The TDMA burst-mode link analyser with least-squares algorithm

The burst-mode link analyser with the decompositional least-squares algorithm (BMLA/LS) is an attractive and novel computational approach for impulse response measurement and equalization of a QPSK/TDMA satellite communication channel. Analysis and simulation confirms it to be superior to the current BMLA/radar approach. This paper presents a brief and systematic introduction to the methodology of the subject, and to software simulation, system design and hardware implementation.     

Equalization of tdma links using in-service impulse response measurement

This paper describes a method of measuring the frequency response of a digital radio link given the received (modulated) message signal, suitable for the incremental commissioning and in-service monitoring of links in a QPSK/TDMA system. The method applies least squares estimation techniques to samples of the I and Q baseband components in order to derive a linear equivalent model for the channel. Results obtained by computer simulation of an INTELSAT V channel demonstrate that the method can provide a better basis for linear equalization of a non-linear channel than the frequency response measurements provided by conventional techniques. A mathematical formulation and an implementation overview are also given.     

The BMLA/LS technique and its application in A mobile satellite communication channel

The burst mode link analyser with least squares algorithm (BMLA/LS) is a real-time impulse or frequency response extraction (IRE/FRE) technique of a satellite communication channel, originated and explored by the author. This paper summarizes some key technical points of the BMLA/LS and presents its novel simulation results and applications in a land-mobile DBPSK/DQPSK satellite channel at K_a-band, including the channel additive white Gaussian noise (AWGN) effect on IRE/FRE by BMLA/LS, and the instantaneous IRE/FRE in a Rician channel with multi-path reflection, in a shadowed channel by trees and in a combinational fading channel, and their comparisons and conclusions.     

A soft-output bidirectional decision feedback equalizationtechnique for TDMA cellular ratio

Issues encountered in the design of reliable narrowband time-division multiple access (TDMA) digital cellular mobile communication systems are considered. In particular, the problem of compensating for the harsh multipath fading environment in systems whose transmission bandwidth is commensurate with the coherence bandwidth of the fading channel is considered. A TDMA channel characterization parameter, the slot-normalized fade rate, is introduced, and an adaptive bidirectional equalization technique, which estimates the location of a deep fade within a time slot, is proposed. The simulation results show that the carrier-to-noise ratio requirement is only 15.5 dB when this equalization technique is used. This is achieved without diversity, and with low complexity. An equivalent equalized land mobile radio channel model and the analytical solution for the optimal bit likelihood calculation for π/4-shift quadrature differential phase-shift keying (QDPSK) modulation are also derived under certain channel conditions. The results are used as soft decisions for the convolutional decoder     

COMPUTER SIMULATION TECHNIQUES FOR A MOBILE SATELLITE COMMUNICATION CHANNEL

This paper gives a brief and systematic presentation of the computer simulation techniques of satellite communications, especially for a mobile satellite channel, based on a summarization of the author's exploration and experience in this area. It includes the equivalent complex baseband notation (ECBN) method, a very convenient mathematical form for software simulation; development of a multi-functional and expandable channel simulation system on a computer from L-band to Ka-band; channel impulse or frequency response estimation in real time by the technique burst-mode link analyser with least-squares algorithm (BMLA/LS), modelling and simulation of the channel fast fading due to multipath and shadowing; acquisition of the equivalent normalized binary signal-to-noise ratio (E_b/N₀) data at the channel input; and bit-error rate (BER) evaluation by the directly error-counting approach and the computational approach at the channel output in simulation.     

Bit-error rate of TDMA links under cochannel interference inoverlaid cellular networks

This paper presents a series of numerical experiments conducted to investigate the error performance of a simulated digital time-division multiple-access (TDMA) radio link between a portable handset and the base station serving it, subject to interference coming from other portable handsets. The experiments take radio channel impairments such as fading, shadowing, and distance loss into account. Analytical expressions for the bit-error probability in uncoded digital radio transmission bursts under interference from nearby analog FM or digital minimum-shift keying (MSK) transmitters are reviewed. Numerical methods are used to extend analytical estimates of symbol error probability of a link with a single interferer, without channel impairments, to the case where the interference is due to a random distribution of transmitters whose signals are subject to fading, shadowing, and distance power loss. Specifically, three links, each based on one of three different modulation methods, MSK, quadrature phase-shift keying (QPSK) (proposed in the North American Digital Cellular Standard IS-54), and Gaussian MSK (GMSK) [used in the Global Standard for Mobile communications (GSM)] are simulated, and the bit-error rate (BER) results reported. The BER results generated thus indicate the sensitivity of the digital radio link to the user density for a given radio frequency (RF) bandwidth and, where applicable, to fading depth and postmodulation intersymbol interference (ISI) removal method. Possible extensions and applications of the simulation model to the problem of resource sharing between coexisting networks are suggested     

The Effects of Time Delay Spread on Portable Radio Communications Channels with Digital Modulation

Frequency-selective fading caused by multipath time delay spread degrades digital communication channels by causing intersymbol interference, thus resulting in an irreducible BER and imposing a upper limit on the data symbol rate. In this paper, a frequencyselective, slowly fading channel is studied by computer simulation. The unfiltered BPSK, QPSK, OQPSK, and MSK modulations are considered first to illustrate the physical insights and the error mechanisms. Two classes of modulation with spectral-shaping filtering are studied next to assess the tradeoff between spectral occupancy and the performance under the influence of time delay spread. The simulation is very flexible so that different channel parameters can be studied and optimized either individually or collectively. The irreducible BER averaged over fading samples with a given delay profile is used to compare different modulation/detection methods, while the cumulative distribution of short-term BER is employed to show allowable data symbol rates for given values of delay spread. It is found that both GMSK and QPSK with a raised-cosine Nyquist pulse are suitable for a TDM/TDMA digital portable communications channel.     

Quadrature Overlapped Raised-Cosine Modulation

A variation of an old but neglected pulse shaping technique, raised-cosine, is investigated. By overlapping raised-cosine pulses in each of two data streams and then by quadrature combining them, a simple QPSK/MSK type modulation results. This quadrature overlapped raised-cosine (QORC) modulation exhibits a hybrid structure of QPSK and MSK modulations. The power spectral density of QORC is shown to take on the form of the product of the power spectral densities of MSK and QPSK. The obvious consequences are that the power spectral density main lobe retains the width of the spectral density main lobe of QPSK, but the sidelobes drop off much faster(1/f^{6}). A simple QORC modulator can be implemented similar to an MSK modulator. Several correlation type receivers are investigated and their performances calculated. Computer simulation results are used to compare end-to-end system performance of QORC and staggered QORC (SQORC) with MSK, QPSK, and staggered QPSK (SQPSK) for both linear and nonlinear satellite channels. The performance of QORC and SQORC compares very favorably with QPSK, SQPSK, and MSK. QORC performs particularly well in the presence of a nonlinear channel. The effect of phase equalization of the channel filter was investigated with outstanding performance improvement. The simulation results show that sidelobe regeneration caused by the channel nonlinearity is much less for SQORC than it is for the other modulation formats considered.     

Performance of a TDM/TDMA portable radio link for interference,noise, and delay spread impairments

Presents the measured/simulated performance of a time division multiplexing/time division multiple access (TDM/TDMA) portable radio link for noise, interference and delay spread impairments. The radio link transmits short TDMA bursts of 82 symbols at 450 kbits/sec using 4QAM. This modulation is also referred to 4PSK, but phase shift modulation is constant envelope. Nyquist filtering of quadrature signal components yields quadrature amplitude modulation which, of course, also contains π/2 phase variations at the sampling instants. Demodulation is performed by a low-overhead digital coherent demodulator with 2-branch selection diversity. The port or base has two receivers for diversity but the portable or handset uses only a single receiver to perform selection diversity. Signal-to-noise ratio and signal-to-interference ratio performance of the link was measured for a stationary channel and for a channel with different fading rates. In a Rayleigh fading environment, increasing the fading rate causes only a small performance degradation. The results also show that selection diversity is effective against interference in a slow Rayleigh fading environment. A separate set of experiments were performed to measure the effects of delay-spread on the link. Selection diversity is effective in reducing the word-error ratio floor caused by frequency-selective fading. As a result, relatively high data rates can be supported by a multipath fading channel without using adaptive equalization. Thus, a portable radio communications system using low complexity hardware design incorporating selection diversity can achieve good performance     

A retransmission scheme for circuit-mode data on wireless links

The cellular radio link is characterized by deep fades leading to long error bursts (lasting hundreds of milliseconds). Data transmission over such links results in large packet error rates (in the range 10^-3 to 10^-1). We have designed a radio link protocol (RLP) to achieve high throughput on such links. The RLP is based on frequent, complete or partial feedback of the receiver state. Performance results for the US digital cellular TDMA standard show that the scheme can provide the equivalent of 9.6 kb/s service per full-rate TDMA channel above a carrier to interference ratio of 18 dB     

Adaptive decision feedback equalization for digital satellitechannels using multilayer neural networks

This paper introduces an adaptive derision feedback equalization using the multilayer perceptron structure of an M-ary PSK signal through a TDMA satellite radio channel. The transmission is disturbed not only by intersymbol interference (ISI) and additive white Gaussian noise, but also by the nonlinearity of transmitter amplifiers. The conventional decision feedback equalizer (DFE) is not well-suited to detect the transmitted sequence, whereas the neural-based DFE is able to take into account the nonlinearities and therefore to detect the signal much better. Nevertheless, the applications of the traditional multilayer neural networks have been limited to real-valued signals. To overcome this difficulty, a neural-based DFE is proposed to deal with the complex PSK signal over the complex-valued nonlinear MPSK satellite channel without performing time-consuming complex-valued back-propagation training algorithms, while maintaining almost the same computational complexity as the original real-valued training algorithm. Moreover, a modified back-propagation algorithm with better convergence properties is derived on the basis of delta-bar-delta rule. Simulation results for the equalization of QPSK satellite channels show that the neural-based DFE provides a superior bit error rate performance relative to the conventional mean square DFE, especially in poor signal-to-noise ratio conditions     

一种短波数据链的动态TDMA协议

针对短波资源有限、传输速率低的特点,提出了基于短波数据链的动态TDMA协议（HF-DTDMA）,该协议能对短波信道资源进行自适应动态分配,提高信道利用率,满足短波网络业务量实时变化的需求。通过建立短波数据链的仿真场景,分析了动态TDMA协议的性能,并且与固定TD-MA机制的性能进行了对比分析,动态TDMA协议在吞吐量和传输时延上性能明显提升。仿真结果也表明,提出的动态TDMA协议在时效性和吞吐率上能满足短波数据链的设计要求。     

Blind FSR-LPTV equalization and interference rejection

We address the problem of synthesizing blind channel identification and equalization methods for digital communications systems, aimed at counteracting the presence of cochannel or adjacent-channel interference. Owing to the presence of the interfering signal, the minimum mean-square error equalizer turns out to be linear periodically time-varying, which is implemented by resorting to its Fourier series representation. Moreover, by exploiting the cyclic conjugate second-order statistics of the channel output, we propose a new weighted subspace-based channel identification method, which is asymptotically immune to the presence of high-level interference. Computer simulation results confirm the effectiveness of the proposed identification/equalization technique.     

Adaptive equalization for TDMA digital mobile radio

Adaptive equalization for a TDMA (time-division multiple-access) digital cellular system is discussed. A survey of adaptive equalization techniques that includes their performance characteristics and limitations and their implementation complexity is presented. The design of adaptive equalization algorithms for a narrowband TDMA system is considered. It is concluded that, on the basis of implementation complexity and performance in the presence of multipath distortion and signal fading, MLSE (maximum-likelihood sequence estimation) and DFE (decision feedback equalization) are viable equalization methods for mobile radio     

Adaptive equalization of indoor radio channels for 156-Mb/s,QPSK data transmission

Indoor radio communication in the 20–60 GHz band using TDMA with differentially encoded QPSK is consideed. A burst-type transmission, based on a basic time slot consisting of a preamble for synchronization and equalizer training and of an information data section, is adopted. We employ fractionally spaced decision feedback equalization and give the relevant analytical and simulated performance results in terms of Doppler frequency. An upper limit is determined for the channel variations which can be tracked by the equalizer. Performance results are obtained for equalization with and without carrier phase recovery. Calculated and simulated probability of error show that error propagation degrades the performance by about 6 dB at a bit error rate of 10^–3 for a channel without any diversity. However, the effect of past decision errors is negligible for dual diversity. Numerical stability, required accuracy, and hardware complexity are also discussed.     

A study of analog decision feedback equalization for ADC-Based serial link receivers

High-speed serial link receivers based on analog-to-digital converters (ADCs) provide better programmability with different channel characteristics and the possibility of employing powerful signal equalization techniques in the digital domain. However, complexity and power consumption are still major issues in adopting such receivers in high-speed applications when compared to traditional binary or mixed-signal approaches. Embedded decision feedback equalization (DFE) before ADC quantization can relax the design requirements of both the ADC and post-ADC digital processing. This paper studies the impact of embedded analog DFE on voltage margin improvement of an ADC-based receiver through worst-case analysis. An analytical expression for the link bit-error-rate (BER) with analog DFE is derived and validated through simulations. An empirical study is conducted that evaluates the achievable BER of embedded analog DFE as a function of the channel inter-symbol interference (ISI) and ADC resolution. A channel-dependent parameter is introduced and employed to quantify the BER improvement achieved by embedding analog DFE in a receiver. A prototype receiver with embedded DFE is designed and laid out in a 130 nm CMOS process and achieves 4.64-bits peak ENOB and 4.08 pJ/conv.-step FOM at a 1.6-GS/s sampling rate. The BER performance of the receiver over high-loss FR4 channels at 1.6 Gb/s is evaluated and used to validate the simulation results.     

Performance of unequalized frequency-hopped TDMA with convolutional coding on dispersive fading channels

Increasing demand for wireless personal communications has stimulated research on new digital radio technologies that are optimized for various service applications and environments. This paper discusses the performance of a slow-frequency-hopped time-division multiple-access (SFH-TDMA) technique, which has been proposed as a high-tier extension of a low-complexity TDMA architecture optimized for low-power pedestrian applications. The SFH-TDMA technique considered uses QPSK modulation and rate-1/2 convolutional coding. Numerical results for a wide range of fading rates are obtained through analytical calculation of the effective signal-to-noise ratio combined with a simulation approach which incorporates measured multipath channels and actual frequency correlation among contiguous hopping channels. The results indicate that the SFH-TDMA technique can tolerate root-mean-square (rms) delay spread up to several bit periods without a need for adaptive equalization, but also point to the need for fast power control when the fading is slow and the rms delay spread is much smaller than the bit period. This work is targeted toward understanding the implications to local exchange networks of wireless technology alternatives that could provide access to those networks.     

Adaptive Modulation/TDMA/TDD with Convolutionally Coded Type-II Hybrid ARQ for Wireless Communication Systems

This paper proposes a time divisionmultiple access/time division duplex (TDMA/TDD)-basedadaptive modulation system (AMS) with aconvolutionally coded Type-II Hybrid automatic repeatrequest (ARQ) protocol to achieve high throughput datatransmission in wireless communication systems. First,this paper discusses the cause of throughputlimitation of the conventional fixed-rate quaternaryphase shift keying (QPSK) with ARQ protocol andsuggests that the effective means to improvethroughput performance are the reduction ofre-transmission probability under bad channelcondition and the increase of transmission bit rate perTDMA/TDD slot under good channel condition. Based onthese strategies, this paper discusses an AMS with aconvolutionally coded type-II hybrid ARQ protocol(type-II hybrid ARQ/AMS) and evaluates itsperformance by computer simulation. The resultsconfirm that the proposed type-II hybrid ARQ/AMS candrastically improve throughput performances comparedto the conventional fixed-rate QPSK with selectiverepeat ARQ protocol (SR-ARQ/QPSK) or a simplecombination of SR-ARQ and AMS (SR-ARQ/AMS) under anychannel conditions, especially under severe channelconditions.     

Imrpoving QPSK Transmission in Band-Limited Channels with Interchannel Interference Through Equalization

This paper describes the use of equalization in conjunction with channel filtering to improve QPSK transmission subject to both intersymbol interference (ISI) and interchannel interference (ICI). Performance bounds are computed using the nonclassical Gaussquadrature rule (GQR) method. The signal-to-noise ratio (SNR) gain due to linear equalization over nonequalization is thereby obtained and presented. The performance of a linear equalizer thus obtained is compared with the Viterbi algorithm sequence estimator (VASE). In the absence of bounds for the VASE receiver under the channel conditions considered, simulation results are used to make the comparison. With a possible difference in the accuracies of the performance thus obtained it is shown that the VASE provides improved performance over the linear equalizer under the channel conditions considered.     

TDMA-based adaptive modulation with dynamic channel assignment forhigh-capacity communication systems

This paper proposes a time-division multiple-access (TDMA)-based adaptive modulation scheme with dynamic channel assignment (AMDCA) to achieve high-capacity communication systems under dynamically changing propagation path and traffic conditions. The proposed method measures the received carrier-to-noise plus interference power ratio (C/(N+I)) of each TDMA slot to search available slots as well as to find out the optimum modulation parameters for each terminal, thereby effectively combining the effect of mitigating both spatially distributed electric field strength variation by the slow adaptive modulation and spatially and temporally distributed traffic variation by the dynamic channel assignment (DCA). Computer simulation confirms that the proposed AMDCA system can achieve approximately three times higher system capacity than the conventional fixed channel assignment (FCA) using QPSK