Introduction to CDMA

The purpose of this paper is to introduce the basic concepts of code division multiple access (cdma) to those who are not familiar with the subject area. It briefly covers the two most important forms of spread spectrum modulation, frequency hopping (fh) and direct sequence (ds), before concentrating on cdma based on ds. The jamming margin equation is developed to give an estimate of spectral efficiency in a single-cell environment. Then the improvements in spectral efficiency based on coding and voice activation are discussed. The main advantages of cdma over more conventional access techniques in the cellular environment are discussed. Finally, the main limiting factors on cdma capacity, i.e. hand-off and power control are discussed.     

The role of satellites in personal communication services

This paper aims at providing a complete analysis of the issues relevant to satellite systems in the perspective of the third generation of mobile systems. The analysis considers, on the one hand, the possible satellite system evolutionary paths and, on the other hand, the satellite system requirements related to technological issues. As regards the satellite evolutionary paths, the paper provides a view of the satellite systems already in operation or currently being developed and outlines the guidelines followed by future satellite systems (in that respect, particular emphasis is given to small satellites). The analysis is performed by also taking into account the potential market and the regulatory issues. As regards the satellite system requirements related to technological issues, some key features such as integration with the terrestrial cellular networks, satellite system capacity, handover, synchronization, are discussed in the light of the technological innovations which are being proposed for the third generation of mobile systems. The above-mentioned issues are dealt with by referring both to time division multiple access (TDMA) and to code division multiple access (CDMA), i.e., the third-generation candidate access techniques     

CDMA versus FDMA channel capacity in mobile satellite communication

At present a major effort is under way to define the most efficient modulation/multiple access system in mobile satellite communication. Where the emphasis is on digital voice modulation, the proposed multiple access methods almost always imply frequency division multiple access (FDMA). This analysis presents a comparison between FDMA and code division multiple access (CDMA), for the operation of both multiple access methods in the mobile satellite communication environment. The mobile satellites under consideration use multiple-beam or scan-beam antennas and employ frequency reuse of the allocated L-band frequency spectrum. As CDMA is better at absorbing Doppler and multipath effects, and permits higher rate coding, in general (practical considerations aside) it appears to be the more capable system.     

Code-based all optical routing using two-level coding

Code-based all optical routing employing a two-level-coding scheme is proposed. The first level of coding is employed to establish connections between users within a local area network; a second level of coding provides routing/interconnectivity between networks. The limitations due to physical-layer impairments, such as relative intensity noise (RIN) of the optical source, the signal-dependent shot noise, optical beat interference (OBI), and thermal noise at the receiver, which are some of the fundamental issues in the design of practical optical code division multiple access (CDMA) systems, are analyzed for the two-level scheme. The throughput in terms of "packets/time slot" offered by the scheme is also compared with that of the wavelength division multiple access (WDMA) system. The system capacity in WDMA is limited by the number of available wavelengths; optical CDMA, on the other hand, has many codes (user addresses), but the throughput is limited by multiple user interference, OBI, and RIN. System designs that overcome these effects and thereby improve the throughput are suggested.     

Optimization of Coded Spread Spectrum System Performance

Error correction coding techniques significantly improve performance of spread spectrum communication systems in environments containing jamming, multipath, and unregulated multiple access. This paper investigates the optimization of spread spectrum system performance for time-varying unknown interference. Noncoherent frequency hopping (FH) spread spectrum modulation, and hybrid FH-PN incorporating a direct sequence PN modulation on each hopped frequency are studied. For FH or FH-PN, the data modulations considered are differential phase-shift-keying (DPSK), differential quadriphase-quadriphase-shift-keying (DQPSK), and multiple-frequency-multiple-frequency-shift-keying (MFSK). Both block and convolutional error correction coding techniques are studied as a means of improving the spread spectrum performance.     

OFDMA上行链路系统的频偏估计算法

介绍了正交频分复用多址接入（OFDMA）的优缺点,以及目前应用到的领域。对OFDMA上行链路载波频偏（CFO）估计研究的发展背景和方法分类进行了简要汇总,并对当前估计方法进行了总结分类;给出了带频偏OFDMA上行链路的系统框图和信号模型;且对现有的几种典型频偏估计算法进行了分析和讨论。总结和展望了OFDMA上行链路频偏估计方法的研究方向和关键问题。     

Cross layer scheduling for real-time traffic in multiuser MIMO-OFDMA systems

A novel cross layer scheduling algorithm is proposed for real-time (RT) traffic in multiuser downlink multiple-input multiple-output orthogonal frequency division multiple access (MIMO-OFDMA) wireless systems. The algorithm dynamically allocates resources in space, time and frequency domain based on channel state information (CSI), users' quality of service (QoS) requirements and queue state information (QSI). To provide higher data rate and spectrum efficiency, adaptive modulation and coding (AMC) is employed. The proposed algorithm can improve cell throughput and increase the number of users that can be supported while guaranteeing users' QoS requirements and fairness among all users. Simulation results indicate that the proposed algorithm can achieve superior performance.     

Performance of a cellular hybrid C/TDMA mobile radio systemapplying joint detection and coherent receiver antenna diversity

For future mobile radio systems, an appropriately chosen multiple access technique is a critical issue. Multiple access techniques presently under discussion are code division multiple access (CDMA), time division multiple access (TDMA), and hybrids of both. In the paper, a hybrid C/TDMA system using joint detection (JD-C/TDMA) with coherent receiver antenna diversity (CRAD) at the base station (BS) receiver is proposed. Some attractive features of the JD-C/TDMA system are the possibility to flexibly offer voice and data services with different bit rates, soft capacity, inherent frequency and interferer diversity, and high system capacity due to JD. Furthermore, due to JD, a cluster size equal to 1 can be realized without needing soft handover. The single cell E_b/N₀ performance and the interference situation in a cellular environment of the uplink of a JD-C/TDMA mobile radio system with CRAD is investigated in detail. It is shown that the cellular spectrum efficiency is remarkably high, taking values up to 0.2 bit/s/Hz/BS in the uplink, depending on the actual transmission conditions     

Communications satellites: orbiting into the 90s

Engineering advances in satellite communications are discussed. These include sophisticated switchboards, narrow beams, source coding for higher capacity, larger networks, and the use of higher and lower frequency bands and lower orbits. One of the most popular new 14/11-14/12 GHz commercial services has been time-division multiplexing of multiple carriers operating at low to medium bit rates. Multiple-carrier, low-burst-rate TDMA (time-division multiple access), in which several TDMA carriers are shared among ground terminals, is widely used with very small-aperture terminals (VSAT) on the customer's premises. NASA's (US National Aeronautic and Space Administration) ACTS and Italy's Italsat both plan to use signal regeneration at 30/20 GHz. Onboard switching and multiplexing will not only minimize noise and boost power, but also trim the cost of the entire satellite network     

Information theoretic considerations for cellular mobile radio

We present some information-theoretic considerations used to determine upper bounds on the information rates that can be reliably transmitted over a two-ray propagation path mobile radio channel model, operating in a time division multiplex access (TDMA) regime, under given decoding delay constraints. The sense in which reliability is measured is addressed, and in the interesting eases where the decoding delay constraint plays a significant role, the maximal achievable rate (capacity), is specified in terms of capacity versus outage. In this case, no coding capacity in the strict Shannon sense exists. Simple schemes for time and space diversity are examined, and their potential benefits are illuminated from an information-theoretic stand point. In our presentation, we chose to specialize to the TDMA protocol for the sake of clarity and convenience. Our main arguments and results extend directly to certain variants of other multiple access protocols such as code division multiple access (CDMA) and frequency division multiple access (FDMA), provided that no fast feedback from the receiver to the transmitter is available     

Code division multiple access versus frequency division multipleaccess channel capacity in mobile satellite communication

A comparison between frequency-division multiple access (FDMA) and code-division multiple access (CDMA) when both methods operate in the mobile satellite communication environment is presented. The mobile satellites under consideration use multiple beams or scan beam antennas and employ frequency reuse of the allocated L-band frequency spectrum. Because CDMA can better absorb Doppler and multipath effects and permits higher rate coding, it appears in general, with practical considerations set aside, to be the more capable system     

Space-Time/Frequency Coding for MIMO-OFDM in Next Generation Broadband Wireless Systems

With the advent of next generation (4G) broadband wireless communications, the combination of multiple-input multiple-output (MIMO) wireless technology with orthogonal frequency division multiplexing (OFDM) has been recognized as one of the most promising techniques to support high data rate and high performance. In particular, coding over the space, time, and frequency domains provided by MIMO-OFDM will enable a much more reliable and robust transmission over the harsh wireless environment. In this article we provide an overview of space-time (ST) coding, space-frequency (SF) coding, and space-time-frequency (STF) coding for MIMO-OFDM systems. Performance results show that STF coding can achieve the maximum diversity gain in an end- to-end MIMO-OFDM system over broadband wireless channels. Furthermore, for orthogonal frequency division multiple access (OFDMA), we propose a multiuser SF coding scheme that can achieve the maximum diversity for each user while minimizing the interference introduced from all the other users.     

Design and analysis of synchronous dynamic frequency hopping code division multiple access communication system

An optical dynamic frequency hopping code division multiple access communication system is proposed. In this system, an electrically controlled tunable optical filter (TOF) is used to encode the modulated broadband light source. The code depends on the function set to the controller. Two-dimensional code, named functional code, is also proposed based of shifted sine function. The function defines the dynamic coding pattern of the central wavelength of the transmitted narrowband optical signal. Thus, the system will allow for an easy reconfiguration of the transmitter without the need for sophisticated encoder. At the receiver, a synchronized TOF with the same function is used as a decoder. The system is modeled and analyzed taking into account the multiple access interference, phase induced intensity noise, and thermal noise. The performance of this system is shown to be better compared with a fast frequency hopping system and a spectral amplitude coding system that uses either a Hadamard code, a modified quadratic congruence code (MQC), or a modified frequency hopping code (MFH).     

Performance analysis of a hybrid DS/SFH CDMA system usinganalytical and measured pico cellular channels

This paper presents the throughput and delay analysis of a packet-switched code division multiple access (CDMA) network based on the hybrid direct sequence (DS)/slow frequency hopping (SFH) spread-spectrum multiple access (SS MA) technique with Q-, B-, and D-PSK modulation using analytical and measured pico cellular channels. The performance of the hybrid DS/SFH, DS, and SFH multiple access techniques have been compared in a pico cellular personal communications network (PCN) environment. Multipath and multiple access interference are considered. The performance is evaluated for a given delay spread and a fixed bandwidth. The effects of forward error correction (FEC) coding and diversity techniques, such as selection diversity and maximal ratio combining on the performance, are also investigated     

On-chip error correcting techniques for new-generation flash memories

In new-generation flash memories, issues such as disturbs and data retention become more and more critical as a consequence of reduced cell size and decreased oxide thickness. Furthermore, the progressive increase in the cell count within a single die tends to decrease device reliability. In particular, reliability issues turn out to be more critical in multilevel (ML) flash memories, due to the reduced spacing between adjacent programmed levels. It is therefore deemed that the use of on-chip error correction codes (ECCs) will gain widespread acceptance in large-capacity flash memories. ECCs for flash memories must have very fast and compact encoding/decoding circuitry so as to have a minimum impact on memory access time. The area penalty due to check cells must also be minimized. Moreover, specific codes must be developed for ML storage. This paper presents error control coding techniques and schemes for new-generation flash memories, focusing on ML devices. The basic concepts of error control coding are reviewed, and the on-chip ECC design procedure is analyzed. Dedicated codes such as polyvalent ECCs, able to correct data stored in ML memories working at a variable number of bits per cell, and bit-layer organized ECCs are described.     

自适应调制与编码系统及其在WCDMA中的应用

本文主要讨论自适应调制与编码系统(Adaptive Modulation and Coding system）的原理，介绍AMCS实现的几个关键技术，包括RCPT（速率适配凿孔turbo）码、高阶调制、H-ARQ和MIMO（多输入多输出）系统。介绍了在WCDMA的高速下行分组接入业务（HSDPA）中使用的自适应调制编码机制，并讨论了可能增加的设备复杂度。     

Time division multiple access methods for wireless personalcommunications

Time division multiple access (TDMA) is a classic approach to multiple access in digital cellular wireless communications systems. The authors summarize a number of frequency and time slot allocation techniques for enhancing the capacity and flexibility of TDMA-based systems. They also describe how the problems of fading, delay spread, time variability and interference affect TDMA systems, and how they may he countered and even exploited by appropriate techniques of detection, diversity, coding, adaptive equalization and slow frequency hopping (FH). It is worth emphasizing that the use of one of these techniques, slow random FH, results in a system that is in effect a hybrid of TDMA and code division multiple access (CDMA)     

CSMA local radio networks with BPSK modulation in Rayleigh fading channels

Calculation of the throughput of a carrier sense multiple access (CSMA) packet radio network with capture over slow Rayleigh-fading channels is presented. The effects of capture on the throughput of the system are related to the modulation and coding technique of the transmission system, the general distribution of terminals in the area and the length of the transmitted packets. BPSK modulation and BCH coding are considered.<>     

OFDMA及其在IEEE802．16-2005中的应用

正交频分多址（OFDMA）是以正交频分复用（OFDM）调制为基础的新一代多址技术,具有很高的带宽利用率。文章介绍了它的基本原理,然后简单介绍了它在移动宽带无线接入中的应用,主要是关于信道编码,链路自适应技术,多天线技术。     

Optimization of satellite access lower layers for the transport of IP datagrams

This paper presents a novel solution optimizing satellite access lower layers and system performances called variable size packets (VSPs) designed for the transport of Internet protocol (IP) datagrams or any packet type from the network layers characterized by a highly variable size. VSP objective is to maximize the efficiency of layers 1 and 2 through a joint optimization of access and transport mechanisms mainly for satellite systems addressing residential or corporate terminals market through a multiple-access scheme. It is particularly attractive for the return link of next-generation satellite systems to be IP-based, as a possible evolution of the digital video broadcast-return channel via satellite (DVB-RCS) standard. The VSP solution relies on the definition of multiple encapsulation sizes, and is associated to a waveform and an access adapted to variable size packets' transport. It uses the traffic's variable size nature to increase encapsulation efficiency, coding, and spectral efficiency performances, and system flexibility. It is well adapted to systems using an adaptive waveform leading to system capacity's increase and better performances. This paper describes the VSP principle, advantages and possible high-level system tradeoffs. It then focuses on one critical issue, the coding, and presents a suitable coding scheme's choice with regards to performances and implementation complexity.