Implementation and Field Test of a New Channel Estimation ... - LRTS

Implementation and Field Test of a New Channel Estimation Technique for DVB-T System Sili Lu† , Assia Semmar† , Xianbin Wang‡ , Yiyan Wu‡ , Jean-Yves Chouinard† and Paul Fortier†

Abstract The European standard for digital terrestrial transmission, DVB-T, has been designed for fixed, portable and mobile reception. This versatility is due in part to Coded Orthogonal Frequency Division Multiplexing (COFDM). OFDM can provide large data rates with sufficient robustness to radio channel impairments. During the deployment of terrestrial DTV broadcasting systems, very often the channel characteristics have to be captured for the DTV coverage study, reception problem diagnosis and service planning. The software implementation of a new channel characterization technique using frequency domain pilot time domain correlation (FPTC) method for DVB-T systems is presented in this paper. This technique exploits the pilot sequence embedded in the DVB-T signal, which is derived from the frequency domain pilots and known to the receiver. The major advantages of this proposal, compared to other DTV channel characterization techniques, are its wide amplitude dynamic range, robustness to frequency synchronization errors, and avoidance of interruptions to the broadcasting service. Index Terms Channel estimation, OFDM, DVB-T I. I NTRODUCTION Orthogonal Frequency Division Multiplexing (OFDM) has been used in the DVB-T standard, in which a large number of orthogonal, overlapping, narrowband subchannels, or subcarriers, are transmitted in parallel over the available bandwidth[1][2]. The channel characterization of DVB-T system is often done in the frequency domain[3][4]. However, the channel information can only be reliable if the DTV receiver can successfully achieve reliable frequency and time synchronizations. Unfortunately, this is difficult under severe multipath conditions and strong co-channel and adjacent channel interferences. Another way for the † S. Lu, A. Semmar, J.-Y. Chouinard and P. Fortier are with the Department of Electrical and Computer Engineering, Laval University, Quebec, Quebec, Canada, G1K 7P4. Email: {kenlu,assia,chouinar,fortier}@gel.ulaval.ca ‡ X. Wang and Y. Wu are with the Communications Research Centre Canada, 3701 Carling Ave., Box. 11490, Station H, Ottawa, Ontario, Canada, K2H 8S2. Email: {xianbin.wang,yiyan.wu}@crc.ca

channel estimation is to send a training sequence occupying the same spectrum, but the regular broadcasting service has to be interrupted. These observations motivates the study of new in-service DTV channel characterization methods. A simple channel characterization technique using the frequency domain pilot time domain correlation (FPTC) method for DVB-T has been proposed by X. Wang et al. [5]. This new technique is based on the time domain correlation between the received signal and the time domain pilot sequence embedded in the OFDM signal, which is derived from the frequency domain pilots and known to the channel estimation device. It is shown that the time domain correlation function of the received signal and the pilot sequence can be approximated as the convolution between the channel impulse response and the time domain autocorrelation function of the pilot sequence. Channel impulse response can then be obtained. II. DVB-T R EFERENCE SIGNALS OVERVIEW In the DVB-T standard, the symbols in an OFDM frame contain data and reference information. Since the OFDM signal comprises many separately-modulated carriers, each symbol can in turn be considered to be divided into cells, each corresponding to the modulation carried on one carrier during one symbol. An OFDM frame contains the following reference signals: scattered pilot cells, continual pilot cells, and Transmission Parameter Signalling (TPS) carriers. The pilots can be used for synchronization and channel estimation. The value of the scattered or continual pilot information is derived from a PRBS (Pseudo Random Binary Sequence), one for each of the transmitted carriers. Reference information is transmitted in scattered pilot cells in every symbol. Scattered pilot cells are always transmitted at a “boosted” power level. In addition to the scattered pilots described above, 177 continual pilots in the DVB-T 8K mode are inserted according to a certain pattern. Here “continual” means that they occur on all symbols: a detailed list of carrier indices for continual pilot carriers is given in [6]. The continual pilots are all modulated according to the reference sequence and are also transmitted at the “boosted” power level.

  
   
          

III. FTPC I MPLEMENTATION P ROGRAM A. FPTC algorithm

Rrp (n) ≈ h(n)σp2 + w1 (n)

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where w1 (n) is the noise and interference and σp2 is the variance of the reference signal sp (t). The cross-correlation function of the reference signal, Rrp (n) can be used to estimate the channel impulse response h(n). A detailed mathematical description of the FPTC channel estimation method is presented in [5]. DE6FHG I  !" #

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The principle of the proposed time domain channel estimation mehtod is shown in Figure 1. At the transmitter, the complex data in the frequency domain can be divided into two different sets, the data symbol to be transmitted and the pilot symbols for channel synchronization and estimation. Therefore, the DVB symbol s(t) consists of two parts, sd (t) and sp (t), which are transformed from data set and pilot set, respectively. At the receiver, the received signal is passed through a correlator matched to the reference signal sp (t), and the output of the correlator is the estimated channel response. In [5], the correlator output is given by

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Principle of the proposed FPTC technique [5].

B. Channel estimation program The data analysis (channel estimation) program block chain is illustrated in Figure 2. The measurements provide digital recordings of DVB-T 8K mode signals. According to the standard, 1/12 of the OFDM subcarriers are used for pilots, which could be exploited for synchronization and channel estimation. The data analysis program performs the following tasks:

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