Stream transmission method and device

[0001] In general, the present invention relates to the field of communication systems. More specifically, the present invention relates to the transmission of data streams by a transmitter of communication system.

[0002] Bell Labs Layered Space-Time (BLAST) is a prior art technique for sending multiple data streams from a single transmitting device and may use successive cancellation to recover the transmitted symbols. Each data stream is transmitted with the same average power, and in order to perform successive cancellation, the optimal cancellation order of the data streams at the receiver needs to be determined. The prior art determination of the optimal ordering requires either channel knowledge at the transmitter to solve the ordering problem or a computationally complex algorithm in the receiver to determine the optimal ordering. Having channel knowledge at the transmitter requires a large amount of feedback in mobile channels, and transmitting each data stream with the same power requires a successive cancellation algorithm at the receiver to determine the ordering of the data streams. The consequence of both solutions is a burdensome, complex computation being performed by the transmitter or receiver, respectively.

[0003] Thus, there is a need for a new and unique data stream transmission method.

[0004] One form of the present invention is a method of operating a transmitter comprising a determination of an unequal weighting of a plurality of stream weights. In one aspect, the unequal weighting is accomplished by a determination of a mean square error for each stream weight of a plurality of stream weights, a determination of a first stream weight of the plurality of stream weights having the largest mean square error, an increase in a power of the first stream weight, and a decrease in a power of each stream weight of the plurality of stream weights excluding the first stream weight. In a second aspect, the unequal weighting is accomplished by an initialization of a first stream weight, an increase in the first stream weight by a product of a power variable and an increment, and a determination of a plurality of stream weights excluding the first stream weight as a function of a corresponding mean square error.

[0005] The foregoing form as well as other forms, features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiment, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof.

[0006] FIG. 1 illustrates one embodiment of a transmitter in accordance with the present invention;

[0007] FIG. 2 illustrates a flowchart representative of a stream weighting determination method in accordance with the present invention;

[0008] FIG. 3 illustrates a flowchart representative of a first embodiment of the stream weighting method of FIG. 2 in accordance with the present invention; and

[0009] FIG. 4 illustrates a flowchart representative of a second embodiment of the stream weighting method of FIG. 2 in accordance with the present invention.

[0010] FIG. 1 illustrates one embodiment of a transmitter in accordance with one embodiment of the present invention. In the illustrated embodiment, a transmitter 10 comprises a new and unique stream weight module 11, a plurality of conventional mixers 12₁-12_n, a plurality of conventional transmitting units 13₁-13_n, and a plurality of conventional antennas 14₁-14_n. The stream weight module 11 provides a plurality of stream weights ₁-_n to the mixers 12₁-12_n, respectively. The mixers 12₁-12_n conventionally mix the stream weights ₁-_n with a plurality of data streams₁-_n, respectively, whereby the transmitting units 13₁-13_n and the antennas 14₁-14_n conventionally provide a plurality of transmission signals (not shown).

[0011] The stream weight module 11 can be implemented in hardware (analog and/or digital), software, or a combination of hardware and software. A functional description of the stream weight module 11 will now be provided herein in the context of an execution of a flowchart 20, a flowchart 30, and a flowchart 50 by the stream weight module 11. The stream weight module 11, as will be appreciated by those having ordinary skill in the art from the subsequent description herein, can be employed within various communications systems, such as, for example, a orthogonal frequency division multiplexing (OFDM) system, a spread OFDM system, a single carrier system, a code division multiple access (CDMA) system, and an interleaved frequency division multiple access (IFDMA) system. Those having ordinary skill in the art will further appreciate a sequential operation of various stages of the stream weight module 11 (e.g., in a software implementation) and a concurrent operation of various stages of the stream weight module 11 (e.g., in a hardware implementation).

[0012] Because the determination of the unequal weightings for the stream weight module 11 requires some assumptions about the type of receiver that receives the signal transmitted from the transmitter 10, the received signal is now described. The signal from the transmitter 10 received at a receiving unit may be expressed in the frequency domain according to the following equation 1: $\begin{array}{cc}Y\left(k\right)=\underset{l=1}{\overset{n}{}}{}_l{H}_l\left(k\right)x_l\left(k\right)+N\left(k\right)& \left[1\right]\end{array}$

[0013] where k is an integer that may indicate the subcarrier number (or frequency bin), Y(k) is an M_R1 vector where M_R is the number of receive antennas, H_l(k) is the M_R1 channel vector for stream l, x_l(k) is the frequency-domain data symbol for stream l on subcarrier k (for single-carrier, x_l(k) is the Fast Fourier Transform (FFT) of N time-domain symbols), and N(k) is additive noise that may have covariance matrix ²I where ² is the noise power. The noise power is important in determining the best unequal stream weights as described below.

[0014] The determination of an unequal weighting of stream weights by the stream weight module 11 requires the transmitter to assume a type of demodulation used by the receiver. If the receiver performs successive cancellation (as is known in the art), the received symbols can be estimated according to the following equation 2: $\begin{array}{cc}{r}_l\left(k\right)=w_l^H\left(k\right)\left\{Y\left(k\right)-\underset{p=1}{\overset{l-1}{}}{H}_l\left(k\right){\hat{x}}_l\left(k\right)\right\}& \left[2\right]\end{array}$

[0015] where w_l(k) is given by the following equation 3: $\begin{array}{cc}{w}_l\left(k\right)={{}_l\left({}^{2}I+\underset{p=l}{\overset{n}{}}{}_p^2{H}_p\left(k\right)H_p^H\left(k\right)\right)}^{-1}{H}_l\left(k\right)& \left[3\right]\end{array}$

[0016] and for single carrier, {circumflex over (x)}_l(k) is given by the following equation 4:

[0017] where IFFT denotes an inverse FFT, and the slice operation refers to a symbol decision operation.

[0018] The idea of one embodiment of the present invention is to find the unequal stream weights that improve the mean square error performance of the successive cancellation receiver and allow the receiver to demodulate the data streams in an order known by the receiver to thereby thus avoid a need by the receiver to determine an optimal stream demodulating order. The stream demodulation order is set in equation 2. In one embodiment, the stream demodulation ordering is from data stream₁ to data stream_n. Other predetermined orderings can also be used in alternative embodiments, such as, for example, the stream demodulation ordering can be from data stream_n to data stream₁.

[0019] Each mean square error equation subsequently described herein assumes the successive cancellation operations described above.

[0020] FIG. 2 illustrates the flowchart 20 as a representation of a stream weighting method in accordance with the present invention. The stream weight module 11 initiates a stage S22 of the flowchart 20 upon an operational activation of the transmitter 10. During the stage S22, the stream weight module 11 determines an equal weighting for the stream weights ₁-_n. In one embodiment, the equal weighting is dynamically determined, such as, for example, by an execution of one or more equations in real time by the stream weight module 11. In another embodiment, the equal weighting is pre-determined and stored whereby the stream weight module 11 retrieves the appropriate weight value for the stream weights ₁-_n in real time.

[0021] Upon completion of the stage S22, the stream weight module 11 proceeds to a stage S24 of the flowchart 20. In one embodiment, the stage S22 is completed by the stream weight module 11 within a prescribed time period as determined by the operational specification of a communication system employing the stream weight module 11. During the stage S24, the stream weight module 11 determines an unequal weighting of the stream weights ₁-_n whereby at least two or more of the stream weights ₁-_n have unequal weight values. In one embodiment, the unequal weighting is dynamically determined, such as, for example, by an execution of one or more equations in real time by the stream weight module 11. In another embodiment, the unequal weighting is pre-determined and stored whereby the stream weight module 11 retrieves the appropriate weight values for the stream weights ₁-_n in real time.

[0022] During a stage S26 of the flowchart 20, the stream weight module 11 ascertains a need for reiterating a determination of the unequal weighting of the stream weights ₁-_n. In one embodiment, the stream weight module 11 proceeds to stage S24 to perform an additional determination of the unequal weighting when the current unequal weighting determination fails to achieve an optimal power allocation of the transmitted signals as determined by the operational specification of a communication system employing the stream weight module 11. Conversely, the stream weight module 11 proceeds to terminate the flowchart 20 when the current unequal weighting determination achieves an optimal power allocation of the transmitted signals.

[0023] In the embodiments where the unequal weighting of stream weights ₁-_n is pre-determined and stored, stage S22 may be omitted and stage S24 can determine the unequal weighting of stream weights ₁-_n from a lookup table operation. Additionally, a receiver may determine the unequal weighting of stream weights ₁-_n and provide such to stream weight module 11.

[0024] Those having ordinary skill in the art will appreciate various benefits of flowchart 20, such as, for example, improved receiver performance as measured by a bit error rate and simplified successive cancellation demodulation because the data streams will be decoded in order (i.e., the demodulation ordering of the streams does not need to be determined by the receiver).

[0025] FIG. 3 illustrates a flowchart 30 as a representation of one embodiment of flowchart 20. During a stage S32, the stream weight module 11 determines an equal weighting of the stream weights ₁-_n. In one embodiment of the stage S32, the equal weighting of the stream weights ₁-_n are determined in accordance with the following equation 5:

[0026] where i1 to n, and n is the number of data stream₁-data stream_n.

[0027] Upon completion of the stage S32, the stream weight module 11 proceeds to a stage S34 of the flowchart 30 to establish an initial value for an increment s, and a statistical nature of channel vector H_l(k) (for l1, . . . n). The statistical nature of H_l(k) is used because the true channel in equation 1 may not be known by the transmitter 10 and thus when the unequal power weights are determined, a model for channel vector H_l(k) may be employed. In one embodiment of the stage S34, the initial value of the increment s is an arbitrarily small number (e.g., 0.001). In another embodiment of the stage S34, the initial value of the increment s is determined in accordance with the following equation 6: $\begin{array}{cc}s=\frac{1}{4\sqrt{n}}& \left[6\right]\end{array}$

[0028] In one embodiment of the stage S34, the statistical nature of channel vector H_l(k) (for l1, . . . n) is determined whereby the stream weights ₁-_n can be averaged over at least one realization of channel vector H_l(k). Specifically, the elements of channel vector H_l(k) can be formed at each k by generating independent unit variance complex Gaussian random variables to approximate independent fading as is known in the art. In another embodiment of stage S34, channel vector H_l(k) is generated as a FFT of some number (e.g., a L number) of time taps where the time taps span the expected delay spread of the channel and are generated as independent complex Gaussian random variables having some variance (e.g., 1/L).

[0029] Upon completion of the stage S34, the stream weight module 11 proceeds to a stage S36 of the flowchart 30 to determine a mean square error (MSE) for each stream₁-stream_n in accordance with the following equation 7: $\begin{array}{cc}{\mathrm{MSE}}_l=\underset{k=0}{\overset{N-1}{}}\frac{1}{1+{}_l^2{H}_l^H\left(k\right)Q_l^{-1}\left(k\right)H_l\left(k\right)}& \left[7\right]\end{array}$

[0030] where Q_l(k) is determined in accordance with the following equation 8: $\begin{array}{cc}{Q}_l\left(k\right)={}^{2}I+\underset{p=l+1}{\overset{n}{}}{}_p^2{H}_p\left(k\right)H_p^H\left(k\right)& \left[8\right]\end{array}$

[0031] From equation 8, the noise power ² of the receiver is needed to determine Q_l(k). Thus, the noise power ₂ of the receiver is vital to a determination of the unequal weighting of stream weights ₁-_n.

[0032] Upon completion of the stage S36, the stream weight module 11 proceeds to a stage S38 of the flowchart 30 to determine the stream weight from the stream weights ₁-_n that has the largest MSE as determined during the stage S36 in accordance with equations 7 and 8. The stream weight having the highest MSE is referred to as stream weight _max.

[0033] Upon completion of the stage S38 of the flowchart 30, a power of the stream weight _max is increased by increment s during a stage S40 of the flowchart 30 and the remaining stream weights are decreased in power during a stage S42 of the flowchart 30 to realize the following equation 9: $\begin{array}{cc}\underset{l=1}{\overset{n}{}}{}_l^2=1& \left[9\right]\end{array}$

[0034] An unequal weighting of the stream weights ₁-_n is achieved upon a completion of the stage S42 of the flowchart 30. The stream weight module 11 thereafter proceeds to a stage S44 of the flowchart 30 to ascertain a need for an additional iteration of the stages S36-S42 by ascertaining whether the MSE of each stream weight ₁-_n according to equations 7 and 8 are approximately equal, such as, for example, when the MSE of each stream weight ₁-_n is within 0.001 of each other. If so, the stream weight module 11 terminates the flowchart 30. Otherwise, the stream weight module 11 proceeds to stages S36-S42 for an additional determination of an unequal weighting of stream weight ₁-_n.

[0035] In an alternative embodiment of the flowchart 30, the stage S32 may be omitted.

[0036] FIG. 4 illustrates a flowchart 50 as a representation of a second embodiment of flowchart 20 (FIG. 2). During a stage S52 of the flowchart 50, the stream weight module 11 initializes stream weight _n. In one embodiment of the stage S52, stream weight _n is initialized in accordance with the following equation 10: $\begin{array}{cc}{}_n=\frac{1}{2\sqrt{n}}& \left[10\right]\end{array}$

[0037] Upon completion of the stage S52, the stream weight module 11 proceeds to a stage S54 of the flowchart 50 to establish an initial value for an increment s and a statistical nature of channel vector H_l(k) (for l0, . . . n) as previously described herein in a connection with stage S34 of flowchart 30 (FIG. 3). Additionally, a resolution value E is established during stage S54. In one embodiment, the resolution value E is 0.001.

[0038] Upon completion of the stage S54, the stream weight module 11 proceeds to a stage S56 of the flowchart 50 to determine a weighting of the stream weights ₁-_n1. In one embodiment, the weighting of the stream weights ₁-_n1 are determined in accordance with the following equation 11: $\begin{array}{cc}{}_l^2=\frac{1}{{\mathrm{MSE}}_n}\underset{k=0}{\overset{N-1}{}}\frac{1}{H_l^H\left(k\right)Q_l^{-1}\left(k\right)H_l\left(k\right)}& \left[11\right]\end{array}$

[0039] where MSE_n and Q_l(k) are determined in accordance with equations 7 and 8, respectively.

[0040] Upon completion of the stage S56, the stream weight module 11 proceeds to a stage S58 of the flowchart 50 to determine a power variable c. In one embodiment, the power variable c is determined in accordance with the following equation 12: $\begin{array}{cc}c=\mathrm{sign}\left(1-\underset{l=1}{\overset{n}{}}{}_l^2\right)& \left[12\right]\end{array}$

[0041] where sign(x)1 if x>0, sign(x)0 if x0, and sign(x)1 if x<0.

[0042] Upon completion of the stage S58, the stream weight module 11 sequentially proceeds to a stage S60 of the flowchart 50 to increase the weighting of stream weights _n by a product of power variable c and increment s, a stage S62 of the flowchart 50 to divide the increment s by a factor of two, and a stage S64 of the flowchart 50 to determine the weighting of the stream weights ₁-_n1 as previously described in connection with stage S56.

[0043] An unequal weighting of the stream weights ₁-_n is achieved upon a completion of the stage S64 of the flowchart 50. The stream weight module 11 thereafter proceeds to a stage S66 of the flowchart 50 to ascertain a need for an additional iteration of stages S58-S64 by comparing increment s and resolution value E. Stream weight module 11 proceeds to stage S58 when increment s is greater than resolution value E. Otherwise, the stream weight module 11 terminates the flowchart 50.

[0044] The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.