Blind and Semiblind Channel Estimation
Visa Koivunen, Mihai Enescu, Marius Sirbu · 2003
CONTENTS 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 8.2 Time-Varying Channel Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 8.3 Signal Properties Used in Blind Equalization . . . . . . . . . . . . . . . . . . . . . . . 264 8.4 Receivers for MIMO Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270 8.5 Blind Receivers for GSM/EDGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276 8.6 Blind Receivers for CDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280 8.7 OFDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 8.8 Semiblind Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288 8.9 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290 In digital wireless communications, the impairment caused by time-varying multipath fading must be compensated for. The transmitted signal propagating via multiple paths experiences various delays due to the differing lengths of the paths. The resulting intersymbol interference (ISI) may distort the received signal so severely that the transmitted symbol sequence cannot be recovered. The effects of the ISI can be mitigated in the receiver with either direct equalization or channel identification together with, for example, maximum likelihood sequence estimation (MLSE). Typically, no prior knowledge of the channel impulse response is available and the channel is time-varying. Hence, it needs to be estimated and the estimates updated on a regular basis. The estimation is commonly performed by transmitting a known sequence of training symbols in a periodic fashion or by using a separate pilot signal. The receiver generates a local copy of the training data and the channel coefficients or equalizer are estimated. In the presence of Gaussian noise, the estimators derived are typically linear. This type of periodic transmission of training data or pilot signals significantly reduces the effective data rates. For example, in Methods, and approximately 20% of the data in a time frame is allocated for the training sequence.