Ifdma - a Promising Multiple Access Scheme for Future Mobile Radio Systems.

IFDMA - A Promising Multiple

Access Scheme for Future Mobile Radio Systems

R. Pon arun kumar.

Abstract

The Interleaved Frequency Division Multiple Access (IFDMA) scheme is based on compression, repetition and subsequent user dependent frequency shift of a modulated signal. IFDMA is a promising candidate MA scheme for non adaptive transmission in the uplink of future mobile radio systems.

In this paper it is shown that IFDMA can be regarded as unitary precoded OFDMA with interleaved subcarriers. It combines the advantages of single and multi carrier transmission such as low peak to average power ratio, orthogonality of the signals of different users even for transmission over a time dispersive channel. It is of low complexity for user separation and channel equalization that doesn't require any channel state information at the transmitter. It combines the advantages of both CDMA and OFDMA.

I. INTRODUCTION

Currently, research on beyond 3rd and 4th generation

(B3G/4G) mobile radio systems is in progress worldwide. A future mobile radio system will provide packet oriented data services carrying multi-media contents. Thus, on the one hand, properties like high spectral efficiency as well as high flexibility and granularity in terms of different data rates from

a few kbit/s up to several Mbit/s are essential. On the other hand, low cost implementation and high power efficiency are important, especially for mobile terminals. Moreover, high user mobility has to be taken into account. The choice of the multiple access (MA) scheme has a great impact on the achievable features of a future cellular mobile radio system. The candidate MA schemes can be classified in single carrier based and multi carrier based MA schemes. Prominent representatives of single carrier based MA schemes are Time Division Multiple Access (TDMA), which is used in 2nd generation (2G) mobile radio systems like the Global System for Mobile Communications (GSM), and Direct Sequence Code Division Multiple Access (DS-CDMA), which is used in 2G mobile radio system like IS-95 as well as in 3rd generation (3G) mobile radio systems like IMT- 2000/UMTS. Typically, single carrier based MA schemes provide low complexity for signal generation and, for the uplink, low envelope fluctuations of the transmit signal. DSCDMA and TDMA provide high frequency diversity since a large bandwidth is used for transmission. However, especially for high data rates, single carrier based MA schemes suffer from high computational complexity at the receiver for channel equalization or, in case of DS-CDMA, user separation. Recently, also multi carrier based MA schemes have been receiving wide interest. The most prominent multi carrier scheme, Orthogonal Frequency Division Multiplexing (OFDM), and its MA derivate Orthogonal Frequency Division

Multiple Access (OFDMA), are currently used, e.g., in WLAN standards HIPERLAN/2 and IEEE802.11a, and they are also key technologies of IEEE802.16e (WIMAX) and 3GPP LTE. Moreover, they are considered as promising schemes, e.g., in the European Union research project WINNER . For multi carrier based MA schemes like OFDMA, Inter-Symbol Interference (ISI) can be avoided by application of a guard interval and orthogonality of different users' signals is maintained even for transmission over time dispersive channels. Thus, low computational effort at the receiver for compensation of the channel and for user separation is achieved which enables high data rate transmission with acceptable computational effort. Moreover, OFDMA provides low computational complexity due to efficient implementation using the Fast Fourier Transform (FFT) algorithm and high spectral efficiency due to the concept of overlapping but mutually orthogonal narrowband subcarriers. However, multi carrier based MA schemes suffer from high envelope fluctuations of the transmit signals. Thus, expensive linear power amplifiers are required which, especially for the uplink, results in undesirable additional costs for the mobile terminals. Moreover, due to the high envelope fluctuations, a power back-off is necessary which reduces the power efficiency of the amplifier and results in a waste of battery power. A reduction of the envelope fluctuations can be obtained by techniques like clipping, windowing or coding at the expense of additional implementation effort, out of band radiation or additional overhead. Further interesting MA schemes can be obtained from the combination of techniques from single and multi carrier transmission. One concept is the combination of multi carrier modulation and spread spectrum techniques that results in Multi-Carrier CDMA (MC-CDMA). Another concept is the application of block transmission with cyclic prefix (CP), which is common for OFDM(A), also to single carrier based MA schemes. It enables the combination of properties of multi carrier based MA schemes like low complexity for user separation and channel equalization and properties of single carrier based MA schemes like low envelope fluctuations. In the remainder of this article, in a first step, different multiple access schemes based on a combination of techniques from single and multi carrier transmission are briefly summarized and classified using a new framework. In a second step, a promising MA scheme which entirely combines the

TABLE I: Overview over block transmission based multiple access schemes

advantages of CDMA and OFDMA is discussed in detail. The scheme has been introduced in as Interleaved Frequency Division Multiple Access (IFDMA). Throughout this article, IFDMA is derived from a CDMA perspective as well as from an OFDMA perspective and the properties of the scheme and its applicability for B3G mobile radio systems are discussed.

II. INTEGRATION OF SINGLE CARRIER AND MULTICARRIER CHARACTERISTICS

Since both, single and multi carrier based MA schemes, provide interesting advantages, the integration of characteristics of both is receiving more and more interest for investigation towards B3G/4G mobile radio systems. The schemes discussed throughout this section which are all based on block transmission with CP, are classified using a new framework. It is assumed that, in general, the schemes can