Institute for Systems Research Technical Reports
Permanent URI for this collectionhttp://hdl.handle.net/1903/4376
This archive contains a collection of reports generated by the faculty and students of the Institute for Systems Research (ISR), a permanent, interdisciplinary research unit in the A. James Clark School of Engineering at the University of Maryland. ISR-based projects are conducted through partnerships with industry and government, bringing together faculty and students from multiple academic departments and colleges across the university.
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Item Design and Performance of Trellis Codes for Wireless Channels(1995) Al-Semari, Saud A.; Fuja, T.E.; ISRSignal fading is one of the primary sources of performance degradation in mobile radio (wireless) systems. This dissertation addresses three different techniques to improve the performance of communication systems over fading channels, namely trellis coded modulation (TCM), space diversity and sequence maximum a posteriori decoding (MAP).In the first part, TCM schemes that provide high coding gains over the flat, slow Rayleigh distributed fading channel are presented. It is shown that the use of two encoders in parallel used to specify the in-phase and quadrature components of the transmitted signal results in large performance improvements in bit error rates when compared with conventional TCM schemes in which a single encoder is used. Using this approach which we label ﲉ-Q TCM codes with bandwidth efficiencies of 1, 2, and 3 bits/sec/Hz are described for various constraint lengths. The performance of these codes is evaluated using tight upper bounds and simulation.
In the second part, the use of space diversity with three different combining schemes is investigated. Expressions for the cutoff rate parameter are shown for the three combining schemes over the fully interleaved Rayleigh-distributed flat fading channel. Also, tight upper bounds on the pairwise error probability are derived for the three combining schemes. Examples of I-Q TCM schemes with diversity combining are shown. The cutoff rate and a tight upper bound on the pairwise error probability are also derived for maximal ration combining with correlated branches.
In the last part the problem of reliably transmitting trellis coded signals over very noisy channels is considered. Sequence maximum a posteriori (MAP) decoding of correlated signals transmitted over very noisy AWGN and Rayleigh channels is presented. A variety of different systems with different sources, modulation schemes, encoder rates and complexities are simulated. Sequence MAP decoding proves to substantially improve the performance at very noisy channel conditions especially for systems with moderate redundancies and encoder rates. A practical example for coding the CELP line spectral parameters (LSPs) is also considered. Two source models are used. Coding gains of as much as 4 dB are achieved.
Item I-Q TCM: Reliable Communication Over the Rayleigh Fading Channel Close to the Cutoff Rate(1996) Al-Semari, Saud A.; Fuja, Tom E.; ISRThis paper presents some trellis codes that provide high coding gain over the frequency non-selective slowly Raleigh distributed fading channel. It is shown that the use of two encoders in parallel - used to specify the in-phase and quadrature components of the transmitted signal - results in greater minimum time diversity than the conventional design in which a single encoder is used. Using this approach - which we label ﲉ-Q TCM - codes with bandwidth efficiencies of 1,2, and 3 bits/sec/Hz are described for various constraint lengths. The performance of these codes is bounded analytically and approximated via simulation; the results show a large improvement in the BER when compared with conventional TCM schemes when perfect channel state information (CSI) is available to the receiver. Indeed, when this approach is applied to channels with independent Rayleigh fading, the resulting coding gain is close to that implied by the cutoff rate limit, even for only moderately complex systems.The proposed codes are also simulated under less ideal assumptions. For instance, results for a 1 bit/sec/Hz IQ-TCM code without CSI show a significant gain over conventional coding. Finally, simulations over channels with correlated fading were undertaken; it is concluded that an interleaver span of 4v yields performance close to what is achieved with ideal interleaving.
Item I-Q TCM: Reliable Communication over the Rayleigh Fading Channel Close to the, Cutoff Rate(1995) Al-Semari, Saud A.; Fuja, Tom E.; ISRThis paper presents some trellis codes that provide high coding gain over the frequency non-selective slowly Rayleigh distributed fading channel. It is shown that the use of two encoders in parallel - used to specify the in-phase and qudrature components of the transmitted signal - results in great minimum time diversity than the conventional design in which a single encoder is used. Using this approach - which we label ﲉ-Q TCM - codes with bandwidth efficiencies of 1,2 and 3 bits/sec/Hz are described for various constraint lengths. The performance of these codes is bounded analytically and approximated via simulation; the results show a large improvement in the BER when compared with conventional TCM schemes. Indeed, when this approach is applied to channels with independent Rayleigh fading, the resulting coding gain is close to that implies by the cutoff rate limit, even for only moderately complex systems.The proposed codes are also simulated over channels with correlated fading; it is concluded that an interleaver span of 4v yields performance close to what is achieved with ideal interleaving.
Item Performance Analysis of Coherent TCM Systems with Diversity Reception in Slow Rayleigh Fading(1996) Al-Semari, Saud A.; Fuja, Tom E.; ISRCoherent trellis coded modulation (TCM) systems employing diversity combining are analyzed. Three different kinds of combining are considered: maximal ratio, equal gain, and selection combining. For each combining scheme, the cutoff rate parameter is derived assuming transmission over a fully- interleaved channel with flat, slow, Rayleigh fading; in addition, tight upper bounds on the pairwise error probabilities are derived. These upper bounds are expressed in product form to permit bounding of the BER via the transfer function approach. In each case it is assumed that the diversity branches are independent and that the channel state information (CSI) can be recovered perfectly.Also included is an analysis of maximal ratio combining when the diversity branches are correlated; the cutoff rate and a tight upper bound on the pairwise error probability are derived. It is shown that, with double diversity, a branch correlation coefficient as high as 0.5 results in only slight performance degradation.