S. Hara

An overview of multi-carrier CDMA

We review new multiple access schemes based on a combination of code division and multi-carrier (also known as OFDM: orthogonal frequency division multiplexing) techniques, such as multi-carrier code division multiple access (MC-CDMA), multicarrier DS-CDMA and multitone CDMA (MT-CDMA). We discuss their advantages and disadvantages, and present some computer simulation results on the down-link bit error rate (BER) performance in a frequency selective slow Rayleigh fading channel. Code division and OFDM based multiple access schemes have drawn a lot of attention in the field of wireless personal and multimedia communications because of the need to transmit at a high data rate in a mobile environment.

DS-CDMA, MC-CDMA and MT-CDMA for mobile multi-media communications

We present a comparison between direct sequence code division multiple access (DS-CDMA), multi-carrier (MC-) CDMA and multitone (MT-) CDMA schemes in terms of the transmitter and receiver structure. We discuss their down-link bit error rate performance in a frequency selective Rayleigh fading channel by computer simulation.

Design and performance of quasi-synchronous multi-carrier CDMA system

Multicarrier (MC)-CDM/CDMA is one of the promising code division-based multiple access/multiplexing techniques for fourth-generation mobile communications systems, where high data rate services will be provided even for high speed-cruising mobiles. Investigations so far made in Japan have shown that the MC-CDM scheme has superiority over the direct sequence (DS)-CDM scheme in the downlink; on the other hand, the MC-CDMA scheme is not so advantageous over the DS-CDMA scheme in the uplink. This is because the code orthogonality among users can be kept in the downlink, whereas it is totally distorted by frequency selective fading in the uplink. First, we show a new signal design for MC-CDM/CDMA transmission. The new signal format has head/tail guard intervals in one symbol duration, for accurate DFT window synchronization in downlink and uplink, and establishment of quasi-synchronous transmission in uplink. Then, we show the bit error rate performance for MC-CDM with minimum mean square error (MMSE)-based combining in downlink and MC-CDMA with MMSE-based multiuser detection in uplink.

A novel FSK demodulation method using short-time DFT analysis for LEO satellite communication systems

This paper proposes a novel frequency-shift keying (FSK) demodulation method using short-time discrete Fourier transform (ST-DFT) analysis for low-Earth-orbit (LEO) satellite communication systems. The ST-DFT-based FSK demodulation method is simple and robust to a large and time-variant frequency offset because it expands the received signal in a time-frequency plane and demodulates it only by searching the instantaneous spectral peaks with no complicated carrier-recovery circuit. Two kinds of demodulation strategies are proposed: a bit-by-bit demodulation algorithm and an efficient demodulation-algorithm frequency-sequence estimation (FSE) based on the Viterbi algorithm. In addition, in order to carry out an accurate ST-DFT window synchronization, a simple DFT-based ST-DFT window-synchronization method is proposed.

Robustness of OFDM signal against temporally localized impulsive noise

We present the theoretical analysis of the bit error rate (BER) performance of single-carrier modulation (SCM) and orthogonal frequency division multiplexing (OFDM) systems under two types of temporally localized impulsive noise environments, namely stationary and bursty impulsive noise. The robustness of OFDM system against impulsive noise is then discussed and compared with that of the SCM system at the same transmission rate. In addition to the common knowledge on OFDM, we show that OFDM system is sometime less robust to impulsive noise than SCM system.

An uplink/downlink asymmetric slot allocation algorithm in CDMA/TDD-based wireless multimedia communications systems

We propose an uplink/downlink asymmetric slot allocation algorithm for CDMA/TDD-based wireless multimedia communications systems. We evaluate the average delay performance of the proposed algorithm by computer simulation, assuming a practical multicell environment and a multimedia service model which consists of several kinds of uplink/downlink symmetric and asymmetric services. Our simulation results show that the proposed algorithm, which can flexibly cope with uplink/downlink traffic asymmetry, improves the average delay performance as compared with symmetric slot allocation and fixed asymmetric slot allocation algorithms.

An efficient slot allocation algorithm to accommodate multimedia traffic in CDMA/TDD-based wireless communications systems

We propose two time slot allocation algorithms for code division multiple access with time division duplex (CDMA/TDD) systems to efficiently accommodate multimedia traffic. Assuming a practical multiple cell environment and a multimedia service model which consists of several kinds of circuit-switched and packet-switched services with different qualities of service (QoS), we evaluate the average delay of the CDMA/TDD system with the proposed algorithms, and compare the performance with that of CDMA with frequency division duplex (CDMA/FDD) and time division multiple access with TDD (TDMA/TDD) systems. Our computer simulation results show that the CDMA/TDD system with one of the proposed algorithms, which can effectively avoid interference among users with different QoS, can improve the average delay performance as compared with the other systems.

A simple null-steering adaptive array antenna in OFDM-based WPAN/WLAN

In a situation where different standard-based wireless PANs/LANs operate sharing the same frequency band in neighboring areas, it is important to suppress interference from/to other networks. This paper proposes a null-steering adaptive array antenna to suppress interference for terminals in OFDM-based wireless networks. Taking advantage of the characteristics of OFDM waveform, the array antenna has a simple structure and weight control algorithm. Computer simulation results show that the proposed null-steering array antenna can achieve good BER performance and outperforms a beam-steering array antenna in a scattering-rich indoor environment.

Packet communications with slotted ALOHA in a mobile cellular system

We present a simple uplink access technique for packet data communications in a mobile environment. This technique is an application of a random-access communication scheme with slotted ALOHA to a wireless cellular system. All base stations connected to a control station use a common frequency and have synchronized slots. Mobile terminals can transmit packets at any time and do not require slot reservation, handover control, or transmission power control. This system can perform base station diversity reception with selective combining or with maximal ratio combining (MRC) in the uplink. Computer simulation shows that throughput is much improved when base stations use adaptive array antennas. In addition, the effects of the limitation of retransmissions on throughput and service fairness for all mobile terminals are analyzed.

An RF-adaptive array antenna incorporated in a MIMO receiver under interference

In this paper, incorporation of an RF-adaptive array antenna (RF-AAA) is proposed on each receive branch in a multiple-input multiple-output (MIMO) receiver to achieve interference suppression. A criterion is shown for maximizing the output signal-to-interference-plus-noise ratio (SINR) in the RF-AAA with amplitude and phase adjusts. The performance is roughly evaluated with no amplitude adjust and a four-bit phase shift. Computer simulation verifies that the RF-AAA improves the capacity of the MIMO system and the bit-error rate (BER) performance of its receiver by suppressing interference, as the number of RF-AAA elements increases. However, diversity gain is not obtained as the number of RF-AAA elements increases, and the impact of interference suppression is seriously degraded as the angular spread of the interference increases.