Differential closed-loop transmission feedback in wireless communication systems
A method and apparatus for providing channel feedback is provided herein. During operation a covariance matrix at time t (R) is calculated as a function of a received downlink signal. Matrix C t is also calculated and is based on a previous quantized covariance matrix (R q t−1 ), the covariance matrix (R) at time t, and a forgetting factor (γ) that is applied to R q t−1 . The C t is then used to create a DERC feedback message (signal or waveform) and may be transmitted with pilots on a proper feedback channel to a base unit. The base unit receives the feedback (C t ) as a DERC waveform on a proper feedback channel. The base unit uses non-coherent or coherent detection to detect the DERC values send by the remote unit and uses the DERC values with a previous quantized covariance matrix estimate, a forgetting factor, and a weighting value to compute a covariance matrix estimate to use for beamforming. The base unit then uses the covariance matrix estimate to determine appropriate channel beamforming weights, and instructs transmit beamforming circuitry to use the appropriate weights.
1. A mobile unit comprising:
a transceiver configured for receiving a request to feed back channel information, the request sent from a base station; and
a processing unit configured for:
calculating a covariance matrix at time t (R) as a function of a received downlink signal;
calculating matrix C t to feed back to the base station, wherein C t is based on a previous quantized covariance matrix (R q t−1 ), the covariance matrix (R) at time t, and a forgetting factor (γ) that is applied to R q t-1 ; and
transmitting, via the transceiver, C t to the base station;
wherein R q =Q(R/trace(R)) where Q( )is a quantization function.
2. The mobile unit of claim 1 wherein C t is based on a difference between the current value of R and a previous value of the quantized covariance matrix R q t−1 .
3. The mobile unit of claim 2 wherein for main diagonal elements of C t (where (A) n,m means element n,m of matrix A), {C t } nn =α sgn((R) nn −Re(γ{R q t−1 } nn )) where α is a scaling factor and sgn(x) is +1 if x>0 and is −1 if x<0;
wherein Re(x) is the real part of x.
4. The mobile unit of claim 2 wherein for off-diagonal elements of C t (m>n), {C t } nm =α sgn(Re((R) nm )−Re(γ{R q t−1 } nm ))+jα sgn(Im((R) nm )−Im(γ{R q t−1 } nm )) where Re(x) is the real part of x and Im(x) is the imaginary part of x;
wherein (A) n,m means element n,m of matrix A;
wherein α is a scaling factor; and
wherein sgn(x) is +1 if x>0 and is −1 if x<0.