Patent Publication Number: US-8995584-B1

Title: Multi-stream demodulation scheme using multiple detectors

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 13/735,781, now U.S. Pat. No. 8,638,887, entitled “Multi-Stream Demodulation Scheme Using Multiple Detectors,” filed Jan. 7, 2013, which is a continuation of U.S. patent application Ser. No. 12/562,687, now U.S. Pat. No. 8,351,549, entitled “Multi-Stream Demodulation Scheme Using Multiple Detectors,” filed on Sep. 18, 2009, which claims the benefit of U.S. Provisional Patent Application No. 61/102,163, entitled “Near-ML MIMO Hard Demodulation Based on Multiple Candidates from Heterogeneous Detectors,” filed on Oct. 2, 2008. The disclosures of all of the above-identified applications are hereby expressly incorporated by reference herein in their entireties. 
    
    
     FIELD OF TECHNOLOGY 
     The present disclosure relates generally to multi-antenna wireless systems and, more particularly, to a multi-stream demodulation. 
     BACKGROUND 
     An ever-increasing number of relatively inexpensive, low power wireless data communication services, networks and devices have been made available over the past number of years, promising near wire speed transmission and reliability. Various wireless technology is described in detail in the 802 IEEE Standards, including for example, the IEEE Standard 802.11a (1999) and its updates and amendments, the IEEE Standard 802.11g (2003), and the IEEE Standard 802.11n now in the process of being adopted, all of which are collectively incorporated herein fully by reference. These standards have been or are in the process of being commercialized with the promise of 54 Mbps or higher data rate, making them a strong competitor to traditional wired Ethernet and the more common “802.11b” or “WiFi” 11 Mbps mobile wireless transmission standard. 
     Generally speaking, transmission systems compliant with the IEEE 802.11a and 802.11g or “802.11a/g” standards as well as the IEEE 802.11n standard achieve their high data transmission rates using Orthogonal Frequency Division Multiplexing (OFDM) encoded symbols. Generally speaking, the use of OFDM divides the overall system bandwidth into a number of frequency sub-bands or channels, with each frequency sub-band being associated with a respective sub-carrier. Data upon each sub-carrier may be modulated with a modulation scheme such as QAM, phase shift keying, etc. Thus, each frequency sub-band of the OFDM system may be viewed as an independent transmission channel within which to send data, thereby increasing the overall throughput or transmission rate of the communication system. 
     Generally, transmitters used in the wireless communication systems that are compliant with the aforementioned 802.11a/802.11g/802.11n standards as well as other standards such as the 802.16 IEEE Standard, perform multi-carrier OFDM symbol encoding (which may include error correction encoding and interleaving), convert the encoded symbols into the time domain using Inverse Fast Fourier Transform (IFFT) techniques, and perform digital to analog conversion and conventional radio frequency (RF) upconversion on the signals. These transmitters then transmit the modulated and upconverted signals after appropriate power amplification to one or more receivers, resulting in a relatively high-speed time domain signal with a large peak-to-average ratio (PAR). 
     Likewise, the receivers used in the wireless communication systems that are compliant with the aforementioned 802.11a/802.11g/802.11n and 802.16 IEEE standards generally include an RF receiving unit that performs RF downconversion and filtering of the received signals (which may be performed in one or more stages), and a baseband processor unit that processes the OFDM encoded symbols bearing the data of interest. Generally, the digital form of each OFDM symbol presented in the frequency domain is recovered after baseband downconversion, conventional analog to digital conversion and Fast Fourier Transformation of the received time domain analog signal. Thereafter, the baseband processor performs frequency domain equalization (FEQ) and demodulation to recover the transmitted symbols. The recovered and recognized stream of symbols is then decoded, which may include deinterleaving and error correction using any of a number of known error correction techniques, to produce a set of recovered signals corresponding to the original signals transmitted by the transmitter. 
     In wireless communication systems, the RF modulated signals generated by the transmitter may reach a particular receiver via a number of different propagation paths, the characteristics of which typically change over time due to the phenomena of multi-path and fading. Moreover, the characteristics of a propagation channel differ or vary based on the frequency of propagation. To compensate for the time varying, frequency selective nature of the propagation effects, and generally to enhance effective encoding and modulation in a wireless communication system, each receiver of the wireless communication system may periodically develop or collect channel state information (CSI) for each of the frequency channels, such as the channels associated with each of the OFDM sub-bands discussed above. Generally speaking, CSI is information defining or describing one or more characteristics about each of the OFDM channels (for example, the gain, the phase and the SNR of each channel). Upon determining the CSI for one or more channels, the receiver may send this CSI back to the transmitter, which may use the CSI for each channel to precondition the signals transmitted using that channel so as to compensate for the varying propagation effects of each of the channels. 
     To further increase the number of signals which may be propagated in the communication system and/or to compensate for deleterious effects associated with the various propagation paths, and to thereby improve transmission performance, it is known to use multiple transmit and receive antennas within a wireless transmission system. Such a system is commonly referred to as a multiple-input, multiple-output (MIMO) wireless transmission system and is specifically provided for within the 802.11n IEEE Standard now being adopted. Various other standards and projects, such as the 802.16 standard, or WiMAX, and Long Term Evolution (LTE project), etc., support MIMO techniques. Generally speaking, the use of MIMO technology produces significant increases in spectral efficiency and link reliability of IEEE 802.11, IEEE 802.16, and other systems, and these benefits generally increase as the number of transmit and receive antennas within the MIMO system increases. 
     In addition to the frequency sub-channels created by the use of OFDM, a MIMO channel formed by the various transmit and receive antennas between a particular transmitter and a particular receiver may include a number of independent spatial channels. As is known, a wireless MIMO communication system can provide improved performance (e.g., increased transmission capacity) by utilizing the additional dimensionalities created by these spatial channels for the transmission of additional data. Of course, the spatial channels of a wideband MIMO system may experience different channel conditions (e.g., different fading and multi-path effects) across the overall system bandwidth and may therefore achieve different SNRs at different frequencies (i.e., at the different OFDM frequency sub-bands) of the overall system bandwidth. Consequently, the number of information bits per modulation symbol (i.e., the data rate) that may be transmitted using the different frequency sub-bands of each spatial channel for a particular level of performance may differ from frequency sub-band to frequency sub-band. 
     The transmitters and receivers in the wireless communication system may each be capable of using a variety of modulation schemes. For example, some modulations schemes may provide a higher bit rate than other schemes (e.g., 64-QAM vs. 16-QAM). Typically, modulation schemes that provide a higher bit rate may be more sensitive to channel impairments, such as noise, as compared to modulation schemes with a lower bit rate. 
     Different modulation/demodulation schemes have various degrees of complexity, and complexity of a given demodulation technique may affect the complexity of a receiver. Higher complexity of a demodulation technique generally requires more complexity on the receiver, e.g., more, larger, and/or more expensive processing modules and/or components. Higher complexity of a demodulation technique may also lead to an increase in processing time. 
     SUMMARY 
     The present disclosure describes methods and apparatus for demodulating a signal transmitted via a multiple input multiple output (MIMO) communication channel. 
     In one embodiment, a method includes receiving, at a communication device, a data symbol vector comprising a plurality of data symbols that are received via a plurality of antennas, wherein the received data symbol vector corresponds to a transmitted data symbol vector comprising a plurality of transmitted data symbols, and generating, at the communication device, a plurality of candidate vectors for the transmitted data symbol vector using a plurality of non-maximum likelihood (non-ML) detection techniques. The method also includes selecting, at the communication device, one candidate vector from the plurality of candidate vectors using a maximum likelihood (ML) technique. 
     In another embodiment, a method includes receiving, at a communication device, a data symbol vector comprising a plurality of data symbols that are received via a plurality of antennas, wherein the received data symbol vector corresponds to a transmitted data symbol vector comprising a plurality of transmitted data symbols, and generating, at the communication device, a plurality of candidate vectors for a first group of data symbols in the transmitted data symbol vector using a plurality of non-ML detection techniques. Additionally, the method includes selecting, at the communication device, one candidate vector from the plurality of candidate vectors using a hard ML technique, and calculating likelihood values for data symbols in a second group of transmitted data symbols in the transmitted data symbol vector using a soft ML technique based on the selected candidate vector. 
     In yet another embodiment, a communication device comprises a multi-stream demodulator configured to demodulate a received data symbol vector comprising a plurality of data symbols received via a plurality of antennas, wherein the received data symbol vector corresponds to a transmitted data symbol vector. The multi-stream demodulator includes a plurality of non-ML detectors configured to generate a plurality of candidate vectors for the transmitted data symbol vector, and a selector configured to select one candidate vector from the plurality of candidate vectors using an ML technique. 
     In still another embodiment, a communication device comprises a multi-stream demodulator configured to demodulate a received data symbol vector comprising a plurality of data symbols received via a plurality of antennas, wherein the received data symbol vector corresponds to a transmitted data symbol vector. The multi-stream demodulator includes a plurality of non-ML detectors configured to generate a plurality of candidate vectors for a first group of data symbols in the transmitted data symbol vector, and a selector configured to select one candidate vector from the plurality of candidate vectors using a hard ML technique. Additionally, the multi-stream demodulator is configured to calculate likelihood values for data symbols in a second group of transmitted data symbols in the transmitted data symbol vector using a soft ML technique based on the selected candidate vector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an example wireless communication system; 
         FIG. 2A  is a block diagram of an example transmitting device; 
         FIG. 2B  is a block diagram of another example transmitting device; 
         FIG. 3A  is an example constellation set for a 4-QAM scheme; 
         FIG. 3B  is an example constellation set for a 16-QAM scheme; 
         FIG. 4  is a block diagram of an example receiving device having multiple detectors and a minimum-distance selector; 
         FIG. 5  is a flow diagram of an example multi-stream demodulation method; and 
         FIG. 6  is a flow diagram of an example multi-stream demodulation method that uses hard and soft demodulation techniques. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. Furthermore, when individual elements are designated by references numbers in the form Nn, these elements may be referred to in the collective by N. For example,  FIGS. 2A and 2B  illustrate transmitting devices  200   a  and  200   b  that may be referred to collectively as transmitting device(s)  200 . 
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram of an example wireless communication system  100  in which multiple devices, e.g., a transmitting device  102  and a receiving device  106  may communicate with each other via a shared wireless communication channel  104 . Each of the devices  102  and  106  may be, for example, a stationary device, such as a base station, or a mobile station equipped with a set of antennas  110 - 114  and  120 - 124 , respectively. Although the wireless communication system  100  illustrated in  FIG. 1  includes two devices, the wireless communication system  100  may, of course, include any number of devices, each equipped with the same or a different number of antennas (e.g., 1, 2, 3, 4 antennas and so on). 
     Furthermore, although the wireless communication system  100  illustrated in  FIG. 1  includes a transmitting device  102  and a receiving device  106 , devices in the wireless communication system  100  may generally operate in multiple modes (e.g., a transmit mode and a receive mode). For example, if a given device is a mobile station in a WiMAX communication network or a lap top computer having an IEEE Standard 802.11n compliant wireless transceiver, the device may operate in both a transmit mode and a receive mode. Accordingly, in some embodiments, antennas  110 - 114  and  120 - 124  may support both transmission and reception. Alternatively, or additionally, a given device may include separate transmit antennas and separate receive antennas. 
     Different numbers of spatial streams  130  may be transmitted between the antennas  110 - 114  and  120 - 124  in various embodiments and/or configurations of the transmitting device  102  and/or the receiving device  106 . Typically, the number S of spatial streams  130  associated with a shared communication channel  104  is less than or equal to minimum of the number N T  of transmit antennas  110 - 114  and the number N R  of receive antennas  120 - 124  (i.e., S=min(N T ,N R )). The streams  130  may be defined in a variety of ways, e.g., according to various multiple-input and multiple-output (MIMO) modes or schemes, including those known in the art. For example, the transmitting device  102  may use the antennas  110 - 114  to improve channel diversity by transmitting multiple copies of the same symbol via several streams. Alternatively, the transmitting device  102  may transmit different symbols via each of the antennas  110 - 114  to increase throughput. As yet another alternative, the transmitting device  102  may operate in a mixed MIMO mode to improve both channel diversity and throughput. 
     The transmitting device  102  and the receiving device  106  may communicate using a single frequency or multiple frequencies. When the transmitting device  102  and the receiving device  106  communicate using multiple frequencies, the communication channel  104  may include multiple carriers or subcarriers, each associated with a particular frequency at which the devices  102  and  106  may communicate. In some embodiments, the wireless communication system  100  may use an OFDM technique, and the subcarriers may be selected to be mutually orthogonal (i.e., to minimize cross-talk between each pair of subcarriers). However, the wireless communication system  100  could also use any other frequency division multiplexing technique. 
     The receiving device  106  may be configured to perform a multi-stream demodulation techniques to be described in more detail below. However, before describing the multi-stream demodulation techniques in detail, it is helpful to briefly explain how example signals may be modulated. 
       FIGS. 2A-2B  are block diagrams of example transmitting devices  200 . The transmitting devices  200   a,b  may be utilized in the wireless communication system  100  as the transmitting device  102 , for example. It will be understood, however, that the wireless communication system  100  may alternatively use other types of transmitting devices. 
     Referring to  FIG. 2A , the transmitting device  200   a  may generally convert a sequence of information bits into signals appropriate for transmission through a wireless channel (e.g., channel  104 ). More specifically, the transmitting device  200   a  may include an encoder  220   a  (e.g., a convolution encoder) that encodes information bits, and a modulator  230   a  that modulates the encoded bits into data symbols, which are mapped and converted to signals appropriate for transmission via transmit antennas  210   a - 218   a . The transmitting device  200   a  may include various additional modules that, for ease of explanation, are not shown in  FIG. 2A . For example, the transmitting device  200   a  may include an interleaver that interleaves the encoded bits to mitigate burst errors. The transmitting device  200   a  may further include an analog radio frequency (RF) front end for performing frequency upconversion, various filters, power amplifiers, and so on. 
     The modulator  230   a  may include a bit-to-symbol mapper  232   a  that maps encoded bits into multiple data symbols, and a symbol-to-stream mapper  234   a  that maps the multiple data symbols into multiple parallel data streams. For example, the modulator  230   a  may generate S parallel data streams that may be represented by a data symbol vector x=[x 1 , x 2 , . . . , x S ], and each individual symbol x s  in the data symbol vector x may be a symbol representative of W bits (b s,1 , b s,2 , . . . , b s,W ). 
     For ease of explanation, in the examples presented herein, streams and symbols have a one-to-one correspondence. That is, a single stream is associated with a single symbol and vice versa. Accordingly, the words “streams” and “symbols” may be used interchangeably. However, it should be understood that a given stream, for example, may have a number of associated symbols and vice versa. Also, in some embodiments, the same symbol may be transmitted on multiple streams. 
     In some embodiments, the modulator  230   a  may use QAM to map bits to symbols in a QAM signal constellation set, where the symbols are differentiated from one another by phase and/or magnitude. For example,  FIG. 3A  illustrates a 4-QAM constellation set  300   a  in an in phase (I) and quadrature phase (Q) plane. In this case, constellation points  302   a - 302   d  are distinguishable only by phase. Each constellation point  302  represents a different two-bit symbol:  302   a  may represent “00,”  302   b  may represent “01,”  302   c  may represent “10,” and  302   d  may represent “11.” However, other bit to symbol mappings may be utilized. 
     Similarly,  FIG. 3B  illustrates a 16-QAM constellation set  300   b , where four-bit sequences correspond to symbols. Here, both the amplitude and the phase of constellation points  304  may vary.  FIG. 3B  illustrates a mapping, where each constellation point  304  represents a different four-bit symbol:  304   a  may represent “1101,”  304   b  may represent “1100,”  304   c  may represent “1110,”  304   d  may represent “1111,” and so on. However, as discussed in reference to  FIG. 3B , other mappings may be utilized. 
     In general, a W-bit symbol x s  may be mapped according to an M-QAM signal set, where M=2 W . Thus, as illustrated in  FIG. 3A , if the modulator  230   a  uses the 4-QAM modulation scheme, or a quadrature phase-shift keying (QPSK) modulation scheme, the modulator  230   a  will have a signal alphabet size of M=2 2 =4 (i.e., 4 constellation points), and will map 2-bit pairs into the 4 constellation points. If the modulator  230   a  uses a 64-QAM scheme, the modulator  230   a  will have an alphabet size of M=2 6 =64 (i.e., 64 constellation points) and will map 6-bit segments into the 64 constellation points. 
     It should be noted that although the transmitting device  200   a  described with reference to  FIG. 2A  includes a common encoder chain (an encoder  220   a , an interleaver (not shown), a modulator  230   a , etc.), a transmitting device  200  may include different encoder chains for different streams. For example, as illustrated in  FIG. 2B , each stream generated by the transmitting device  200   b  may correspond to a separate encoder chain. Other transmitting devices, such as those supporting the WiMAX standards, for example, may support both a single-encoder option and a two-encoder option for a two-transmit-antenna configuration. In general, the number of encoders and/or encoder chains may be less than or equal to the number of transmitted streams S. 
       FIG. 4  is a block diagram of an example receiving device  400  that demodulates and decodes signals from one or more transmitting devices using a multi-stream demodulation scheme. The receiving device  400  may be utilized in the wireless communication system  100  as the receiving device  106 , for example. The receiving device  400  may receive and demodulate multiple streams transmitted by a device such as the transmitting device  200   a  of  FIG. 2A  or the transmitting device  200   b  of  FIG. 2B  or some other transmitting device. 
     Generally, the receiving device  400  may receive information via multiple receive antennas  402 - 408  and demodulate and decode the received information to estimate the information that was sent by a transmitting device. The receiving device  400  processes received information utilizing a model, such as:
 
 y=Hx+z , where
 
               y   =     [           y   1               y   2             ⋮             y     N   R             ]       ;     H   =     [           h     1   ,   1             h     1   ,   2           …         h     1   ,   S                 h     2   ,   1             h     1   ,   2           …         h     2   ,   S               ⋮       ⋮       ⋮       ⋮             h       N   R     ,   1             h       N   R     ,   2           …         h       N   R     ,   S             ]       ;                   x   =     [           x   1               x   2             ⋮             x   S           ]       ;     z   =     [           z   1               z   2             ⋮             z     N   R             ]         ,         
in which y represents, in vector from, a received signal, H represents a MIMO communication channel, x represents, in vector form, a transmitted signal, z represents a noise vector, and N R  is the number of receive antennas. More precisely, y r  is a received signal at antenna r, and z r  is noise at antenna r. Although N R  is illustrated in  FIG. 4  as being three, any number of multiple antennas may, in general, be utilized.
 
     The MIMO communication channel H includes channel gain parameters h r,s  representing channel gain in a stream s at a receive antenna r. In at least some of the embodiments, each channel gain parameter h r,s  is a complex number that incorporates an amplitude factor and a phase shift factor. In other words, each h r,s  parameter may represent an attenuation coefficient associated with a certain propagation path as used in, for example, a Rayleigh fading channel model. The receiving device  400  may estimate the parameters h r,s , and parameters associated with the noise z using any suitable technique, including known techniques. 
     With continued reference to  FIG. 4 , the receiving device  400  includes a multi-stream demodulator  442  that may generate, based on the received symbols y 1 , y 2 , . . . , y N     R   , estimates {tilde over (x)} 1 , {tilde over (x)} 2 , . . . , {tilde over (x)} S  (represented as {tilde over (x)} in vector form) of the transmitted symbols x 1 , x 2 , . . . , x S . Generally speaking, in order to generate the estimate {tilde over (x)} of the transmitted vector x, the multi-stream demodulator  442  may consider Q different candidate vectors {tilde over (x)} 1 , {tilde over (x)} 2 , . . . , {tilde over (x)} Q , where Q is less than the number of all possible candidate vectors for the transmitted vector x, and select one candidate vector {tilde over (x)} that minimizes the probability of error with respect to the transmitted vector x. Because the multi-stream demodulator  442  outputs a vector {tilde over (x)}, and not likelihood values associated with the vector {tilde over (x)}, the multi-stream demodulator  442  is called a “hard” demodulator. By contrast, demodulators that output likelihood values associated the vector {tilde over (x)} are called “soft-decision demodulators,” or simply “soft demodulators.” 
     As illustrated in  FIG. 4 , the multi-stream demodulator  442  includes various modules and/or components. In particular, the multi-stream demodulator  442  includes a minimum-distance selector  454  and one or more detectors  458 . In general, the detectors  458  (or a single detector  458 ) generate Q different candidate vectors {tilde over (x)} 1 , {tilde over (x)} 2 , . . . , {tilde over (x)} Q , where Q is less than the number of all possible candidate vectors, and the minimum-distance selector  454  selects one candidate vector {tilde over (x)} using, for example a maximum-likelihood technique, or using another minimum-distance criterion. 
     Each of the detectors  458  may comprise one of a variety of suitable detectors, including non-ML detectors, for example. In some embodiments, the detectors  458  may include linear detectors  458   a , such as a simple Zero Forcing (ZF) Equalizer, a Minimum Mean Square Error (MMSE) detector, a Maximum-Ratio Combining (MRC) detector, and so on. Generally speaking, the linear detectors  458   a  may generate estimate {tilde over (x)} of the transmitted vector x using an equation such as: 
                       x   ~     =         [         x   ~     1     ,   …   ⁢           ,       x   ~     S       ]     T     =       des   ⁡     (     x   _     )       =       [       dec   ⁡     (       x   ~     1     )       ⁢           ⁢   …   ⁢           ⁢     dec   ⁡     (       x   ~     S     )         ]     T           ,           (   1   )               
where  x =Gy, dec(  x   s )= xεΩ   arg min |x−  x   s |,s=1, . . . , S, where Ω is the set of constellation points, and where G may be different for different types of linear detectors. For example, G=(H H H) −1 H H  in the case of a zero-forcing detector; G=(H H H+σ z   2 I) −1 H H , where I is an identity matrix and σ z   2  is the noise power, in the case of an MMSE detector; and G=H H (diag(H H H)) −1 , where diag(A) is matrix A with zero non-diagonals, in the case of an MRC detector.
 
     In addition to, or instead of linear detectors, the detectors  458  illustrated in  FIG. 4  may include non-linear detectors  458   b , such as Ordered Serial Interference Cancellers (OSIC), K-Best detectors, etc. It will be understood that the possible detectors recited above are mentioned only as examples, and that the multi-stream demodulator  442  is not limited to any particular detectors, or to any particular type of detectors. It will be further understood that although  FIG. 4  illustrates one linear detector  458   a  and one nonlinear detector, the multi-stream demodulator  454  may include any combination of linear and nonlinear detectors. For example, the multi-stream demodulator  442  may include only linear detectors  458   a , only nonlinear detectors  458   b , an equal number of linear and non-linear detectors  458 , an unequal number of linear and non-linear detectors  458  and so on. Still further, it will be understood that each detector  458  may generate 1, 2, or more of the Q candidate vectors {tilde over (x)} 1 , {tilde over (x)} 2 , . . . , {tilde over (x)} Q , and different detectors  458  may generate a different number of candidate vectors. 
     As described above, the Q candidate vectors {tilde over (x)} 1 , {tilde over (x)} 2 , . . . , {tilde over (x)} Q , generated by the detectors  458  may be used by the minimum-distance selector  454 . The minimum-distance selector  454  selects one candidate vector {tilde over (x)} from the Q candidate vectors {tilde over (x)} 1 , {tilde over (x)} 2 , . . . , {tilde over (x)} Q , using, for example, a maximum-likelihood technique, or using another minimum-distance criterion. In particular, the minimum-distance selector  454  may select the candidate vector {tilde over (x)} that minimizes a distance value corresponding to the Euclidean distance between the received data symbol vector y and a channel matrix H multiplied with the candidate vector x. That is, the minimum-distance selector  454  may select the candidate vector using an equation, such as: 
     
       
         
           
             
               
                 
                   
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     It should be noted that unlike conventional maximum-likelihood (ML) detectors, the minimum-distance selector  454  illustrated in  FIG. 4  does not consider all possible candidate vectors in determining {tilde over (x)}. Instead, the minimum-distance selector  454  considers only a subset of all possible candidate vectors that is generated by the detectors  458 . Because the detectors  458  effectively eliminate certain candidate vectors from consideration, they reduce the complexity of the maximum likelihood technique at the cost of little or no performance degradation. As a result, the receiving device  400  may be able to demodulate and decode signals faster, with fewer and/or less costly processing components, and so on. 
     The multi-stream demodulator  442 , in some embodiments, or in some modes of operation, may include additional modules and/or components. For example, the multi-stream demodulator  442  may include a symbol slicer  452  that performs suitable symbol slicing operations (e.g., QAM slicing), including known symbol slicing operations. The multi-stream demodulator  442  may also include a channel estimator  448  to estimate the MIMO communication channel H and a noise estimator  446  for generating noise estimate information, including, for example, the variance of the noise σ z   2 . Moreover, the multi-stream demodulator  442 , in some embodiments, or in some modes of operation, may not include one or more of the modules illustrated in  FIG. 4  or, alternatively, may not use each of the modules in demodulating the received signals. Further, it will be appreciated that some of the modules illustrated in  FIG. 4  may be combined. Still further, the multi-stream demodulator  442  and/or the receiving device  400  may include additional components and/or modules that, for clarity, are not shown in  FIG. 4 . For example, the receiving device  400  may include a deinterleaver that rearranges scattered bits and restores the proper bit sequence, an analog RF front end that performs frequency downconversion, various filters, power amplifiers, and so on. 
     Different components and/or modules of the receiving device  400  may be implemented as hardware, a processor executing software instructions, a processor implementing firmware instructions, or some combination thereof. For example, some or all of the components may be implemented on one or more custom integrated circuits, application-specific integration circuits (ASICs), etc. In this case, the receiving device  400  optionally may include bypass busses (not shown) to bypass some of the components if the currently active MIMO mode does not require certain operations, such as processing multiple presentations of a symbol encoded according to a space-time encoding scheme. 
       FIG. 5  is a flow diagram illustrating an example multi-stream method  500  for demodulating a received data symbol vector using multiple detectors. For ease of explanation,  FIG. 5  will be described with reference to  FIG. 4 . It will be understood, however, that the method  500  for demodulating and decoding a received data symbol vector y and estimating the corresponding transmitted data symbol vector x using multiple detectors may be utilized with systems and devices other than those illustrated in  FIG. 4 . 
     Referring to  FIG. 5 , when a receiving device (such as the receiving device  400  illustrated in  FIG. 4 ) receives a data symbol vector y (block  510 ) including S symbols (where the symbols may be received at substantially the same time via separate spatial streams, for example), the receiving device may select one or more detectors (such as detectors  458  illustrated in  FIG. 4 ) to generate 1, 2, or more of the Q candidate vectors {tilde over (x)} 1 , {tilde over (x)} 2 , . . . , {tilde over (x)} Q  for the transmitted data symbol vector x (block  520 ). The selection of suitable detectors may be performed in a variety of ways. Generally speaking, the detectors may be selected in such a way as to minimize computational complexity and/or maximize accuracy, for example. The selection of detectors may depend on a number of operational parameters and conditions, such as spatial interference associated with the channel H, presence of error propagation, signal-to-noise ratio (SNR) and so on. For example, an OSIC detector may generally perform better than linear detectors in the absence of error propagation, while linear detectors may be preferable otherwise. However, it should be understood that conditions such as error propagation may be difficult to determine, or predetermine, so other criteria for detector selection may be used. Furthermore, it should be understood that because conditions may change with time, different detectors may be selected by the same multi-stream demodulator at different points in time. It should also be noted that the selection of detectors (block  520 ) may be omitted altogether, and the multi-stream demodulator my use all available detectors. 
     After one or more detectors are selected (if detector selection is utilized), the selected detectors may be used to generate Q candidate vectors {tilde over (x)}, {tilde over (x)} 2 , . . . , {tilde over (x)} Q  for the transmitted data symbol vector x (block  530 ). A given detector can generate more than one candidate vector. For example, an OSIC detector may generate multiple candidate vectors, each of which may be a different vector according to different ordering of symbols. For example, in a MIMO system with three receive antennas and three transmit antennas (such as the system illustrated in  FIG. 1 ), an OSIC detector may generate six candidate vectors that include different orderings of the same three symbols. 
     After Q candidate vectors {tilde over (x)} 1 , {tilde over (x)} 2 , . . . , {tilde over (x)} Q  are generated (block  530 ), a minimum-distance selector may be used to select one candidate vector {tilde over (x)} which has minimum distance as (2) from the Q candidate vectors {tilde over (x)} 1 , {tilde over (x)} 2 , . . . , {tilde over (x)} Q , using a maximum-likelihood technique, or using another minimum-distance criterion (block  540 ). The minimum-distance selector may use a variety of suitable maximum likelihood techniques to select the “best” candidate vector X from the Q candidate vectors {tilde over (x)} 1 , {tilde over (x)} 2 , . . . , {tilde over (x)} Q , including those known in the art. The “best” candidate vector {tilde over (x)} may be, for example, the vector {tilde over (x)} that most closely resembles (or has the highest probability of most closely resembling) the transmitted vector x. It should be noted that the “best” candidate vector {tilde over (x)} is not necessarily associated with any particular detector or group of detectors. In fact, as conditions change, the “best” candidate vectors {tilde over (x)} may be chosen by the same multi-steam demodulator from different detectors at different times. 
     The method  500  for demodulating and decoding a received data symbol vector using multiple detectors described in reference to  FIG. 5  may be used alone, or in combination with other demodulation and decoding techniques. For example, the method  500  for demodulating and decoding a received data symbol vector using multiple detectors may be used in combination with a soft ML demodulation technique. For example, the method  500  for demodulating and decoding a received data symbol vector using multiple detectors may be used with modulation techniques described in the U.S. patent application Ser. No. 12/469,382, entitled “MULTI-STREAM SOFT DEMODULATION USING HARD DECISION SCHEMES,” filed on May 20, 2009, the disclosure of which is hereby expressly incorporated herein by reference. 
       FIG. 6  is a flow diagram illustrating an example multi-stream method  600  for demodulating and decoding a received data symbol vector using a combination of hard and soft demodulation schemes and using multiple detectors. For ease of explanation,  FIG. 6  will be described with reference to  FIG. 4 . It will be understood, however, that the method  600  for demodulating and decoding a received data symbol vector using a combination of hard and soft demodulation schemes and using multiple detectors may be utilized with systems and devices other than those illustrated in  FIG. 4 . 
     Referring to  FIG. 6 , when a receiving device (such as the receiving device  400  illustrated in  FIG. 4 ) receives a data symbol vector y (block  610 ) including S symbols (where the symbols may be received at substantially the same time via multiple spatial streams, for example), the receiving device may group the transmitted data symbols x s  (block  615 ), e.g., into one group (x 1 , . . . , x K ) and another group (x K+1 , . . . , x S ). The receiving device may then determine the optimum, or near-optimum vector 
                   [             x   ^     1             ⋮               x   ^     K           ]           
that substantially minimizes the squared distance ∥y−Hx∥ 2  for a given x K+1 , . . . , x S  (block  620 ). The optimum (or near-optimum) vector
 
                   [             x   ^     1             ⋮               x   ^     K           ]           
may be determined using a hard-deciding technique, such as the multi-stream method  500  for demodulating and decoding a received data symbol vector using multiple detectors described in reference to  FIG. 5 . In particular, one or more detectors (or a single detector) may generate Q different candidate vectors {tilde over (x)} 1 , {tilde over (x)} 2 , . . . , {tilde over (x)} Q  for the vector
 
                   [             x   ^     1             ⋮               x   ^     K           ]           
(block  620   a ), and a minimum-distance selector may select one candidate vector {tilde over (x)} as an optimum, or near-optimum vector
 
                   [             x   ^     1             ⋮               x   ^     K           ]           
using a maximum-likelihood technique, or using another minimum-distance criterion (block  620   b ).
 
     After determining the optimum, or near-optimum vector 
                   [             x   ^     1             ⋮               x   ^     K           ]           
that substantially minimizes the squared distance ∥y−Hx∥ 2  for a given x K+1 , . . . , x S  (block  620 ), the receiving device may use the vector
 
                   [             x   ^     1             ⋮               x   ^     K           ]           
to calculate the minimum squared distance  {circumflex over (x)}     1     , . . . , {circumflex over (x)}     K     , x     K+1     , . . . , x     S     min ∥y−Hx∥ 2  for that vector
 
                   [             x   ^     1             ⋮               x   ^     K           ]           
(block  630 ) for the given x K+1 , . . . , x S . After computing the distance for all the candidate values of x K+1 , . . . , x S , the receiving device may use this calculated squared distance to calculate log-likelihood ratios (LLRs) for x K+1 , . . . , x S  (block  640 ), e.g., using techniques described in the U.S. patent application Ser. No. 12/469,382, entitled “MULTI-STREAM SOFT DEMODULATION USING HARD DECISION SCHEMES,” filed on May 20, 2009.
 
     Example multi-stream demodulation and decoding techniques that use multiple detectors have been described above, but other implementations are also possible. For example, the multi-stream demodulation and decoding techniques have been described in reference to a wireless communication model where y=Hx+z. However, multi-stream demodulation and decoding techniques that use multiple detectors may be used with systems using other communication models. As one example, a receiving device may use the described the multi-stream demodulation and decoding techniques in conjunction with techniques for QR decomposition of matrix H, such as known QR decomposition techniques. Such a receiving device could use the following model for example:
 
 {tilde over (y)}=Q*y=Q*Hx+Q*z=Rx+{tilde over (z)}   (3)
 
where * denotes a complex conjugate.
 
     The model (3) has a similar form as the y=Hx+z model, except that the matrix R is an upper triangular matrix. Q matrix is a unitary matrix and the R matrix may be represented as 
             R   =       [           r   11           r   12             0         r   22           ]     .           
This feature of the matrix R may further lower the complexity of the multi-stream ML demodulation techniques described above because computations at the elements of zero in R are not required. Similar to a QR decomposition technique, other linear processing techniques may be applied to y.
 
     At least some of the various blocks, operations, and techniques described above may be implemented using hardware, a processor executing firmware instructions, a processor executing software instructions, or any combination thereof. When implemented using a processor executing firmware or software instructions, the software or firmware may be stored in any computer readable memory such as on a magnetic disk, an optical disk, or other storage medium, in a RAM or ROM or flash memory, processor, hard disk drive, optical disk drive, tape drive, etc. Likewise, the software or firmware may be delivered to a user or a system via any known or desired delivery method including, for example, on a computer readable disk or other transportable computer storage mechanism or via communication media. Communication media typically embodies computer readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism. The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, radio frequency, infrared and other wireless media. Thus, the software or firmware may be delivered to a user or a system via a communication channel such as a telephone line, a DSL line, a cable television line, a fiber optics line, a wireless communication channel, the Internet, etc. (which are viewed as being the same as or interchangeable with providing such software via a transportable storage medium). The software or firmware may include machine readable instructions that are capable of causing one or more processors to perform various acts. 
     Although the forgoing text sets forth a detailed description of numerous different embodiments, it should be understood that the scope of the patent is defined by the words of the claims set forth at the end of this patent. The detailed description is to be construed as exemplary only and does not describe every possible embodiment because describing every possible embodiment would be impractical, if not impossible. Numerous alternative embodiments could be implemented, using either current technology or technology developed after the filing date of this disclosure, which would still fall within the scope of the claims.