In this paper, we investigate noncoherent detection, i.e. detection assuming the absence of channel state information at the receiver, of on-off keying in an intensity modulation and direct detection (IM/DD) free-space optical (FSO) system. To partially recover the performance loss associated with symbol- by-symbol noncoherent detection, we consider the application of multiple-symbol detection (MSD), in which block-wise decisions are made using an observation window of several bit intervals. Specifically, we develop a fast search algorithm for optimal MSD and propose a reduced-complexity decision metric suitable for suboptimal MSD; performance results confirm that the optimal and suboptimal metrics perform comparably well. Significantly, the complexity of our receiver, on a per bit-decision basis, is only logarithmically dependent on the observation window size. We also present the framework for a decision-feedback receiver and obtain performance expressions for the ideal case of error-free feedback; these expressions serve as an upper bound to the performance of MSD. Analytical and simulation results indicate that as the observation window size increases, the performance of the MSD receiver approaches that of detection with channel state information. The conclusion is reached that the proposed implementation provides an attractive low-complexity mechanism for performing noncoherent MSD in FSO systems.

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.

We employ a photon-counting signal model of a multiple-input multiple-output (MIMO) free-space optical (FSO) system and investigate detection assuming the absence of channel state information (CSI) at the receiver, in moderate to strong atmospheric turbulence. The considered modulation format is on-off keying with repetition coding across the transmitters. To partially recover the performance loss associated with symbol-by-symbol detection without CSI, we consider the application of multiple-symbol detection (MSD) to equal gain combined (EGC) statistics. We develop a fast search algorithm for EGC-MSD and propose a suboptimal closed-form decision metric suitable for reduced-complexity implementation; performance results confirm that the true and suboptimal metrics perform comparably well. Significantly, the complexity of our receiver, on a per bit-decision basis, is only logarithmically dependent on the observation window length N, and is effectively independent of the size of the MIMO array. We also present the framework for a decision-feedback receiver and obtain performance expressions for the ideal case of error-free feedback; these expressions serve as an upper bound to the performance of EGC-MSD. Analytical and simulation results indicate that the system effectively realizes the diversity gains expected from a MIMO configuration and that the performance of the EGC-MSD receiver approaches the EGC with CSI lower bound with increasing N.

In this paper, we investigate noncoherent detection, i.e. detection assuming the absence of channel state information at the receiver, of on-off keying in a free-space optical system. To partially recover the performance loss associated with symbol- by-symbol noncoherent detection, we consider the application ofmultiple-symboldetection(MSD), in which block-wise decisions are made using an observation window ofNbit intervals. Specifically, we develop a fast search algorithm for optimal MSD and propose a reduced-complexity decision metric suitable for suboptimal MSD; performance results confirm that the optimal and suboptimal MSD metrics perform equally well. Significantly, the complexity of our MSD receiver, on a per bit-decision basis, is effectively independent of the length of the observation window. Furthermore, bit-error-rate results clearly indicate that the performance of the MSD receiver approaches the coherent detection lower bound with increasingN.

Free-space optics (FSO) have received increased attention recently for last-mile, line-of-sight wireless links. In this paper, we employ a photon-counting signal model of an FSO system and investigate noncoherent detection, i.e. detection assuming the absence of channel state information at the receiver, of on-off keying (OOK). To partially recover the performance loss associated with symbol-by-symbol noncoherent detection, we consider the application of multiple-symbol detection (MSD), in which block-wise decisions are made using an observation window of N bit intervals. We develop a fast search algorithm for optimal MSD, which corresponds to a fixed bit-wise implementation complexity effectively independent of N. Furthermore, we obtain performance expressions for an ideal decision-feedback receiver, which serve as an upper bound to the performance of the MSD receiver. Analytical and simulation results indicate that the performance of the MSD receiver approaches the coherent detection lower bound with increasing N.

Free-space optics (FSO) have received increased attention recently for last-mile wireless links. In this paper, we investigate noncoherent detection, i.e. detection assuming the absence of channel state information at the receiver, of on-off keying (OOK) in an FSO system. To partially recover the performance loss associated with conventional symbol-by- symbol noncoherent detection, we consider the application of decision-feedback detection (DFD), in which symbol-by-symbol decisions are made using previous decisions and an observation window of N received statistics. Analytical and simulation results indicate that the performance of a DFD receiver approaches the coherent detection lower bound with increasing window length. Consequently, as the proposed receiver exhibits a complexity independent of N, the conclusion is reached that DFD is an effective approach for noncoherent detection in an FSO system.

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 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.

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.

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.