Low-latency polyphase filter scheme based on the IIR filter for the OFDM repeater

Abstract

Recently, the orthogonal frequency division multiplexing (OFDM) technique has been recommended as an access scheme for high-speed wireless broadband internet service due to its high-data-rate and robustness for multipath fading channels. When OFDM operates in the time division duplex (TDD) style, a guard interval is usually inserted so as to prevent the system from inter-symbol interference. Here, the length of the guard interval is nominally determined by observing the maximum delay spread. Due to the usage of the wireless repeater, the outage in this shadowed region can be avoide, and the service range can be extended. However, the problem to be resolved is shortening the processing time for relaying the data from the base station to the terminal or vice versa. Otherwise, the uplink and downlink slot can overlap either at the base station or the terminal, resulting in major performance degradation. Therefore, the primary concern in the realization of the wireless repeater is how to effectively reduce the processing time needed for down- and up-conversion of the signal to be relayed. With the TDD system, strict requirements relating to processing delay must be taken into account to avoid interference. This kind of interference results in outage because it causes the deterioration of signal quality. The primary concern is how to diminish the processing delay intrinsically occurring at the repeater so that the TDD system works properly. The conventional down and up-conversion processes are usually deployable to the repeater systems; these consist of multiple decimation/interpolation filters which cause delays. To reduce the delay without any major degradation in signal quality, IIR filter could be used instead of FIR filter. For the stable IIR filter, the filter design method using the balanced reduction scheme (BRS) introduced not only sustains the stability, but also reveals good approximation in the magnitude as well as the linear-like phase responses associated with a prototype FIR filter. Due to the inherently large amount of group delay representing system delay, it is not suitable for the low-latency repeater focused on in this paper. We adopted composed of concatenating IIR filter and allpass filter (CIAF) to each branch filter of polyphase filter (PF). The magnitude response of each branch fi Iter of PF may be identical to that resulting from the original branch filter. In our approach, the constraint on the design of each IIR filter in CIAF lies whether the stability is guaranteed. To accomplish this, referring to [5], the design of a minimum-phase IIR filter is utilized. This feature is quite preferable in the sense of minimizing overall processing delay for our applications. In order to resolve phase distortion from the nonlinearity of the phase response of magnitude response shaping filter, phase response equalization filter (PREF) is a filter used for making the overall phase response nearly linear. To achieve PREF, we employed an all-pass filter whose coefficients are updated by the complex least mean squares algorithm. The performances of the proposed method had been compared with the system requirements according to KT's Wibro repeater development specifications and BRS. According to performance results, the system delay of proposed method overwhelms one of the BRS. Even though error vector magnitude (EVM) and adjacent channel leakage ratio (ACLR) of proposed method are inferior to those of BRS, they might be suitable because the corresponding EVM and ACLR obey the requirements. In conclusion, our proposed method is quite appropriate for delay-sensitive repeaters and can be applicable to any system required to have low-latency.