Riaz Esmailzadeh

TDD-CDMA for the 4th generation of wireless communications

This article discusses an evolutionary TDD mode of CDMA-based path for 3.5G/4G systems. This technology has already been the basis of two 3G standards: TD-CDMA and TD-SCDMA with a synchronous uplink. Several techniques have been developed that allow TDD-CDMA-based systems to transmit at rates usually associated with 3.5G to 4G, up to 10-20 Mbit/s with wide area coverage. The present TDD-CDMA-based 3G standards are evolving to provide these higher rates, while fully retaining the coverage and mobility associated with, and expected from, the present mobile communications systems. This article discusses how TDD-CDMA specific techniques facilitate delivering services at data rates associated with 3.5G and 4G. We review how TDD-CDMA can provide for asymmetric uplink and downlink transmissions, facilitate deployment of advanced antennas for improved downlink coverage, and enable provision of advanced receiver techniques in base stations and mobiles. We also discuss how these techniques affect systems capacity in full packet-switched IP-based systems. We then discuss 4G TDD CDMA systems: those with different modulation techniques for uplink and downlink communications. These are generally based on a multicarrier mode of CDMA, and may incorporate OFDM technique.

Pre-RAKE diversity combining in time division duplex CDMA mobile communications

A pre-RAKE system is proposed for the time division duplex code division multiple access systems for portable communications. Since the up and down links are time slots on the same carrier, they have the same channel impulse response during a short period of time. Instead of building a RAKE receiver in the portable unit the base station can pre-RAKE the signal before transmission in the down link using the channel impulse response estimated from the up link. When the pre-RAKE signal is convolved with the channel impulse response, the function of the RAKE receiver is automatically performed. The mobile or portable unit can use a conventional matched filter and still achieve the diversity gain of a RAKE receiver.

Downlink performance of a CDMA system with distributed base station

High-speed and high-capacity communication is required in 4G mobile communication. However the currently used frequency band (2 GHz) will be insufficient to meet such a demand, thus more bandwidth, that seems to be allocated at higher frequencies (3 GHz or 5 GHz), is required. As the frequency band becomes higher, the propagation loss increases; thus mobile terminals (MT) need to strengthen transmission power. However, since there is a limitation on power consumption in an MT, the higher frequency band leads to reduction of cell size. Thus more base stations (BS) are needed to maintain the same area as a conventional system, boosting the infrastructure cost. To solve this problem, a system is proposed that uses some simple relay sub-base stations (SBS) for uplink and downlink and maintains the same coverage area as a conventional system. Due to the imbalance of uplink and downlink traffic, it is expected that capacity requirements are much higher in the downlink. We describe a CDMA (code division multiple access) based distributed base station (BS+SBS) system that fulfills coverage requirements. We evaluate the downlink performance of the proposed system through computer simulation and demonstrate an overall increased system capacity.

Power control in packet switched time division duplex direct sequence spread spectrum communications

A packet switched time-division duplex direct-sequence spread-spectrum (DS-SS) communication system is proposed for operation in mobile and wireless indoor CDMA communications with Rayleigh fading characteristics. A power control method is then proposed for this system capable of providing effective estimates of the fading state of the channel and using this information to adaptively control the uplink transmission power for equalized received power at the base station. The performance of this power control scheme is evaluated by examining the distribution of the received power compared to a reference level for typical indoor communication system parameters.<<ETX>>

Time division duplex transmission of direct sequence spread spectrum signals in multipath channels

The time division duplex (TDD) mode of transmission of direct sequence spread spectrum (DS-SS) signals is proposed for systems operating in multipath channels. It is shown that the TDD systems are simpler, can facilitate efficient power control, consume less power and can operate with simpler mobile unit receivers. In the proposed TDD method, both reverse and forward links use the same frequency band and thus have a highly correlated fading pattern. The reciprocity of fading patterns of forward and reverse links provides for a non-feedback power control technique to improve system performance under multipath fading. In systems where the multipaths are created artificially by using multiple antennas, such as indoor distributed antenna systems, the characteristic of the channel as seen by each antenna can be estimated at base station. The best situated antenna can then be selected and used for transmission, creating in effect a pre-selection diversity combining system at the base station. The function of power control is still carried out at mobile units. Computer simulation results show the performance of a power controlled two-path selection diversity to be within 4 dB of static channel performance at bit error rate (BER) of 10/sup -4/.<<ETX>>

Path diversity for FFH/PSK spread-spectrum communication systems

A fast frequency hopping (FFH) method which uses path-diversity combining is proposed. Diversity techniques are realized when a signal is received from diverse independent paths, each of which carries identical information but suffers from independent fading variations. The improvement of communication performance of FFH systems is possible as the delayed paths are used and path-diversity combination is realized. The advantages of this method, operating in Rayleigh fading channels are confirmed by theoretical analyses. The improvement is in order of 2/spl sim/3 dB at bit error rate (BER) of 10/sup -3/. This method can be also effective against interferences from other users in a code division multiple access (CDMA) environment. The performance of this system in a CDMA environment is evaluated by theoretical analyses and is shown to be superior to non-combining method. At BER of 10/sup -3/ the required E/sub b//N/sub 0/ of the proposed system is 5 dB lower. If E/sub b//N/sub o/ is fixed, a greater number of users can be accommodated using the proposed system. >

Evaluation of Asymmetric TDD Systems Employing AMC and HARQ by Considering MCS Selection Errors

In this paper, we evaluate the performance of asymmetric Time Division Duplex (TDD) system that employs Adaptive Modulation and Coding (AMC) and Hybrid ARQ, with consideration of the effect of control delays in TDD. Channel reciprocity characteristic in TDD allows utilization of open loop channel estimation to choose appropriate modulation and coding scheme (MCS) level for AMC. However, control delay in AMC and HARQ depends on TDD time slot allocation formats. Large control delay in AMC will result in false MCS selection due to the poor channel correlation between measured channel state from the received signals and instantaneous channel state of actual transmission with the MCS selected based on the measured channel state. We present an analytical approach to calculate the probability of MCS level selection error in different channel conditions for different asymmetric time slot allocations. From the theoretical and simulation results, it is shown that the instantaneous throughput per slot depends not only on maximum Doppler frequency but also on asymmetric slot allocations. Average delay time that yields error free packet reception in the downlink increases as the number of continuous downlink slots increases.

Transmission power control using perceptual quality metrics

In this paper a novel method of controlling transmission power in mobile communication systems is described. Our paper concentrates on the delivery of high quality voice communications as an example of how networks may be controlled to deliver services of a desired quality. We argue that future communications networks must use metrics closely associated with the control parameter. For example, if voice quality is to be controlled, a voice quality metric should be used for controlling the network. Other proxy metric, e.g. frame error rate, will result in a less efficient network operation. We show preliminary results on how a WCDMA voice communication system may be power controlled using perceptual quality metrics directly related to the quality of the delivered voice.

Time Division Duplex-CDMA

The 3rd generation of mobile operations is underway, including FDD (Frequency Division Duplex) and TDD (Time Division Duplex) modes in wideband CDMA. This paper describes the features of TDD-CDMA including TDD Multi-Carrier systems.

Analysis of Performance Degradation due to Channel Estimation Error in Pre-Rake TDD/CDMA

In recent years, Pre-Rake combining technique has become a hot topic of research as it decreases the complexity of the portable mobile unit, while achieving the same multipath diversity effect of the Rake receiver. The technique is based on precoding of the transmitted signal relying on knowledge of the channel estimation response before transmission. This a priori channel state information is available in Time Division Duplexing (TDD) systems, since the same channel is used both in uplink and downlink. In practice, the error in channel estimation in Pre-Rake system occurs due to time variability in mobile radio channel. Most previous works on Pre-Rake in TDD CDMA have not taken into consideration the effect of imperfect channel estimation. In this paper, we present Pre-Rake performance under imperfect channel estimation due to time variability in TDD system, depending on Doppler Frequency and compare it with the ideal Pre-Rake system. Numerical analysis and computer simulations were carried out to obtain the system error probability.