Iterative channel estimation and decoding for convolutionally coded anti-jam FH signals

Department of Electrical and Computer Engineering, The Ohio State University, Columbus, Ohio, United States
IEEE Transactions on Communications (Impact Factor: 1.99). 03/2002; 50(2):321 - 331. DOI: 10.1109/26.983327
Source: IEEE Xplore


An iterative algorithm for joint decoding and channel estimation
in frequency-hopping (FH) networks is proposed. In the proposed
algorithm, soft decoder outputs are used in the iterative estimation of
the time-varying variance of the additive interference resulting from
the sum of the thermal noise, partial-band noise jamming, and other-user
interference. The soft outputs are also used in the estimation of the
independent random carrier phases and multiplicative Rayleigh fading
coefficients in different frequency dwells. The estimation process is
further enhanced through the insertion of known symbols in the
transmitted data stream. The proposed iterative symbol-aided
demodulation scheme is compared with the coherent scenario, where the
channel state information is assumed to be known a priori at the
receiver, for both convolutionally coded and turbo coded FH systems. The
proposed iterative channel estimation approach is suited for slow FH
systems where the channel dynamics are much slower than the hopping
rate. This observation motivates the consideration of another robust
approach for generating the log-likelihood ratios for fast hopping
systems in additive white Gaussian noise channels. Simulation results
that demonstrate the excellent performance of the proposed algorithms in
various scenarios are also presented

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Available from: Hesham El Gamal, Mar 26, 2015
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    • "For convenience, we call this LLR calculation the genie LLR since interference statistics are known. When the receiver does not know ς 2 k , a robust method was proposed in [4] to obtain the prior LLRs. The maximumlikelihood (ML) estimate of ς 2 k for P(y k |b i = 0, ˆ H k ) is used on the numerator in (5), and that for P(y k |b i = 1, ˆ H k ) is used on the denominator in (5). "
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    ABSTRACT: Orthogonal frequency division multiplexing (OFDM) systems are vulnerable to narrow-band jamming signals. We jointly tackle two problems: channel estimation in the presence of unknown interference, and decoding with imperfect channel knowledge. In this paper, we propose robust, yet simple, receiver algorithms consisting of both channel estimation and information decoding. The receiver conducts threshold tests to detect interference followed by pilot erasure and channel estimation. Then, channel estimation error and unknown interference statistics are dealt with by the robust log-likelihood ratio (LLR) calculations for soft iterative decoding. The proposed receiver algorithm does not require any statistical knowledge of interference and its complexity is linear against the length of codewords. Simulation results show that the bit-error-rate (BER) performance of the proposed system is only 2~3 dB away from a genie system where channel information and interference parameters are perfectly known. We also demonstrate that soft decision feedback from a decoder to enhance channel estimation achieves additional 0.5 ~ 1dB improvement.
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    • "The effect of jamming on the information capacity in wideband regime was studied in [13] and [14], and their results showed the benefit of having impulsive training that randomly changes its position in a transmission block. While the studies in [10]–[14] focused on the design from the legitimate user's point of view, very few existing results look at the design of smart jamming in trainingbased transmissions. Recently, jamming strategies, which make use of the legitimate user's CSI, were proposed to attack the channel estimation in singular-value-decomposition-based MIMO systems, as well as systems using space-time block codes [15]. "
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    ABSTRACT: We consider training-based transmissions over multiple-input-multiple-output (MIMO) fading channels in the presence of jamming. Each transmission block consists of a training phase and a data transmission phase. From an information-theoretic viewpoint, we study the optimal energy allocation between the two phases for both the legitimate user of the channel and the jammer. For a fixed jamming strategy, we derive a closed-form solution of the optimal transmit energy allocation for the legitimate user and show that the optimal training length is equal to the number of transmit antennas. On the other hand, if the jammer has optimized its strategy, the best choice for the training length is shown to be larger than the number of transmit antennas and approaches half of the block length at low signal-to-jamming-and-noise ratio (SJNR). From the jammer's perspective, we derive closed-form solutions of the optimal jamming energy allocation. Numerical results demonstrate 30%-50% performance gains by using optimal designs in various scenarios. We also model the energy allocation problem as a zero-sum game and prove the existence and uniqueness of the Nash equilibrium when the training length is fixed. Furthermore, we extend our analysis to the case where the channel state information (CSI) is available at the transmitter. We show that many results found for systems with no transmitter CSI are also valid for systems with full transmitter CSI.
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    • "Some other relevant studies undertaken in the literature on coded, coherent, or FHMA systems operating in jamming environments include [9], [27], [28]. In [9], taking advantage of the turbo processing principle, the authors developed an iterative scheme for joint decoding and channel estimation in convolutionally coded and turbo-coded FH networks in the presence of additive thermal noise, additive partial-band noise jamming (PBNJ), additive MAI, and multiplicative flat Rayleigh fading. Unintentional or intentional interference was identically modeled as Gaussian processes, with their variance being estimated at the receiver. "
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    ABSTRACT: In the first part of the paper, the capacity of a general multiuser code division multiple access (CDMA) jamming channel was analyzed for noncooperative and cooperative users in uplink and downlink static and Nakagami fading channels when the receiver has or lacks jammer state information. The results were applied to a unified channel model encompassing a variety of multiuser spread spectrum systems contaminated by jamming. It was found that the jammer should spread its energy evenly over all degrees of freedom in order to minimize the average capacity. In the second part of the paper, using a standard orthogonal frequency division multiplexing (OFDM) model, the performance of a coded version of a multicarrier frequency-hopping (MC-FH) CDMA system in static and Rayleigh fading jamming uplink channels is analyzed. The MC-FH system under study is a hybrid of the OFDM and frequency hopping concepts, and is also a practical example of the model developed in the first part. It is demonstrated that in the cases where the receiver knows the jammer state and the receiver lacks the jammer state, spreading and contracting the jamming power over the system bandwidth, respectively, will give rise to the worst performance for the communicators. Optimal decorrelator weights for the receiver soft outputs in different channels, valuable for practical system design, are also obtained.
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