The present invention relates generally to data communication, and more particularly, to data communication in multi-channel communication system such as multiple-input multiple-output (MIMO) systems.
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.
MIMO techniques are adopted in wireless standards, such as 3GPP, for high data rate services. In a wireless MIMO system, multiple antennas are used in both transmitter and receiver, wherein each transmit antenna can transmit a different data stream into the wireless channels whereby the overall transmission rate is increased.
There are two types of MIMO systems, known as open-loop and closed-loop. 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 prevent 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. 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.
The SVD (singular value decomposition) type of beamforming technique is widely used in closed-loop MIMO systems. Using SVD, a MIMO channel can be decomposed into several independent channels for data transmission resulting in no interferences between different data streams at the receiver.
Since the MIMO channels can be decomposed into independent channels with different eigenvalues, the transmission rates for each channel can be selected based on the channel eigenvalues, as described in S. A. Mujtaba, “TGn Sync Proposal Technical Specification”, a contribution to IEEE 802.11, 11-04-889r1, November 2004 (incorporated herein by reference).
The algorithm to select the transmission rates can be adapted to the channel conditions (i.e., link adaptation algorithm). However, in a beamforming system with uneven power loadings, the signal-to-noise-ratio (SNR) is also tightly related to the power loadings in all the channels, as shown in D.-S. Shiu, G. J. Fochini, M. J. Gans, and J. M. Kahn, “Fading correlation and its effect on the capacity of multi-element antenna systems”, IEEE Trans. Communication, vol. 48, pp. 502-513, March 2000.
Using the link adaptation algorithm based only on the channel eigenvalues causes significant performance degradations, especially for the beamforming systems supporting even transmission rates for all channels.
In one embodiment the present invention provides an apparatus and method for closed-loop signaling over multiple channels in a telecommunication system. Channel condition for each channel is obtained, and transmission rate per channel is determined according to channel condition. The information bit streams is transmitted via the multiple channels over a plurality of transmitter antennas according to the transmission rates.
In order to increase system capacity, a link adaptation algorithm according to the present invention is utilized in selecting channel transmission rates. According to an embodiment of the present invention, a method to determine the transmission rates for each channel in a beamforming system selects the transmission rates based on the channel conditions (i.e., link adaptation algorithm).
The present invention further provides a general criterion for determining the SNR for transmission rate selections in a beamforming MIMO system. For a beamforming MIMO system with uneven power loading, the present invention provides better link adaptation quality than transmission rate selections based on channel eigenvalues. For a beamforming system supporting even transmission rates for all channels, the present invention together with uneven power loadings provides significant performance improvements over the prior art.
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.
In the MIMO system 100 of
The power loading unit 104 of the MIMO system 100 implements adaptive power loading for different transmit channels according to the present invention. In one embodiment, where the SNR thresholds for peak rate transmission are known, the power loading unit 104 performs channel power loading.
For the MIMO system 100 having a channel H with Nt transmit antennas and Nr receiving antennas, without V processing at the transmitter TX, the received signal y can be represented as:
y=HPx+n (1)
where x is the Nt×1 transmitted signal vector, P is a diagonal matrix with loading power αi along the diagonal, and n is the additive noise in the channel.
The channel H comprises a Nr×Nt matrix wherein each element hij of the matrix represents the channel response from jth transmit antenna to ith receiving antenna. By applying SVD to H, H can be expressed as:
H=UDVH (2)
wherein U and V are unitary matrices (i.e., U is a Nr×Nt matrix, and VH is a Nt×Nt matrix), and D is a Nt×Nt a diagonal matrix with the elements equal to the square-root of eigenvalues of the matrix (HHH), where (•)H is the Hermitian operation.
For simplicity of explanation of the example embodiments of the present invention herein, it is assumed that Nss=Nt. Hence, in the following description, the matrix dimensions are related to Nt, not Nss. As those skilled in the art will recognize, the present invention applies to the generalized case where Nt>=Nss.
As shown in
y=HVPx+n (3)
And with UH processing at the receiver RX, the received signal after processing Xp can be expressed as:
Xp=UHy=DPx+UHn (4)
wherein the transmitted data x can be completely separated after this operation since D and P are diagonal matrices.
The eigenvalues in every channel play important roles in determining the signal-to-noise ratios (SNR), which is commonly used for transmission rate selections. Since the MIMO channels can be decomposed into independent channels with different eigenvalues, the transmission rates for each channel can be selected based on the channel eigenvalues, as described in S. A. Mujtaba, “TGn Sync Proposal Technical Specification”, a contribution to IEEE 802.11, 11-04-889r1, November 2004.
The algorithm to select the transmission rates can be adapted to the channel conditions (link adaptation algorithm). However, in a beamforming system with uneven power loadings, the SNR is also tightly related to the power loadings in all the channels, as shown in the reference D.-S. Shiu, G. J. Fochini, M. J. Gans, and J. M. Kahn, “Fading correlation and its effect on the capacity of multi-element antenna systems”, IEEE Trans. Communication, vol. 48, pp. 502-513, March 2000. It can be shown that in said reference, the capacity for a beamforming system can be expressed as the sum of multiple AWGN (additive white Gaussian noise) channels by:
where λi and pi2 are the eigenvalue and transmitted power corresponding to the decomposed channels, respectively, and N0 is the noise power.
From relation (5) above, it is observed that the transmitted power plays an important role in determining the system capacity, since other parameters, λi and N0, are related to channel conditions and cannot be controlled. In fact, the signal to noise ratio for each channel is linearly proportional to the product of power loadings and channel eigenvalues. From relation (4) and (5), the SNR for each channel, SNRi, can be expressed as:
where we assume the total transmitted power is fixed, i.e.,
Under the assumption that, before the power scaling operation, the power for each data stream, Pdata, is identity, the power loading αi, can be shown by:
Therefore, the criterion for transmission rate selection should be determined by the product of power loading and channel eigenvalue, since the SNR for ith channel can be expressed as:
The procedure of rate selection according to the present invention includes the steps of:
The selection of Ri is a direct mapping from a pre-defined table. Based on the measurements and system testing results, this table defines the required SNRs to support certain transmission rates. Once the SNR is estimated, the corresponding transmission rate from the table may be selected.
In general, the transmission rate is changed by changing the modulation scheme and coding rate for the transmitted data. In case of beamforming systems supporting even transmission rate for all the channels, the rate selection procedure can include the steps of: (i) finding the transmission rate in each channel Ri from steps 1-4 above, and (ii) select final rate R=minimum of Ri.
In another embodiment, the link adaptation/rate selection may be implemented at the receiver RX (
The present invention provides a general criterion in determining the SNR for transmission rate selections in a beamforming system. For a beamforming system with uneven power loading, the present invention provides better link adaptation quality than the algorithm based on channel eigenvalues. For a beamforming system supporting even transmission rates for all channels, the present invention together with uneven power loadings has significant performance improvements over the prior art systems.
As those skilled in the art recognize, the embodiments described herein are examples of generalized case of Nt>Nss where in that case, x is Nss×1, P is Nss×Nss, V is Nt×Nss, UH is Nss×Nr, etc., according to the present invention.
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.