IFDMA for Uplink Mobile Radio Communication Systems

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.

The first 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.