Harish Ramchandran

A comparison of two iterative equalization-and-decoding techniques in frequency-hop spread-spectrum communications using Reed-Solomon coding

Two methods of packet-level iterative equalization-and-decoding are evaluated for SFH (slow-frequency-hop) spread-spectrum communication with Reed-Solomon coding. During; each iteration of the first technique, the receiver uses MLSE equalization with priors, and code-symbol erasures for bounded-distance errors-and-erasures decoding are generated by the parity-bit method. During each iteration of the second technique, the receiver uses MAP equalization with priors, and the soft equalizer outputs are used for bounded distance successive-erasures decoding. Successful decoding of some of the packets code words in earlier iterations is used to update the equalizers priors for the next iteration. The performance and decoding complexity of the two techniques are compared for several multipath channels.

Comparing List Viterbi Algorithms with and without tail bits

In this paper we evaluate the impact of tail bits on the performance of list Viterbi algorithms (LVA). The LVA provides an ordered list of decoded bit sequences. The outer cyclic redundancy check (CRC) block code, concatenated with the convolutional code, is used to select the output sequence. For the conventional Viterbi algorithm (VA), tail-biting outperforms tail bit-aided decoder with additional complexity. However, for the concatenated system, tail-biting algorithm applied to the LVA does not perform as well. Since tail bits incur rate loss, we propose an alternative algorithm where the tail bit-aided LVA with puncturing is employed. The tail-punctured LVA, while retaining the original code rate, provides good performance with reduced complexity. A set of simulation results are presented comparing these algorithms for different constraint lengths, code rates and channel conditions.

Packet-level iterative detection for SFH communications with Reed-Solomon coding in partial-band interference

A SFH system using Reed-Solomon coding and the parity-bit method is considered with a receiver employing packet-level iterative detection and decoding, and the performance of the system in a channel with partial-band interference is evaluated. It is shown that the packet-level iterative reception technique results in substantially better performance than one-shot detection and decoding at a minimal cost in increased computation at the receiver. The use of an inner differential encoder is also examined, and its inclusion is shown to result in improved performance with packet-level iterative reception. Furthermore, alternative approaches to the parity-bit method are evaluated. Finally, the performance is compared with that of several other choices for coding and either one-shot or iterative decoding in SFH systems

Packet-level iterative errors-and-erasures decoding for SFH spread-spectrum communications with Reed-Solomon codes and differential encoding

A packet-level iterative detection technique that employs errors-and-erasures decoding has been described previously for SFH communications using Reed-Solomon coding. The technique enhances the performance of the SFH system in intersymbol-interference channels with only a minimal increase in complexity over one-shot errors-and-erasures decoding. In this paper, the performance of iterative EE decoding is considered for a SFH system with differentially encoded transmissions. It is shown that the use of differential encoding improves the performance of packet-level iterative detection in an AWGN channel with only a modest increase in detection complexity, and it also improves the performance in an intersymbol-interference channel in many instances. The packet size, the target probability of error, and the channel impulse response are considered, and the effect of each on the performance gain and the complexity is examined

Design and performance of an all-Internet Protocol mobile satellite system

Summary Third and Fourth Generation (3G and 4G) terrestrial systems provide higher speed Internet Protocol multimedia services to end-users with differentiated QoS across applications. To facilitate this, terrestrial architectures are moving towards an end-to-end all-IP architecture that unifies all services, including Voice over IP bearer. In parallel, Mobile Satellite Systems (MSS) are being designed to complement and/or co-exist with terrestrial coverage depending on spectrum sharing rules and operator choice. The challenge for MSS designers is to address the different physical layer characteristics while maintaining interoperability and compatibility with terrestrial services and reuse 3G Partnership Project core networks. To this end, MSS have been developed that mirror terrestrial Second Generation, 2.5G and 3G systems and are already operational in different parts of the world using different orbital constellations from low Earth orbits to geostationary orbits and both proprietary and standardized air interfaces. This paper describes architectures and methods for mobile satellite systems to achieve services similar to 3G and Fourth Generation terrestrial systems, focusing on the European Telecommunications Standards Institute Geostationary-Mobile Radio GMR-1 3G satellite air interface standard. Copyright

High Penetration Alerting Signal Design for Mobile Satellite Communications Systems

This paper presents a novel practical transmission signal design that enables a mobile satellite network to alert user terminals operating in a heavily power attenuated environment, such as inside buildings or in shadowed areas. In particular, we describe in detail a waveform design employing an orthogonal sequence modulated with π/2-BPSK and encoded with Reed-Solomon codes. The transmit waveform performance is characterized in terms of its Power Spectral Density (PSD) and Peak-to-Average Power Ratio (PAPR). The receiver performance is evaluated in terms of the symbol error rate and message error rate under the AWGN and Rician channels. Here we have examined the performance of conventional sequence detection as well as computationally efficient joint sequence detection with simple erasure decoding.

Iterative equalization and decoding for high-data-rate frequency-hop spread-spectrum communications

In this paper, a packet-level iterative equalization-and-decoding technique is evaluated for slow-frequency-hop spread-spectrum communications with Reed-Solomon coding. The receiver employs MAP equalization with priors during each iteration, and the soft equalizer outputs are used for bounded-distance successive-erasures decoding. Successful decoding of some R-S codewords of the packet in earlier iterations is used to update the priors for equalization in subsequent iterations. The performance of this iterative technique is compared with the performance of one-shot equalization and successive-erasures decoding, and both are compared with the performance of a system using MLSE equalization and the parity-bit method for erasure generation. The decoding complexity of each approach is also considered, and the effect of limits on the maximum decoding delay is examined.

Equalization and decoding in SFH spread-spectrum communications

There is an increasing need for tactical mobile radio networks that support link data rates of hundreds of kilobits per second to several megabits per second. If a radio network employing slow-frequency-hop (SFH) spread-spectrum modulation operates at these high data rates, however, the channel is likely to exhibit frequency-selective fading within each frequency slot of the SFH system. The frequency selectivity manifests itself as intersymbol interference at the receiver, and adaptive equalization must be employed in the receiver for each dwell interval in order to compensate for the intersymbol interference. We examine the performance of a SFH spread-spectrum system with Reed-Solomon coding in a channel that exhibits frequency-selective fading within each frequency slot. The performance is evaluated for a receiver that employs maximum likelihood sequence estimation in an equalizer that is retrained on a hop-by-hop basis. The use of parity bits for erasure generation is also investigated.