Michael L. B. Riediger

Fast multiple-symbol detection for free-space optical communications

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.

Multiple-symbol detection for photon-counting MIMO free-space optical communications

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.

Blind detection of on-off keying for free-space optical communications

We investigate blind detection, i.e. detection assuming the absence of instantaneous channel state information (CSI) and a statistical channel description at the receiver, of on-off keying (OOK) in a free-space optical (FSO) system. Using an observation window encompassing many consecutive independent bits, our blind detector capitalizes on the fact that the atmospheric turbulence is constant over the observation window and that the expected number of transmitted 1s and 0 s are equal. To improve receiver performance beyond that of the initial blind detector, we also consider the use of decision-aided threshold estimation and a second detection stage, respectively. Provided that a sufficiently large observation window is employed, simulation results indicate that the proposed receiver can attain performance comparable to that of the lower performance bound given by detection with CSI.

Reduced-Complexity Multiple-Symbol Detection for Free-Space Optical Communications

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 of multiple-symbol detection (MSD), in which block-wise decisions are made using an observation window of N 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 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 increasing N.

Decision-Feedback Detection for Free-Space Optical Communications

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.

Differential Group Space-Time Block Coded CPM

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.

Scalar multiple-symbol differential detection of MPSK with diversity reception

M.K. Simon and M.S. Alouini (2001) derived the structure of the optimal N-length differential detector for differentially encoded M-ary phase shift keying (MPSK) signals over a slow fading diversity channel. It was demonstrated that as N increases, the error performance of this differential detector approaches that of maximal-ratio combining (with differential encoding). Unfortunately, efficient implementation of this optimal receiver is not possible, and it exhibits high computational complexity. We show that the complexity can be reduced dramatically, if the receiver first performs coherent combining of the received multi-channel baseband signals, followed by single-channel multiple-symbol differential detection (MSDD). Our main contribution is the introduction of a coherent combiner that is capable of cophasing the multi-channel signals, without violating the fundamental nature of differential detection; i.e., avoiding explicit phase estimation. The proposed suboptimal coherent combining MSDD (CC-MSDD) receiver essentially consists of two differential detectors, one operating across diversity branches and one operating across time. Due to the dramatic reduction in complexity, we are able to use a very large observation window, N=64, with the CC-MSDD. With this window size, the asymptotic performance of the CC-MSDD is only 0.5 dB away from maximal-ratio combining with differential encoding. This translates into a 1.5 dB gain in power efficiency over conventional equal gain combining, for a second order diversity system.

A Probabilistic Subpacket Retransmission Scheme for ARQ Protocols

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, a cyclic redundancy check (CRC) was employed at the subpacket level. In this paper, we propose a probabilistic subpacket retransmission ARQ (PSR-ARQ) scheme for wireless systems, which does not employ CRC bits at the subpacket level. Rather, we utilize the demodulators 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.

Multiple-Symbol Detection for Shot-Noise Limited Free-Space Optical Communications

Free-space optics (FSO) have received increased attention recently for last-mile, line-of-sight wireless links. In this research, 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 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 many bit intervals. Significantly, we develop a fast decoding algorithm for optimal MSD, which avoids the complexity of an exhaustive search over all candidate sequences. The conclusion is reached that the proposed MSD technique provides an attractive low complexity mechanism for performing optimal noncoherent detection of OOK in an FSO system.

Two-Stage Blind Detection of Differential Space-Time Block Coded Continuous Phase Modulation

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.