TDMA System Research Articles

Benefits and feasibility of the partial loading approach in cellular mobile radio systems

This paper analyzes the feasibility and potential benefits of following a novel design strategy for cellular systems known as partial loading. This strategy seeks low channel occupancy per cell at the capacity limit, as opposed to the high channel occupancy per cell of more conventional design strategies. By doing so, a considerable flexibility for the radio resource management is obtained, which can be exploited both to increase system capacity and to ease the accommodation of highly fluctuating traffic. A comprehensive system capacity analysis has been performed, based on an analytical model, and it is shown that the introduction of flexibility through partial loading has a capacity cost associated with it. However, it is shown that when that flexibility is properly exploited by adaptive radio link control techniques, such as link adaptation, frequency hopping, or call admission control policies, that cost is more than compensated for, making the partial loading approach a very attractive strategy for the evolution of current TDMA systems and the design of future ones.

Blind equalization for short burst wireless communications

In this paper, we propose a dual mode blind equalizer based on the constant modulus algorithm (CMA). The blind equalizer is devised for short burst transmission formats used in many current wireless TDMA systems as well as future wireless packet data systems. Blind equalization is useful for such short burst formats, since the overhead associated with training can be significant when only a small number of bits are transmitted at a time. The proposed equalizer overcomes the common problems associated with classic blind algorithms, i.e., slow convergence and ill-convergence, which are detrimental to applying blind equalization to short burst formats. Thus, it can eliminate the overhead associated with training sequences. Also, the blind equalizer is extended to a two branch diversity combining blind equalizer. A new initialization for fractionally spaced CMA equalizers is introduced. This greatly improves the symbol timing recovery performance of fractionally spaced CMA equalizers with or without diversity, when applied to short bursts. Through simulations with quasi-static or time-varying frequency selective wireless channels, the performance of the proposed equalizer is compared to selection diversity and conventional equalizers with training sequences. The results indicate that its performance is far superior to that of selection diversity alone and comparable to the performance of equalizers with short training sequences. Thus, training overhead can be removed with no performance degradation for fast time-varying channels, and with slight performance degradation for static channels.

Bilinear approach to multiuser second-order statistics-based blind channel estimation

We present a bilinear approach to multiple-input multiple-output (MIMO) blind channel estimation where products of the channel parameters are first estimated from the covariance of the received data. The channel parameters are then obtained as the dominant eigenvectors of the outer-product estimate. Necessary and sufficient identifiability conditions are presented for a single channel and extended to the multichannel case. It is found that this technique can identify the channel to within a subspace ambiguity, as long as the basis functions for the channel satisfy certain constraints, regardless of the left invertability of the channel matrix. One important requirement for identifiability is that the number of channel parameters is small compared with the channel length; advantageously, this is exactly the situation in which this algorithm has significantly lower complexity than competing (parametric, multiuser) blind algorithms. Simulations show that the technique is applicable in situations where typical identifiability conditions fail: common nulls, a single symbol-spaced channel, and more users than channels. These simulations are for the almost flat faded situation when the propagation delay spread is a fraction of the transmission pulse duration (as might be found in current TDMA systems). Comparisons are made, when possible, to a subspace method incorporating knowledge of the basis functions. The bilinear approach requires significantly less computation but performs better than the subspace method at low SNR, especially for multiple users.

Medium access and radio link control algorithms for mobile multimedia CDMA

A mobile multimedia CDMA system with multiple links between a user and radio ports is introduced in this paper. We propose a centralized reservation access control scheme with transmission scheduling and allocation algorithm supporting various types of multimedia traffic. Two types of allocation algorithms, a local and a global algorithm, are also proposed. The allocation algorithms allocate a set of ports to configure multiple radio links and transmission slot / power to each scheduled transmission requests. We evaluate the performance of the proposed CDMA system and control algorithms, and compare it with a distributed random access CDMA system and a dynamic TDMA system. It is shown that the global algorithm is the best and the random transmission method the worst in performance. It is also noted that the local algorithm performs well compared with the global algorithm. The proposed CDMA system shows better performance than the TDMA system in terms of delay and throughput.

Maximum likelihood blind channel estimation in the presence of Doppler shifts

Transmitter/receiver motion in mobile radio channels may cause frequency shifts in each received path due to Doppler effects. Most blind equalization methods, however, assume time-invariant channels and may not be applicable to fading channels with severe Doppler spread. We address the problem of simultaneously estimating the Doppler shift and channel parameters in a blind setup. Both deterministic and stochastic maximum likelihood methods are developed and iterative solutions proposed. The stochastic maximum likelihood solution is based on the modified version of the Baum-Welch (1970) algorithm, which originated in the study of hidden Markov models. The proposed methods are well suited for short data records appearing in TDMA systems. Identifiability and performance analysis issues are discussed, and Cramer-Rao bounds are derived. In addition, some illustrative simulations are presented.

Adaptive antennas for GSM and TDMA systems

Adaptive antennas have generated great interest in previous years, and at present several manufacturers and operators are separately or jointly performing field tests to gain more detailed knowledge of the potential of the technology. This article summarizes many of the experiences that have been collected since 1996 during field trials carried out for both GSM and TDMA (IS-136) by Ericsson in cooperation with two major network operators, Mannesmann Mobilfunk and AT&T Wireless Services.

A frequency synchronization for TDMA system

A frequency synchronization algorithm utilizing the peak phase error detection and frequency offset smoother is proposed for burst data transmission. The basic idea of frequency offset estimator (FOE) is to use a curve fitting method. The proposed peak phase error detection avoids a large phase error giving a bad value for FOE. For the purpose of controlling the automatic frequency control (AFC), frequency offset smoother by a simple filter is used. Simulation results show that the proposed algorithm is adequate for frequency synchronization of burst data transmission.

Blind receivers for nonlinearly modulated signals in multipath

Classical blind channel equalization methods may not be suitable for systems with nonlinear modulation (e.g., continuous phase-modulated signals) because the input to the channel may not be i.i.d. due to the modulator's memory. Moreover, the nonlinear characteristics of the modulator further complicate the channel estimation task. In this correspondence, we derive blind maximum likelihood channel estimation algorithms for nonlinearly modulated signals using a hidden Markov model (HMM) formulation. The proposed method implements a stochastic maximum likelihood approach and is well suited for short data records appearing in TDMA systems. Cramer-Rao bounds are derived, and some illustrative simulations are presented.

Frequency-hopping codes for multimedia services in mobile telecommunications

In this paper, "multilength" frequency-hopping (FH) codes are constructed by modifying the generalized prime codes (GPCs). The new codes have asymptotically optimal cardinality as well as zero autocorrelation sidelobes and cross-correlation functions of no greater than one. They are suitable for the integration of multimedia services (e.g., data, voice, image, and video) with different quality-of-service (QoS) requirements and signaling rates in future telecommunication systems, such as third-generation cellular mobile radio systems, where rate and service quality are dynamically matched to users' needs through the assignment of different-length FH patterns. The use of the new codes in multirate multimedia systems with FH code-division multiple access (FH-CDMA) is illustrated. Providing frequency and time diversities, the applications of the GPCs to an FH time-division multiple-access (FH-TDMA) and FH/time-hopping TDMA (FH/TH-TDMA) system are also discussed. Finally, the use of the codes in optical CDMA systems as multiwavelength optical orthogonal codes is studied.

Improved blind channel identification using a parametric approach

In wireless communications, the channel has a known manifold structure due to the sensor array response, pulse-shaping function, and sampling phase. By exploiting this information, the method presented in this article achieves improved performance over unstructured blind channel estimation schemes. The approach is subspace-based, and can take advantage of multiple channel estimates if available, e.g., from different time slots in TDMA systems. Its performance is numerically simulated and compared against the original unstructured channel subspace method.

Comparative study of switching methods for the on-board processor in a mobile satellite communications system

An on-board processor makes efficient use of the channel capacities in a satellite communications system employing multibeam technology, and an on-board processor has been developed for the Engineering Test Satellite VIII (ETS-VIII), which is the experimental satellite for a multibeam mobile satellite communications system. The multicarrier time division multiple access (MC-TDMA) system used in this communications system, however, sometimes causes internal congestion in the on-board processor. This paper describes the results of computer simulation studies of the traffic capacity obtained using three baseband switching methods and also describes the results of studies of the on-board processor design for each of the methods. The computer simulation results indicate that the probability of internal congestion due to the TDMA system is very small when the simultaneous-slot switching method is used, and the design study results reveal that an on-board processor using the simultaneous-slot switching method will be compact and lightweight and will consume little power. © 1998 John Wiley & Sons, Ltd.

Determining the call admission region for real-time heterogeneous applications in wireless TDMA networks

The call admission problem for a wireless packet-switched network supporting homogeneous applications-such as a cellular voice network-is an old one and has been extensively investigated in the past. The article investigates the call admission region for a TDMA (wireless) system supporting heterogeneous real-time VBR applications with distinct QoS requirements and traffic characteristics. The QoS is defined in terms of a maximum tolerable packet delay and dropping probability; packets may be dropped due to delay violations or channel-induced errors. The call acceptance region is investigated under the assumption that each user's per-frame resource (slot) requests are communicated to the scheduler (resource allocation authority). The call acceptance region is shaped by the QoS that can be delivered by the uplink scheduling policy. Some of the mechanisms employed to inform the scheduler of the users' requests along with the scheduling policies are discussed. While these scheduling policies identify QoS points in the call admission region, they cannot reveal the entire region. An approach is outlined on how to precisely determine the call admission region (largest set of QoS points delivered under any scheduling policy) for a TDMA system. This approach is shown to lead to the precise description of the reduced call admission region in the presence of channel errors. In order to demonstrate the performance improvement provided by an error control scheme, a simple QoS-sensitive forward error control protocol operating over on erroneous channel is employed, leading to an enlargement of the calculated call admission region. Finally, it is shown that feasible scheduling policies exist which do deliver at least the minimum QoS requirement of the applications whose associated QoS vector falls within the determined call admission region.

Blind Multiuser Detection with Array Observations

Cochannel interference is usually a major limitation to the performance of mobile wireless systems. Examples of different forms of cochannel interference include multi-access interference in CDMA systems and cochannel interference resulting from frequency reuse in TDMA systems. In order to mitigate the interference from other users we present a blind multiuser receiver which utilizes array observations and performs both spatial and temporal processing of the received signal. The presented technique is completely blind in the sense that no signature sequences, channel state or spatial location needs to be known a priori, nor use of a training sequences, channel state or spatial location needs to be known a priori, nor use of a training sequence is required for the adaptation. The diversity introduced by the array observations can be efficiently combined with the use of CDMA signature sequences. After initial convergence, a reliable estimate of the combined temporal and spatial signature for each user is provided that can be employed by a multiuser receiver of lower complexity.

A space-time-filtered Viterbi receiver for CCI/ISI reduction in TDMA systems

In this paper, we present a hybrid space-time-filtered Viterbi receiver using multiple antennas for co-channel interference (CCI) reduction and intersymbol interference (ISI) equalization in a slow Rayleigh fading channel. In this approach, a space-time filter is first applied at the antenna outputs to maximize the signal-to-interference-plus-noise ratio (SINR), and the scalar output is then sent to a Viterbi equalizer. We propose a closed-form solution to jointly determine the weight vector for the space-time filter and the channel vector for the Viterbi equalizer. We also examine the need for a whitening filter prior to the Viterbi equalizer and show that it only marginally improves the performance. Simulation results are provided to validate our approach and to compare the performance of our receiver with that of different existing receivers.

Spectrum and technology of a wireless local loop system

This article shows that the wireless local loop (WLL) may not necessarily provide more capacity than mobile cellular systems if an FDMA or TDMA system is used. CDMA has proven to have higher capacity than the other multiple access schemes due to the nature of CDMA systems. In industrial countries, the requirements of WLL are quality and service features. In developed countries, the requirement is a low-cost high-capacity system, even if voice quality must be sacrificed. Therefore, the system design aspects are different.

Performance of a Dynamic TDMA System for Mobile Wireless Access.

A dynamic TDMA system can utilize voice activityand allow the integration of voice and data traffic.This can be achieved by allocating frequency channelsand time slots on demand. In this approach, upon the arrival of a talkspurt or a data packet,the base station is requested to assign a time slot foreach transmission. Message requests and assignments ofmobile users are carried over a Control channel, while the voice and traffic are transmittedover a Traffic channel. Time slot assignments are madefrom a “pool” of Traffic channels. A numberof slots in the pool will be shared by voice and data, with voice having priority over data, andthe remaining will be used by data only. Voice slots arereserved for the duration of the talkspurt whereas datapackets are assigned on a per-slot basis. Data packets can be buffered whereas voicetraffic can only tolerate limited delay beyond whichtalkspurts will be clipped off. The Control channeluplink access is based on Slotted Aloha so that mobile users have autonomous access to base stations.This paper presents the performance of the dynamic TDMAsystem outlined here. The analysis aims at assessing thecapacity gained by using voice activity and voice/data integration, in terms of theimpairments introduced to voice quality (e.g., speechclipping and/or delay) and the delays to data packets.The analysis has been based on a discrete time Markov model operating on a frame-by-frame basis thatprovides the joint distribution of the number of activevoice and data users in the system. The analysis alsoevaluates the delays of message requests via the uplink control channel. In evaluating theclipping probability, we combine the impact of both theaccess delays at the control channel as well as theunavailability of time slots in the pool. Performance results indicate that the capacity gain mayexceed 80% and the speech clipping can be kept below 1%.Also, data packets may be transmitted with limiteddelays even when all capacity is allocated for voice users. The proposed approach may be used toenhance the capacity of the existing TDMA cellularsystems and to provide integration of voice and dataservices.

On MMSE real-time antenna array processing using fourth-order statistics in the US cellular TDMA system

The antenna array processing problem in the reverse link of the current US digital cellular communication system is studied and higher-than-second-order-statistics (HOS) baseband processing is proposed as a possible candidate solution. The remarkable difference of our approach as compared to other existing similar techniques is the idea of the minimization of the mean squared error using fourth-order cumulants alone and nonblind criteria. A recursive Jacobi total least squares algorithm is used in the adaptive implementation to mitigate the effects of high error variance in the estimates of the cumulants based on sample statistics. The method is shown to be very effective in a fast fading environment with multiple cochannel interferers.

A study of the effect of microwave oven interference on the performance of digital radio communications systems

With increasing use of wireless communication systems and electronic equipment using high-frequency devices, a poor electromagnetic environment for wireless communication systems is expected. This paper discusses the microwave oven, which is a widely used ISM apparatus. The effect of the interference waves generated by a microwave oven on the bit error characteristics of various wireless communication systems (PDC, PHS, ISM band wireless LAN) is studied. First, based on the measured interference waves generated from various microwave ovens, the statistical nature of the interference waves is discussed. It is found that a high level interference wave with periodicity is generated from microwave ovens and that the level and duration of the interference wave increase as the oscillation frequency of the magnetron is approached. Next, the bit error rates of π/4 shift QDPSK and DS/CDMA systems are simulated on a computer. It is found that the bit error rate is changed substantially by the effect of the interference wave. Further, for the case where the TDMA system is used as a multiple access system, a channel assignment method based on the periodicity of the interference wave is proposed. It is clear that the bit error rate can be improved significantly by using the proposed channel assignment method. © 1997 Scripta Technica, Inc. Electron Comm Jpn Pt 1, 80(12): 58–68, 1997

A survey of MAC protocols proposed for wireless ATM

Wireless ATM (W-ATM) networks have been studied extensively. The extension of ATM network services to the wireless environment faces many interesting problems. The original ATM network was designed for high-speed noiseless, reliable channels. None of these characteristics are applicable to the wireless channel. One of the most critical aspects of a W-ATM network is the medium access control (MAC) protocol used by mobile terminals (MTs) to request service from the base station (BS), which has to consider the quality of service (QoS) of the specific application. In this article the authors analyze some MAC protocols, particularly those for TDMA systems, and discuss their advantages and disadvantages.

The influence of relative humidity on the size of atmospheric aerosol

The relative humidity of ambient air is very often over 75% throughout the year in Taiwan. Thus, the influence of relative humidity on the growth of hygroscopic aerosols in the atmosphere is particularly important. In this study, a TDMA system (Tandem Differential Mobility Analyzer ) equipped with newly developed a SMPS (Scanning Mobility Particle Sizer) was used to observe the sizes of submicron ammonium sulfate aerosols, from 63.8nm to 224.7nm, at five controlled relative humidities, 50%, 75%, 81%, 85% and 89%. It is the first time that the hygroscopic growths and deliquescence points of submicron aerosols of ammonium sulfate have ever been closely observed by use of the SMPS. The expermental results showed that the abrupt size changes of aerosols at their deliquescence points can be more precisely observed than in previous studies. It was also shown that there was no apparent growth in an ammonium sulfate aerosol until the relative humidities was over 75%. At relative humidities between 81% and 89%, the growth ratio of size was about a factor of 1.3 to 1.6 and the water content of ammonium sulfate aerosols by mass was about 47% to 66%.