We advocate in this paper the adoption of phase forward (PF) as a technique to overcome non-linear power amplifier distortion in two-way cooperative communication systems that employ PSK modulations. The performance of this new signal relay technique, measured in terms of the amount-of-fading (AoF) and bit-error-rate (BER), is analyzed and compared against a benchmark system that employs amplify-and-forward (AF) at the relay. It was discovered that not only is PF superior to AF when the amplifier at the relay is non-linear, it can also outperform AF when the amplifier is linear. The conclusion is reached PF is a useful alternative to existing signal forwarding techniques for two-way cooperative communication.

In amplify-and-forward (AF) relay systems, multi-hop transmissions over multiple frequency selective fading channels result in an accumulation of delay spread, which could lead to strong intersymbol interference (ISI) at the destination since there is no proper channel compensation at the relay nodes. If orthogonal frequency division multiplexing (OFDM) with fixed-length guard interval (GI) is used in these systems, then the length of the GI has to be extended to the longest possible channel delay spread to avoid ISI. However, using such a long GI lowers the effective data transmission throughput. To this end, we propose a novel OFDM system with adaptive GI for AF relay networks to reduce the overall transmission overhead by dynamically choosing the suitable GI length. More specifically, the GI is adaptively selected at the transmitter from a variable-length orthogonal code set, which provides a GI sequence with proper length, depending on the accumulated channel duration. By exploiting the orthogonal property between different GI sequences, the proposed adaptive GI scheme can be implemented without any extra control signal transmitted by the source to notify the destination about the GI used. This differs from existing adaptive GI studies which require such an additional control signal and thus introduce extra system overhead. Numerical results show that the proposed adaptive scheme (without additional control signal) can achieve the same symbol error rate (SER) performance as the conventional adaptive GI approaches (with control signal). This implies that the proposed scheme can further save the control signaling overhead without any SER performance loss.

We propose in this paper the idea of multiply-and-forward (MF) as a mean to overcome nonlinear power amplifier (NLPA) distortion in two-way cooperative communication systems using MPSK modulation. Compared to amplify-and-forward (AF), of which the relay broadcasts a scaled sum of the signals received from individual sources, the relay in the proposed MF system broadcasts a scaled product of the individually received signals instead. Through signal analysis and simulation, we show that the bit error rate (BER) of the MF protocol is unaffected by NLPA distortion and is able to deliver a second order diversity effect. In contrast, AF exhibits a high irreducible error floor in the presence of NLPA distortion. Actually, MF with an NLPA attains the same BER as AF with a linear power amplifier.

We study in this paper the performance of a one-way cooperative transmission system using differential PSK (DPSK) modulation with differential detection. As opposed to previous works which consider either a decode-and-forward (DF) or an amplify-and-forward (AF) relay, we adopt in this paper a phase-forward (PF) relay, whereby each forwarded symbol has constant modulus and a phase equals the phase of the corresponding relay's received symbol. The rationale for adopting this relaying strategy is to avoid potential non-linear amplifier distortion in an amplify-and-forward relay, as well as the implicit information loss/quantization in a DF relay. Through analysis and simulation, we found that this PF-DPSK cooperative transmission scheme has a lower bit-error rate (BER) than that of its DF counterpart. Furthermore, by adopting a threshold-based selective forwarding approach, it can attain a BER similar to that of AF. Finally, we anticipate that PF is most useful in two or multi-way relaying, in which any non-linear amplifier distortion on an AF signal will manifest into significant inter-modulation.

We propose the concept of Phase-Only Forward (PF) relays for cooperative communication involving constant envelope modulations. The technique enables the relaying nodes to maintain constant envelope signaling without the need to perform decoding and signal re-modulation. The bit-error-probability (BEP) of this PF technique, in conjunction with constant envelope CPFSK modulation in a time-selective Rayleigh fading environment and discriminator detection, was analyzed. Both cases of with and without antenna selection at the relay were considered. In the former case, a clever phase-adjustment scheme that allows the relay to maintain phase continuity was proposed. Our results indicate that without antenna selection, PF has a lower BEP than decode-and-forward (DF). It also delivers the same performance as amplify-and-forward (AF) when fading is quasi-static. In a time-selective fading environment, PF is not as efficient as AF without antenna selection. However with dual-antenna selection at the relay, PF has practically the same performance as AF even in a time-selective fading environment. We thus believe that PF is a cost-effective alternative to AF and DF, since signal re-modulation and expensive linear amplifiers are no longer needed at the relay to support non-constant envelope transmission. Finally, this phase-forward concept can be generalized to other continuous-phase modulation schemes as well as to PSK.

Various terrestrial digital television broadcasting standards have been developed in the last two decades for both fixed residential and mobile receivers. Since personal mobile terminal integrated with multiple functionalities is becoming more popular, it is highly desirable to come up with a universal platform for mobile TV reception which can detect the system parameters of DTV signals automatically and adjust its receiving mechanism correspondingly. In this paper, we propose a progressive automatic method for parameter extraction of OFDM-based mobile broadcasting signals. Classification of ATSC and OFDM based DTV signals can be easily achieved with the unique pilot of ATSC signals. Based on a combination of cyclostationarity test and conventional correlation, the proposed algorithm can estimate system parameters of the received OFDM signal, including the sampling rate, the number of subcarriers and the CP length (CP ratio), without any prior information.

In this paper we propose the idea of Phase-Superposition-Phase-Forward (PSPF) relaying for 2-way 3-phase cooperative network involving constant envelop modulation with discriminator detection in a time-selective Rayleigh fading environment. A semi-analytical expression for the bit-error-rate (BER) of the system is derived and comparison with one-way relay is made. Preliminary results indicate that 2-way relaying with PSPF suffers only a moderate loss in energy efficiency (of 1.25 dB) when compared to its one-way relaying counterpart. We attribute the loss to non-optimal power allocation amongst the three nodes in the system. On the other hand, PSPF improves the transmission efficiency by 33% and it offers a way to avoid expensive linear power amplifiers and complicated signal processing at the relay. It is thus a viable alternative to some of the 2-way relaying techniques in the literature.

In this paper we proposed two extensions to the recently proposed Phase Forward (PF) relaying strategy for cooperative communication. One extension is the replacement of CPFSK modulation by the more practical Gaussian filtered minimum shift keying (GMSK) modulation scheme. The second extension is the incorporation of antenna selection at the relay. Semi-analytical expressions for the bit error probabilities (BEP) of the two extension systems are derived in the paper. It is found that PF-GMSK can attain similar BEP performance as PF-CPFSK, despite the fact that it is a more bandwidth efficient modulation. Furthermore, it is better than GMSK with decode-and-forward (DF). It is also found that PF, with antenna selection at the relay, can actually outperform amplify-and-forward (AF) without antenna selection in a time-selective fading environment. It also leads to a lower outage probability when compared to time-diversity in an operating environment with shadowing.

We derive in this paper the bit error probability and cutoff rate of an Amplify-and-Forward (AF) relay system operating in Rayleigh fast fading. In our model, we consider the relay thermal noise to be non-negligible and derive a closed form expression to approximate the bit error probability. Our approximated closed form expression matches well with simulation result. Using Chernoff bound, we upper bound its pairwise error probability and we also derive the cutoff rate of the AF relay system.

In the IEEE 802.16 WiMAX standard, fixed codebooks are used for the purpose of transmitter beamforming. Transmission of the codebook indices is accomplished via digital limited feedback uplink. We propose an extension to the feedback scheme that can improve BER performance at the cost of slight increase in feedback rate. Namely, for each feedback period, we deploy an additional analog feedback symbol to encapsulate channel information that is otherwise lost in the codebook quantization process. This analog/digital hybrid feedback scheme essentially converts the fixed codebook into an adaptive one that matches the instantaneous channel condition, allowing us to properly perform beamforming in a time-selective fading channel. As verified in our simulation, the hybrid feedback scheme offers better BER performance compared to the fixed codebook pure digital feedback scheme.

In this paper, we investigate the performance of a single-relay assisted cooperative link, where the source, relay, and destination terminals are equipped with multiple transmit and receive antennas. We focus on the so-called transparent amplify-and-forward (T-AaF) for MIMO relay channels, in which the relay requires neither channel state information (CSI) nor synchronization of symbols or carrier. Specifically, we propose and analyze new selection schemes, where the relay, based on the received signal energies, selects the "best" received signal on the S → R hop and forwards it to the destination terminal with either repetitive or selective transmission. Through the derivation of pairwise error probability (PEP) expressions and asymptotic order of diversity (AOD) analysis, we demonstrate that the proposed selection schemes preserve full diversity order, while maintaining low complexity implementation, which makes them attractive from a practical point of view. Furthermore, we show that adopting selective transmission on the R → D hop provides better performance, better throughput, and significantly better energy efficiency than repetitive transmission. A Monte Carlo simulation study is also presented to corroborate the analytical results and to provide detailed performance comparisons between the two schemes under consideration.

Reliable spectrum sensing in low signal-to-noise ratio (SNR) condition is one of the major technical challenges in cognitive radio network. In this paper, we propose a robust spectrum sensing algorithm for unsynchronized low SNR OFDM signal by mitigating the impacts of time and frequency offsets with multiple processing time window and sliding frequency correlator, respectively. In addition, considering that the noise's statistics are unknown to the devices and varying over time, our proposed algorithm is based on a ratio threshold and hence it is not sensitive to the noise power level. Our theoretical and simulation results show that this algorithm works effectively at very low SNR, while being insensitive to time and frequency offsets, and requires no information of the noise's statistics.

We propose in the paper the concept of phase forward (PF) as a possible relay strategy for cooperative communication involving CPFSK modulation in a "fast" fading environment. The technique enables the relay nodes to maintain constant envelop signaling without the need to perform decoding and signal regeneration. To further reduce complexity, we adopt noncoherent discriminator detection at the destination node. A semianalytical expression for the bit error probability (BEP) of this phase forward non-coherent CPFSK cooperative transmission scheme is derived in the paper. The analysis is general in the sense that it can accommodate different Doppler frequencies and signal-to-noise ratios in the various links. It was found that PF can provide a strong diversity effect, especially when the signal-to-noise ratio in the source-relay link is substantially stronger than those in the source-destination and the relay-destination links. Furthermore, from a BEP stand point, phase forward is better than conventional decode-and-forward (DF) under typical channel conditions. It can also attain the same performance as amplify-and-forward (AF) when fading is static. Finally, we found that the concept of PF applies equally well to PSK modulations.

In this paper, we consider a single-relay cooperative scenario, where the source, relay, and destination terminals are equipped with multiple transmit and receive antennas. We focus on the so-called transparent amplify-and-forward (T-AaF) for MIMO relay channels, in which the relay requires neither channel state information (CSI) nor synchronization of symbols or carrier. Specifically, we propose and analyze new selection schemes where the relay, based on the received signal energies, select the "best" received signals on the S rarr R hop and retransmits them to the destination terminal with either repetitive or selective transmission. Through the derivation of pairwise error probability (PEP) expressions and asymptotic order of diversity (AOD) analysis, we demonstrate that the proposed selection schemes maintain full diversity order. Furthermore, we show that selective transmission on R rarr D hop has better performance, better throughput, and significantly better energy efficiency than repetitive transmission.

In this paper, we investigate distributed differential space-time block coding (STBC) for cooperative communications over frequency-selective fading channels. We carefully exploit the unitary structure of the orthogonal STBC to design a low complexity differential STBC receiver for multi-carrier broadband cooperative networks. We consider amplify-and-forward relaying and assume a single-relay scenario. Under the assumption of perfect power control for the relay terminal and high signal-to- noise ratio for the underlying links, our performance analysis demonstrates that the considered scheme is able to exploit fully the spatial diversity. We further present Monte Carlo simulation results to confirm our analytical observations.

A recent study demonstrated that the technique of subpacket retransmission can substantially improve the throughput of an ARQ system. To select the subpackets suitable for retransmission, acyclicredundancycheck(CRC) was employed at the subpacket level. In this paper, we propose aprobabilisticsubpacketretransmissionARQ (PSR-ARQ) scheme for wireless systems, which does not employ CRC bits at the subpacket level. Rather, we utilize the demodulator's decision statistics to assess the reliability of the decoded subpackets. In the event of an erroneous large packet, the subpacket with the lowest bit-wise posterior probability is retransmitted. Using updated subpacket reliability metrics, individual subpacket retransmissions are continued until the overall CRC of the large packet is verified. By avoiding the use of CRC bits at the subpacket level, our PSR-ARQ scheme exhibits a much lower transmission overhead relative to the benchmark scheme. This translates into a significant improvement in power efficiency (on the order of 6 dB) at a high throughput level.

Generalized selection combining (GSC), whereby the receiver selects M out of N received replicas of the same signal for combining, is an effective mean to achieve reliable transmission in fading channel. However, in order to perform coherent combining, the selected signals must be individually tracked for sufficiently long time before accurate channel estimates can be produced for combining purpose. This tracking operation, unfortunately, is incompatible with the inherently antenna switching that takes place inside the GSC receiver. In this paper, we extend the switching rate analysis of M out of N GSC receivers in (J. Cavers et al., 2007) to the case of independent but statistically non-identical branches. Despite the fact that non-identical branches introduces a correlation between the difference of the M-th and the M+1-th strongest signals, u'(t) , and its derivative, u'(t) , we were able to derive an analytical expression for the switching rate of the GSC receiver under this condition (independency between u'(t) and u'(t) is crucial in obtaining the simple results in (J. Cavers et al., 2007)). Our numerical results agree with the intuition that having non- identical branches reduces the switching rate. The more dissimilar the branches are, the larger the reduction. While this lowering of the switching rate allows the GSC receiver more time to dwell on the selected signals and hence producing more accurate channel estimates for coherent combining, the bit-error rate of GSC, unfortunately, is higher when the branches are not identical.

The limited availability of spectrum and the inefficiency of its usage necessitate new research on spectral opportunistic communication technologies including cognitive radio (CR). Such new systems are characterized by the coexistence of the heterogeneous wireless systems. In this paper, a radiation footprint minimization technique is proposed through encoded in-band pilot tones for Orthogonal frequency division multiplexing (OFDM) system. A transmission power negotiation signaling between the transmitter and receiver is established through the encoded pilot tones. Electromagnetic interference to the primary and other cognitive radios can be minimized with an automatically reduced radiation footprint. In addition, system performance of the receiver can be guaranteed in the process of mutual interference minimization. The transceiver structure and the inband pilot tone detection algorithm are investigated. The principle and performance of the system are validated through numerical simulations.

In this paper, the performance of a closed-loop transmit beamforming system employing the ";unconventional"; technique of analog channel state information (CSI) feedback is studied. The use of analog or unquantized symbols for CSI feedback leads to a low feedback rate of only one symbol per transmit antenna per frame, and does not result in an unrecoverable distortion as in the traditional digital CSI feedback approach. The BER of differential BPSK and 8PSK in the downlink of the proposed system is evaluated via computer simulation. The results show that analog feedback can perform substantially better than digital feedback under some operating conditions. Another contribution of the paper is the use of position dependent modulation (PDM) to counter the varying beamforming quality across the data frame. It was shown that PDM can substantially increase the downlink throughput (relative to optimal fixed modulation) at given target BERs.

A new cross-layer signaling interface using overlay watermarks for orthogonal frequency division multiplexing (OFDM) system is proposed in this paper. The major advantage of the new proposal is the multiple functionalities the overlay watermark provides, which includes a cross-layer signaling interface, a transceiver identification for position-aware cross-layer design, as well as its basic role as a training sequence for channel estimation. Location information of a mobile can be easily derived for efficient routing when a unique watermark is associated with each individual mobile transceiver. In addition, a new data pipe can be created by modulating the overlay watermarks for the transmission of cross-layer signaling and other interlayer interactive information. We also study the channel estimation and watermark removal techniques at the physical layer for the proposed overlay OFDM. Our channel estimator iteratively estimates the channel impulse response and the combined signal vector from the overlay OFDM signal. Cross-layer design that leads to low power consumption and more efficient routing are investigated.

In this paper, an expression for the joint probability density function (pdf) of the order statistics for a set of arbitrarily correlated Nakagami-m fading channels is derived. This result is useful in analyzing the performance of a variety of diversity schemes which involve branch selections. The derivation also yields a computationally efficient method for obtaining the marginal pdf of the p-th order statistic for a set of arbitrarily correlated Nakagami-m random variables. As an illustration, the derived result is applied to the performance analysis of the generalized selection combining (GSC) scheme with arbitrarily correlated branches.

Motivated by the desirable attributes of unitary space-time (ST) codes and constant envelope modulations, we introduce an ST continuous phase modulation (CPM) methodology and its companion noncoherent receiver. We consider M-ary full-response CPMs, with a modulation index of 1/M and multiple transmit antennas. Through careful selection of an underlying differential group code, the defining characteristics of the CPM transmissions (constant envelope and spectral bandwidth) are effectively maintained. Using a set of interference suppressed statistics and taking advantage of the underlying ST differential encoding, the receiver performs conventional differential detection (CDD). Results for M = 2 with four transmit antennas show that the proposed CDD performs within 4 dB of the coherent detection lower bound, at a symbol-error-rate of 10-. Furthermore, due to the underlying unitary structure of the ST-CPM, the CDD achieves a diversity order comparable to that of coherent detection at high SNR.

We present a class of suboptimal multiple-symbol differential detectors (MSDDs) for differential unitary space-time (ST) codes in a time-selective Rayleigh flat-fading channel. These noncoherent detectors, termed Fano ST-MSDDs, employ the well-known Fano algorithm as their decoding engines. Their bit-error rate (BER) performance is excellent, as there is no irreducible error floor, even at very high fade rates. Compared with the sphere decoder, which is optimal, our best detector, termed an on-demand bi-Fano ST-MSDD, suffers only a 0.3 dB degradation in power efficiency. However, its computational complexity, as a function of the signal-to-noise ratio (SNR), is consistently lower than that of the sphere decoder and does not experience the exponential growth in complexity in the latter when the SNR decreases. It can thus be concluded that the Fano algorithm is a powerful detection strategy for differential ST codes in fading channels. Our main contribution is in making the modifications and improvements to the Fano algorithm that enables this realization. The innovations include the bi-Fano, edge-bi-Fano, and on-demand bi-Fano search strategies, as well as a new dynamic threshold adjustment scheme for avoiding unnecessary back-and-forth node evaluations during backtracking

In this paper, we investigate the performance of differentially detected differential phase-shift keying (DPSK) modulation with postdetection maximal ratio combining, in nonselective Rayleigh fading channels with multiple asynchronous cochannel interferers. The approach is based on an analytical technique we have presented earlier in the literature. Exact bit-error probability (BEP) results for binary DPSK and quaternary DPSK are derived. More specifically, we look into the effects of symbol-timing offsets between the interfering signals and the desired signal on the error performance. Our results show that when all the interfering signals are synchronous with the desired signal, the impairment caused by the cochannel interference to the desired user is maximum. On the other hand, when all the interfering signals are half-symbol-duration-delayed with respect to the desired user, they introduce the minimum impairment. Based on these findings, upper and lower bounds on the BEP are derived in simple closed form. Our explicit BEP results also show that the error probabilities of different transmitted symbols of the desired user are affected differently by the interfering signal

Many investigations of hybrid selection (HS) diversity assume maximal ratio combining (MRC) of the selected branches. However, a coherent detector needs to dwell on the received signal for some time before it can produce accurate channel estimates for fading compensation, a requirement that appears inconsistent with the branch switching that occurs in a selection diversity receiver. Motivated by this observation, we derive in this letter analytical results on the switching rate and average dwell time of a selection diversity receiver where M out of a total of N independent branches are selected for combining. We show that the switching rate can be many times the Doppler frequency, while the average dwell time can be a small fraction of the reciprocal Doppler frequency. The brevity of the dwell times suggests difficulty in obtaining channel state information, which in turn calls into question performance analyses of idealized HS/MRC structures. Our results also suggest that HS/MRC should be frame-based, rather than continuously acting in time

We develop in this paper an Alamouti-type space- time (ST) block code that employs orthogonalM-ary FSK (MFSK) modulation. It is demonstrated that every transmitted symbol in the proposed ST-MFSK scheme can be treated as an implicit pilot symbol, and consequently the receiver can attain close to ideal coherent bit-error performance even in the absence of an explicit channel sounding signal. In addition to employing the popular block-wise static fading model for performance evaluation, we also study the implications of a more realistic fading model whereby the fading gains are allowed to vary from one subinterval to another within a ST code block. It was discovered that an iterative channel estimation strategy has to be adopted to mitigate the cross-talk associated with this refined channel model. The iterative channel estimator, in conjunction with a companion minimum mean square error detector, or a companion maximum likelihood detector, enable the receiver to obtain a bit-error performance similar to that obtained under the block-wise static fading model. In comparison with differential ST-MPSK, the two approaches yield similar error performance in a static fading environment. However, the error performance of differential ST-MPSK deteriorates rapidly as the fade rate increases whereas the performance of ST-MFSK is almost independent of the fade rate. In terms of implementation complexity and delay of the receiver, the proposed coherent ST- MFSK scheme and differential ST-MPSK are very similar, as both approaches employ digital filters of similar lengths. The findings in this paper suggest that despite its larger bandwidth requirement, ST-MFSK is a very attractive alternative space-time signaling format.

We introduce a space-time (ST) continuous phase modulation (CPM) and its companion noncoherent receiver. Through careful selection of an underlying differential code, the defining characteristics of the CPM transmissions are effectively maintained. Using a set of interference suppressed statistics and taking advantage of the ST differential encoding, the receiver first performs low complexity block detection. Channel estimates are then obtained at a rate notably higher than the CPM symbol rate, enabling a second-stage coherent sequence detector to refine the initial decisions. Simulation results for two CPM formats, with two and four transmit antennas, validate the proposed ST-CPM scheme.

In a recent study [7], the authors successfully incorporated minimum-shift keying (MSK) into an Alamouti-type space-time (ST) transmission system, operating in a fading channel environment. This work extends the investigation to include an enhanced receiver and a more realistic channel model that does not limit the fading to be constant over each ST block. The proposed receiver first detects an underlying differential ST π / 2 -shifted BPSK code present in the ST-MSK signal, by performing low-complexity non-coherent multiple-symbol differential detection with decision feedback. The relatively reliable initial data estimates are then used to obtain high-quality channel estimates, at a rate notably higher than the MSK symbol rate. The availability of these channel estimates enables a second-stage coherent sequence detector to significantly refine the initial data decisions. In fast fading, results show that the proposed receiver performance is within 2 dB of the coherent detection lower bound, at a bit-error-rate of 10-5. When compared to a linearly modulated ST π / 2 -shifted BPSK code, the proposed ST-MSK scheme achieves a greater order of diversity and hence better performance in a fast fading environment. We attribute this to the additional time-diversity effect associated with the phase coding evident in MSK.

Many investigations of hybrid selection (HS) diversity assume maximal ratio combining (MRC) of the selected branches. However, a coherent detector needs to dwell on the received signal for some time before it can produce accurate channel estimates for fading compensation, a requirement that appears inconsistent with the branch switching that occurs in a selection diversity receiver. Motivated by this observation, we derive in this letter analytical results on the switching rate and average dwell time of a hybrid selection diversity receiver where M out of a total of N branches are selected for combining. As one would expect, the switching rate is proportional to the Doppler frequency, while the average dwell time is proportional to the duration of a fade cycle. More importantly, the results in the paper suggest caution in interpreting the results of performance analysis of idealized HS/MRC structures. They also shed some light on the demodulator design in these systems

This paper is concerned with the efficient transmission of channel state information (CSI) in the uplink, and data in the downlink, of a closed-loop transmit diversity system. Two ideas were explored: 1) the employment of analog CSI feedback whereby estimates of the downlink complex gains were fed back periodically to the base-station at a rate of only 1 analog symbol per estimate per frame, and 2) the use of position dependent modulation (PDM) to counter the signal-to-noise ratio variation across a downlink data frame. Simulation results indicated that as far as the bit-error-rate (BER) is concerned, analog feedback is superior to digital feedback because it does not experience unrecoverable quantization errors in the feedback channel estimates. The simulation also indicated that at a given target BER, PDM can attain a substantially higher throughput than fixed modulation

We present in the paper a multiple-symbol differential detector (MSDD) for differential space-time (ST) codes in Rayleigh flat fading channel. The detector, termed a Fano ST-MSDD, uses the Fano algorithm as its decoding engine and is capable of delivering excellent error performance at moderate implementation complexity over a wide range of fading rates. Essentially, the detector is an "intelligent" decision feedback detector (DFD) that uses a running threshold and the accumulated path metric as navigation tools when it roams the decoding tree. In the static channel, our best Fano ST-MSDD scheme with a detection window size of N=6 is able to narrow the original 3 dB gap between ideal coherent and conventional ST differential detection by 1 dB. In a fast fading channel with a Doppler frequency of three percent the ST symbol rate, the error curve of our best TV-10 Fano ST-MSDD is able to "track" that of the ideal coherent detector (with a 3-dB gap in between) and there is no irreducible error floor. All these performances become even more remarkable when we consider the rather moderate implementation complexity reported in the paper. Because of its close relationship with a DFD, the Fano ST-MSDD has a similar complexity as the DFD at large signal-to-noise ratio (SNR). Actually, we found that the complexity of the Fano ST-MSDD is a relatively stable function of the SNR

We investigate in this paper an Alamouti-type space-time block code (STBC) using orthogonal M-ary frequency-shift-keying (MFSK). The signal is coherently detected using the channel estimates derived from the unmodulated (or implicit pilot) component of the orthogonal signal. An exact bit error probability expression for BFSK and tight union bound for MFSK are derived for the proposed coherent ST-FSK receiver. It is observed that the receiver can maintain close to ideal coherent performance even at very fast fading rates. In comparison with differential STBC, the two approaches yield similar error performance in a static fading environment. However, the error performance of differential STBC deteriorates rapidly as the fade rate increases. In terms of implementation complexity and delay, the proposed coherent ST-FSK receiver is similar to differential STBC, as both approaches employ digital filters of similar lengths. The findings in this paper suggest that despite its larger bandwidth requirement, ST-FSK is a very attractive alternative space-time signaling format.

Multiple-symbol differential detection (MSDD) is a robust maximum-likelihood (ML) receiver technique for frequency-nonselective fast Rayleigh fading channels. However, its complexity grows exponentially with the block size N and that makes it impractical to implement. Recently, multiple-symbol differential sphere decoder (MSDSD) is developed to alleviate this problem but its complexity at low signal-to-noise ratio (SNR) grows exponentially with decreasing SNR. In this paper, we explore the potential of the Fano algorithm as an efficient MSDD receiver. The bit-error performance and the complexity of the Fano-MSDD are evaluated and compared with other reduced complexity techniques. Our preliminary results indicate that Fano-MSDD is more attractive than decision-feedback differential detection (P.Y. Kam and CH Teh, 1983) and (R Schober and WH Gerstacker, 2000) from both the error performance and the complexity points of view. When compared to the MSDSD, the Fano-MSDD suffers a moderate degradation in power efficiency. However, its computational complexity is very steady even at low SNR. This translates into a dramatic saving in complexity over the MSDSD when SNR is low. Even at large SNR, the Fano-MSDD still provides a small edge over the MSDSD in terms of complexity. We believe that with some fine tuning of the decoder parameters, such as the bias and the threshold's step size, it is possible to extract better performance from the Fano decoder than it is now.

We consider noncoherent block detection of Alamouti-type space-time (ST) modulations, employing PSK constellations in quasi-static Rayleigh-fading channels with L receive antennas. The proposed detector, termed an eigen-assisted (EA) receiver, constructs a sample-correlation matrix from the L length-N received signals, determines its two principal eigenvectors, and uses these eigenvectors to reconstruct the two transmitted length-N patterns. Scalar differential encoding is performed at the transmit antennas, and as a result, the transmitted data can be recovered from the reconstructed patterns using scalar multiple-symbol differential detection. In other words, ST-differential encoding is not required at the transmitter and the constellation expansion typically observed with nonbinary signaling is avoided; a highly desirable result under a peak power constraint. Furthermore, the performance of the proposed EA-receiver is only 0.25 dB away from the coherent detection (with differential encoding) lower bound for the modulations considered. For quadrature phase-shift keying at a bit-error rate of 10/sup -4/, our EA-receiver with N=64 outperforms a decision feedback detector by 1 dB (L=1) and conventional ST-differential detection by more than 2.5 dB (L=2). Note that the complexity of our receiver, per symbol decision, is essentially independent of N and is comparable to that of a conventional ST-differential detector. The conclusion is reached that the proposed encoder/receiver pair is a promising alternative to currently known noncoherent techniques employing Alamouti-type ST-modulations.

A feature-suppressed orthogonal frequency-division multiplexing (OFDM) system and the corresponding channel estimation and intersymbol interference (ISI) mitigation techniques are investigated in this paper. Cyclic prefix (CP) and pilot tones, which are commonly used in civilian OFDM systems for ISI mitigation and channel estimation, create distinctive waveform features that can be easily used for synchronization and channel estimation purposes by intercepting receivers. As a result, CP and pilot tones are eliminated in the proposed feature suppressed OFDM system to reduce the interception probability. Instead, a set of specially designed OFDM symbols, driven by different pseudorandom sequences, are employed as preambles to avoid unique spectral signature. These preambles are inserted into the OFDM data symbol stream periodically and in a round-robin manner. In addition, a random frequency offset is introduced to each preamble to further mask the multicarrier signature. New challenges arising from these feature suppression efforts are studied, including robust channel estimation and demodulation techniques in the presence of frequency offset and severe interference. Based on our interference analysis, an iterative ISI and intercarrier interference (ICI) estimation-cancellation-based technique is proposed for both channel estimation and OFDM data demodulation. Our channel estimator performs joint frequency offset and channel impulse response estimation based on the maximum-likelihood (ML) principle. To reduce its complexity, we employ a number of techniques, which include approximation of the ML metrics, as well as fast Fourier transform pruning. The performances and feasibility of the proposed feature suppressed OFDM system and the channel estimator are analyzed and verified through numerical simulations.

A coherent symbol-by-symbol (SBS) diversity receiver for m-ary phase-shift keying (MPSK) and differentially encoded MPSK (MDPSK) signals transmitted over a nonselective Rayleigh fading channel is presented. It incorporates a new adaptive filter for channel estimation that does not require any prior knowledge of the fading process model. It estimates the fading gains through decision-feedback and recursive least squares adaptation of the filter coefficients. A novel forgetting-factor adaptation algorithm that enables the filter to react quickly to randomly changing fading statistics caused by shadowing and acceleration/deceleration is introduced. Simulations show that the receiver performs better than that of Adachi's ( IEEE Trans. Veh. Technol. vol. 47 p. 909, 1998), either without shadowing or under slow lognormal shadowing.

We consider the issue of non-coherent detection of Alamouti-type space-time (ST) modulations, employing phase- shift keying constellations and L receive antennas, in static Rayleigh fading channels. We observe that the two dominant eigenvectors of the received signal's correlation matrix are linear combinations of the two transmitted signal patterns. Consequently, we propose a receiver that recovers the transmitted patterns through linear combinations of these eigenvectors. Through the use of two scalar multiple-symbol differential detectors (MSDD), our receiver is able to perform low complexity sequence detection of the data contained in the two reconstructed patterns. The overall complexity of our receiver is 3 N , where N is the size of the MSDDs. This corresponds to a dramatic reduction from the exponential complexity of the optimal sequence detector for ST modulations. Our results show that for QPSK, N=64 and 3 receive antennas, the proposed receiver's performance is within 0.2-dB of the ideal coherent lower bound. Furthermore, with the proposed receiver processing methodology, there is no need for ST differential encoding; scalar differential encoding is sufficient. Keywords-Alamouti modulation, non-coherent detection, scalar- MSDD, eigen-decomposition.

Hybrid ARQ protocols are capable of improving the robustness and throughput of a mobile wireless communication system when a feedback channel is available. In this paper a type-II hybrid ARQ protocol is implemented with a punctured Reed-Solomon (RS) code to result in a rate-compatible system. It is proposed to use side-information provided by pilot symbols to flag symbol erasures, and perform erasure decoding of rate-compatible RS codes. Throughput simulation results show that when using errors-only decoding, rate-compatible punctured RS codes outperform their constituent punctured RS codes. When comparing errors-only decoding to erasure decoding of the rate-compatible RS codes, a power savings is observed when erasure decoding is employed. The specific gain achieved however, is dependant on the average signal-to-noise ratio and constituent set of punctured RS codes.

Multiple-symbol differential detection (MSDD) and pilot symbol-aided detection (PSAD) are two detection schemes that have been proposed for detecting data transmitted over fast fading channels. When the parameters of these detectors are chosen properly, they can provide an error performance that is close to ideal coherent detection. In this paper, we apply these two detection methods to a multiuser receiver for direct sequence CDMA. The multi-user receiver is implemented in the form of a multi-channel linear equalizer (LE), followed by either a bank of MSDD or a bank of PSAD. With this receiver structure, the front end equalizer is not required to perform any channel estimation. Fading gain estimation and compensation are done by the MSDD or the PSAD. It was found that both MSDD and PSAD perform substantially better than conventional differential detection when the signal-to-noise ratio (SNR) is large (greater than 20 dB) and/or when the fade rate is high (greater than 1 percent the symbol rate).

We consider an orthogonal frequency division multiplexing (OFDM) system with successive interference cancellation to reduce interchannel interference. The system operates in a Rayleigh frequency selective channel and pilot symbols are inserted in each OFDM frame for the purpose of channel estimation. Interchannel interference (ICI) is caused by the time variations in the channel gain within one OFDM frame. The ICI leads to an error floor, which is a function of the Doppler frequency. It was found that by performing successive interference cancellation at the receiver, we were able to significantly reduce the error floor caused by ICI. Furthermore, we introduce a simple channel estimator based on pilot symbols to estimate the multipath fading channel information required to perform the SIC.

This paper presents a method for the exact calculation of the union bound on the error performance of trellis-coded modulation (TCM) in fading channels. Previous analyses have approximated the error expression to allow the use of the transfer function, or restricted their attention to the most common error events, or both. In contrast, we combine the transfer function, which counts all error events, with exact calculation of event probabilities, and produce an exact expression for the union bound. Although computationally intensive, the technique produces the tightest bound yet published. In addition, it is very general and is applicable to coherent, differential, and pilot-based detection, and to phase-shift keying (PSK)- and quadrature amplitude modulation (QAM)-types of constellations.

This paper is concerned with the subject of coding and detection for DS/CDMA systems operating under very fast Rayleigh fading. Specifically, we compare the performance of various chip level detectors, including the chip level pilot-assisted coherent detector, chip level differential detector, chip level non-coherent detector, and the chip level maximum likelihood detector. The true union bounds on the bit error rate of these detectors were computed, with the fade rate, coding rate, and the spreading factor as parameters. It was found that these detectors are very robust against fast fading. A reasonable level of data integrity can be maintained even if the fade rate is as large as 10% of the chip rate. Among the various detectors, the pilot-aided chip level coherent detector has the best performance while the chip level differential detector is the worst. Surprisingly enough, the optimal pilot-aided detector, under the assumption of ideal interleaving, uses a transmission format that consists of an alternating pattern of pilot and coded data symbols. It was also found that chip level differential detection performs better than its symbol level counterpart only when the fade rate exceeds 6% of the chip rate.

We present the true union bound of a DS/CDMA system that employs convolutional coding and a quadratic detector. In our study, the total bandwidth expansion factor caused by coding and spreading is fixed and the channel is assumed to exhibit Rayleigh flat fading. The objective is to assess the performance of various quadratic detectors as a function of the spreading factor and the fade rate. Specifically, four detectors were considered: chip level differential detection, chip level maximum likelihood sequence estimation, noncoherent detection, and perfect coherent detection. It was found that in the presence of fast Rayleigh fading, different detectors behave differently when the spreading factor varies. For example, the chip level differential detector and the ideal coherent detector favour a small spreading factor and a low rate code, while the MLSE prefers a high rate code and a high spreading factor. For the noncoherent detector, there actually exists an optimal system configuration at certain fade rates.

This paper focuses on a reference symbol assisted multiuser interference cancelling receiver for a quasi-synchronous DS/CDMA system using BPSK modulation. The channel is assumed Rayleigh and the propagation delay differences between the users are assumed to be a fraction of the symbol time. The receiver employs two structures: a decorrelator structure for channel estimation based on pilot symbols, and a successive interference cancellation scheme (SIC) for data detection. We refer to this receiver as a hybrid receiver and its performance will be compared with that of a pure SIC receiver for both channel estimation and data detection. To facilitate channel estimation in the presence of propagation delay differences, we shortened the correlation interval for the pilot symbols but increased the pilot power accordingly. The numerical results indicate that as far as the bit error rate is concerned, a one-stage hybrid receiver is as good as a two-stage pure SIC receiver. Although the decorrelation receiver has to perform matrix inversions, the matrix updates can be obtained by a computational efficient order recursive procedure. Moreover, the matrix calculations can be spread over a frame, allowing us to keep the computational load per symbol on a moderate level. Therefore, we believe our hybrid receiver scheme offers a good performance at an acceptable level of implementation complexity.

We present a coherent detection technique for continuous phase modulation (CPM) operating in the Rayleigh flat fading channel. The technique is based on the idea of inserting periodically data dependent pilot symbols that force the CPM signal to pass through known phase states. This transmission format enables the receiver to extract from the received signal the channel fading gains at regularly spaced instants. When coupled with proper channel estimation filters, very accurate channel state information (CSI) can be estimated at the receiver for fading compensation. Moreover, the accuracy of the CSI can be further refined by adopting a multiple-pass decoding approach. The paper discusses (a) the pilot symbol encoding technique required to force a M-level CPM scheme with a modulation index of p/M, p is an integer, to return periodically to a set of known phase states, (b) the optimal channel estimation filters, (c) a trellis-based precoding technique that can reduce the bit error rate in M-level CPM systems by close to 50%, and (d) a multiple-pass channel estimator/demodulator. Analytical and simulation results are presented for minimum shift keying (MSK), Gaussian MSK, and four-level continuous phase frequency shift keying with a modulation index of 1/4. It is observed that our pilot symbol-assisted CPM schemes exhibit no irreducible error floor even at a channel fade rate of three percent the symbol rate. The implicit phase coding in CPM and the accurate CSI provided by the pilot symbols lead to a diversity effect in the bit error rate curves of these modulation schemes.

Code division multiple access (CDMA) based on direct sequence (DS) spread spectrum (SS) modulation has been advocated for next generation personal/cellular communications. The topic of code acquisition is an important design issue in a CDMA system that supports multimedia services. Although in isolation acquisition is a physical layer design issue, the performance of this subsystem has a strong impact on the overall system design. As an example, the system designer has to decide for bursty traffic applications whether it is more desirable to acquire on a per packet basis or is it better to transmit a low rate signal to keep the communication link in synchronism (both in terms of chip timing and power control) in between data bursts. We present an acquisition procedure, based on a differentially encoded Barker sequence as the preamble, for CDMA packet radio systems that is able to provide frame and chip synchronization simultaneously. We also introduce a two-step algorithm and a windowing technique which we found effective in overcoming the partial correlation problem. With simulation studies, we found that the new scheme outperforms other conventional schemes both under a frequency selective fading channel and a AWGN channel.

Code Division Multiple Access (CDMA) is a promising technology for future digital cellular and personal communications. To despread the pseudo noise (PN) code and thus understanding the message sent, synchronization (acquisition fluid tracking) is necessary in priori. As a matter of fact, the throughput of a packet mode CDMA system is directly related to the performance of its frame synchronization algorithm. In this paper, we propose a new acquisition technique for packet mode CDMA operating in the Rayleigh fading channel. The technique is based on on-off keying (during the preamble section) followed by a Markov chain search. The length of the preamble is a design parameter and the performance measure is the throughput of packets. We have tested our new scheme on a flat fading and a selective fading channel, based on a fixed blocking probability criterion. Specifically for the latter case, we adopt a RAKE receiver to combat the multipath effect, and introduce a windowing technique which we found effective in overcoming the temporary increase in noise level caused by partial correlation of the PN sequence used. Results in both cases show that the proposed on-off keying technique is more efficient than many others in the literature such as the coincidence detector or simple Markov search.>

Although the mobile communication channels are time-varying, most systems allocate the combined rate between the speech coder and error correction coder according to a nominal channel condition. This generally leads to a pessimistic design and consequently an inefficient utilization of the available resources, such as bandwidth and power. This paper describes an adaptive coding system that adjusts the rate allocation according to actual channel conditions. Two types of variable rate speech coders are considered : the embedded coders and the multimode coders and both are based on code excited linear prediction (CELP). On the other hand, the variable rate channel coders are based on the rate compatible punctured convolutional codes (RCPC). A channel estimator is used at the receiver to track both the short term and the long term fading condition in the channel. The estimated channel state information is then used to vary the rate allocation between the speech and the channel coder, on a frame by frame basis. This is achieved by sending an appropriate rate adjustment command through a feedback channel. Experimental results show that the objective and the subjective speech quality of the adaptive coders are superior than their non-adaptive counterparts. Improvements of up to 1.35 dB in SEGSNR of the speech signal and up to 0.9 in informal MOS for a combined rate of 12.8 kbit/s have been found. In addition, we found that the multimode coders perform better than their embedded counterparts.>

This work presents an exact expression for the pairwise error event probability of trellis-coded modulation (TCM) transmitted over Rayleigh-fading channels. It includes phase shift keying (PSK) and multilevel quadrature amplitude modulation (QAM) codes, as well as coherent and partially coherent (e.g. differential, pilot tone, etc.) detection. Due to the form of the exact pairwise error event probabilities, this calculation technique cannot be used with the transfer function technique to obtain an upper (union) bound on the overall bit error probability. For this reason, the authors estimate the bit error probability by considering only a small number of short error events. Through simulations, they found that the estimation is usually very accurate at high signal-to-noise ratios but not as accurate at lower signal-to-noise ratios. They study several coded modulation schemes this way. Among the results are the fact that TCM provides significant improvement in the error floor when detected differentially, and an asymmetry in the pairwise error event probability for 16 QAM.>

The error performance of multiple-symbol differential detection of uncoded QPSK signals transmitted over correlated Rayleigh fading channels is studied. The optimal detector is presented, along with an exact expression for the corresponding pairwise error event probability. It is shown that multiple-symbol differential detection is a very effective strategy for eliminating the irreducible error floor associated with a conventional differential detector. In all of the cases investigated, a detector with an observation interval as small as two symbols is sufficient for this purpose. It is also found that the error performance of a multiple-symbol differential detector is not sensitive to the mismatch between the decoding metric and the channel fading statistics.>

The authors derive an exact and easily computed expression for the pairwise error event probability of interleaved coded PSK modulations transmitted over channels with correlated Rayleigh flat fading and additive white Gaussian noise. Both coherent and differential detection are considered. In the case of coded DPSK, it is found that full interleaving does not necessarily provide the best error performance, especially when the fading is relatively fast and when the autocorrelation function of the channel fading process exhibits an oscillating behavior. For coherent detection or for differential detection in channels with relatively slow fading, increasing the interleaving depth always improves the error performance. In these cases, an interleaving depth equivalent to one-fifth to one-quarter the duration of a fade cycle is almost as good as full interleaving.>

The performance of 8-PSK and 8-DPSK trellis codes is presented for a class of fast fading, land mobile satellite communication channels. The fading model is Rician but, in addition, the line-of-sight path is subjected to a fast lognormal attenuation that represents tree shadowing. The fading parameters used in this study represent the degree of shadowing and are based on measured data. The primary application considered is for digital speech transmission and thus, bit error probabilities in the order of 10/sup -3/ are emphasized. Sensitivity of the bit error probability to amplitude fading, amplitude and phase fading, and decoding delay is presented. Performance is determined via digital computer simulation. Optimal four- and eight-state codes are determined and optimality is found to be dependent on the presence of lognormal shadowing.>

A comparison of different classes of encoded continuous phase modulation (CPM) schemes is presented. In terms of joint power-bandwidth performance, it is found that coded eight-level systems are better than coded four-level systems and that partial-response signalling is better than full-response signalling. In general, the price paid for better performance is increased receiver complexity. Heuristic design rules have been used to find good codes for CPM. In most cases the coding gains found are identical to published results that were generated by exhaustive search, indicating that the choice of design rules is quite appropriate. In the present study bandwidth efficiency is considered from both the 99% and 99.9% power-bandwidth points of view. It is found that partial-response signalling is even more jointly power and bandwidth efficient for the latter than for the former bandwidth criterion.>