as the Eureka 147 Digital Audio/Multimedia Broadcasting. (DAB/DMB) system. The iD4 technique improves the differential detection performance in fast fading ...
A NOVEL ITERATIVE DECISION-DIRECTED DIFFERENTIAL DETECTION TECHNIQUE FOR DIFFERENTIAL OFDM SYSTEMS Liang Zhang, Louis Thibault, Zhihong Hong Advanced Audio Systems, Broadcast Technology Communications Research Centre Canada Ottawa, ON, Canada Abstract—We propose a novel Iterative Decision-Directed Differential Detection (iD4) technique for Orthogonal Frequency Division Multiplexing (OFDM) systems employing Differential Phase Shift Keying (DPSK) modulation in each subcarrier, such as the Eureka 147 Digital Audio/Multimedia Broadcasting (DAB/DMB) system. The iD4 technique improves the differential detection performance in fast fading channels by estimating and compensating the time-domain Channel Phase Variation (CPV). Computer simulations show that significant performance improvement is achieved by using iD4 over Conventional Differential Detection (CDD), and the maximum Doppler spread, at which the OFDM receiver can achieve satisfactory reception, is greatly extended. Taking the DAB/DMB system operating in mode IV at L-band as an example, iD4 increases the maximum receiver vehicle speed from 95 kilometers per hour (km/h) to 190 km/h, in a Typical Urban (TU) channel environment. Keywords- OFDM, Decision feedback, differential detection,
π/4-DQPSK, fast fading, channel estimation
I.
INTRODUCTION
OFDM is an effective technology to provide high data rate wireless transmission because of its inherent robustness against multipath fading. It is used in many current wireless communication systems, including DAB/DMB systems, IEEE 802.11, 802.16 systems, Digital Video Broadcasting (DVB) and 3GPP Long Term Evolution (LTE) systems. The basic idea of OFDM is to divide a wideband frequency-selective channel into many narrowband nearly flat faded subchannels, each being characterized by a single complex multiplicative gain. For coherent OFDM systems, accurate estimate of these subchannel gains is critical for good signal detection. However, estimation of the channel gains requires both the transmission of pilot symbols and additional signal processing complexity, which is undesirable for many mobile applications, especially for handheld devices. To achieve simple receiver implementation, one solution is to use differential modulation to avoid the channel estimation. This is proposed in the Eureka 147 DAB/DMB OFDM system [1], where DPSK modulation is employed in each subcarrier. The simplicity of the receiver design of a differential system is obtained at the price of performance degradation.
Firstly, because of the noisy reference symbol (the previous received symbol), a conventional differential detector suffers from noise boosting in the decision metric as compared to a coherent detector with perfect channel estimate. In case of π/4DQPSK in AWGN channel, this results in an SNR loss of 2.2 dB. Secondly, for time-varying channels, the channel variation between adjacent symbols will introduce significant performance degradation. For OFDM systems, the subchannel bandwidth is usually very small, resulting in a long OFDM symbol interval. This could cause significant channel variation between adjacent OFDM symbols, which causes severe performance degradation. In addition, Inter-Carrier Interference (ICI) is introduced by OFDM due to the loss of orthogonality caused by the Doppler spread of the time-varying channels. Fast channel variation is usually caused by the motion of the receiver, and is usually proportional to the receiver speed. In [2], it is shown that the receiver speed is limited to 95 km/h for DAB transmission with mode IV at L-band, in a Typical Urban (TU) [3] channel environment. This suggests that satisfactory DAB service reception is not available for most vehicle receivers running on freeways. Therefore, better detection techniques need to be developed in order to extend this speed limit. The major contributions of this paper are: 1) we propose a new technique to improve the detection performance of receivers for differentially modulated OFDM systems in fast fading channels; 2) we also prove by simulation that using a 4tap Wiener filter can achieve near optimal performance with relatively low complexity. For the rest of this paper, DAB system is always assumed since it is the most popular differential OFDM system and it is being deployed in many countries worldwide. This paper is organized as follows: in Section II, we briefly go over the differential OFDM system model. Section III describes the limitation of an OFDM receiver with CDD. In Section IV, we present a novel iterative decision-directed differential detection (iD4) technique to improve the reception performance of DAB/DMB receivers in fast fading channels. Simulation results are reported in Section V. Finally, conclusions are drawn in Section VI.
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II.
DIFFERENTIAL OFDM SYSTEM MODEL
The block diagram of a DAB transmitter is plotted in Fig. 1. Service data is first encoded by a Rate-CompatibleConvolutional Code (RCPC) and passed through a convolutional time-domain interleaver. The encoded bit sequence is then mapped into a QPSK symbol sequence a, which is divided into K-symbol data blocks, K being the number of active subcarriers. After a rearrangement of the relative position of these K QPSK symbols in one block (frequency-domain interleaving), π/4-DQPSK encoding is performed on each subcarrier. A frequency-domain data vector (X) of length N is assembled with the K π/4-DQPSK symbols and (N−K) zeros, where N is the first power of 2 value which is larger than K. An length-N time-domain vector, x, is generated with an N-point IFFT operation as x ( n ) = IFFT { X ( k )} ,
n = 0,1, " N − 1
(1)
A cyclic prefix (CP) of length G is added to x to form a time-domain OFDM symbol of length N+G. The OFDM symbols are assembled into DAB frames, whose structure is shown in Fig. 2. It is shown that one DAB frame consists of a NULL symbol, followed by a Phase Reference Symbol (PRS) and multiple data OFDM symbols. The PRS provides the reference symbol for π/4−DQPSK encoding/decoding. III.
CHALLENGES FOR DIFFERENTIALLY MODULATED OFDM SYSTEMS IN FAST FADING CHANNELS
The received symbol on the kth subcarrier of the nth OFDM symbol after the OFDM demodulation can be expressed as, y n ( k ) = x n ( k ) ⋅ H n ( k ) + n I ( k ) + n0 ( k )
(2)
where xn(k) is the transmitted symbol, Hn(k) is the channel gain, nI(k) is the ICI and n0(k) is the AWGN noise. The simplest detection for π/4−DQPSK is the CDD, which is performed as
is assumed with a uniformed distribution of incoming signal incident angle over [0, 2π), the correlation is calculated as, R k ( Δ t ) := E ⎡⎣ H k ( t ) ⋅ H k* ( t + Δ t ) ⎤⎦ = J 0 ( 2π f d Δ t ) Δ t = Ts
(6)
= J 0 ( 2π f d T s ) where fd is the Doppler spread and Ts is the OFDM symbol duration. It shows that the correlation is determined by the fdTs, i.e. the Doppler spread normalized by the subchannel bandwidth (scaled by a small factor (1+G/N)).
For a mobile receiver, fd can be calculated as v (7) c where fc is the RF frequency, v is the vehicle speed and c is the speed of light. For a system with a given fc, the correlation between adjacent OFDM symbols is determined by the vehicle speed. The higher the vehicle speed, the smaller the correlation. Smaller correlation results in larger variance in the phase variation (θn,k), and therefore more significant performance degradation for the CDD. fd = fc ⋅
In Fig. 3, we report the performance of a DAB receiver employing CDD in fast fading channels using DAB mode IV at L-band (fc≅1.5 GHz). Significant performance degradation is observed when the receiver moves at high speed. The target coded BER requirement of 10−4 for adequate audio services can only be achieved for fdTs values less than 0.08, which corresponds to a maximum vehicle speed of 95 km/h. This suggests that most vehicles on freeways cannot obtain good DAB reception for CDD-based receivers. IV.
ITERATIVE DECISION-DIRECTED DIFFERENTIAL DETECTION
(5)
Multiple symbol differential detection [4] can be employed to improve the performance of the OFDM receiver in fast fading channels. It is however very complex to implement, especially when soft-decision is required for subsequent error correction. In [5], a decision-feedback differential detection technique is proposed to improve the performance of singlecarrier systems. Application of this technique to OFDM system is not straightforward. Furthermore, its additional complexity does not make it an attractive option either. A decision-directed coherent detection technique of MDPSK is proposed in [6] and is extended to OFDM systems in [7]. This technique, however, suffers significant error propagation due to the fact that one decision error would cause all the following feedback decisions to be in error.
(4) and (5) show the two sources of performance degradation suffered by DQPSK in fast fading channels: the phase variation between two adjacent symbols, θn,k, and the boosted noise power.
In this section, we describe a novel detection technique, labeled Iterative Decision-Directed Differential Detection (iD4), based on the estimation and compensation of the CPV term in (4). If the CPV is known to the receiver, performance of the differential detection could be greatly improved by performing phase compensation.
bn ( k ) = y n ( k ) ⋅ y n* − 1 ( k )
(3)
Taking (2) into (3), we have bn ( k ) ≈ a n ( k ) ⋅ H n ( k ) ⋅ H n* − 1 ( k ) e
jθ n , k
+ nn ( k )
(4)
th
where an an(k) is the QPSK data carried in the k subcarrier before the differential encoding, θn,k is the channel phase variation between the (n−1)th and nth symbol on the kth subchannel and nn(k) is the combined ICI and AWGN noise, which has a variance of, σ d2 ( k ) = H n ( k ) ⋅ σ n2− 1, k + H n − 1 ( k ) ⋅ σ n2, k 2
2 n,k
where σ
2
is the variance of nn(k) in (4).
The channel phase variation (CPV) is a random number whose variance increases with faster time-varying channels. It can be characterized by the correlation between the two adjacent channel gains, Hn(k) and Hn+1(k). When a TU channel
The block diagram of the iD4-based receiver is shown in Fig. 4. Assuming perfect synchronization in the DAB receiver, a time-domain sequence, y, is received for each DAB frame.
978-1-4244-4148-8/09/$25.00 ©2009 Crown This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2009 proceedings.
From the Ns OFDM symbols for each DAB frame, a CDD is first performed as (3). The receiver then performs the standard DAB data decoding process (deinterleaving and Viterbi decoding) to generate a first decision vector, a, of the data carried in this received DAB frame. This decision vector is used to regenerate estimates of the transmitted OFDM symbols, cˆn , through convolutional encoding, time-interleaving, QPSK modulation, frequency-interleaving and π/4−DQPSK encoding, where th th cˆn (k ) is the regenerated symbol in the k subcarrier of the n OFDM symbol in this DAB frame.
DFT-filtering is an efficient method to perform this lowpass filtering [8]. Following (12), a K-point DFT is performed over the CPV vector to obtain its cepstrum-domain response
η n = DFT {Δ n , Intp }
(13)
The LPF is realized by a windowing function in the cepstrum-domain as, ⎧
⎪ηˆ , ηn = ⎨ n
0≤n
