A spread spectrum radio frequency communication system includes a Forward Error Correction (FEC) algorithm to encode digital data to provide a plurality of symbol groups, the FEC algorithm using a Reed Solomon FEC code, an interleaving algorithm to map each one of the plurality of symbol groups into a corresponding one of a plurality of coherent subbands, and a Walsh encoder to encode each one of the plurality of symbol groups.

Frequency domain direct sequence spread spectrum with flexible time frequency code

In frequency selective channels, some frequencies may be in deep fade. Inherent frequency diversity in multilevel frequency shift keying is exploited by a coding technique called permutation code, which ensures that the frequencies occur equally often. Information bits are uniformly spread over the frequency band. The performance of permutation codes and 4FSK in terms of bit error rate is compared with convolutional codes over 50 ns and 100 ns delay spread channel.

Permutation coding and MFSK modulation for frequency selective channel

The maximum likelihood (ML) multi-user detector is developed for fast frequency hopping/multiple frequency shift keying (FFH/MFSK) multiple access (MA) systems. The channels are modeled with Rayleigh fading and additive, white Gaussian noise (AWGN). The FFH system has L chips per symbol, and each user has a unique L-chip hopping pattern over 2/sup K/ frequencies. The M users are assumed to be chip-synchronized at the receiver, and the receiver performs non-coherent detection. The ML multiuser detector simultaneously performs diversity combining and joint symbol detection, but the computational requirements are prohibitive if the 2/sup KM/ symbol combinations are searched exhaustively. We propose a reduced-complexity multiuser detection algorithm for FFH/MFSK MA systems, and we demonstrate through simulations that the new algorithm performs better than existing multiuser detection algorithms on channels with AWGN and Rayleigh fading.

Maximum likelihood multi-user detection for fast frequency hopping/multiple frequency shift keying systems

Iterative decoding (ID) aided fast frequency hopping (FFH), M-ary frequency shift keying (MFSK) using clipped combining in multiple access (MA) channels is investigated. All users' data are convolutionally encoded and the encoded bits are interleaved and converted to M-ary symbols, which are transmitted using FFH-MFSK modulation. The soft metrics to be passed from the demodulator to the decoder are derived assuming a Rayleigh fading channel. We also propose a novel multiuser detection (MUD) scheme which employs joint soft decoding as well as successive interference cancellation (SIC), the receiver exploiting the soft information fed back by the decoder to the demodulator in order to cancel the interference imposed by reliable symbols. Our simulation results show that the proposed scheme is capable of combatting multiuser interference and outperforms the conventional ID by about 3dB.

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Iterative Decoding and Soft Interference Cancellation in Fast Frequency Hopping Multiuser System Using Clipped Combining

This paper presents an iterative decoding method in the coded MFSK (multilevel frequency shift keying)/FH-SSMA (frequency hopping-spread spectrum multiple access) system. The kernel of the system, which is a symbol APP (a posteriori probability) calculator, accepts channel values and a priori information from the channel decoder and generates reliability values of the received symbols. The channel decoder also makes reliability outputs using the reliability values from the symbol APP calculator. Iterative decoding is performed by repeating this process. Simulation results reveal that the channel coded system with iterative decoding in the FH-SSMA system can reduce the BER significantly, thereby accommodating more users.

Iterative decoding in convolutionally and turbo coded MFSK/FH-SSMA systems

In this contribution we present an exhaustive treatment of various coding and decoding techniques for use in fast frequency-hopping/multiple frequency shift keying multiple-access systems. One of the main goals is to show how reliability information on each received bit can be derived to enable soft-decision decoding. Convolutional codes as well as turbo codes are considered applying soft-decision, erasure, and hard-decision decoding. Their performance is compared to that of previously proposed Reed-Solomon with either errors-only or errors-and-erasures decoding. A mobile radio environment yielding a frequency-selective fading channel is assumed. It is shown that the application of turbo codes and convolutional codes with soft decision decoding can allow for a comparable number of simultaneously transmitting users to Reed-Solomon codes with errors-and-erasures decoding. Furthermore, the advantage of soft decisions is shown, which can be applied to a widely and growing range of channel codes. The pertinent technique of calculating soft decisions is described in the paper.

P. Robertson

Papers

Soft-decision and erasure decoding in fast frequency-hopping systems with convolutional, turbo, and Reed-Solomon codes