We study a problem of covert communication over binary symmetric channels (BSC) in an asynchronous setup. Here, Alice seeks to communicate to Bob over a BSC while trying to be covert with respect to Willie, who observes any communication through possibly a different BSC. When Alice communicates, she transmits a message (using a codeword of length n) at a random time uniformly distributed in a window of size Awslots. We assume that Bob has side information about the time of transmission leading to a reduced uncertainty of Abslots for Bob, where $A_{b}\lt A_{w}$. In this setup, we seek to characterize the limits of covert communication as a function of the timing advantage. When Awis increasing exponentially in n, we characterize the covert capacity as a function of Awand Ab. When Awis increasing sub-exponentially in n, we characterize lower and upper bounds on achievable covert bits and show that positive covert rates are not feasible irrespective of timing advantage. Using numerical work, we illustrate our results for different network scenarios, and also highlight a tradeoff between timing advantage and channel advantage (between Bob and Willie).

It is well known that modern battlefields are subject to intense jamming by enemy forces to deny use of the electromagnetic spectrum. To combat jamming, the Line of Sight (LoS) wireless communication is typically employs Frequency Hopping (FH) technology. The FH radio links are invariably duplex in nature employing either Time Division Duplexing (TDD) or Frequency Division Duplexing (FDD) techniques. The choice of appropriate duplexing technique has a direct bearing on resilience to combat jamming (and hence resultant effective throughput) as well as impacts achievable ranges. This paper examines the impact of the choice of duplexing method under these conditions. In particular we examine the Jammer to Signal (J/S) strength required to disrupt the Radio link under each of these duplexing schemes. We also provide useful insight into maximum throughput achievable for various FEC schemes under jamming conditions.

We study one shot frame synchronization in a discrete memoryless channel where, a sync frame of known length Nvsymbols is transmitted at a random time v. For a random misalignment 7 between the transmitter clock and the receiver clock, we seek to characterize the sync frame (length) necessary for error-free frame synchronization. We assume that the random times v and 7 are independent and have a general distribution. Using a variable length sync word and a multi-stage sequential decoder for the setup, we derive necessary and sufficient scaling of the average sync frame length for asymptotic error-free frame synchronization. In particular, we show that the average sync frame length must scale with the entropy of v and/or logarithm of cardinality of the sample space of 7. We illustrate our results for the AWGN channel and also derive the scaling needed of the sync frame energy for the asynchronous setup.

We study the problem of frame synchronization over a discrete memoryless channel (DMC) in an asynchronous setup. A sync frame is transmitted at a random time V with a known distribution {αv} and entropy H. We seek to characterize the minimum average length or energy of the sync frame necessary for error-free frame synchronization, as H tends to infinity. We present a variable length sync frame, where the length of the sync frame is adapted based on H and {αv}, for the general arrival distribution and show error-free frame synchronization when the average sync frame length Ñ scales as Ω((H/α(Q)), where α(Q) is the synchronization threshold of the DMC. We then generalize the framework and study a tradeoff between Ñ and α(Q) for optimal frame synchronization and characterize the scaling needed of both Ñ and α(Q) with H. We illustrate our results with the AWGN channel and discuss the adapting sync frame length and symbol power for optimal frame synchronization. Finally, using numerical work and simulations, we evaluate the results under relaxed assumptions, including the imperfect knowledge of arrival distribution and symbol timing error.

We study a problem of sequential frame detection over a finite state Markov channel (FSMC). We consider an asynchronous framework where a sync frame of length N symbols is transmitted uniformly over a large interval of known size A slots. In this setup, we study the scaling needed of the sync frame length N with the asynchronism interval length A for error-free frame synchronization. We study the problem when channel state information (CSI) is known at the transmitter and the receiver, and compute a synchronization threshold, α, that relates the average sync frame length N and A as N > log2(A)/α for asymptotic frame synchronization. Our discussion includes the description of a variable length and adaptive code word for FSMC that achieves the optimal delay performance.

A cognitive radio network (CRN) with a cooperative spectrum sensing scheme is considered. This CRN has a primary user and multiple secondary users, some of which are malicious secondary users (MSUs). Energy detection at each SU is performed using a p -norm detector with p≥2 , where p=2 corresponds to the standard energy detector. The MSUs are capable of perpetrating spectrum sensing data falsification (SSDF) attacks. At the fusion center (FC), an algorithm is used to suppress these MSUs which could be either an adaptive weighing algorithm or one of the following: Tietjen-Moore (TM) test or Peirce’s criterion. This is followed by computation of a test statistic (TS) which is a random variable. In this paper, we assume TS to have either a Gamma or a Gaussian distribution and calculate the threshold accordingly. We provide closed-form expressions of probability of false alarm and probability of miss-detection under both assumptions. We show that Gaussian assumption of TS is more suited in presence of an SSDF attack when compared with the Gamma assumption. We also compare the detection performance for various values of p and show that p=3 along with the Gaussian assumption is the best amongst all the cases considered.

We consider the problem of detecting the presence of a spatially correlated multichannel signal corrupted by additive Gaussian noise (i.i.d across sensors). No prior knowledge is assumed about the system parameters such as the noise variance, number of sources and correlation among signals. The non-parametric detection statistics were formed based on the statistical covariances obtained through Bartlett decomposition of sample covariance matrix. They are designed such that the detection performance is immune to the uncertainty in the knowledge of noise variance. The analysis presented verifies the invariability of threshold value and identifies a few specific scenarios where the proposed statistics have better performance compared to generalised likelihood ratio test (GLRT) statistics. The sensitivity of the statistic to correlation among streams, number of sources and sample size at low signal to noise ratio are discussed.

We address the problem of designing a communication system for deploying under disaster situations where the pre-disaster communication infrastructure has been uprooted. The disaster communication system is required to aid in rescue effort by providing text, voice, and video communication links between the field worker and the operation center. It is also required to provide some communication services to the affected citizens so that the overwhelming demands made on the communication infrastructure during the times of disaster are mitigated to some extent. The other highlights of the design are its low-cost nature by exploiting the economies of scale and the fact that basic mobile phones with text-only features are widely available in developing countries.

Multiple antennas are widely used in modern high speed wireless data communication systems. Though multiple antennas provide diversity as well as multiplexing gain, there is a fundamental tradeoff between the two under slow fading channel conditions, that any coding scheme can achieve. This is captured in the diversity multiplexing tradeoff (DMT) framework at asymptotically high signal to noise ratios (SNR). The framework has been further generalized for the non-asymptotic i.e finite SNR case. In this paper we analyze the DMT for the rate-adaptive SIMO (single input multiple output) receive diversity schemes, namely SIMO-MRC (maximal-ratio combining), SIMO-SC (selection combining) and SIMO-EGC (equal gain combining). We obtain closed form expressions of the DMT at finite SNR. These expressions enable comparison of performance of the above schemes at finite SNR. The results of this comparative study are presented here.

The exact time-of-arrival (TOA) information of the multi-path signals is crucial for optimal channel estimation in vehicular OFDM systems. Super-resolution algorithms such as ESPRIT (Estimation of Signal Parameters via Rotational Invariance Technique) have been applied to retrieve this information from embedded pilots in the received OFDM symbols. These algorithms have high applicability for vehicular wireless environs characterized by high Doppler frequencies which leads to faster auto-correlation averaging. In this paper, we propose to improve upon the classical ESPRIT algorithm by incorporating the iterative reduced rank Hankel approximation (RRHA) technique. We show that this allows achieving of a lower MSE (mean square error) compared to the classical ESPRIT algorithm. Further, we portray that the degree of improvement brought forth increases with the increased number of RRHA-iterations.

This paper addresses the issue of peak-to-average power ratio (PAPR) reduction in multiple-input and single-output systems with a number of power allocation schemes. We propose a constellation-based power allocation scheme that is capable of achieving a lower PAPR when compared to the optimal power allocation scheme. Although this method of power allocation improves the PAPR performance, there is a slight degradation in bit error rate (BER) performance. However, the degradation in BER performance can be compensated through the use of co-ordinate interleaving technique. We also propose a constrained-optimal power allocation scheme with which it is possible to control the PAPR

Orthogonal space-time block codes (OSTBCs) yield full diversity gain even while requiring only a linear receiver. Such full-rate (rate-one) orthogonal designs are available for complex symbol constellations only for N=2 transmit antennas. In this paper, we propose a new family of full-rate space-time block codes (STBCs) using a single parameter feedback for communication over Rayleigh fading channels for N=3,4 transmit antennas and M receive antennas. The proposed rate-one codes achieve full diversity, and the performance is similar to maximum receiver ratio combining. The decoding complexity of these codes are only linear even while performing maximum-likelihood decoding. The partial channel information is a real phase parameter that is a function of all the channel gains, and has a simple closed-form expression for N=3,4. This feedback information enables us to derive (channel) orthogonal designs starting from quasi-orthogonal STBCs. The feedback complexity is significantly lower than conventional closed-loop transmit beamforming. We compare the proposed codes with the open-loop OSTBCs and also with the closed-loop equal gain transmission (EGT) scheme which uses equal power loading on all antennas. Simulated error-rate performances indicate that the proposed channel orthogonalized STBCs significantly outperform the open-loop orthogonal designs, for the same spectral efficiency. Moreover, even with significantly lower feedback and computational complexity, the proposed scheme outperforms the EGT technique for M>N

Linear dispersion codes (LDC) designed for multiple input multiple output (MIMO) wireless systems are examples of space-time codes which try to maximize the mutual information between the transmitter and receiver. However, such codes are typically designed for flat fading channels. It is possible to extend LDC designs for frequency selective fading channels using orthogonal frequency division multiplexing (OFDM). Additionally, the multipath diversity available as frequency diversity could be potentially exploited by properly designed space-frequency LDC codes for MIMO-OFDM systems. In this paper, the design criteria to obtain such space-frequency LDC along with the error-rate performance of such codes in frequency selective fading channels are provided.

Closed-loop, rate-one, channel orthogonalized space-time block codes (CO-STBCs) for three and four transmit antennas using a single (real) phase feedback term have been proposed . These codes achieve full diversity and result in maximum likelihood (ML) decoding with only linear processing at the receiver similar to OSTBCs. In this paper, we propose a closed-loop STBC for five and six transmit antennas with rate 3/4, where the one parameter feedback angle can be evaluated in closed form. These codes achieve full diversity and are delay optimal. Simulation results comparing the error-rate performance of the CO-STBC with open-loop OSTBC as well as some quasiorthogonal space-time block codes are also provided. While the 1- to 5-dB gain accrued by the closed-loop scheme (over the open-loop methods) is not surprising, the novel contribution of this work is in the approach taken to derive the feedback parameter as a single phase term, which is purely a function of the channel gains.

Spatial correlation is introduced when antennas are not well separated, which typically leads to performance degradation in space diversity systems for flat fading wireless channels. However, in a frequency-selective environment with orthogonal frequency division multiplexing (OFDM), multipath diversity can help in overcoming this performance degradation. This is due to transformation of a highly spatially correlated channel impulse response to less spatially correlated channel frequency response inherently by an OFDM system in the presence of multipath diversity . In this letter, we numerically evaluate the minimum antenna separation required for a target spatial correlation in the frequency domain in situations with low multipath diversity. Performance results for two highly spatially correlated receive antennas with such spacing are provided for low multipath diversity channels and it is found to be within 1 dB (at BER=10/sup -3/) of spatially uncorrelated reception for QPSK modulation.

Tarokh et al. (1998) introduced the concept of space-time trellis coding as a means to combat fading by introducing redundancy, both in space and time. If the wireless channels are static or fade very slowly, the channel state information can be sent back to the transmitter using a low bit rate feedback path. In such a situation, it may be possible to mitigate the effects of signal cancellation due to simultaneous transmission, by using some sort of pre-processing technique at the transmitter (which incorporates the channel knowledge). However, we assume that no such feedback channel exists. Therefore, we address the issue of feedforward only pre-processing schemes at the transmitter. For fading channels, the error-rate performance depends on the effective code length, which is the length of the shortest error event path. The performance also depends on the minimum product distance which is computed from the maximum likelihood path and the other error paths with non-zero distance from it.