Turbo Equalization of GMSK Signals Using Noncoherent Frequency Detection

Tomoya OKADA  Yasunori IWANAMI  

IEICE TRANSACTIONS on Electronics   Vol.E85-C   No.3   pp.473-479
Publication Date: 2002/03/01
Online ISSN: 
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Issue on Signals, Systems and Electronics Technology)
Category: Digital Transmission
GMSK,  ISI,  limiter-discriminator detection,  iterative equalization,  

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In this paper, we propose a turbo equalization scheme for GMSK signals with frequency detection. Although the channel is AWGN, there exists severe ISI (Inter-Symbol Interference) in the received signal due to the premodulation Gaussian baseband filter in the transmitter as well as the narrowband IF filter in the receiver. We regard these two filters as a real number inner convolutional encoder. The ISI equalizer for this inner encoder and the outer decoder for a RSC (Recursive Systematic Convolutional) code, are connected through a random (de-)interleaver. These inner and outer decoders generate the reliability values in terms of LLR (Log Likelihood Ratio), using MAP or SOVA algorithm with SISO (soft input and soft output). Moreover iterative decoding with the limitation of LLR values are employed between two decoders to achieve a turbo equalization for GMSK frequency detection. Through computer simulations, the proposed system shows the BER=10-5 at Eb/N0=8.8 dB, when we take BT=0.6 (IF filter bandwidth multiplied by symbol duration) with the iteration number of 3. This means 3.1 dB improvement compared with the conventional scheme where the inner ISI equalizer is concatenated with the outer hard decision Viterbi decoder.