Reconfigurable Outer Block Interleaving over Correlated Rayleigh Fading for 3GPP Turbo Coding C. Chaikalis and J. M. Noras F. Riera-Palou University of Bradford Philips Research School of Engineering Prof. Holstlaan 4 - WY82 BD7 1DP, Bradford, UK 5656 AA Eindhoven, The Netherlands Abstract- Turbo coding in correlated Rayleigh fading channels is improved by outer block interleaving, but with added complexity and delay. Choosing the wrong number of columns can increase the BER, so a reconfigurable block interleaver with column number optimised for different operating environments is useful. Simulation results are presented for the four block interleaver configurations specified for the 3GPP mobile standard in the case of correlated Rayleigh fading. Using different data frame lengths in various scenarios, the advantages of adapting to an optimum number of columns are demonstrated. I. INTRODUCTION During the past few years, turbo codes have attracted considerable attention because of the large coding gains they can achieve in an additive white Gaussian noise (AWGN) channel [1]. For wireless communication systems, turbo codes have also been shown to provide impressive coding gains in fading channels. Especially in correlated Rayleigh fading channels, performance can be greatly improved if outer block interleaving is used [2], [3], [4]. Turbo encoding alone may not cope with the errors induced in a correlated fading channel. These tend to produce burst errors, whereas turbo codes are more effective with random errors, so outer block interleaving is essential. Furthermore, according to [2] perfect interleaving over a long period for correlated Rayleigh fading can approximate the BER performance of the uncorrelated Rayleigh fading channel. Turbo codes have been adopted as a channel coding scheme in a number of mobile systems, one being 3GPP (third generation partnership project) for high data rates. This paper considers the different outer block interleaver lengths specified in 3GPP, and proposes interleaver lengths which are optimum in terms of performance and complexity for correlated Rayleigh fading channels, validated by computer simulation. The terms "reconfiguration" and "software radio" are interrelated: the software radio concept is general, while the reconfiguration concept is more specific. Generally, and in terms of channel coding, two reconfiguration types can be defined: • Mobile standards reconfiguration • Single standard reconfiguration The first type involves common functionalities (i.e. channel coding blocks) of the radio transceiver between different mobile standards operating worldwide in order to achieve interoperability in a single terminal. The second type deals with reconfiguration within a single standard, which means the adjustment of the parameters of a radio transceiver according to the specific operating conditions and the required performance and complexity [5]. In this paper the second type is considered: the number of columns of the outer block interleaver-deinterleaver is adjusted according to the operating environment, the frame length and the corresponding BER and FER performance. II. OUTER BLOCK INTERLEAVING IN FADING ENVIRONMENT In [6] it is shown that the number of columns is the critical parameter in the design of outer block interleavers for turbo codes over correlated Rayleigh fading channels. The higher the mobile speed, the larger number of columns is needed [6]. In [7] a reconfigurable block interleaver for block codes over correlated Rayleigh fading is presented. A formula is derived to calculate the interleaving depth according to the Doppler frequency. It is also mentioned that the BER (Bit Error Rate) will be higher unless the right number of columns is employed. This is because burst errors (due to correlated Rayleigh fading channel) are spread into multiple uncorrectable error blocks destroying more bits, which otherwise would be correct. In this paper it will be shown that we draw the same conclusion for turbo codes. III. DATA TRANSFER IN 3GPP The way that the information is transferred over the radio interface from the MAC (Medium Access Control) sub-layer of layer 2 to the physical layer defines a transport channel [8], [9]. The characteristics of a transport channel are determined by its TF (transport format) or TF set, specifying the physical layer processing to be applied to the particular transport channel. Thus, data exchange between the MAC and the physical layer is defined in terms of transport block sets, that is the number of input bits in the turbo encoder. On a transport channel, one transport block set can be transmitted every TTI (time transmission interval). A transport block set consists of one or several transport blocks, and a TTI is defined as the maximum allowed time to transmit a transport block set. In other words, TF is a format offered by the physical layer to 0-7803-7700-1/03/$17.00 (C) 2003 IEEE 294