EQUALIZATION IN FILTER BANK BASED MULTICARRIER SYSTEMS Ari Viholainen, Juuso Alhava, Janne Helenius, Jukka Rinne, and Markku Renfors Telecommunications Laboratory, Tampere University of Technology P.O. Box 553, FIN-33101 Tampere, Finland, Email: mr@cs.tut.fi ABSTRACT In this paper we consider filter bank based multicar- rier systems, which provide better spectral shaping than DFT-based Orthogonal Frequency Division Multiplex- ing (OFDM) or Discrete MultiTone (DMT) systems for the subchannels, as well as for the overall signal. Due to their better performance in case of narrowband in- terference, such techniques have received attention es- pecially in the context of Very high-speed Digital Sub- scriber Line (VDSL) system development. However, the channel equalization for such systems is still an open problem. In this paper, it is demonstrated that filter bank based multicarrier systems are very sensitive to the non- linear phase response of the channel. Alternative ways to perform the channel equalization are discussed. Also measurement results from real subscriber lines are pre- sented and, based on these results, a two-step equaliza- tion approach is proposed. 1. INTRODUCTION Multicarrier modulation techniques have received wide attention in the context of digital subscriber line system development. In multicarrier modulation, the available channel bandwidth is subdivided into a number of narrow subchannels, which are partly overlapping in spectrally efficient systems. The order of utilized mod- ulation for each subchannel depends on the Signal-to- Noise Ratio (SNR) of the corresponding subchannel. In this way the transmission capacity of the transmission medium, twisted pair, can be utilized efficiently. In prac- tice, multicarrier modulation has been implemented us- ing Discrete Fourier Transform (DFT). A conventionalDFT-based multicarrier system, DMT, has been adopted for Asymmetric Digital Subscriber Line (ADSL) standards [1]. However, DMT is rather sensitive to narrowband interferences due to the large sidelobes of the DFT. Therefore, so-called Discrete Wavelet MultiTone (DWMT) technique has been pro- posed for VDSL standardization [2]. Both the DMT and DWMT can be considered as filter bank based transmul- tiplexer (TMUX) system. In DWMT, the idea is to make the filter bank more selective in order to limit the effect of narrowband interference to the subchannels which are in the frequency range of the interference. In a previous paper [3], we presented efficient solu- tions for VDSL systems based on the idea of Perfect- Reconstruction (PR) Cosine-Modulated Filter Banks 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 -80 -60 -40 -20 0 20 H 0 (e jω ), F 0 (e jω ) H 1 (e jω ), F 1 (e jω ) H 2 (e jω ), F 2 (e jω ) H 3 (e jω ), F 3 (e jω ) 0 20 40 60 -0.2 0 0.2 0.4 h 0 (n) * f 0 (n) 0 20 40 60 -0.2 0 0.2 0.4 h 1 (n) * f 1 (n) 0 20 40 60 -0.2 0 0.2 0.4 h 2 (n) * f 2 (n) 0 20 40 60 -0.2 0 0.2 0.4 h 3 (n) * f 3 (n) Fig. 1: Amplitude and overall impulse responses for a cosine-modulated transmultiplexer. (CMFBs). In this paper, we analyze the differences be- tween CMFB-based multicarrier systems and DFT-based multicarrier systems (OFDM, DMT) from the channel equalization point of view. A scheme based on phase equalization of the received signal before the receiver (analysis) filter bank is proposed. 2. MULTICARRIER SYSTEMS BASED ON COSINE-MODULATED FILTER BANKS 2.1 Cosine-Modulated Transmultiplexer Systems Conventional filter bank based TMUX system is de- scribed more detailed in these proceedings [4]. More- over, cosine-modulated TMUX approach together with further references are presented in [4]. As an example, Fig. 1 shows the amplitude responses of the subchannel filters for a 4-channel CMFB, as well as the overall im- pulse responses for the subchannels. 2.2 Data Modulation and Transmultiplexers When CMFBs are used for data transmission in the TMUX configuration, each subchannel in the transmitter end takes f s /M real symbols per second resulting in the total symbol rate of f s . In the modulation domain, each subchannel has a bandwidth of (1 + ρ)f s /(2M ) and the subchannel spacing is f s /(2M ). In [3] we have shown that the subchannel signals can be considered to be offset-QAM type of signals.