Frieder Sanzi

A comparative study of iterative channel estimators for mobile OFDM systems

We investigate two iterative channel estimators for mobile orthogonal-frequency division multiplexing. The first estimator is based on iterative filtering and decoding whereas the second one uses an a posteriori probability (APP) algorithm. The first method consists of two cascaded one-dimensional Wiener filters, which interpolate the unknown time-varying two-dimensional frequency response in between the known pilot symbols. As shown, the performance can be increased by feeding back the likelihood values at the output of the APP-decoder to iteratively compute an improved estimate of the channel frequency response. The second method applies two APP estimators, one for the frequency and the other one for the time direction. The two estimators are embedded in an iterative loop similar to the turbo decoding principle. As shown in detail, this iterative estimator is superior and its performance is independent of whether the chosen time-frequency pilot grid satisfies the two-dimensional sampling theorem or not. The bit-error rate as a function of the signal-to-noise ratio is used as a performance measure. In addition, the convergence of the iterative decoding loop is studied with the extrinsic information transfer chart.

An adaptive two-dimensional channel estimator for wireless OFDM with application to mobile DVB-T

An adaptive channel estimator is proposed and investigated to improve the performance of the receiver for pilot aided wireless and mobile OFDM systems. The estimator consists of a two-dimensional Wiener filter which is implemented as a cascade of two one-dimensional filters. We propose an efficient algorithm for adaptation to time varying channels of the second filter. The method is applied to the Terrestrial Digital Video Broadcasting System (DVB-T), which was originally specified for fixed receivers and which is in the process of being extended for mobile reception. The results are shown after inner decoding. Depending on the channel conditions, the signal-to-noise ratio gain can be up to 1.6 dB. The method provides a compatible improvement to DVB-T receivers.

Error Floor Removal for Bit-Interleaved Coded Modulation with Iterative Detection

The performance of bit-interleaved coded modulation with iterative detection (BICM-ID) depends strongly on the chosen constellation mapping. In this paper, we present an EXIT chart analysis of a number of mappings that have been designed especially for BICM-ID. This analysis reveals that all these mappings inevitably yield a bit error rate floor, which cannot be overcome by increasing the number of iterations or other measures taken at the receiver side. In order to remove this error floor, we introduce an inner code based on differential encoding with code doping. This coding scheme is of low complexity and introduces no additional redundancy. Two detection schemes of different complexity and similar performance are presented. This enhanced BICM scheme is applied to V-BLAST and a new concept to iteratively mitigate the multi-stream interference is presented

Two-dimensional iterative APP channel estimation and decoding for OFDM systems

We present a channel estimation method for coherent detection of multicarrier signals which is based on the a posteriori probability calculation algorithm (APP estimator). A two-dimensional channel estimation is performed by applying a concatenation of two one-dimensional APP estimators. The combination with an outer soft in/soft out channel decoder allows one to further improve the bit error rate by means of iterative estimation and decoding. The robustness of the new channel estimator permits one to reduce the number of pilot symbols which are required as phase references for coherent detection. It is readily applicable to current multicarrier systems (e.g. DVB-T) without changing the transmission format. A combination with an inner convolutional code is suggested to further improve the performance.

Totally blind APP channel estimation for mobile OFDM systems

A new two-dimensional blind channel estimation scheme for coherent detection of orthogonal frequency-division multiplexing (OFDM) signals in a mobile environment is presented. The channel estimation is based on the a posteriori probability (APP) calculation algorithm. The time-variant channel transfer function is completely recovered without phase ambiguity with no need for any pilot or reference symbols, thus maximizing the spectral efficiency of the underlying OFDM system. The phase ambiguity problem is solved by using a 4-QAM (quadrature amplitude modulation) scheme with asymmetrical arrangement. The results clearly indicate that totally blind channel estimation is possible for virtually any realistic time-variant mobile channel.

Totally blind APP channel estimation with higher order modulation schemes

A new two-dimensional blind channel estimation scheme for coherent detection of OFDM signals in a mobile environment is presented. The channel estimation is based on the a posteriori probability (APP) calculation algorithm. The time-variant channel transfer function is completely recovered without phase ambiguity with no need for any pilot or reference symbols. The two-dimensional channel estimation is performed by applying a concatenation of two one-dimensional APP estimators for frequency and time direction in combination with an iterative estimation and decoding loop. The phase ambiguity problem is solved by using higher order modulation schemes with asymmetrical arrangement. The proposed approach maximizes the spectral efficiency by avoiding any reference or pilot symbols and minimizes the BER by using coherent demodulation. We investigate the performance of our algorithm with respect to the BER and study the convergence of the iterative estimation and decoding loop using extrinsic information transfer (EXIT) charts.

Iterative channel estimation and decoding with product codes in multicarrier systems

A two-dimensional channel estimation scheme for coherent detection of multicarrier modulated signals is presented which is based on the a posteriori probability (APP) calculation algorithm. An iterative estimation and decoding loop over inner estimator/product code and outer soft in/soft out APP decoder allows to further reduce the bit error rate. Heavily punctured recursive systematic convolutional codes are introduced as high-rate component codes of the inner product code. The proposed system outperforms the reference system which applies conventional two-dimensional Wiener filters for channel estimation.

Multicarrier code division multiplex with iterative MAP symbol-by-symbol estimation

We consider a multicarrier code division multiplex (MC-CDM) scheme. At the receiver side the maximum a posteriori symbol-by-symbol estimator (MAPSSE) is used for the detection of the CDM signal. Therefore the influence of the spreading factor on the overall bit error rate (BER) is investigated. We concatenate the MAPSSE with the channel decoder which allows for iterative decoding. This system can be considered as a serially concatenated iterative decoding scheme whereby the inner decoder is replaced by the MAPSSE. Therefore the bit error rate can be further reduced by means of iterative decoding. A combination with an inner recursive systematic convolutional (RSC) component code with rate 1 is suggested to further improve the performance. The applications are broadcasting and, with some extensions, two-way communications.

Generalized 8-PSK for totally blind channel estimation in OFDM

Channel estimation in OFDM systems can conveniently be done by inserting a stream of pilot symbols at the transmitter and by using FIR interpolation filters at the receiver. The drawback of this method is the decrease in spectral efficiency due to the pilot symbols. Alternatively, blind channel estimation makes pilot symbols unnecessary. Most blind channel estimation approaches are based on higher order statistics and converge slowly, making them unsuitable for mobile environments. Moreover, the channel estimate suffers from a phase blindness, which can only be resolved by pilot symbols. The concept of totally blind channel estimation makes pilots completely unnecessary, and even works in rapidly time varying environments. This is achieved by using two different modulation schemes, such as QPSK and 3-PSK, on adjacent subcarriers. We further develop the concept of totally blind channel estimation by applying a regular 8-PSK and a generalized 8-PSK to achieve totally blind channel estimation without the need for any pilot symbols. We enhance the original receiver design by applying a two-dimensional a posteriori probability (APP) calculation algorithm. We evaluate our system at high Doppler frequencies with COST 207 channels on the basis of extrinsic information transfer (EXIT) and BER charts.

Generalized PSK for improved iterative decoding and demodulation of coded DPSK systems

The transmission of coded DPSK signals over a time-varying channel is considered. Coded DPSK is similar to a serially concatenated coding scheme where the inner encoder is replaced by a differential modulator. At the receiver side, the a posteriori probability (APP) calculation algorithm is applied for differential demodulation, followed by an outer APP channel decoder. The likelihood values at the output of the channel decoder are fed back to the differential demodulator in an iterative decoding loop. Such a system shows large coding gain for transmission over a time-varying flat fading channel if the receiver has perfect channel state information. However, perfect channel knowledge is normally not available at the receiver side. Therefore, joint channel estimation and demodulation has to be applied, leading to a dramatic performance degradation for conventional DPSK. In order to maintain the large coding gain even without any channel knowledge, we propose a novel concept for DPSK by applying regular and generalized PSK symbols in an alternating manner. We evaluate the proposed system on the basis of extrinsic information transfer (EXIT) and bit error ratio (BER) charts.