Justin C.-I. Chuang

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.

M-PSK and M-QAM BER computation using signal-space concepts

In this paper, we introduce a simple geometric approach that is based on signal-space concepts to efficiently evaluate the performance of M-ary phase-shift keying (M-PSK) and M-ary quadrature amplitude modulation (M-QAM) schemes over an additive white Gaussian noise channel. In particular, new bit error rate approximations are derived and shown to be in excellent agreement with Monte Carlo simulation results.

Transmitter diversity for OFDM systems and its impact on high-rate data wireless networks

Transmitter diversity and down-link beamforming can be used in high-rate data wireless networks with orthogonal frequency division multiplexing (OFDM) for capacity improvement. We compare the performance of delay, permutation and space-time coding transmitter diversity for high-rate packet data wireless networks using OFDM modulation. For these systems, relatively high block error rates, such as 10%, are acceptable assuming the use of effective automatic retransmission request (ARQ). As an alternative, we also consider using the same number of transmitter antennas for down-link beamforming as we consider for transmitter diversity. The investigation indicates that delay transmitter diversity with quaternary phase-shift keying (QPSK) modulation and adaptive antenna arrays provides a good quality of service (QoS) with low retransmission probability, while space-time coding transmitter diversity provides high peak data rates. Down-link beamforming together with adaptive antenna arrays, however, provides a higher capacity than transmitter diversity for typical mobile environments.

Performance issues and algorithms for dynamic channel assignment

Using dynamic channel assignment (DCA) algorithms to select communications channels as needed, time-division multiple access (TDMA) or frequency-division multiple access (FDMA) systems can serve dynamic and nonuniform traffic demands without frequency planning as long as quality is sufficient and equipment is available. Here, performance issues and algorithms for DCA in a TDMA portable radio system are considered. A fixed number of traffic servers (time slots) per radio port is assumed: therefore, the system capacity is hard-limited by the equipment availability, and the DCA efficiency is compared mainly through the signal-to-interference ratio in both the uplink and downlink directions. >

Performance evaluation of distributed measurement-based dynamic channel assignment in local wireless communications

The diverse environments emerging for wireless communication applications could render the centralized prediction-based channel assignment methodology, conventionally employed in cellular radio networks, impractical. The distributed measurement-based approach seems to be a more practical solution. We evaluate and compare several distributed measurement-based algorithms for dynamic channel assignment (DCA). Their performance is also compared with a centralized prediction-based algorithm. It is found that a simple aggressive algorithm with the use of a threshold, known as the least interference algorithm (LIA), performs the best.

Burst coherent demodulation with combined symbol timing, frequency offset estimation, and diversity selection

A low-overhead burst coherent demodulation method that jointly estimates symbol timing and carrier frequency offset and then performs diversity selection is studied. It coherently demodulates individual bursts of TDMA (time division multiple access) symbols by operating solely on random data within the burst without requiring training sequences. Its performance is robust against frequency offset between transmitter and receiver, thereby eliminating the need for a highly stable frequency reference. The performance of this demodulation method in a fading channel can be further improved by using a diversity selection technique based on a quality measure derived as part of the joint timing/frequency offset estimation process. Simulations and experiments have confirmed that two-branch diversity using this method can provide reliable speech communication using TDMA with a transmission rate of 450 kb/s for a portable radio channel with an RMS delay spread of 555 ns or less. >

Autonomous adaptive frequency assignment for TDMA portable radio systems

A simple autonomous procedure performed by each radio port (base station) to determine its own transmitting frequency is proposed. This procedure consists of signal strength measurements and an algorithm which selects the frequency with minimum interference from other ports. The algorithm converges rapidly while adapting to changes of operational conditions, such as installing new ports, which changes the system configuration, or adding new buildings, which causes different shadow fading. This method is significantly superior to a random assignment method both in resultant channel quality and in traffic-handling capability. For a regular-grid configuration, this method performs nearly as well as an a priori optimal frequency assignment method. It is also found that this assignment algorithm is robust against short-term signal fluctuations and it can be performed completely autonomously by each radio port. >

Low-overhead symbol timing and carrier recovery for TDMA portable radio systems

Simulation and experimental results for a hardware implementation of a low-overhead digital coherent burst demodulator for time-division multiple-access (TDMA) radio systems using short bursts are analyzed. This implementation is based on digitizing a receiving signal after a low-frequency IF amplifier and performing all demodulation functions using digital signal processing techniques. Demodulation with very low overhead for TDMA is made possible by storing a burst in memory. A novel forward loop plus backward loop structure performs carrier recovery. Symbol-timing and carrier-frequency-offset estimations are performed by block processes, using the error signal resulting from differential demodulation. Bursts as short as 32 symbols can be demodulated without overhead. Since the demodulator is entirely digital and has the potential for low-power VLSI implementation, it is suitable for a portable radio system. Other advantages of this implementation include the availability of a channel quality measure derived from symbol timing estimation and the possibility of providing differential detection when it is desirable. >

Uplink power control for TDMA portable radio channels

Power control based on signal-to-interference ratio (SIR) has been proposed as a technique for managing co-channel interference in frequency reuse radio systems. Recently, new autonomous power control methods were introduced to achieve near-optimum performance without difficult centralized control proposed earlier. The achievable performance from preliminary studies appears promising for providing significant increase in spectrum efficiency. However, the implementation of the SIR-based power control algorithms remains challenging. In this paper, implementation of power control that indirectly depends on SIR is discussed. As an example, a simple closed-loop power control algorithm for the portable transmitter is introduced for TDMA portable radio systems. While it may appear specific for the system considered, the underlying principle and parameters required (i.e., error indicator, received power level, and signal quality indicator) are common to the implementation of digital demodulation circuitry. Computer simulations indicate that SIR level is maintained at a level suitable for sustaining desirable performance. Furthermore, when the power-control updating period is short, as in the specific system considered, moderate-rate short-term fading can be tracked and mitigated. >

Transmitter diversity for OFDM systems and its impacts on high-rate wireless networks

Transmitter diversity and down-link beamforming can be used in high-rate wireless networks with OFDM for capacity improvement. In this paper, we compare the performance of delay, permutation and space-time coding transmitter diversity for OFDM systems, and its impact on high-rate packet data wireless networks, where relatively high block error rates, such as, 10%, on the wireless links are acceptable using ARQ. As an alternative, we also consider using the same number of transmitter antennas for down-link beamforming in the direction of the desired mobiles. Our investigation indicates that the delay transmitter diversity with QPSK modulation and adaptive antenna arrays can provide good quality of service (QoS) with a low retransmission probability while space-time coding transmitter diversity can provide high peak-rate. However, down-link beamforming together with adaptive antenna arrays, if it is applicable, appears to be a better alternative to transmitter diversity.