Equalizer for single carrier FDMA receiver

#The present invention relates generally to equalization of received signals in a mobile communication system and, more particularly, to an equalizer for a receiver in a single carrier frequency division multiple access (SC-FDMA) system.

#Orthogonal Frequency-Division Multiple Access (OFDMA) is an attractive technique for sharing an available radio resource with multiple users in a high-speed wireless data communication system. Since each subcarrier of an OFDMA signal is simply scaled by a complex-valued scalar after passing through a time dispersive channel, demodulation can be performed for each subcarrier individually, and hence equalization is not needed in the receiver. Moreover, through the use of a cyclic prefix, orthogonality among different subcarriers is preserved even if they are not completely synchronized so long as the relative time delay is limited. This property is particularly desirable for uplink communications because users assigned to different subcarriers are typically only coarsely time aligned.

#A major drawback of OFDMA is the high peak-to-average-power ratio (PAR), or equivalently, the high crest factor (CF) (square root of PAR) of the transmitted waveform, which can cause undesired out-of-band radiation and/or inefficient power amplification in mobile terminals. Because of this limitation, Single-Carrier Frequency Division Multiple Access (SC-FDMA), whose transmitted waveforms have considerably lower peak-to-average-power ratio (PAR) than those of OFDMA, has recently been selected by 3GPP as the standard access method for the uplink (UL) in Evolved UTRA. The low PAR property of SC-FDMA signals enables the mobile terminals to transmit at higher efficiency while reducing undesired out-of-band emissions.

#FIG. 1 illustrates a receiver for a single carrier FDMA system according to one exemplary embodiment of the present invention.

#FIG. 2 illustrates a first embodiment of an equalizer for the receiver shown in FIG. 1.

#FIG. 3 illustrates a second embodiment of an equalizer for the receiver shown in FIG. 1.

#Referring now to the drawings, FIG. 1 illustrates the main functional elements of an SC-FDMA receiver 10 according to one exemplary embodiment. The sampled signals from two or more antennas (not shown) are first converted into the frequency domain by a time-to-frequency converter 12. The time-to-frequency converter 12 converts the samples into the frequency domain using an N-point Fast Fourier Transform (FFT). After conversion into the frequency domain, a subcarrier mapping function 14 extracts the received signal on the subcarriers of interest from the FFT output, where NC is the number of subcarriers. The received signal Ri for the ith antenna may be represented by the signal vector Ri=(Ri[1], Ri[2], . . . , Ri[Nc])T, for i=1, 2, . . . , r. A typical baseband model of the received signal vector Ri for the ith antenna is given by: Ri=D(Hi)S+Vi,  (1) where Hi=(Hi[1], Hi[2], . . . , Hi[Nc])T denotes a vector of channel coefficients over the desired frequency sub-carriers for the ith antenna, D(Hi) denotes a diagonal matrix with elements of Hi as the diagonal elements, S=[S[1], S[2], . . . , S[Nc])T represents the NC-point FFT of the time-domain, modulated symbols Z=[Z[1], Z[2], . . . , Z[Nc])T such that ESSH=I, and Vi=(Vi[1], Vi[2], . . . , Vi[Nc])T denotes the noise vector at the ith antenna, which is assumed to be a zero-mean, Gaussian-distributed random vector. The noise vector components are assumed to be uncorrelated across different subcarriers, i.e., EVi[k]Vj*[m]=0 if k≠m for any i and j.

#A channel estimator 16 processes the received signals Ri to generate estimates Ĥi of the channel coefficients and provides the channel estimates Ĥi to a signal combiner 18. The signal combiner 18 combines the received signals Ri from each antenna i using the channel estimates to produce a combined signal vector R=(R[1], R[2], . . . , R[Nc])T. The signal combiner 18 may comprise, for example, a maximal ratio combiner (MRC) or Interference Rejection Combiner (IRC). In the case when the noise is uncorrelated across antennas and identically distributed across subcarriers, i.e., E[ViVjH]=σi2Iδ(i−j), an MRC can be used. For an MRC, the signal combiner 18 combines the signal vectors Ri from each antenna according to:

#R = ∑ i = 1 r ⁢ D ⁡ ( G i ) ⁢ R i σ i , ( 2 ) where Gi≡(Gi[1], Gi[2], . . . , Gi[Nc])T represent the combining weight for the ith antenna, and D(Gi) comprises a diagonal matrix with the elements of Gi on its diagonal. The combining weights Gi may be calculated according to: