Adaptive coded modulation for fading channels

We apply coset codes to adaptive modulation in fading channels. Adaptive modulation is a powerful technique to improve the energy efficiency and increase the data rate over a fading channel. Coset codes are a natural choice to use with adaptive modulation since the channel coding and modulation designs are separable. Therefore, trellis and lattice codes designed for additive white Gaussian noise (AWGN) channels can be superimposed on adaptive modulation for fading channels, with the same approximate coding gains. We first describe the methodology for combining coset codes with a general class of adaptive modulation techniques. We then apply this methodology to a spectrally efficient adaptive M-ary quadrature amplitude modulation (MQAM) to obtain trellis-coded adaptive MQAM. We present analytical and simulation results for this design which show an effective coding gain of 3 dB relative to uncoded adaptive MQAM for a simple four-state trellis code, and an effective 3.6-dB coding gain for an eight-state trellis code. More complex trellis codes are shown to achieve higher gains. We also compare the performance of trellis-coded adaptive MQAM to that of coded modulation with built-in time diversity and fixed-rate modulation. The adaptive method exhibits a power savings of up to 20 dB     

Performance of pulse-position modulation on measured non-directedindoor infrared channels

We examine the performance of pulse-position modulation (PPM) on measured channels with intersymbol interference (ISI). We summarize the bit-error-rate performance of unequalized systems and review the performance of maximum-likelihood sequence detection (MLSD) for PPM over ISI channels with additive white Gaussian noise. We evaluate the performance of PPM links over 46 experimentally measured indoor infrared channels. Detailed results are presented for 2, 4, 8, and 16-PPM at bit rates of 10 Mb/s and 30 Mb/s, and these techniques are compared to on-off keying. Our results show that when MLSD is employed, 16-PPM provides the best average-power efficiency among the modulation techniques considered in this study     

Modulation and coding for linear Gaussian channels

Shannon's determination of the capacity of the linear Gaussian channel has posed a magnificent challenge to succeeding generations of researchers. This paper surveys how this challenge has been met during the past half century. Orthogonal minimum-bandwidth modulation techniques and channel capacity are discussed. Binary coding techniques for low-signal-to-noise ratio (SNR) channels and nonbinary coding techniques for high-SNR channels are reviewed. Recent developments, which now allow capacity to be approached on any linear Gaussian channel, are surveyed. These new capacity-approaching techniques include turbo coding and decoding, multilevel coding, and combined coding/precoding for intersymbol-interference channels     

Combined Transmitter Diversity and Multi-Level Modulation Techniques

Orthogonal transmitter diversity such as frequency diversity and time diversity is quite simple to implement and, with optimum signal combining, can take full advantage of fading multipath channels. However, such a scheme has a bandwidth efficiency that decreases inversely with the number of diversity branches making it less attractive in wireless communications applications. This paper considers combined orthogonal transmitter diversity and multi-level linear modulation techniques. The idea is to view the signal constellations of the modulation scheme in an augmented signal space formed by the modulation signal dimension and the number of branches of the transmitter diversity scheme. This augmented signal space provides a good spread for the modulation signal points and can be quite efficient for high-level linear modulation techniques. The obtained results show that this combined scheme, not only improves the system performance on both additive white Gaussian noise and fading multipath channels, but also improves the bandwidth efficiency of orthogonal transmitter diversity.     

Optimisation of switching levels for adaptive modulation in slowRayleigh fading

Multi-dimensional minimisation techniques are used to optimise for specific applications, the upper bound bit error rate (BER) and bits per symbol (BPS) performances of adaptive modulation schemes over Rayleigh channels. This is achieved by modifying the switching criteria for selecting the appropriate modulation scheme     

Demodulation level selection in adaptive modulation

A novel, uneven protection, modulation scheme is proposed for the transmission of control symbols in adaptive modulation. Theoretical and simulated results for transmissions over Rayleigh channels are presented and compared with those obtained using other techniques. A 5 dB signal-to-noise (SNR) improvement is recorded for computer data transmission     

QPSK, Staggered QPSK, and MSK--A Comparative Evaluation

Bandwidth- and power-efficient quadrature carrier modulation techniques are identified, evaluated, and compared. The modulation techniques include quadrature phase shift keying (QPSK), staggered QPSK (SQPSK), and minimum (frequency) shift keying (MSK). High quality QPSK, SQPSK, and MSK transmitters and receivers were used in the evaluation, along with a computer simulation program which modeled the hardware. Both linear and nonlinear channels were included in the comparative investigation. The agreement between simulated and measured results is shown to be excellent. In the comparison of modulation techniques, the effects of channel filtering (band limiting), nonlinearities, detection techniques, and phase equalization are addressed. The application-oriented results are intended to be an aid in selecting modulation and detection implementations, and in predicting the performance of linear and nonlinear communication systems. For systems with imperfections, data are presented for determination of passive detection filters which can outperform "matched" integrate and dump (I&D) filters.     

Equalization of microwave digital radio channels with a hybridacousto-optic and digital processor

Digital radio transmission systems use complex modulation schemes that require powerful signal processing techniques to correct channel distortions and to minimize bit-error rates (BERs). Combined analog and digital processors are investigated for minimizing the mean square error (MSE) of the radio receiver. The analog filters are implemented using acousto-optic (AO) processing since rapidly adaptable, inverse channel filters can be produced for either minimum or nonminimum phase channels. A specific architecture is identified and a laboratory system is tested to verify the ability of the processor to track and correct time-varying channels. Computer simulations are used to show that hybrid analog and digital equalization allows an increase in the modulation capacity of radio, relative to all digital equalization, while maintaining similar equipment signatures     

A two-symbols/branch MBCM scheme based on 8-PSK constellation under fading channels

A two-symbols/branch scheme of multiple block coded modulation (MBCM) is investigated under fading channels. Compared with a conventional scheme of block coded modulation (BCM), this two-symbols/branch MBCM scheme greatly increases the minimum squared Euclidean distance (MSED), minimum symbol distance (MSD), and minimum product distance (MPD). These three distances determine the bit-error-rate (BER) performance under either Gaussian or fading channels. A pilot symbol assisted fading compensation, as well as the techniques of symbol interleaving and branch weighting, are employed to combat the effect of channel fading. Through computer simulations, it is shown that large coding gains are obtained under both Rayleigh and Rician fading channels.     

Optical coherent broad-band transmission for long-haul anddistribution systems using subcarrier multiplexing

Signal multiplexing techniques for coherent optical transmission are compared, and appropriate application for a coherent subcarrier multiplexing (SCM) system is discussed. Optical frequency modulation (FM) using direct modulation of a distributed-feedback laser diode (DFB-LD) and a heterodyne detection is shown to be feasible. A transmission system using a local laser in the transmitter is unaffected by polarization and is cost effective. Phase noise can be suppressed by a phase-noise-canceling circuit (PNC) in a heterodyne receiver. This circuit can also effectively compensate for the frequency of instability of light sources. A theoretical simulation of a coherent SCM system showed that 100 channels of 30-MHz FM signal or 15 channels of 155-Mb/s signal can be distributed to 10000 subscribers using single-stage or double-stage optical amplifiers     

Features and Performance of 12PM3 Modulation Methods for Digital Land Mobile Radio

A number of continuous-phase modulation methods suitable for digital and mobile radio applications has been grouped in a common class, which has been named 12PM3 (12-state, phase modulation with correlation over three consecutive bits). This paper introduces the definition of this class of modulation schemes, and presents some of its features. With reference to the scheme that gives the best spectral compactness, some effects of typical implementation imperfections of the modulation circuits are discussed. The performance of some 12PM3 modulators associated with a frequency discrimination receiver is then computed with reference to both AWGN (additive white Gaussian noise) channels and the typical propagation conditions envisaged for SCPC/FDMA digital land mobile radio systems in urban environment. The adopted pre and postdiscrimination filters of the receiver are optimized for the best performance. Different decision techniques are considered, namely 2-TH (2-threshold), 4-TH, mixed 2-TH/4-TH and MLSE (maximum-likelihood sequence estimation) techniques.     

A comparison of two combining techniques for equal gain, trelliscoded diversity receivers

Trellis coded modulation is widely used for digital transmission over fading channels. Classical diversity techniques are also frequently employed to combat fading. In this paper two different strategies for equal gain combining are compared, One scheme is based on an interleaved code combining technique. The alternative scheme is based on averaged diversity combining. The well known transfer function bounding technique for trellis codes is used to obtain: expressions for the bit error rate performance of the two trellis coded diversity receivers over a slowly fading Rayleigh channel. The analysis of interleaved code combining is a straightforward modification of the analysis for multiple trellis coded modulation. The analysis of averaged diversity combining is accommodated through a more involved, novel modification of the branch labeling of the error state diagram. The analytic techniques presented in this paper are supported by simulation results using a TCM scheme based on QPSK modulation and a rate-l/2 linear convolution code     

Decoding with approximate channel statistics for bandlimited nonlinear satellite channels

Expressions for the cutoff rate of memoryless channels and certain channels with memory are derived assuming decoding with approximate channel statistics. For channels with memory, two different decoding techniques are examined: conventional decoders in conjunction with ideal interleaving/deinterleaving, and maximum likelihood decoders that take advantage of the channel memory. As a practical case of interest, the cutoff rate for the band-limited nonlinear satellite channel is evaluated where the modulation is assumed to be M-ary phase shift keying (MPSK). The channel nonlinearity is introduced by a limiter in cascade with a traveling wave tube amplifier (TWTA) at the satellite repeater while the channel memory is created by channel filters in the transmission path.     

Codierte Modulation in modernen Übertragungssystemen

Modern communication systems offer high speed and reliable data transmission services. The quality of these services is achieved by combining coded multi-level modulation techniques and modern digital signal processing (Viterbi-decoding) in the receiver. In this paper we first present an introduction to Trellis Coded Modulation (TCM) and then some new results on unversal techniques adapted for the AWGN-channel as well as for the fading channel. It is shown that TCM is well suited for communication channels with time variant characteristics (e.g. mobile communication channels), especially when our new codes are applied.     

Terminals for a high-speed optical pulse code modulation communication system: II. Optical multiplexing and demultiplexing

Techniques are described for optically multiplexing and demultiplexing individual pulse code modulation channels in order to develop the terminal capability for a high-capacity optical communication system. It is shown that, using these techniques, an information capacity in excess of 10¹⁰bit/s can be achieved on the output beam of a single laser.     

DCTPSK--Differentially Detected Controlled Transition PSK

An efficient modem is introduced for system applications which favor differential detection techniques. In differentially detected controlled transition PSK (DCTPSK), pulse-shaping techniques are used to achieve bandwidth and power efficiency in nonlinear channels. The baseband waveshapes, eye diagrams, and signal space diagram of DCTPSK are given; its bit error rate performance is calculated and its power spectral density is obtained. In a multicarrier adjacent channel interference environment, the compact spectrum of DCTPSK enables it to utilize the available bandwidth and power more efficiently than some well-known modulation techniques such as DQPSK and DMSK.     

Space-time bit-interleaved coded modulation for OFDM systems

Space-time coding techniques significantly improve transmission efficiency in radio channels by using multiple transmit and/or receive antennas and coordination of the signaling over these antennas. Bit-interleaved coded modulation gives good diversity gains with higher order modulation schemes using well-known binary convolutional codes on a single transmit and receive antenna link. By using orthogonal frequency division multiplexing (OFDM), wideband transmission can be achieved over frequency-selective fading radio channels without adaptive equalizers. In this correspondence, we combine these three ideas into a family of flexible space-time coding methods. The pairwise error probability is analyzed based on the correlated fading assumption. Near-optimum iterative decoders are evaluated by means of simulations for slowly varying wireless channels. Theoretical evaluation of the achievable degree of diversity is also presented. Significant performance gains over the wireless local area network (LAN) 802.11a standard system are reported.     

On efficient spectrum utilization from the standpoint of communication theory

This paper reviews the concepts of spectrum efficiency, the parameters for assessing system performance, and the techniques for improving spectrum utilization. The rationale of possible bandwidth reduction and power savings by combined modulation techniques is discussed and special attention is given to combined amplitude and phase modulation (APSK) systems. It is believed, based on the available results of several analyses over additive Gaussian noise channels, that certain combined modulation systems can have significant power savings over single-parameter modulation systems when the number of bits per symbol of the signal is large and can achieve more efficient use of the spectrum for signal-to-noise ratios greater than certain levels.     

Joint MAP equalization and channel estimation forfrequency-selective and frequency-flat fast-fading channels

This paper presents a new fractionally-spaced maximum a posteriori (MAP) equalizer for data transmission over frequency-selective fading channels. The technique is applicable to any standard modulation technique. The MAP equalizer uses an expanded hypothesis trellis for the purpose of joint channel estimation and equalization. The fading channel is estimated by coupling minimum mean square error techniques with the (fixed size) expanded trellis. The new MAP equalizer is also presented in an iterative (turbo) receiver structure. Both uncoded and conventionally coded systems (including iterative processing) are studied. Even on frequency-flat fading channels, the proposed receiver outperforms conventional techniques. Simulations demonstrate the performance of the proposed equalizer     

Error probability performance of L-branch diversity reception ofMQAM in Rayleigh fading

New exact expressions involving hypergeometric functions are derived for the symbol-error rate (SER) of M-ary quadrature amplitude modulation (MQAM) for L branch diversity reception in Rayleigh fading and additive white Gaussian noise (AWGN). The diversity combining techniques considered are maximum ratio combining (MRC) and selection combining (SC). MRC with identical channels and dissimilar channels are analyzed