MSAT voice modulation considerations

The challenge for Mobile satellite (MSAT) voice services is to provide near toll quality voice to the user, while minimizing the power and bandwidth resources of the satellite. The options for MSAT voice can be put into one of two groups: Analog and Digital. Analog, nominally narrowband single sideband techniques, have a shown robustness to the fading and shadowing environment. Digital techniques, a combination of low rate vocoders and bandwidth efficient modems, show the promise of enhanced fidelity, as well as easier networking to the emerging digital world. The problems and tradeoffs to designers are many, especially in the digital case. Processor speed vs. cost and MET power requirements, channel coding, bandwidth efficiency vs. power efficiency etc. While the list looks daunting, in fact an acceptable solution is well within the technology. The objectives are reviewed that the MSAT voice service must meet, along with the options that are seen for the future

An implementation of a reference symbol approach to generic modulation in fading channels

As mobile satellite communications systems evolve over the next decade, they will have to adapt to a changing tradeoff between bandwidth and power. This paper presents a flexible approach to digital modulation and coding that will accommodate both wideband and narrowband schemes. This architecture could be the basis for a family of modems, each satisfying a specific power and bandwidth constraint, yet all having a large number of common signal processing blocks. The implementation of this generic approach, with general purpose digital processors for transmission of 4.8 kilobits per sec. digitally encoded speech, is described

Separable concatenated codes with iterative map decoding for Rician fading channels

Very efficient signalling in radio channels requires the design of very powerful codes having special structure suitable for practical decoding schemes. In this paper, powerful codes are obtained by combining comparatively simple convolutional codes to form multi-tiered 'separable' convolutional codes. The decoding of these codes, using separable symbol-by-symbol maximum a posteriori (MAP) 'filters', is described. It is known that this approach yields impressive results in non-fading additive white Gaussian noise channels. Interleaving is an inherent part of the code construction, and consequently, these codes are well suited for fading channel communications. Here, simulation results for communications over Rician fading channels are presented to support this claim

Data-aided fading estimation technique for shadowed mobile satellite fading channels

A novel data-aided fading estimation technique that employs both pilot and data symbols is proposed to significantly reduce the bandwidth redundancy of the pilot-symbol-aided (PSA) systems using receivers with low complexity and latency in the shadowed mobile satellite fading channels. The shadowed mobile satellite fading channels are modeled as the sum of a lognormally distributed direct component and a Rayleigh distributed multipath component, and the PSA system employs 16-ary quadrature-amplitude-modulation (16QAM) for transmission. Monte Carlo computer simulation has been used to assess the technique on the bit-error-rate (BER) performances of the system in the light shadowed, the average shadowed and the heavy shadowed Rician fading environments. The results have shown that the proposed technique requires a very low bandwidth redundancy to provide satisfactory BER performances, and can substantially lower the error floors of the PSA systems.postprin

Bandwidth-efficient pilot-symbol-aided technique for multipath-fading channels

Pilot-symbol-aided (PSA) transmission is one of the effective methods to combat multipath fading in digital mobile communications systems. In the PSA systems, redundant bandwidth and power are required to transmit the pilot symbols. In this paper, we propose a novel fading estimation technique that requires a very low bandwidth redundancy in the PSA systems. The proposed technique uses simple linear interpolation on the pilot symbols and the detected data symbols to obtain the estimates of the channel fading effects. Monte Carlo computer simulation has been carried out to study the effects of the technique on the bit error rate performance of 16-ary quadrature amplitude modulation in the flat and the frequency-selective Rayleigh-fading channels corrupted with cochannel interference and additive white Gaussian noise. Results have shown that the proposed technique can, at the expense of a little power efficiency, significantly improve the bandwidth efficiency of the PSA systems using receivers with low com plexity and latency.published_or_final_versio

Combining PSA fading estimation techniques for TCM and diversity reception in Rician fading channels

In this paper, a novel pilot-symbol-aided (PSA) technique is proposed for fading estimation in the land mobile satellite fading channels. The proposed technique combines the fading estimates obtained from a bandwidth-efficient technique and a conventional technique according to the signal-to-noise ratios (SNRs) of the fading estimates. To enhance the transmission quality, trellis-coded modulation (TCM) and diversity reception are employed in the system, and the combined estimates are subsequently used to correct the channel fading effects, to weight the signals from different diversity branches, and to provide channel state information of the Viterbi decoder. Monte Carlo computer simulation has been used to study the bit-error-rate (BER) performance of the proposed technique on trellis-coded 16-ary quadrature amplitude modulation in the frequency non-selective Rician fading channels. Results have shown that the proposed PSA technique requires a very low bandwidth redundancy to provide satisfactory BER performance at low SNRs, and thus is suitable for use with TCM and diversity reception to achieve both bandwidth and power-efficient transmission.postprin

Combining pilot-symbol-aided techniques for fading estimation and diversity reception in multipath fading channels

A novel pilot-symbol-aided (PSA) fading estimation technique that combines the estimates from a conventional PSA technique and a bandwidth-efficient PSA technique to achieve better performances is proposed for digital signals in multipath fading channels. The conventional technique has better performances at low signal-to-noise ratios (SNRs), while the bandwidth-efficient technique is superior at high SNRs. Monte Carlo computer simulation has been used to assess the effects of the proposed combining technique on the bit-error-rate (BER) performances of 16-ary quadrature-amplitude-modulation (16QAM), with and without two-branch diversity reception, in a flat Rayleigh fading channel. Results have shown that the combining technique has the advantages of both of the conventional technique and the bandwidth-efficient technique and is more preferred for use with diversity reception. © 2006 Springer Science + Business Media, Inc.postprin

An Analysis of Pilot Symbol Assisted Modulation for Rayleigh Fading Channels

Abstract-Proposals have appeared recently for the use of pilot symbols to mitigate the effects of rapid fading in mobile communications. Unlike the more familiar pilot tone systems, pilot symbol assisted modulation (PSAM) does not affect the transmitted pulse shape or the peak-to-average power ratio, and implementation is straightforward. This paper puts PSAM on a solid analytical basis, a feature missing from previous work. It presents closed form expressions for the BER in BPSK and QPSK, for a tight upper bound on SER in 16QAM, and for the optimized receiver coefficients. The error rates obtained are lower than for differential detection for any combination of SNR and Doppler spread, and the performance is within 1 dB of a perfect reference system under slow fading conditions, and within 3 dB when the Doppler spread is 5% of the symbol rate

Channel Estimation in OFDM systems

Orthogonal frequency division multiplexing (OFDM) provides an effective and low complexity means of eliminating intersymbol interference for transmission over frequency selective fading channels. This technique has received a lot of interest in mobile communication research as the radio channel is usually frequency selective and time variant. In OFDM system, modulation may be coherent or differential. Channel state information (CSI) is required for the OFDM receiver to perform coherent detection or diversity combining, if multiple transmit and receive antennas are deployed. In practice, CSI can be reliably estimated at the receiver by transmitting pilots along with data symbols. Pilot symbol assisted channel estimation is especially attractive for wireless links, where the channel is time-varying. When using differential modulation there is no need for a channel estimate but its performance is inferior to coherent system.In this thesis we investigate and compare various efficient pilot based channel estimation schemes for OFDM systems. The channel estimation can be performed by either inserting pilot tones into all subcarriers of OFDM symbols with a specific period or inserting pilot tones into each OFDM symbol. In this present study, two major types of pilot arrangement such as blocktype and comb-type pilot have been focused employing Least Square Error (LSE) and Minimum Mean Square Error (MMSE) channel estimators. Block type pilot sub-carriers is especially suitable for slow-fading radio channels whereas comb type pilots provide better resistance to fast fading channels. Also comb type pilot arrangement is sensitive to frequency selectivity when comparing to block type arrangement. The channel estimation algorithm based on comb type pilots is divided into pilot signal estimation and channel interpolation. The pilot signal estimation is based on LSE and MMSE criteria, together with channel interpolation using linear interpolation and spline cubic interpolation. The symbol error rate (SER) performances of OFDM system for both block type and comb type pilot subcarriers are presented in the thesis