly hypothesized symbol. This embodiment can be extended across multiple symbols where we hypothesize all combinations of multiple symbols. [0081] In an alternative embodiment, the STAP filter weights are determined based on a combination of “known” symbols and hypothesized symbols. The known symbols may be obtained by feeding back previously detected symbols, or from a priori known pilot reference symbols. Utilizing the known symbols allows extension of the length of the training sequence without requiring additional hypothesized symbols. It also anchors the hypothesized STAP solutions to a partially known training sequence, which makes it more likely that the correctly hypothesized solution will stand out. The above embodiments can be repeated for each symbol. These procedures can also be utilized to detect initial symbol(s), and then utilize an update procedure to compute the STAP weights for the remaining symbols. In other words, the STAP weights of the previous symbol can be used to detect the current symbol which can then in turn be used to update the STAP tap weights for the next symbol. [0082] III. Combined STAP and MUD [0083] The STAP receiver shown in FIG. 12 is limited in several ways. First, it can only effectively null M-1 high level signals (including temporally resolvable multipath components) where M is the number of antennas used. Therefore, it is only effective at extracting the M strongest signal components. Another embodiment of our invention combines MUD and temporal interference cancellation techniques and thus, removes much of the interfering signals before applying the STAP receiver. This approach frees up STAP degrees of freedom to operate on the remaining interference more effectively. [0084]FIG. 13 depicts a single stage of a system [0085] A single stage of the STAP/VSIC system includes a STAP processor [0086] When a user signal is received by the antenna array, the user signal from each antenna in the array is first downconverted to baseband in a processor (not shown) and sampled. For each antenna, the resulting signal, r [0087] The STAP/VSIC system approach can also be extended to vectorized parallel interference cancellation. We shall refer to this system as the STAP/VPIC system. In these embodiments, the system would take the form of the PIC detector shown in FIG. 9 with the conventional detector replaced by the one of the above described embodiments of a STAP processor. [0088] Another embodiment of our invention combines STAP with interference cancellation techniques. In this embodiment, the system jointly solves for the ATRF tap weights and STAP tap weights. For example, the system minimizes the error associated with the cost function between the transmitted symbol replica and the sum of the STAP filter outputs and ATRF filter outputs. FIG. 14 depicts one illustrative embodiment of our invention. We shall refer to this system as the J-STAPSIC system. [0089] The illustrative system of FIG. 14 comprises a plurality of J-STAPSIC processors arranged in successive stages [0090] An illustrative embodiment of a k [0091] In the k [0092] Although the invention has been shown and described with respect to exemplary embodiments thereof, it should be understood by those skilled in the art that various changes, omissions and additions may be therein and thereto, without departing from the spirit and the scope of the invention. Referenced by
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