Time Division Multiple Access System Research Articles

Trellis-based soft-output adaptive equalization techniques for TDMA cellular systems

The concatenation of an equalizer and a Viterbi (1967) decoder is a powerful means for improving receiver performance in wireless communication systems. A soft-output equalizer increases the impact of this combination by enabling the use of soft-decision Viterbi decoding. It is well known that the maximum a posteriori (MAP) algorithm provides optimal reliability information, but at the cost of substantial complexity. This paper contains the results of an investigation into the design and performance of soft-output adaptive equalization techniques based on suboptimum trellis-based soft-output decoding algorithms. It is shown that the performance improvement relative to hard output equalizers is substantial, while the cost in terms of complexity is modest. A time-division multiple-access (TDMA) cellular system is used as the basis for comparisons. Simulation results and a complexity analysis are presented.

Packet dropping analysis for reservation voice TDMA systems

We analyze the packet dropping performance in an ideal reservation time-division multiple-access (TDMA) multiplexing voice system with the focus on the probability distribution of the number of packets dropped in a particular talkspurt. Our analysis method is based on the discrete three-state Markov speech model which corresponds to the voice source equipped with a fast speech activity detector (SAD). The numerical results for a system of 14 slots/frame reveal that the probability of losing several packets in a talkspurt is much higher than expected (by geometric distribution) and thus not negligible.

Spatial-temporal equalization for IS-136 TDMA systems with rapid dispersive fading and cochannel interference

In this paper, we investigate spatial-temporal equalization for IS-136 time-division multiple-access (TDMA) cellular/PCS systems to suppress intersymbol interference and cochannel interference and improve communication quality. This research emphasizes channels with large Doppler frequency (up to 184 Hz), delay dispersion under one symbol duration, and strong cochannel interference. We first present the structure of the optimum spatial-temporal decision-feedback equalizer (DFE) and linear equalizer and derive closed-form expressions for the equalizer parameters and mean-square error (MSE) for the case of known channel parameters. Since the channel can change within an IS-136 time slot, the spatial-temporal equalizer requires parameter tracking techniques. Therefore, we present three parameter tracking algorithms: the diagonal loading minimum MSE algorithm, which uses diagonal loading to improve tracking ability, the two-stage tracking algorithm, which uses diagonal loading in combination with a reduced complexity architecture, and the simplified two-stage tracking algorithm, which further reduces complexity to one M/spl times/M and one 3/spl times/3 matrix inversion for weight calculation with M antennas. For a four-antenna system, the simplified two-stage tracking algorithm can attain a 10/sup -2/ bit error rate (BER) when the channel delay spread is half of the symbol duration and the signal-to-interference ratio (SIR) of the system is as low as 5 dB, making it a computationally feasible technique to enhance system performance for IS-136 TDMA systems.

RF and ELF Exposures from Cellular Phone Handsets: TDMA and CDMA Systems

The future digital systems for personal communications have now been partially standardised, so it is possible to get a good idea of the exposure power levels and time variation of the power, which may be assumed to be the two biologically relevant parameters. Using GSM and the coming UMTS system the relevant details are discussed, where the main difference is between the highly periodic TDMA (Time Division Multiple Access) power variation and the more smooth, slowly varying CDMA signal (Code Division Multiple Access). Epidemiological and biological studies must reflect the intricate details of the various systems.

Joint receiver for DS-CDMA/TDMA signals using complex multirate filter bank

This paper discusses the problem of coexistence of DS-CDMA (direct sequence code division multiplex access) and TDMA (time division multiple access) systems where their signals are transmitted in the same frequency band. The DS-CDMA signal is robust against narrowband interference, but there is a limit to its immunity against interference. This paper proposes, from such a viewpoint, a method using a complex multirate filter bank, as a way to reject TDMA signals, which act as narrowband interference to the DS-CDMA signal, in the coexistence system. By the proposed method, a notch filter can be composed with an arbitrary frequency resolution at an arbitrary frequency, by constructing the complex version of the conventional multirate filter bank. It is shown that, by eliminating the TDMA signal by the proposed method, the bit error rate of the DS-CDMA signal is improved, indicating that the proposed system is useful in the coexistence problem. © 1999 Scripta Technica, Electron Comm Jpn Pt 1, 82(5): 34–41, 1999

Frequency offset estimator for multipath fading channels

A fast and exact frequency offset estimator (FOE) algorithm using peak phase error detection and frequency offset smoothing is proposed for time division multiple access (TDMA) systems. The proposed peak phase error detection scheme avoids the large phase errors which lead to poor FOE performance. To control the AFC, frequency offset smoothing using a simple filter is utilised. Simulation results show that the proposed algorithm is adequate for the frequency offset estimator of TDMA systems for burst data transmission.

Integrated dynamic channel assignment and power control in TDMA mobile wireless communication systems

With the evolution of analog mobile wireless communications systems into digital second- and third-generation systems, there is growing interest in finding more efficient ways of managing the available resources, in particular radio spectrum and power. In the context of time division multiple access (TDMA) systems, this interest has led to the development of a variety of dynamic channel assignment (DCA) and power control (PC) algorithms. Despite the intense activity in both the DCA and the PC arenas and some proposals for combined DCA with PC, not enough work has been devoted to effectively integrating them. In this paper, the integration of DCA and PC is investigated and a family of integrated algorithms is presented. These algorithms, fully distributed and cost adaptive, achieve capacity levels significantly higher than those of a system with only DCA or PC. With respect to a system without any DCA or PC, several-fold capacity increases are obtained. Furthermore, these capacity levels are attained with user mobility included in the analysis.

A new approach for medium-access control for data traffic and its adaptation to the GSM general packet radio services

The inhibit sense multiple access with polling (ISMA/P) is a packet-based medium-access control scheme for statistical multiplexing of data users over a single slotted channel of a time-division multiple-access (TDMA) cellular system. In ISMA/P, the central base station (BS) asserts control over the users by polling data in smaller blocks. This enables ISMA/P to attain a near ideal, steady, and high throughput and low-mean delay performance, which is not usually achievable by contention-reservation-based MAC schemes. In this paper, we describe the ISMA/P algorithm and provide a detailed simulation study of it over a global system for mobile communications (GSM)-like slotted channel for a traffic model adopted from European Railways applications. This study shows that ISMA/P can achieve a steady throughput of 90% over a large range of load. A number of practical advantages of ISMA/P, such as elimination of channel hogging, incorrect channel sensing, battery saving, etc., are also elaborated.

A new method for reducing the power consumption of portable handsets in TDMA mobile systems: conditional equalization

In time-division multiple-access (TDMA) systems, the autonomy of portable handsets is a major constraint. We propose a new method, conditional equalization, which minimizes the power consumption due to equalization by equalizing only when it is most needed. At each time slot, the receiver estimates the need for equalization with a deterministic criterion. The proposed criteria use thresholds, chosen to provide a compromise between loss in performance and percentage of saved equalization. The performance of our method is theoretically analyzed for two-path Rayleigh fading channels and simulated for the recommended COST 207 channels. Conditional equalization can ensure a degradation in the average signal-to-noise ratio (SNR), providing a fixed bit error rate (BER with an average SNR=9 dB with systematic equalization), of less than 0.5 dB while providing a gain of 43% spared equalization for bad urban (BU), 62% for hilly terrain (HT), 79% for typical urban (TU), and 98% for rural area (RA) models, respectively. Conditional equalization is shown to greatly improve the autonomy of the receiver by drastically reducing the power consumption due to equalization, without a noticeable loss in the performance.

Delivering QoS requirements to traffic with diverse delay tolerances in a TDMA environment

The focus of this paper is on determining the call admission region and scheduling policies for a time-division multiple-access (wireless) system supporting heterogeneous real-time variable bit rate applications with distinct quality of service (QoS) requirements and traffic characteristics. The QoS is defined in terms of a maximum tolerable packet delay and dropping probability. A packet is dropped if it experiences excess delay. The call admission region is established for policies that are work-conserving (WC) and that satisfy the earliest due date (EDD) service criterion (WC-EDD policies). Such policies are known to optimize the overall system performance. In addition to the determination of the call admission region, this study leads also to the construction of scheduling policies that deliver any performance in the region established for WC-EDD policies. Finally, an upper bound on the call admission region that can be achieved under any policy (not limited to the WC-EDD policies) is determined.

TDMA scheduling design of multihop packet radio networks based on latin squares

Many transmission scheduling algorithms have been proposed to maximize spatial reuse and minimize the time division multiple access (TDMA) frame length in multihop packet radio networks. Almost all existing algorithms assume exact network topology information and require recomputations when the network topology changes. In addition, existing work focuses on single channel TDMA systems. In this paper, we propose a multichannel topology-transparent algorithm based on latin squares. The proposed algorithm has the flexibility to allow the growth of the network, i.e., the network can add more mobile nodes without recomputation of transmission schedules for existing nodes. At the same time, a minimum throughput is guaranteed. We analyze the efficiency of this algorithm and examine the topology-transparent characteristics and the sensitivity on design parameters by analytical and simulation techniques.

Performance evaluation of uplink power control algorithm over TDMA systems

Future PCS (personal communication system) cellular networks will mainly be driven by high link quality, high bandwidth utilization, low power consumption and efficient network management. Power control is one of the several major techniques which could help to achieve these goals. By exploiting power control techniques, co-channel interference could be reduced and as many links as possible could be obtained with satisfactory link quality. SIR-based (signal-to-interference ratio-based) power control was proposed as a technique for managing co-channel interference in cellular radio systems. Furthermore, new distributed autonomous feedback power control methods were introduced to achieve excellent performance without the difficult centralized control used in SIR-based methods. Unfortunately, the implementation of those power control algorithms is still challenging owing to the precision of SIR. The main aim of this paper is to investigate an uplink power control algorithm which depends indirectly on the signal-to-noise ratio in the TDMA (time division multiple access) cellular system. Simulation of the prototype hardware implementation of the receiver baseband signal processing based on the PACS (personal access communications system) specification is used as the main approach to explore the performance evaluation of this power control technique. According to simulation results, suitable values of those parameters used in this power control algorithm are derived, and the minimum frequency reuse factor under different propagation environments is also obtained for PACS under power control. © 1998 John Wiley & Sons, Ltd.

Performance evaluation of uplink power control algorithm over TDMA systems

Future PCS (personal communication system) cellular networks will mainly be driven by high link quality, high bandwidth utilization, low power consumption and efficient network management. Power control is one of the several major techniques which could help to achieve these goals. By exploiting power control techniques, co-channel interference could be reduced and as many links as possible could be obtained with satisfactory link quality. SIR-based (signal-to-interference ratio-based) power control was proposed as a technique for managing co-channel interference in cellular radio systems. Furthermore, new distributed autonomous feedback power control methods were introduced to achieve excellent performance without the difficult centralized control used in SIR-based methods. Unfortunately, the implementation of those power control algorithms is still challenging owing to the precision of SIR. The main aim of this paper is to investigate an uplink power control algorithm which depends indirectly on the signal-to-noise ratio in the TDMA (time division multiple access) cellular system. Simulation of the prototype hardware implementation of the receiver baseband signal processing based on the PACS (personal access communications system) specification is used as the main approach to explore the performance evaluation of this power control technique. According to simulation results, suitable values of those parameters used in this power control algorithm are derived, and the minimum frequency reuse factor under different propagation environments is also obtained for PACS under power control. © 1998 John Wiley & Sons, Ltd.

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.

Modeling of interaction of electromagnetic fields from a cellular telephone with hearing aids

The new generation of cellular telephones and other personal communication services (PCSs) poses new problems and challenges in interactions with the human body, Among them is electromagnetic interference (EMI) with medical devices, particularly for systems using time-division multiple access (TDMA). Hearing aids are among the devices affected by the pulse modulation in the audio range associated with the TDMA systems. While EMI in this case does not generally pose a health risk, it constitutes a considerable annoyance, which may prevent the hearing-aid wearers from using some of the new communication devices. Also, just proximity to the devices used by others or the base station may result in signals sufficient to interfere with proper perception of sounds, We have evaluated the electric and magnetic fields in the ear canal at 900 MHz for a typical monopole antenna on a typical handset, an equivalent dipole, and a plane wave. Special care was taken to properly represent the anatomy of the ear, including its canal. The finite-difference time-domain (FDTD) method was used to compute the electric and magnetic fields in the ear canal and around the ear. The fields from the exposure sources in various realistic placements of the hearing aid were compared. The results presented are of importance and use in developing performance standards and practical testing methods for various types of hearing aids.

A method for estimating local traffic load in a simulcast radio system

This paper presents a simple method for estimating the average local traffic load in a simulcast radio system. Such estimation is crucial to achieving efficient resource sharing and dynamic reconfiguration of the simulcast areas. The proposed method is based only on relative power measurements at each radio port (or distributed antenna element) and is applicable to both code-division multiple-access (CDMA) and time-division multiple-access (TDMA) systems. Simulation results are derived to test the method over various system conditions and assumptions. The proposed method is shown to provide excellent results, giving estimates to within /spl plusmn/15% of the actual Erlang traffic, while requiring only an approximate knowledge of the radio propagation environment.

Blind adaptation of antenna arrays using a simple algorithm based on small frequency offsets

With the development of wireless and personal communication systems, there is considerable interest in using antenna array technology to improve the system capacity in both present and future generation wireless systems. A novel and simple algorithm is proposed for blind adaptive extraction of a binary phase-shift keying (BPSK) signal in the presence of interference by cyclostationary signal processing using an antenna array. The algorithm operates in an interference-limited system in which the desired and interfering signals have identical symbol rates, but are modulated on slightly different carrier frequencies. Compared to existing blind algorithms which also exploit the cyclostationarity of the received signal, the new algorithm provides a simpler and faster converging means to estimate the channel phase for diversity combining. Analytical and simulation results are presented and performances are compared with direct matrix inversion (DMI) and existing blind algorithms. Based on the proposed algorithm and the result obtained by Gardner, Schell and Murphy (1992), a time-division multiple-access (TDMA) system with the new channel assignment scheme called cyclic TDMA (CTDMA) is proposed. The simulation results also show that the proposed algorithm is relatively simple and very promising in applications to indoor wireless communication where interference rejection and increased spectrum efficiency are the objectives. Analysis and simulation results are presented to confirm the interference rejection capabilities. The robustness of existing and proposed algorithms to the perturbation of cyclostationarity is also discussed.

A note on scheduling problems arising in satellite communications

We present a brief overview of recent results on scheduling in Time Division Multiple Access (TDMA) satellite communications systems which augment those included in a recent survey. These results address problems arising in Satellite Switched TDMA (SS/TDMA) systems and SS/TDMA systems with intersatellite links, as well as generalizations and relatives of these problems. They also have application in other areas, for example, I/O in parallel computers.

New concepts and technologies for achieving highly reliable and high-capacity multimedia wireless communications systems

The rapid growth of multimedia wireless communications services forces the development of advanced digital wireless systems with high reliability and high speed as well as flexibility for varying traffic conditions. To achieve such advanced wireless systems. New system design concepts different from the conventional ones aiming at increasing system capacity for voice transmission would be required. Since multimedia wireless communications require high quality, high speed, and high flexibility as well as temporary and spatial control of traffic under severe fading environments, the so-called conventional system design concept will be insufficient. New system design concepts and techniques for achieving highly reliable and high-capacity multimedia wireless communications are discussed using both time division multiple access (TDMA) and code division multiple access (CDMA) systems. The demands for high-speed and high-reliability mobile, personal, and multimedia wireless communications services strongly require matching with the trunk network constructed by a broadband optical fiber system or wired system.

Resource allocation for cellular radio systems

High terminal traffic densities are expected in urban multiuser radio systems. An efficient allocation of resources is the key to high system capacity. In this paper, a distributed dynamic resource allocation (DDRA) scheme based on local signal and interference measurements is proposed for multiuser radio networks. It offers "soft capacity" for time division multiple access (TDMA) and frequency division multiple access (FDMA) systems, bounded above by N per base station, where N is the total number of channels in the system. The decisions are made local to a terminal and its base and are essentially independent of the rest of the system. A distributed dynamic channel assignment scheme is used to assign channels to new calls. This scheme assigns a channel that offers the maximum carrier to interference ratio (CIR) to a new call. A distributed constrained power control (DCPC) scheme based on CIR measurements is used for power control. The channel assignment scheme and the power control scheme are coupled to obtain an interactive resource allocation scheme. We compare the capacity of a system which uses the distributed dynamic resource allocation scheme described above with the capacity of a system which uses the channel assignment scheme alone. The system capacity is measured by simulation as the number of terminals that can be served by the system with a CIR above an acceptable minimum. In a 1D cellular system, coupling the channel assignment scheme with power control is discussed. Simulations were also used to show the effect of varying the maximum transmitter power on system capacity.