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Short cycles in a nonbinary low-density parity-check (NB-LDPC) code may be even more harmful to its performance if they do not satisfy the so-called full rank condition (FRC). This is because they may induce low-weight codewords or absorbing sets in that case. Thus, it is important to count the number of short cycles not satisfying the FRC as well as the number of short cycles for analyzing the performance of an NB-LDPC code. In this paper, we first develop a novel message-passing algorithm and identify how it is related to the FRC. We then propose a low-complexity algorithm for counting the number of short cycles not satisfying the FRC in an NB-LDPC code, as well as the number of short cycles. Finally, we propose a low-complexity algorithm for designing an NB-LDPC code with low error floor. Depending on the modulation scheme, the codes constructed by the proposed design algorithm have similar or slightly worse performance, compared with those constructed via the method by Poulliat et al. However, the proposed design algorithm does not require a cycle enumeration algorithm with high complexity, and therefore is feasible even in the case of large code length, say ${\geq}5000$ .

Block turbo codes (BTCs) are constructed by serially concatenating linear block codes and iteratively decoded by letting each component code be decoded in two stages. The Chase algorithm is employed in the first stage to make a list of candidate codewords by generating a fixed number of test sequences (TSs) and algebraically decoding them, regardless of the signal-to-noise ratio or the iteration number. In the second stage, the extrinsic information is generated for iterative decoding. In this letter, we propose a low-complexity decoding algorithm for BTCs. The proposed algorithm first checks whether an algebraic hard-decision decoder outputs a codeword for a given decoder input vector, and then adaptively applies one of the two estimation rules. Based on these two rules, the number of TSs in the proposed algorithm can be made monotonically decreasing with iterations. Numerical results demonstrate that the proposed algorithm has much lower computational complexity with a negligible performance loss, compared with the conventional decoding scheme based on the Chase algorithm.

High data rate at high mobile speed will still be an essential requirement for the future 5G mobile cellular system. High frequency bands above 6 GHz are particularly promising for the 5G system because of large signal bandwidths such high frequencies can offer. By using high gain beamforming antennas, the problem of high propagation loss at high frequencies can be overcome. However, the use of beamforming antennas at such high frequencies requires a significant change in the design of a cellular system. In particular, it requires a significant change in key functions such as cell search, random access, measurement of beams for fast beam adaptation, and various physical control and data channels. In this paper, we propose a new radio frame structure for the future mobile cellular communications system at millimeter wave frequency that addresses such challenges. A testbed was built at Samsung Electronics, Korea, based on the proposed frame structure at 28 GHz with bandwidth of 800 MHz. It attained the downlink (DL) data rate of 7.5 Gbps by delivering four streams of 64 QAM data with code rate of 3/4 to two mobile stations (MSs) located in a close distance to the base station antennas at fixed positions. It also achieved the DL data rate of 1.2 Gbps by delivering single stream of 16 QAM data with code rate of 3/4 to an MS moving at 110 km/h in a single cell of up to 800 m in a line-of-sight environment. Finally, it implemented handover and achieved an average handover interruption time of 21 ms in a three-cell environment, and demonstrated feasibility of mobile cellular communications at millimeter wave frequency.

In this study, the coverage of downlink cellular orthogonal frequency-division multiple-access networks transmitting one quadrature-amplitude modulation symbol per hop is analysed. The coverage probability is first defined by using an upper bound on the bit error rate (BER) of a bit-interleaved coded modulation system. Moreover then, the moment generating functions of the legacy Gaussian and Laplacian metrics (LMs) for soft-decision decoding of an employed error-correcting code are derived for computing the upper bound on the BER and the corresponding coverage probability of the networks. Numerical results demonstrate that due to the impulsive nature of the inter-cell interference for moderate-to-small cell loads, the networks using the LM for soft-decision decoding significantly outperform those employing the legacy Gaussian metric in terms of the achievable cell radius and the amount of the average transmit power reduction.

Trapping sets with short cycles are known to give a detrimental effect on the error floor performance of a low-density parity-check (LDPC) code. Unlike in binary LDPC codes., short cycles in a nonbinary low-density parity-check (NB-LDPC) code may be even more harmful to its performance if they do not satisfy the so-called full rank condition (FRC). This is because they may induce low-weight codewords or absorbing sets in that case. Thus, it is crucial to count the number of short cycles not satisfying the FRC as well as the number of short cycles for analyzing the performance of an NB-LDPC code. In this paper, we first develop a novel message-passing algorithm and identify how it is related to the FRC. We then propose a low-complexity algorithm for counting the number of short cycles not satisfying the FRC in an NB-LDPC code, as well as the number of short cycles.

Multitone-frequency and quadrature-amplitude modulation (MFQAM) is a combination of multitone frequency-shift keying and quadrature-amplitude modulation (QAM). In this study, the authors consider MFQAM with active subcarriers grouped in balance, where the active subcarriers are divided into a specified number of groups and the same QAM symbol is conveyed by every subcarrier in each group. This grouping may make MFQAM have frequency diversity in itself. They first derive the joint probability density function of the correlator outputs of an MFQAM receiver. They then compute the channel capacity of a coded modulation (CM) system employing MFQAM over Rayleigh fading channels. As a result, they demonstrate that for a moderate code rate between 1/3 and 2/3, its normalised throughput is close to the channel capacity limit in the low signal-to-noise ratio region, contrary to a CM system employing the conventional modulation. They also evaluate the frame error rate (FER) performance of a CM system employing MFQAM when a non-binary error-correcting code is employed. Numerical results demonstrate that the normalised throughput of the CM system computed via the FER shows a similar trend to that obtained via the channel capacity.

The extended min-sum (EMS) and improved EMS (I-EMS) algorithms for non-binary low-density parity-check codes over GF(q) significantly reduce the decoding complexity with an acceptable performance degradation, but they suffer from high latency because of many serial computations, including a sorting process. On the other hand, the trellis-based EMS algorithm can greatly reduce the latency, but it does not solve the complexity problem in high-order fields (q ≥ 64). To improve the latency problem with low-complexity advantages, the authors propose heap-based EMS (H-EMS) and heap-based I-EMS (HI-EMS) algorithms that are modifications of the EMS and I-EMS algorithms, respectively. The authors also propose double H-EMS and double HI-EMS algorithms trading off the latency against the performance by heaping messages twice. Numerical results show that the H-EMS algorithm has 2.74–9.52 times lower latency than the EMS algorithm with a negligible performance degradation over a wide range of code rates, whereas the HI-EMS algorithm has 1.20–1.62 times lower latency than the I-EMS algorithm. Furthermore, the proposed algorithms may be employed regardless of the decoding schedules.

Proliferation of all forms of smart devices has driven the demand for high-data-rate Wi-Fi in recent years. Utilizing wide bandwidth in high frequency is the most efficient way of guaranteeing the explosive data demands. In spite of the notable advancements of RF technologies at millimeter wave, the relatively high propagation loss remains problematic. The authors present efficient beamforming technology to mitigate this challenge. In this article, we first discuss the principles and major applications of beamforming technology. Afterward, we present the challenges for optimizing the efficiency of beamforming as well as the solutions to tackle them by proposing enhanced algorithms and advanced design architectures based on our hands-on experience and knowledge. Lastly, experimental results are discussed to deduce insight and vision of future Wi-Fi.

The distribution of the intercell interference (ICI) in conventional cellular networks employing orthogonal frequency-division multiple-access (OFDMA) with quadrature-amplitude modulation (QAM) tends to approach a Gaussian distribution when all available subcarriers in each cell are fully loaded. Recently, it has been also shown that the worst-case distribution of the ICI as additive noise in wireless networks with respect to the channel capacity is Gaussian. Thus, the channel capacity in cellular networks is expected to be further enhanced when the ICI could be designed properly so that it has a non-Gaussian distribution. This observation motivates us to propose, in this paper, a downlink cellular OFDMA network employing a modulation scheme called frequency and QAM (FQAM). We also derive maximum-likelihood metrics for the binary or non-binary error-correcting codes employed in the proposed network and propose their practical sub-optimal versions. Numerical results demonstrate that the distribution of the ICI in the proposed network deviates far from the Gaussian distribution. As a result, the transmission rates for the cell-edge users in the proposed network are significantly improved. In addition, the measurement results using practically implemented FQAM-based OFDMA systems verify that the transmission rates for the cell-edge users can dramatically increase, compared with the conventional QAM-based OFDMA network.

The ever growing traffic explosion in mobile communications has recently drawn increased attention to the large amount of underutilized spectrum in the millimeter-wave frequency bands as a potentially viable solution for achieving tens to hundreds of times more capacity compared to current 4G cellular networks. Historically, mmWave bands were ruled out for cellular usage mainly due to concerns regarding short-range and non-line-of-sight coverage issues. In this article, we present recent results from channel measurement campaigns and the development of advanced algorithms and a prototype, which clearly demonstrate that the mmWave band may indeed be a worthy candidate for next generation (5G) cellular systems. The results of channel measurements carried out in both the United States and Korea are summarized along with the actual free space propagation measurements in an anechoic chamber. Then a novel hybrid beamforming scheme and its link- and system-level simulation results are presented. Finally, recent results from our mmWave prototyping efforts along with indoor and outdoor test results are described to assert the feasibility of mmWave bands for cellular usage.

The performance of M-ary turbo coded synchronous, fast frequency-hopping spread spectrum multiple-access (FHSS-MA) networks with M-ary frequency shift keying (MFSK) and noncoherent detection is analyzed under Rayleigh fading. Results indicate that M-ary turbo codes dramatically enhance the performance of FHSS-MA networks using MFSK compared to binary turbo codes.

A hybrid PN code phase acquisition system based on a least-mean-square adaptive filter, interpreted as a channel estimator is proposed and analyzed for direct-sequence spread-spectrum systems under frequency selective Rayleigh fading channels. Closed form expressions are derived for the filter tap weights and detection/false alarm probabilities. Compared to previously proposed systems, the proposed system achieves smaller mean acquisition times, is more robust to the operating signal-to-noise ratio and allows for multiplication free tap weight updates.

A systematic, granulized piecewise linear approximation to the Jacobian logarithm is developed. This approximation reverts to the Jacobian approximation and the exact Jacobian logarithm at the far extremes. The utility of the derived approximation is demonstrated by applying it to the decoding of Turbo coded, MIMO/BICM-ID systems.

In this paper, union upper bound on convolutional coded bit error rates (BER) is derived for downlink orthogonal frequency division multiple access (OFDMA) networks. According to the numerical results, for the small network loads, the BER performance with Laplacian decoding metric outperforms the BER performance with Gaussian decoding metric under downlink OFDMA networks with Viterbi decoder.

The performance of uplink cellular OFDMA networks transmitting multiple QAM symbols per hop is evaluated under log-normal shadowing and Rayleigh fading without resorting to a Gaussian approximation of the inter-cell interference. The derived interference statistic is observed to deviate significantly from the Gaussian distribution which may be exploited to dramatically enhance the network performance. It is also observed that the network performance is highly dependent on the number and the geometry of the QAM symbols transmitted within a hop, especially for large code frame lengths.

In this paper, in order to effectively apply unitary space-time modulation to the direct-sequence spread-spectrum multiple-access (DSSS-MA) networks, we propose a low-rate, noncoherent, unitary, and space-time modulated DSSS system supporting any number of transmit antennas based on Walsh matrices. The proposed scheme simultaneously performs bandwidth spreading and space-time coding and outperforms those using high-rate, conventional unitary space-time constellations. Furthermore, the proposed scheme allows for a simple detector structure based on fast Walsh transforms.

An efficient sliding window algorithm employing an adaptive-length guard window for turbo decoders is proposed. The proposed algorithm results in significant complexity reductions, especially for small sliding window lengths where the additional computational complexity required for the guard window is critical.

An efficient preamble structure for OFDM systems is proposed which inserts a short embedded repetition pattern (SERP) into conventional preamble structures allowing a two-step detection of the integer frequency offset (IFO). A coarse estimate of the IFO using the SERP is performed in the time domain prior to performing fine IFO detection in the frequency domain based on the original preamble structure. The complexity of the overall IFO detection procedure can be significantly reduced at a price of slight loss in SNR.

Using a Gaussian approximation to the distribution of the message vectors under density evolution, we design irregular weighted nonbinary repeat-accumulate codes over GF(q) with q-ary orthogonal modulation. The resulting codes achieve a frame error rate of 10-1 within 0.56 to 0.91 dB of channel capacity under the AWGN channel.

An approximate upper bound is derived to the maximum likelihood decoding threshold of weighted nonbinary repeat-accumulate (WNRA) codes over GF(q) with q-ary orthogonal modulation and coherent detection over the additive white Gaussian noise channel. As with binary RA codes, WNRA codes show the potential for achieving the channel capacity with decreasing code rate for large enough q.

In this paper, we analyze the performance of soft decision decoded, synchronous, fast frequency-hopping spread-spectrum multiple-access networks using M-ary frequency shift keying in Rayleigh fading channels under the assumption that the number of frequency hopping slots is sufficiently large. The joint probability density function (pdf) of the absolute values of the correlator outputs is derived to compute the channel capacity and evaluate the soft decision decoding performance of the 3GPP Turbo and convolutional codes. Results indicate that soft decision decoding based on the derived pdf drastically outperforms traditional decoders based on the Gaussian approximation of the effect of the multiple access interference (MAI). The sensitivity of the derived soft decision decoder to estimation errors in system parameters and the required computational complexities of the derived soft decision decoding metrics are also investigated.

A very simple design rule for parallel concatenated Turbo codes over GF(q) with q-ary orthogonal modulation is proposed. Unlike previously proposed exhaustive search methods, the proposed design rule allows straightforward and systematic code designs for any value of q ≥ 2 and any constraint length greater than 2.

An iterative algorithm employing decision feedback provided by either an equalizer or a channel decoder is proposed in order to compensate for the phase noise resulting from imperfect oscillators in orthogonal frequency-division multiplexing (OFDM) systems. In the proposed algorithm, the received OFDM symbol is partitioned into subblocks in the time domain and the estimate of the time-average of the phase noise at each subblock is used to compensate for the phase noise. A formula for the signal-to-interference-plus-noise ratio (SINR) after phase noise compensation is derived and its values are evaluated under a variety of conditions in order to demonstrate the efficiency of the proposed algorithm. Numerical results show that the proposed algorithm achieves performance close to that of an OFDM system without phase noise over a wide range of conditions, while requiring only one-half the complexity of the partial intercarrier interference reduction algorithm.

Algorithms based on the QR decomposition of the equivalent space-time channel matrix have been proved useful in the detection of V-BLAST systems. Especially, the parallel detection (PD) algorithm offers ML approaching performance up to 4 transmit antennas with reasonable complexity. We show that when directly applied to STBCs, the PD algorithm may suffer a rather significant SNR degradation over ML detection, especially at high SNRs. However, simply extending the PD algorithm to allow p ≥2 candidate layers, i.e. p -PD, regains almost all the loss but only at a significant increase in complexity. Here, we propose a simplification to the p-PD algorithm specific to STBCs without a corresponding sacrifice in performance. The proposed algorithm results in significant complexity reductions for moderate to high order modulations.

In this paper, we propose a novel class of unitary space-time constellations based on quaternary quasi-orthogonal sequences (QOSs), minimizing the maximum correlation between the signal matrices. The proposed design, owing to the underlying algebraic structure of the QOSs, allows for an efficient decoder structure while offering performance virtually identical to the unitary space-time constellation design by Hochwald et al.

In this paper, a distribution matching technique is proposed to process the extrinsic information produced by practical multiple-antenna detectors with iteratively demodulated, bit-interleaved coded modulation (BICM-ID). It is shown that significant gains may be achieved by appropriately processing the extrinsic information generated by practical multiple-antenna detectors before feeding it to the channel decoder. The validity of the proposed matching technique is shown via simulation results applied to Turbo-coded BICM-ID systems using (QRD)-M and list sphere detectors.

The probability density function (PDF) of the intercell interference for downlink cellular orthogonal frequency division multiple access networks under log-normal shadowing and multipath Rayleigh fading is derived. The derived PDF is observed to highly deviate from the Gaussian distribution, especially at moderate to small network loads. Based on the derived inter-cell interference statistics, the decoder performance can be significantly improved compared to the case when the traditional decoder optimized for the AWGN channel is employed. A simple suboptimum decoding algorithm that closely approximates the derived optimum decoder performance is also proposed.

In this correspondence, we propose a simplified QR-decomposition-M algorithm for coded multiple-input multiple-output systems based on the idea of selective branch extension. The proposed algorithm results in significant complexity reductions without observable performance losses, especially for large modulation orders where complexity reduction is most important.

In digital communication systems employing binary phase-shift keying and non-data-aided carrier phase recovery, a 180°carrier phase ambiguity is inevitable. Here, we propose a simple modification to the standard regular repeat-accumulate (RA) code structure by exploiting the differential encoding inherent to the inner encoder of RA codes resulting in codes that are 180°rotationally invariant. The proposed code structure exhibit performance virtually identical to that of standard regular RA codes with zero carrier phase offset under both zero and 180°carrier phase offsets with negligible additional hardware complexity.

In this paper, efficient symbol timing synchronization algorithms for IEEE 802.11a/g wireless LAN systems in multipath channels are proposed. For improved accuracy, the algorithms utilize an effectively elongated training symbol together with nonlinear soft-limiting of the correlator output. The algorithms allow efficient utilization of the guard interval in multipath channels. Simulation results show that the proposed algorithms significantly outperform conventional algorithms.

The need for real-time computation of the Euclidean norm of a vector arises frequently in many signal processing applications such as vector median filtering, vector quantization and multiple-input multiple-output wireless communication systems. In this correspondence, we examine the properties of a linear combination of the 1-norm and the infinity norm as an approximation to the Euclidean norm of real-valued vectors. The approximation requires only two multiplications regardless of the vector length and does not require sorting of the absolute values of the vector entries. Numerical results show that the considered approximation incurs negligible performance degradations in typical applications.

The authors optimize system parameters including modulation order M, diversity order L, and traffic density lambda to maximize the throughput of frequency-hopping-multiple-access networks with multilevel frequency-shift keying. It is shown that optimal lambda is close to ln 2 with the exception that optimal lambda=1 for the erasures coding channel with M=2. For the erasures coding channel, optimum L is also derived, and for the hard-decision coding channel, optimum L is numerically searched. For the region of M 64, the hard-decision coding channel achieves slightly higher throughput than the erasures coding channel. While the maximum throughput for the hard-decision coding channel monotonously increases as M increases, the maximum throughput for the erasures coding channel shows a local increase at M=2, and it is significantly higher than that for the hard-decision coding channel with M=2. Tradeoffs between system parameters to achieve the maximum throughput under a fixed bandwidth constraint are also presented. It is shown that the optimal set for (M,L,N(=number of available frequencies)) is approximately (G,log2G,G) for both coding channels, where G is the total system bandwidth expansion normalized by bit rate

It is well known that turbo codes provide highly unequal error protection within a transmitted frame. Previous attempts to exploit this fact focused mainly on adding additional redundancy to provide extra protection for the error-prone bit positions. Here, instead, we use the error-detection capability of the cyclic redundancy check (CRC), which is almost always employed in practical systems. Once a frame is declared uncorrectable by the CRC, a process termed the error-prone bit processing procedure is activated in an attempt to correct the probable error patterns which are a priori identified as being error-prone.

The sensitivity of the iterative decoder for repeat-accumulate (RA) codes to carrier phase and channel signal-to-noise ratio estimation errors is investigated, and efficient algorithms to estimate and correct these errors are developed. The behavior of RA codes with imperfect channel estimation is different from that of turbo codes, and correction algorithms specific to RA codes must be formulated. The proposed algorithms use the soft information generated within the iterative decoder, and thus, are not only hardware-efficient, but also offer excellent performance.

The uplink performance of synchronous and asynchronous slow frequency-hop spread-spectrum multiple-access (SFHSS-MA) networks transmitting L bits per hop using binary differential phase shift keying (BDPSK) is analyzed under the additive white Gaussian noise (AWGN) and Rayleigh fading channels. Analytic expressions for the average conditional bit error probabilities given a hop is hit by K' interfering users are derived. Results show that SFHSS-MA networks using BDPSK achieve nearly twice the maximum normalized network throughput compared to networks using BFSK under both AWGN and Rayleigh fading channels.

A nonlinear spectral-line method utilizing the fourth absolute moment of the receiver discrete Fourier transform output is proposed as a non-data-aided fine carrier frequency synchronization algorithm for OFDM receivers. A simple modification of the algorithm resulting in low implementation complexity is also developed. Analytic expressions are derived for the steady-state frequency error variances of the algorithms and verified to be very accurate via computer simulations over AWGN and frequency selective multipath channels. Numerical results show that the proposed algorithms provide reliable and excellent steady-state performance, especially with PSK modulation. Also, the proposed algorithms are insensitive to symbol timing offsets, only requiring a coarse symbol timing recovery.

Optimum values for the modulation order M, code rate r, and the number of frequency-hop slots q maximizing the network throughput are obtained based on simulations for frequency-hopped spread-spectrum multiple-access networks, where L Q-ary Reed-Solomon (RS) code symbols are transmitted per hop, and each Q-ary RS code symbol is transmitted using log/sub M/Q M-ary frequency-shift-keying-modulated signals. Network throughput is evaluated under additive white Gaussian noise and Rayleigh fading channels. For the case when the received RS symbol is not interfered by multiple-access interference (MAI), a closed-form expression for the symbol-error probability is derived, and for the case when the symbol is interfered by MAI, simulated symbol-error probabilities are used. It is shown that the optimum M is four or eight, irrespective of the channel environment and the number of users. The optimum code rate is determined primarily based on the channel environment and does not show much dependence on M or Q. It is also shown that for the case of synchronous hopping under Rayleigh fading at high signal-to-noise ratios, the difference in instantaneous power among the interfering users significantly improves the performance, compared with the case when there is no fading. We also consider the case when the receiver erases the symbols that are interfered and compare the performance with the case of the hard decisions receiver.

A novel frequency-hop spread-spectrum multiple-access network employing M-ary orthogonal Walsh sequence keying with noncoherent demodulation is proposed. The transmitted Walsh sequence is overlaid by a user-specific pseudonoise sequence to reduce the effect of multiple-access hits. Two Gaussian approximations for the multiple-access interference from both the dehopped slot and its neighboring slots are developed and are used to analyze the performance of the proposed network for synchronous and asynchronous hopping under nonfading and Rayleigh fading channels. The effect of imperfect hop timing synchronization at the receiver is also analyzed. It is shown that the proposed network offers significantly improved network throughput compared to networks based on traditional M-ary frequency-shift keying modulation.

An adaptive serial search pseudonoise (PN) code acquisition scheme is proposed, in which the detection threshold is scaled by the instantaneous received power measured prior to PN code correlation. We observe that the proposed scheme achieves significantly improved mean acquisition times compared to the conventional nonadaptive schemes under Rayleigh fading and pulsed Gaussian noise jamming. Furthermore, the proposed scheme is shown to be optimum under pulsed Gaussian noise jamming in the sense that it forces the worst case jamming fraction to unity.

In cellular systems, the interference has been the most critical issue that practically limits the performance of overall networks. Furthermore, the distribution of the inter-cell interference (ICI) in conventional cellular networks employing orthogonal frequency-division multiple-access (OFDMA) with quadrature amplitude modulation (QAM) tends to approach a Gaussian distribution, which is known to be the worst-case distribution of the ICI as additive noise resulting in poor channel capacity. Thus, a dramatic enhancement of the channel capacity for the cellular network is expected when the ICI could be designed properly so that it has a non-Gaussian distribution. In this context, a cellular OFDMA system with a novel modulation scheme, frequency and quadrature-amplitude modulation (FQAM), is proposed in this paper. The statistical distribution of the ICI is shown to deviate far from the Gaussian distribution for the proposed FQAM-based system. Accordingly, it is shown that the non-Gaussian distribution of the ICI incurred with FQAM results in significantly improved transmission rates for the cell-edge users. Also, from the measurement results using practically implemented FQAM-based OFDMA systems, it is verified that the transmission rate for the cell-edge users could be increased significantly over the conventional QAM-based OFDMA system.

The performance of frequency-hop spread-spectrum multiple-access networks using orthogonal M-ary frequency-shift keying modulation with noncoherent detection is analyzed under tone jamming, Rayleigh fading, and additive white Gaussian noise. A simple Gaussian model is derived for the multiple-access interference giving accurate fit to the simulation results with appropriately chosen correction terms. This approximation is then used to analyze the performance of the network under additional tone jamming. Both cases when the jamming tones fade and when they do not are considered.

In this letter, an approximation to the performance of downlink cellular orthogonal frequency division multiple access networks under log-normal shadowing and Ricean fading is evaluated. The interference statistic is shown to significantly deviate from the Gaussian distribution as shown in previous results under Rayleigh fading.

Unlike traditional trained channel equalizers, not much work has been done to theoretically characterize the convergence properties of blind channel equalizers due to their inherent nonlinearity. It is only recently that convergence properties of some well-known algorithms such as the generalised Sato algorithm (GSA) and the constant modulus algorithm (CMA) have been analytically derived. In this paper, the convergence properties of the stop-and-go algorithm proposed by Picchi and Prati (1987) are analyzed. The derived mean squared error and the coefficient trajectories are compared with simulation results to verify the validity of the analytical results.

Uncoded antijamming capability of the multicarrier direct-sequence spread-spectrum system proposed by Kondo and Milstein (see ibid. vol.44, p.238-46, 1996 ) is analyzed. Joint pulse/partial-band noise jamming is considered with various subcarrier demodulator output combining schemes. We find that full-band pulse jamming maximizes the error probability for all cases considered. With Rayleigh fading and maximal-ratio combining, the multicarrier system achieves performance very close to that of the single-carrier system without fading even with a very small number of subcarriers.

Capture phenomena in slotted ALOHA packet radio networks employing spread-spectrum modulation is considered. Probability of capture with both delay and power capture mechanisms are derived and compared with power- and delay-only capture models. Significant improvements in the capture probability is observed. The improvement over the delay-only capture model increases with decreasing arrival-time randomization overhead.

Recently, Liu et al., developed the /spl Sigma//sub 0/-rank criteria for space-time codes. It provides a sufficient condition on codeword and generator matrices defined over finite rings /spl Zopf//sub 2k/(j) to ensure full spatial diversity with 2/sup 2k/-QAM modulation. Here, we generalize the /spl Sigma//sub 0/-rank criteria and derive a sufficient condition on the generator matrices defined over finite rings /spl Zopf//sub 2l/(j) to ensure full spatial diversity with 2/sup 2k/-QAM modulation for any positive integer l/spl les/k. We also show that generator matrices defined over GF(2) satisfying the BPSK stacking construction constraint of Mammons and El Gamal achieve full spatial diversity when used with 2/sup 2k/-QAM modulation.

In this paper, we describe a patent pending, fine carrier frequency synchronization algorithm based on a minimum mean-squared error (MMSE) approach for OFDM systems which is insensitive to symbol timing offsets. In order to obtain these attributes, the proposed algorithm adopts a cost function similar to the CMA (constant modulus algorithm) used for blind channel equalization. Another advantage of the algorithm is that the transmission of a known training pattern is unnecessary, thus, improving the transmission efficiency. A hardware efficient version of the proposed algorithm is also developed. Computer simulations are performed to evaluate the performance with QPSK and 64-QAM modulations under a frequency selective multipath channel.

The optimum arrival-time distribution that maximizes the delay-capture probability in spread-spectrum packet radio networks is derived. It is shown that when the optimum arrival-time distribution is employed, the capture probability converges to a finite value as the number of contending packets increases. Normalized throughput and average number of packet retransmissions are computed for slotted ALOHA multipoint-to-point packet radio networks employing direct-sequence spread-spectrum modulation. It is shown that large performance improvement is obtained by optimizing the arrival-time distribution compared to the uniform arrival-time distribution assumed by Davis and Gronemeyer (1980), especially when error-correction coding is employed. Two practical modifications are derived which are shown to provide performance close to the optimum along with an adaptive network load control scheme.

A demodulator for the Grand Alliance digital HDTV receiver is designed with hardware efficient algorithms and a prototype is implemented. The implemented prototype contains all the required functions for baseband demodulation. Simple and fast polarity compensation algorithm for the bi-stable frequency and phase locked loop (FPLL), hardware efficient algorithms for segment synchronization, symbol timing recovery, channel equalization and the phase/gain noise tracking loop are of special interest. Experimental results are presented and the proper operation of the implemented functions is verified.