Method and system for power loading implementation detection in beamforming systems

[0001] The present invention relates generally to data communication, and more particularly, to power loading in multi-channel data communication systems such as multiple-input multiple-output (MIMO) systems.

[0002] A multiple-input-multiple-output (MIMO) communication system employs multiple transmit antennas in a transmitter and multiple receive antennas in a receiver for data transmission. A MIMO channel formed by the transmit and receive antennas may be decomposed into independent channels, wherein each channel is a spatial sub-channel (or a transmission channel) of the MIMO channel and corresponds to a dimension. The MIMO system can provide improved performance (e.g., increased transmission capacity) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.

[0003] MIMO increases system link robustness and spectral efficiency. To optimize spectral efficiency for MIMO system, many efforts have been made, which can be broadly classified into two categorists: open-loop approaches and closed-loop approaches. The open-loop approaches include spatial multiplexing, space-time coding and the tradeoff therebetween them. The closed-loop approaches focus on maximizing the link capacity, which results in a“water-filling ” solution, and on minimizing the weighted MMSE which provides an“inverse water-filling” solution.

[0004] In an open-loop MIMO system, the MIMO transmitter has no prior knowledge of the channel condition (i.e., channel state information). As such, space-time coding techniques are usually implemented in the transmitter to combat fading channels. In a closed-loop system, the channel state information (CSI) can be fed back to the transmitter from the receiver, wherein some pre-processing can be performed at the transmitter in order to separate the transmitted data streams at the receiver side.

[0005] Such techniques are referred to as beamforming techniques, which provide better performance in desired receiver's directions and suppress the transmit power in other directions. Beamforming techniques are considered for IEEE 802.11n (high throughput WLAN) standard. Closed-loop eigen-beamforming generally provides higher system capacity compared with the closed-loop solution, assuming the transmitter knows the down-link channel. Singular vector decomposition (SVD) based eigen-beamforming decomposes the correlated MIMO channel into multiple parallel pipes.

[0006] When applying the closed-loop approach to MIMO-OFDM (orthogonal frequency division multiplexing), the optimal solution requires a bit loading and power loading per OFDM subcarrier. In order to simplify the complexity, commonly assigned patent applications Ser. No. 11/110,346, filed Apr. 19, 2005, entitled“Power Loading Method and Apparatus for Throughput Enhancement in MIMO systems”, and patent application Ser. No. 11/110,337, filed Apr. 19,2005, entitled“A Method and Apparatus for Quantization and Detection of Power Loadings in MIMO Beamforming System,” incorporated herein by reference, provide adapting coding/modulation and power level across all subcarriers, fixing coding/modulation for all data streams and only adjusting the uneven power level for all OFDM symbols. In such methods, the receiver needs to know the power loadings before decoding the received signals. Therefore, the transmitter needs to acknowledge the receiver about the power loadings used at the transmitter, or the receiver needs to do automatic detection to estimate the power loading values based on the received reference signals.

[0007] In one embodiment the present invention provides a method of automatically detecting the power loadings at the receiver wherein estimation of power loadings based on the received reference signals is unnecessary. The present invention is applicable to power loading methods which are calculated based on the channel eigenvalues. The present invention is also applicable to quantized power loading cases.

[0008] As such, in one implementation, the present invention provides a closed-loop signaling method over multiple channels in a telecommunication system, comprising the steps of: obtaining channel condition for each channel; detecting transmission power loading per channel according to channel condition, wherein estimation of power loadings based on the received reference signals is unnecessary; and decoding a received encoded information bit stream according to the detected power loading per channel.

[0009] The step of detecting transmission power loading further includes the steps of: determining rank-ordered channel eigenvalues based on the channel matrix; determining un-quantized power loadings; and determining quantized power loadings based on the un-quantized power loadings.

[0010] In another implementation the present invention provides a telecommunication system, comprising: a wireless transmitter and a receiver, wherein the transmitter transmits data streams via multiple channels over a plurality of antennas to the receiver by selecting transmission power loading per channel, and the receiver detects transmission power loading per channel according to channel condition, wherein estimation of power loadings based on the received reference signals is unnecessary. The receiver further obtains channel condition for each channel, detects transmission power loading per channel according to channel condition, wherein estimation of power loadings based on the received reference signals is unnecessary, and decodes a received encoded information bit stream according to the detected power loading per channel. Preferably, the receiver detects transmission power loading by further determining rank-ordered channel eigenvalues based on the channel matrix, determining un-quantized power loadings, and determining quantized power loadings based on the un-quantized power loadings.

[0011] These and other features, aspects and advantages of the present invention will become understood with reference to the following description, appended claims and accompanying figures.

[0012] FIG. 1 shows a functional block diagram of a MIMO system implementing un-quantized uneven power loading and detection.

[0013] FIG. 2 shows a functional block diagram of a MIMO system implementing quantized uneven power loadings and detection according to an embodiment of the present invention.

[0014] FIG. 3 shows a functional block diagram of a detector for detecting power loading at a receiver according to an embodiment of the present invention.

[0015] FIG. 4 shows a flowchart of the steps of quantized power loading detection according to an embodiment of the present invention.

[0016] FIG. 1 shows a functional block diagram of a MIMO system 300 which implements un-quantized uneven power loading and detection according to said commonly assigned patent application Ser. No. 11/110,337, filed Apr. 19, 2005, entitled: “A Method and Apparatus for Quantization and Detection of Power Loadings in MIMO Beamforming System.” The system 300 comprises a transmitter TX and a receiver RX. The transmitter TX includes an information bits generation unit 302, a demultiplexer 304, a power loading calculation unit 306, a multiplication unit 308, a V function unit 310, and transmit antennas 312. The receiver RX includes receiver antennas 314, a U^H function unit 316, a channel estimation unit 318, a power loading detection unit 330, an inverse power loading unit 322 and a multiplier 324.

[0017] The system 300 provides adapting coding/modulation and power level across all subcarriers, fixing coding/modulation for all data streams and only adjusting the uneven power level for all OFDM symbols. The receiver needs to know the power loadings before decoding the received signals. Therefore, the transmitter needs to acknowledge the receiver about the power loadings used at the transmitter, or the receiver needs to do automatic detection to estimate the power loading values based on the received reference signals.

[0018] In one embodiment the present invention provides a method of automatically detecting the power loadings at the receiver wherein estimation of power loadings based on the received reference signals is unnecessary. The present invention is applicable to power loading methods which are calculated based on the channel eigenvalues. The present invention is also applicable to quantized power loading cases.

[0019] For channel eigenvalue-based algorithms for power loading calculation (such as a reverse water filling method described in the above mentioned commonly assigned patent applications) the receiver can estimate the power loading based on the channel eigenvalues. The channel eigenvalues are calculated from the estimated channel matrix H at the receiver. Assuming the power loading α_i at the ith channel is a function of channel eigenvalues λ_i, i=1, 2, . . . , N_ss. At the receiver, the power loadings can be estimated by performing the f function operations on the channel eigenvalues, as in relation (1) below:
α_i=ƒ(λ₁,λ₂, . . . ,λ_Nss)  (1)

[0020] This method is also applicable to quantized power loading cases (one such quantized power loading case is described in the above mentioned commonly assigned patent applications).

[0021] In the reverse water filling method described in the above mentioned commonly assigned patent applications, the un-quantized power loadings α_i are calculated according to relation (2) below: $\begin{array}{cc}{\alpha }_i=\frac{P_{\mathrm{total}}}{{\lambda }_i\sum _{j=1}^{N_{\mathrm{ss}}}\frac{1}{{\lambda }_j}}& \left(2\right)\end{array}$

[0022] where N_ss is the number of spatial data streams and P_total is a fixed number representing total power. The quantized power loadings P_i are calculated according to relation (3) below:
P_i=Q[α_i]  (3)
wherein Q(.) is a quantization function.

[0023] According to an embodiment of the present invention, the process for detecting P_i at the receiver includes the steps of:

• • [0024] 1. Calculate rank-ordered channel eigenvalues λ_i, i=1, 2, . . . , N_ss, from the estimated channel matrix H based on sent packets, where λ are the eigenvalues of the matrix HH* and the (·)* is the Hermitian operation.
  • [0025] 2. Calculate α_i based on relation (2) above.
  • [0026] 3. Calculate P_i, which are the nearest quantized values of α_i. Here the same quantizer is used as in the transmitter.

[0027] FIG. 2 shows a functional block diagram of a MIMO system 400 which implements quantized uneven power loadings and detection according to an embodiment of the present invention. The system 400 comprises a transmitter TX and a receiver RX. The transmitter TX includes an information bits generation unit 402, a demultiplexer 404, a power loading quantization unit 406, a power loading calculation unit 407, a multiplication unit 408, a V function unit 410, and transmit antennas 412. The receiver RX includes receiver antennas 414, a U^H function unit 416, a channel estimation unit 418, a power loading detection unit 420 (detailed by example in detector 100 of FIG. 3, described below), a power loading quantization unit 421, an inverse power loading unit 422 and a multiplier 424.

[0028] FIG. 3 shows a block diagram of an embodiment of a power loading detector 100 for detecting P_i at the receiver RX according to an embodiment of the present invention. The detector 100 includes a channel estimation unit 102, an SVD unit 104 , an eigenvalue calculation unit 106, a power loading calculation unit 108 and a quantizer 110. The channel estimation unit 102 estimates the channel H, the SVD unit 104 determines H=UDV^H where U and V are unitary matrices and D is a diagonal matrix with elements equal to the square-root of eigenvalues of the matrix HH*, where (·)* is the Hermitian operation, the eigenvalue calculation unit 106 determines eigenvalues λ_i=D_ii², the power loading calculation unit 106 determines the un-quantized power loading α_i from relation (2) above, and the quantizer 110 determines the quantized power loading P_i according to relation (3) above.

[0029] An alternative method for implementation of the power loading detector is described in the following to reduce the complexity in determining the un-quantized power loading and determining the quantized power loading, including the steps of:

• • [0030] 1. Calculate and rank order channel eigenvalues λ_i, i=1, 2, . . . , N_ss, in descending order from the estimated channel matrix H, where λ are the eigenvalues of the matrix HH* and the (·)* is the Hermitian operation.
  • [0031] 2. For i=1, calculate c_i=λ_2i−1/λ_2i.
  • [0032] 3 . Find the quantized set P=(P₁, P₂, . . . , P_Nss), from a set of power-loading sets pre-defined and fixed by the transmitter, with the smallest |c_i-d_i|where d_i=P_Nss−2(i−1)/P_Nss−2i+1 .
  • [0033] 4. Repeat steps 2 and 3 with i=i+1, if multiple sets of P in step 3 exist and i<Nss.

[0034] For example, in a case where N_ss =2 and 2-bit quantization, then the pre-defined power loading set P={(P₁, P₂): 0.4, 1.6), (0.7, 1.3)} for quantized power loading values, which are under the fixed power constraint. If the transmitter chooses (P₁, P₂) =(0.4, 1.6) as the power loading pair, and the estimated channel has eigenvalues (3.6, 1.2) at the receiver. From step 2, c₁=3.6/1.2 =3. Therefore, the selection is (P₁, P₂)=(0.4, 1.6), because its d₁=(P₂/P₁)=4 is near c₁=3, rather than the other pair with d₁=(1.3/0.7)=1.86. Since only one set of power loading pair exists, no repetitions of step 2 and 3 are necessary.

[0035] FIG. 4 shows a flowchart of an implementation of the abovementioned alternative method of quantized power loading detection, according to the present invention, including the steps of:

[0036] Step 200: Start,

[0037] Step 202: Calculate and rank order channel eigenvalues λ_i, in descending order, for i=1, . . . , N_ss,

[0038] Step 204: set i=1,

[0039] Step 206: Compute $c_i=\frac{{\lambda }_{2i-1}}{{\lambda }_{2i}},$

[0040] Step 208: Find all P=(P₁, . . . , P_NSS) with smallest |c_i-d_i|, from all pre-defined quantized power loading sets, where $d_i=\frac{P_{N_{\mathrm{ss}}-2i+2}}{P_{N_{\mathrm{ss}}-2i+1}},$

[0041] Step 210: Determine if multiple sets of P's exist and i<Nss,

[0042] Step 214: If so, i =i +1, and proceed back to step 206.

[0043] Step 212: Otherwise, if multiple sets of P's still exist, randomly choose one,

[0044] Step 216: The process is completed.

[0045] Because according to the present invention there is no need to transmit the reference signal for power loading detection, system complexity is reduced. Further, the above methods can be implemented at the transmitter for quantized power loading selections.

[0046] The present invention has been described in considerable detail with reference to certain preferred versions thereof; however, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.