In wireline networks, random network coding combined with path diversity provides inherent protection against eavesdropping since the eavesdropper has to compromise nodes along each diverse path, presumably at significant risk and cost per compromise, in order to acquire enough information to decode the source data. This touted ``free security'' does not necessarily apply, however, towireless networks. Since a sophisticated over-the-air eavesdropper at a protected site away from the network can receive transmissions from multiple nodes, without incurring significant additional risk or expense, any inherent protection that network coding provides in this context is unclear. In this paper, we investigate the eavesdropping vulnerability of a mobile ad-hoc network using the current state-of-the-art in random linear network coding and subgraph generation techniques. Specifically, we develop a simulation framework with proper metrics to quantify the eavesdropper's decoding success probability for a network-coded wireless network. Our results, obtained from extensive Monte Carlo simulations, demonstrate that such networks are indeed vulnerable to eavesdropping-and to a remarkable degree. Finally, we experimentally confirm the high degree of eavesdrop vulnerability using packets captured from an operational prototype testbed.

Distributed space-time code constructions are presented that are suitable for asynchronous cooperative networks using different cooperation strategies. The codes presented herein may be used with either an amplify-and-forward or a decode-and-forward protocol, achieving in either scenario full spatial diversity despite arbitrary timing offsets among asynchronous cooperating relays. The new designs are flexible with respect to the number of relays, input alphabet, and transmission rate.

We propose a new class of C-linear distributed space-time codes for cooperative networks using a decode- and-forward strategy. Our codes achieve full spatial diversity regardless of the relays' timing offsets. The proposed codes are effective and flexible, enabling use of different signal constellations, transmission rates, numbers of transmit and receive antennas, and different tradeoffs between decoder complexity and overall performance. Furthermore, we extend our framework to handle the case where relays have multiple antennas and propose few examples of the modified code construction.

General algebraic space-time code constructions are presented for Gray-mapped 2/sup K/-PSK, 2/sup K/-PAM, and 4/sup K/-QAM constellations. The space-time codes achieve the rate-diversity tradeoff - i.e., they allow the transmission of information at the maximum rate possible for the given signaling constellation and the achieved transmit diversity level.

We consider the design of space-time overlays to upgrade single-antenna wireless communication systems to accommodate multiple transmit antennas efficiently. We define the overlay constraint such that the signal transmitted from the first antenna in the upgraded system is the same as that in the single-antenna system. The signals transmitted from the remaining antennas are designed according to space-time coding principles to achieve full spatial diversity in quasi-static flat fading channels. For both binary phase-shift keying (BPSK) and quaternary phase-shift keying modulation systems, we develop an algebraic design framework that exploits the structure of existing single-dimensional convolutional codes in designing overlays that achieve full spatial diversity with minimum additional decoding complexity at the receiver. We also investigate a concatenated coding approach for a BPSK overlay design in which the inner code is an orthogonal block code. This approach is shown to yield near optimal asymptotic performance for quasi-static fading channels. We conclude by offering a brief discussion outlining the extension of the proposed techniques to time-varying block fading channels.

The availability of multiple transmit antennas allows for two-dimensional channel codes that exploit the spatial transmit diversity. These codes were referred to as space-time codes by Tarokh et al. (see ibid., vol.44, p.744-765, Mar. 1998) Most prior works on space-time code design have considered quasi-static fading channels. We extend our earlier work on algebraic space-time coding to block-fading channels. First, we present baseband design criteria for space-time codes in multi-input multi-output (MIMO) block-fading channels that encompass as special cases the quasi-static and fast fading design rules. The diversity advantage baseband criterion is then translated into binary rank criteria for phase shift keying (PSK) modulated codes. Based on these binary criteria, we construct algebraic space-time codes that exploit the spatial and temporal diversity available in MIMO block-fading channels. We also introduce the notion of universal space-time codes as a generalization of the smart-greedy design rule. As a part of this work, we establish another result that is important in its own right: we generalize the full diversity space-time code constructions for quasi-static channels to allow for higher rate codes at the expense of minimal reductions in the diversity advantage. Finally, we present simulation results that demonstrate the excellent performance of the proposed codes.

The authors introduced an algebraic design framework for space-time coding in flat-fading channels . We extend this framework to design algebraic codes for multiple-input multiple-output (MIMO) frequency-selective fading channels. The proposed codes strive to optimally exploit both the spatial and frequency diversity available in the channel. We consider two design approaches: The first uses space-time coding and maximum likelihood decoding to exploit the multi-path nature of the channel at the expense of increased receiver complexity. Within this time domain framework, we also propose a serially concatenated coding construction which is shown to offer a performance gain with a reasonable complexity iterative receiver in some scenarios. The second approach utilizes the orthogonal frequency division multiplexing technique to transform the MIMO multipath channel into a MIMO flat block fading channel. The algebraic framework is then used to construct space-frequency codes (SFC) that optimally exploit the diversity available in the resulting flat block fading channel. Finally, the two approaches are compared in terms of decoder complexity, maximum achievable diversity advantage, and simulated frame error rate performance in certain representative scenarios.

We consider the design of space-time overlays to upgrade single antenna wireless communication systems to efficiently accommodate multiple transmit antennas. We define the overlay constraint such that the signal transmitted from the first antenna in the upgraded system is the same as that in the single antenna system. The signals transmitted from the remaining antennas are designed according to space-time coding principles to achieve full spatial diversity in quasi-static flat fading channels. For both BPSK and QPSK modulated systems, we develop an algebraic design framework that exploits the structure of existing single dimensional convolutional codes in designing overlays that achieve full spatial diversity with minimum additional decoding complexity at the receiver. We also investigate a concatenated coding approach for BPSK overlay design in which the inner code is an orthogonal block code. This approach is shown to yield near optimal asymptotic performance for quasi-static fading channels.

In this paper, we consider the design of space-time overlays to upgrade single antenna wireless communication systems to accommodate multiple transmit antennas efficiently. We define the overlay constraint such that the signal transmitted from the first antenna in the upgraded system is the same as that in the single antenna system. The signals transmitted from the remaining antenna's are designed according to space-time coding principles to achieve full spatial diversity in quasi-static flat fading channels. For both BPSK and QPSK modulated systems, we develop an algebraic design framework that exploits the structure of existing single dimensional convolutional codes in designing overlays that achieve full spatial diversity with minimum additional decoding complexity at the receiver.

The authors previously developed an algebraic approach to space-time code design that unifies most of the known results on trellis space-time codes and opens the door for more sophisticated space-time code constructions. We present algebraic constructions for trellis and block space-time codes for BPSK and QPSK modulated systems. The new designs benefit from the algebraic approach and are general for arbitrary number of transmit antennas in quasistatic fading channels. We also provide simulation results comparing the frame error rate performance of various constructions. These simulation results establish the performance advantage achieved by algebraic space-time codes compared to previously known codes in various scenarios.

We introduce a simple technique for analyzing the iterative decoder that is broadly applicable to different classes of codes defined over graphs in certain fading as well as additive white Gaussian noise (AWGN) channels. The technique is based on the observation that the extrinsic information from constituent maximum a posteriori (MAP) decoders is well approximated by Gaussian random variables when the inputs to the decoders are Gaussian. The independent Gaussian model implies the existence of an iterative decoder threshold that statistically characterizes the convergence of the iterative decoder. Specifically, the iterative decoder converges to zero probability of error as the number of iterations increases if and only if the channel E/sub b//N/sub 0/ exceeds the threshold. Despite the idealization of the model and the simplicity of the analysis technique, the predicted threshold values are in excellent agreement with the waterfall regions observed experimentally in the literature when the codeword lengths are large. Examples are given for parallel concatenated convolutional codes, serially concatenated convolutional codes, and the generalized low-density parity-check (LDPC) codes of Gallager and Cheng-McEliece (1996). Convergence-based design of asymmetric parallel concatenated convolutional codes (PCCC) is also discussed.

The information-theoretic capacity of multiple antenna systems has been shown to be significantly higher than that of single antenna systems in Rayleigh-fading channels. In an attempt to realize this capacity, Foschini (1996) proposed the layered space-time architecture. This scheme was argued to asymptotically achieve a lower bound on the capacity. Another line of work has focused on the design of channel codes that exploit the spatial diversity provided by multiple transmit antennas (Tarokh et al. 1998, Hammons and Gamal 2000). In this paper, we take a fresh look at the problem of designing multiple-input-multiple-output (MIMO) wireless systems. First, we develop a generalized framework for the design of layered space-time systems. Then, we present a novel layered architecture that combines efficient algebraic code design with iterative signal processing techniques. This novel layered system is referred to as the threaded space-time (TST) architecture. The TST architecture provides more flexibility in the tradeoff between power efficiency, bandwidth efficiency, and receiver complexity. It also allows for exploiting the temporal diversity provided by time-varying fading channels. Simulation results are provided for the various techniques that demonstrate the superiority of the proposed TST architecture over both the diagonal layered space-time architecture in Foschini (1996) and the multilayering approach (Tarokh et al. (1999).

Wireless data applications have just begun to gain popularity in the marketplace. The rollout of 3rd generation (3G) wireless networks is anticipated in the next few years. We examine wireless data networks that are complementary to the 3G wireless networks, their technologies and the challenges. In particular, we present some of the industry initiatives in the millimeter wave frequency bands for broadband and satellite delivered fixed wireless data network. Furthermore, we offer our perspectives on how these different networks will evolve and inter-work together to provide a seamless experience for the end users. Fixed data networks need to be competitive with wire-line services, this requires us to push advanced signal processing technologies further. We present the latest advances that we have made in channel coding and signal processing technologies to approach that goal.

The authors previously presented a new space-time architecture combining generalized layered transmission, advanced iterative multi-user detection techniques, and space-time code design that provides superior performance compared to the layered architectures proposed by Foschini (1996) and Tarokh et al. (see IEEE Trans. on Information Theory, vol.IT-44, p.744-65, 1998). We discuss the design of algebraic space-time codes for layered and non-layered architectures. We present new constructions for the quasi-static and block fading channels that extend to the diagonal transmission architecture proposed for differentially coherent space-time coding.

This paper presents a new idea to detect a continuous-wave (CW) burst with unknown frequency using the discrete Fourier transform. Given a CW burst of certain length, an obvious detection scheme would be to compute the DFT power over the whole burst and in the given frequency bins that cover the total frequency uncertainty range. This may result in poor performance when the CW frequency is not exactly in one of the DFT bins. The new scheme divides the CW burst into L sub-bursts and computes the summation of the DFT power in each sub-burst. With very little increase on the DFT computation complexity, the overall detection performance can he greatly improved. The analysis employs a unified approach to derive the expressions of probabilities of false alarm and miss detection for both the conventional and the new schemes over the additive white Gaussian noise (AWGN) channel. Simulations are done for both the AWGN channel and the satellite fading channel.

The design of space-time codes to achieve full spatial diversity over fading channels has largely been addressed by handcrafting example codes using computer search methods and only for small numbers of antennas. The lack of more general designs is in part due to the fact that the diversity advantage of a code is the minimum rank among the complex baseband differences between modulated codewords, which is difficult to relate to traditional code designs over finite fields and rings. We present general binary design criteria for PSK-modulated space-time codes. For linear BPSK/QPSK codes, the rank of (binary projections of) the unmodulated codewords, as binary matrices over the binary field, is a sufficient design criterion: full binary rank guarantees full spatial diversity. This criterion accounts for much of what is currently known about PSK-modulated space-time codes. We develop new fundamental code constructions for both quasi-static and time-varying channels. These are perhaps the first general constructions-other than delay diversity schemes-that guarantee full spatial diversity for an arbitrary number of transmit antennas.

The recently adopted ITU-T G.729 8 kbps codec standard's performance is evaluated for digital cellular channels that are characterized by Rayleigh fading. Two forward error correction schemes (FEC) are studied. They are the convolutional code based FEC and the Nordstrom Robinson based FEC. The effects of interleaving depth are investigated for both FEC schemes. Both flat fading as well as co-channel interference limited models are used in our study.

A family of quaternary (Z/sub 4/-alphabet) sequences of length L=2/sup r/-1, size M/spl ges/L/sup 2/+3L+2, and maximum nontrivial correlation parameter C/sub max//spl les/2/spl radic/(L+1)+1 is presented. The sequence family always contains the four-phase family /spl Ascr/. When r is odd, it includes the family of binary Gold sequences. The sequence family is easily generated using two shift registers, one binary, the other quaternary. The distribution of correlation values is provided. The construction can be extended to produce a chain of sequence families, with each family in the chain containing the preceding family. This gives the design flexibility with respect to the number of intermittent users that can be supported, in a code-division multiple-access cellular radio system. When r is odd, the sequence families in the chain correspond to shortened Z/sub 4/-linear versions of the Delsarte-Goethals codes.

Certain notorious nonlinear binary codes contain more codewords than any known linear code. These include the codes constructed by Nordstrom-Robinson (1967), Kerdock (1972), Preparata (1968), Goethals (1974), and Delsarte-Goethals (1975). It is shown here that all these codes can be very simply constructed as binary images under the Gray map of linear codes over Z/sub 4/, the integers mod 4 (although this requires a slight modification of the Preparata and Goethals codes). The construction implies that all these binary codes are distance invariant. Duality in the Z/sub 4/ domain implies that the binary images have dual weight distributions. The Kerdock and "Preparata" codes are duals over Z/sub 4/-and the Nordstrom-Robinson code is self-dual-which explains why their weight distributions are dual to each other. The Kerdock and "Preparata" codes are Z/sub 4/-analogues of first-order Reed-Muller and extended Hamming codes, respectively. All these codes are extended cyclic codes over Z/sub 4/, which greatly simplifies encoding and decoding. An algebraic hard-decision decoding algorithm is given for the "Preparata" code and a Hadamard-transform soft-decision decoding algorithm for the I(Kerdock code. Binary first- and second-order Reed-Muller codes are also linear over Z/sub 4/, but extended Hamming codes of length n/spl ges/32 and the Golay code are not. Using Z/sub 4/-linearity, a new family of distance regular graphs are constructed on the cosets of the "Preparata" code.>