Abstract
Modern and future wireless communication systems such as UMTS and beyond 3G systems (B3G) are expected to support very high data rates to/from mobile users. This poses important challenges on the handset design as they should be able to attain an acceptable operating bit error rate (BER) while employing a limited set of resources (i.e. low complexity, low power) and often, with tight processing delay constraints. In this chapter we study how channel decoding and equalisation, two widely used mechanisms to combat the deleterious channel effects, can be made adaptable in accordance to the instantaneous operating environment. Simulation results are given demonstrating how receiver reconfigurability is a promising method to achieve complexity/delay efficient receivers while maintaining prescribed quality of service (QoS) constraints.
TopEfficient 3G Turbo Decoder In Indoor/Low Range Outdoor Environment
Turbo Codes
Turbo codes (Berrou, 1996) represent a powerful channel coding technique. Universal Mobile Telecommunications System (UMTS) belongs to the third generation (3G) of mobile communication systems. Turbo codes have been incorporated as a channel coding scheme in UMTS for data rates higher or equal to 28.8 kbps (Holma, 2000). They also provide high coding gains in flat fading channels with the use of outer block interleaving (Woodard, 2000; Hall, 1998). Soft-input/soft-output (SISO) decoder is part of a turbo decoder and two candidate algorithms to be used in a SISO decoder are soft output Viterbi algorithm (SOVA) and log maximum a-posteriori (log-MAP) algorithm (Woodard, 2000; Robertson, 1995; Hagenauer, 1989; Pietrobon, 1998).
Dynamic reconfiguration in cellular networks is a popular topic in published literature (Kotsopoulos, 1991; Kotsopoulos, 1992), while different propagation (mobile channel) issues, tested by experimental procedures, have been presented in Bouzouki (2001), Ioannou (2004) and Kitsios (2005). A reconfigurable turbo decoder can be derived according to the common operations of the two algorithms, optimal in terms of performance and latency (Chaikalis, 2003; Chaikalis, 2004; Chaikalis, in press). We consider just SOVA and log-MAP and not other turbo decoding algorithms like max-log-MAP or MAP, because SOVA is better in terms of delay, while log-MAP is better in terms of performance (Woodard, 2000; Robertson, 1995).
Key Terms in this Chapter
Soft Output Viterbi Algorithm (SOVA): A modified version of well known Viterbi algorithm, which is used for turbo decoding. Generally is less powerful than log-MAP, but less complex.
Turbo Decoder: In the communications literature a turbo decoder represents an iterative procedure used at the receiver to compensate the channel effects.
Adaptive Algorithm: Algorithm used to drive the coefficients of an adaptive filter (such as an adaptive equaliser) to a set of values minimising some prescribed error function.
Adaptive Equaliser: Digital equaliser whose coefficients can vary in response to variations of the channel.
Least Mean Squares (LMS): A computationally simple yet powerful adaptive algorithm based on gradient descent.
Log Maximum A Posteriori (log-MAP): A simplified powerful version of well known MAP algorithm, which is used for turbo decoding and is based on a posteriori probabilities.
Reconfigurable Equaliser: Equaliser whose structural properties like for example its length, can vary in response to changes in the environment conditions.
Universal Mobile Telecommunications Standard (UMTS): A high data rate 3G mobile communications standard already available in market.
Reconfigurable Turbo Decoder: Turbo decoder whose structural properties like for example its decoding algorithm, can vary according to the required quality of service.
Equaliser: In the communications literature, an equaliser is a filter (fixed or adaptive) used at the receiver to compensate the channel effects.