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IFDMA for Uplink Mobile Radio Communication Systems

:	Alexander Arkhipov
:	IFDMA for Uplink Mobile Radio Communication Systems
:	Herbert Utz Verlag
:	9783831609628
:	1
:	CHF 26.60
:

:	Naturwissenschaft
:	English

:	157
:	DRM
:	PC/MAC/eReader/Tablet
:	PDF

An application of interleaved frequency-division multiple-access (IFDMA) for mobile communications has become an active field of research since 1998. The special interest has been given to the uplink of mobile communication since low peak-to-average power ratio makes IFDMA attractive for hand-held mobile devices allowing an application of cheap amplifiers.
The IFDMA signal can be generated simply by compression and repetition of the transmit data sequence, without usage of fast Fourier transformation. Due to the periodic nulls in transmit spectrum, other users may utilize the same frequency band allowing multiple access.
This work contributes to the state of the art in the field of IFDMA in the following areas: data detection, PAPR reduction of multi-carrier systems, an estimation of frequency offsets in multicarrier uplink, where many users are simultaneously active.

Chapter 4 (S. 57-58)

Frequency Offset

4.1 Introduction

The system design of a multi-carrier uplink assumes that the distance between subcarriers of one user is larger than the coherent bandwidth of the transmission channel so that these subcarriers experience non-correlated fading [Kai98]. One of the advantages of IFDMA systems is that the subcarriers of one user are interleaved equidistantly, providing maximum frequency diversity for the received signal. Moreover, the orthogonality of subcarriers enables the BS to avoid MAI, since data symbols of different users are modulated on different subcarriers. However, these advantages come at the cost of sensitivity to frequency offsets. Multi-carrier systems with a block interleaved frequency allocation scheme especially suffer from the performance degradation due to frequency offsets [AS03].

In IFDMA uplink systems, frequency offsets are different for different users. These offsets occur because of a non-zero relative speed between the BS and MT or because of slightly disparate oscillator’s frequencies at the receiver and transmitter [Haa03, PSM94]. Since IFDMA is a special case of OFDMA, IFDMA inherits the sensitivity of OFDMA to frequency offsets. The performance of an OFDMA uplink in the presence of frequency offsets is studied in [TLP00, Ste00]. The frequency offsets cause MAI, which leads to the performance degradation. Different methods to combat MAI in an OFDMA uplink can be divided into two groups.

Theﬁrst group requires the knowledge of the frequency offsets for each user at the receiver. If the frequency offsets of each user are perfectly known at the BS, interference cancelation schemes can be applied [HL05]. Another possibility is to estimate the frequency offset of each user at the BS. These estimates can be used to correct the sampling time instances at the BS receiver or can be sent back to the MTs where they are used to pre-compensate for the frequency offsets before transmission.

A second group of techniques to combat MAI assumes special construction of the OFDMA transmit signal such that the frequency offsets cause a reduced amount of MAI. One promising technique is windowing at the receiver or transmitter, which, however, has been investigated only for the case of an OFDM downlink [Mus96, MW01, SL05, YH03]. Another technique employs a subband frequency allocation scheme, where data of a particular user is transmitted on a speciﬁc subset of adjacent subcarriers as described in [AS03, LH05].

Unfortunately, the techniques of the second group suffer from a number of drawbacks such as low frequency diversity– as in the case of an adjacent subcarrier allocation scheme– or low bandwidth efﬁciency– as in the case of windowing at the transmitter or receiver. Thus, the techniques which require frequency offset estimation at the BS seem to be only an alternative for a future IFDMA uplink. Generally, frequency offset estimation in the uplink of multi-carrier systems is a complicated task, since the signal received at the BS comprises the received signals from many users each of which can have its own frequency offset. Moreover, received signals from different users are distorted by different transmission channels, which complicates the issue. Several interesting research results for frequency offset estimation in OFDMA uplink have been presented in the literature.

	Foreword	8
	Contents	10
	Chapter 1 Introduction	14
	Unknown	1
	1.1 Fourth Generation of Wireless Communication Systems	18
	1.2 State of the Art in the Field of IFDMA	20
	1.3 Goals of the Thesis	23
	1.4 Contents and Important Results	24
	Chapter 2 Fundamentals of Uplink OFDMA	28
	2.1 Introduction	28
	2.2 Orthogonal Frequency-Division Multiplexing	29
	2.2.1 Time and Frequency Domain Properties of an OFDM signal	29
	2.2.2 Discrete-Time Signal Processing in OFDM Transmitter	34
	2.2.3 Multipath Channel Model	37
	2.2.4 Discrete-Time Signal Processing in OFDM Receiver	37
	2.2.5 Technical Assumptions	41
	2.2.6 Mathematical Notation	42
	2.3 Uplink OFDMA and Its Extensions	44
	2.3.1 Uplink Communications	44
	2.3.2 OFDMA	44
	2.3.3 OFDMA-CDM	46
	2.3.4 IFDMA	49
	2.3.5 MSK-IFDMA and GOQPSK-IFDMA	50
	2.3.6 Spectral Properties	53
	2.3.7 M-Modification	54
	2.3.8 Remarks on IFDMA system design	56
	Chapter 3 Peak-to-Average Power Ratio	58
	3.1 Introduction	58
	3.2 PAPR definition	59
	3.2.1 PAPR Upper Bound for IFDMA and OFDMA	59
	3.2.2 PAPR Distribution	62
	3.3 Simulation Results	62
	3.3.1 Simulation Parameters	62
	3.3.2 Results and Discussions	63
	Chapter 4 Frequency Offset	70
	4.1 Introduction	70
	4.2 Simulation Model	72
	4.3 Performance Degradation due to Frequency Offset	75
	4.4 MAI Reduction	77
	4.5 Frequency Offset Estimation in the Frequency Domain	80
	4.6 Frequency Offset Estimation in the Time Domain	83
	4.7 Performance of the IFDMA Uplink System	85
	4.7.1 Simulation Parameters	85
	4.7.2 System Performance	86
	4.7.3 Comparison with Existing Techniques	89
	Chapter 5 Receiver and Optimum MMSE Equalization	92
	5.1 Introduction	92
	5.2 IFDMA/OFDMA-CDM Receiver	93
	5.2.1 Linear Equalizers	94
	5.2.2 Despreading	96
	5.3 Self-Interference in Independent Rayleigh Channel	97
	5.4 MMSE Equalizer for OFDM System with Windowing	101
	Chapter 6 IFDMA Uplink System Evaluation	108
	6.1 Introduction	108
	6.2 System Design	109
	6.2.1 System Parameters	109
	6.2.2 Nonlinear Power Amplifier Model	110
	6.2.3 Out-Of-Band Radiation	112
	6.2.4 Scatter Plots	114
	6.2.5 BER Performance	117
	6.3 Simulation Results and Discussion	118
	Chapter 7 Abstract	124
	Appendix	126
	A.1 DFT Operation at the Receiver	126
	A.2 CFO Coefficients	130
	A.3 Statistical Properties of the Estimate	132
	Bibliography	134
	Abbreviations and Symbols	144
	Abbreviations	144
	Symbols	146
	Variables	147