Papers by Soung-chang Liew
IEEE Transactions on Information Theory
This paper proposes using the uncertainty of information (UoI), measured by Shannon's entropy, as... more This paper proposes using the uncertainty of information (UoI), measured by Shannon's entropy, as a metric for information freshness. We consider a system in which a central monitor observes multiple binary Markov processes through a communication channel. The UoI of a Markov process corresponds to the monitor's uncertainty about its state. At each time step, only one Markov process can be selected to update its state to the monitor; hence there is a tradeoff among the UoIs of the processes that depend on the scheduling policy used to select the process to be updated. The age of information (AoI) of a process corresponds to the time since its last update. In general, the associated UoI can be a non-increasing function, or even an oscillating function, of its AoI, making the scheduling problem particularly challenging. This paper investigates scheduling policies that aim to minimize the average sum-UoI of the processes over the infinite time horizon. We formulate the problem as a restless multi-armed bandit (RMAB) problem, and develop a Whittle index policy that is near-optimal for the RMAB after proving its indexability. We further provide an iterative algorithm to compute the Whittle index for the practical deployment of the policy. Although this paper focuses on UoI scheduling, our results apply to a general class of RMABs for which the UoI scheduling problem is a special case. Specifically, this paper's Whittle index policy is valid for any RMAB in which the bandits are binary Markov processes and the penalty is a concave function of the belief state of the Markov process. Numerical results demonstrate the excellent performance of the Whittle index policy for this class of RMABs.
Practical NOMA-Based Coordinated Direct and Relay Transmission
IEEE Wireless Communications Letters, 2021
This letter puts forth a practical non-orthogonal multiple access (NOMA)-based coordinated direct... more This letter puts forth a practical non-orthogonal multiple access (NOMA)-based coordinated direct and relay transmission (CDRT) scheme, referred to as network-coded multiple access CDRT (NCMA-CDRT). We consider a two-user uplink scenario, where a near user communicates directly to a base station (BS), whereas a far user needs the assistance of a decode-and-forward relay. While prior NOMA-CDRT studies focused extensively on theoretical analysis, its superiority has not been well investigated through practical communication schemes. In particular, successive interference cancellation (SIC), a predominant NOMA decoding technique, does not work well if transmitters with comparable received powers transmit together. We investigate NCMA-CDRT to tackle this practical issue. A distinguishing feature of NCMA-CDRT is the joint use of physical-layer network coding (PNC) and multiuser decoding at both the relay and the BS. We design packet relaying strategies that can forward the PNC packets decoded at the relay to boost system throughput. Experiments on software-defined radio indicate that the total throughput of NCMA-CDRT significantly outperforms the SIC-based NOMA-CDRT. Overall, NCMA-CDRT is a viable solution for cooperative NOMA systems.
Deep Learning for Joint Noncoherent Differential Detection and Channel Decoding
2021 IEEE/CIC International Conference on Communications in China (ICCC), 2021
We propose a deep-learning approach for the joint noncoherent differential detection and channel ... more We propose a deep-learning approach for the joint noncoherent differential detection and channel decoding problem. Conventional receivers adopt a model-based approach for noncoherent differential detection and channel decoding in linear manners. For the noncoherent differential detection, the conventional way is to use symbol-by-symbol differential detection (DD) and multiple symbol differential detection (MSDD). DD is simple to implement, but it suffers from performance degradation compared with coherent detection. The performance degradation can be narrowed by MSDD that detects a block of symbols jointly. However, its complexity increases exponentially with the block size. Furthermore, MSDD is not robust in that when the channel phase changes substantively within the block, its performance could be worse than that of DD. Channel decoding needs to be determined according to the channel coding. This work applies the advantages in deep learning for the design of receivers. In particular, we employ a Deep neural network (DNN) constructed by Long Short-Term Memory (LSTM) units to solve the joint noncoherent differential detection and channel decoding problem. Our simulations show that a DNN can outperform conventional model-based linear receivers. Furthermore, it does so with faster signal processing. This performance improvement points to a new direction for future receiver design.
IEEE Transactions on Wireless Communications, 2020
This paper puts forth a class of new transceiver designs for interleaved frequency division multi... more This paper puts forth a class of new transceiver designs for interleaved frequency division multiple access (IFDMA) systems. These transceivers are significantly less complex than conventional IFDMA transceiver. The simple new designs are founded on a key observation that multiplexing and demultiplexing of IFDMA data streams of different sizes are coincident with the IFFTs and FFTs of different sizes embedded within the Cooley-Tukey recursive FFT decomposition scheme. For flexible resource allocation, this paper puts forth a new IFDMA resource allocation framework called Multi-IFDMA, in which a user can be allocated multiple IFDMA streams. Our new transceivers are unified designs in that they can be used in conventional IFDMA as well as multi-IFDMA systems. Two other wellknown multiple-access schemes are localized FDMA (LFDMA) and orthogonal FDMA (OFDMA). In terms of flexibility in resource allocation, Multi-IFDMA, LFDMA, and OFDMA are on an equal footing. With our new transceiver designs, however, IFDMA has the following advantages (besides other known advantages not due to our new transceiver designs): 1) IFDMA/Multi-IFDMA transceivers are significantly less complex than LFDMA transceivers; in addition, IFDMA/Multi-IFDMA has better Peak-to-Average Power Ratio (PAPR) than LFDMA; 2) IFDMA/Multi-IFDMA transceivers and OFDMA transceivers are comparable in complexity; but IFDMA/Multi-IFDMA has significantly better PAPR than OFDMA.
2020 International Symposium on Networks, Computers and Communications (ISNCC), 2020
We design and implement Publication Chain (Pub-Chain), a decentralized open-access publication pl... more We design and implement Publication Chain (Pub-Chain), a decentralized open-access publication platform built on decentralized and distributed technologies of blockchain and IPFS peer-to-peer file sharing systems. The existing publication platforms have some severe drawbacks. First, instead of promoting widespread knowledge sharing, access to publications on the platforms owned by publishers is often on a fee basis. This drawback of pay wall prevents researchers from standing on the shoulders of giants. Moreover, the peer review process on most all existing publication platforms (including both openaccess and publisher platforms) is prone to be ineffective, since there is no proper incentive to reviewers for performing highqualified reviews. PubChain is an alternative platform to the existing publication venues aiming to address their drawbacks. No central third-party owns the contents (i.e., papers and reviews) of PubChain. Exploiting blockchain technology, we devise an elaborate incentive scheme on PubChain to incentivize key stakeholders (i.e., authors, readers and reviewers) to participate publication activities on PubChain in a substantive manner by earning credits and rewards through self-motivated interactions. We have performed simulations to investigate the robustness of our proposed incentive scheme against fraudulent publications and reviews. We also have implemented a prototype of PubChain to demonstrate its key concepts.
2009 IEEE International Conference on Communications, 2009
With the proliferation of wireless multimedia applications, multicast/broadcast has been recogniz... more With the proliferation of wireless multimedia applications, multicast/broadcast has been recognized as an efficient technique to transmit a large volume of data to multiple mobile stations at the same time. In most multicast systems, the transmitter (e.g. base station) adapts its data rate to the furthest located users, so as to guarantee service quality to as many users as possible. Predictably, the more users in a multicast group, the lower data rate the base station can transmit. On the other hand, grouping more users together leads to a more efficient utilization of spectrum bandwidth, as these users are served simultaneously. This bring the interesting problem that presses for solution: how to group users in a cell into multicast groups and how to allocate a fixed amount of bandwidth resource to the groups, to achieve a good balance between throughput and fairness in multicast systems. In this paper, we formulate the united user grouping and bandwidth allocation strategy into a utility-based optimization problem. One method of signomial programming is used to solve the non-convex optimization problem. Numerical results will show that this suboptimal algorithm performs well even compared to the optimal one. Moreover, through theoretical analysis, we prove that the best user grouping and bandwidth allocation scheme of throughput maximization is to allocate the entire bandwidth to the unique group containing the users located within a ring-shaped region with an optimal outer radius r * .
IEEE Transactions on Wireless Communications, 2016
In this paper, we investigate the optimization of nonuniform linear antenna arrays (NULAs) for mi... more In this paper, we investigate the optimization of nonuniform linear antenna arrays (NULAs) for millimeter wave (mmWave) line-of-sight (LoS) multiple-input multiple-output (MIMO) channels. Our focus is on the maximization of the system effective multiplexing gain (EMG), by optimizing the individual antenna positions in the transmit/receive NULAs. Here the EMG is defined as the number of signal streams that are practically supported by the channel at a finite SNR. We first derive analytical expressions for the asymptotic channel eigenvalues with arbitrarily deployed NULAs when, asymptotically, the end-to-end distance is sufficiently large compared to the aperture sizes of the transmit/receive NULAs. Based on the derived expressions, we prove that, the asymptotically optimal NULA deployment that maximizes the achievable EMG should follow the groupwise Fekete-point distribution. Specifically, the antennas should be physically grouped into K separate uniform linear antenna arrays (ULAs) with the minimum feasible antenna spacing within each ULA, where K is the target EMG to be achieved; in addition, the centers of these K ULAs follow the Fekete-point distribution. We numerically verify the asymptotic optimality of such an NULA deployment and extend it to a groupwise projected arch type (PAT) NULA deployment, which provides a more practical option for mmWave LoS MIMO systems with realistic non-asymptotic configurations. Numerical examples are provided to demonstrate a significant capacity gain of the optimized NULAs over traditional ULAs.
Interference minimum network topologies for ad hoc networks
Wireless Communications and Mobile Computing, 2010
This paper investigates the topology control problem with the goal of minimizing mutual interfere... more This paper investigates the topology control problem with the goal of minimizing mutual interferences in wireless ad hoc networks. It is known that interference is considered as a relationship between link and node in previous works. In this paper, we attempt to capture the physical situation of spaceādivision multiplex more realistically by defining interference as a relationship between any two bidirectional links. We formulate the pairāwise interference condition between any two bidirectional links, and demonstrate that the interference condition is equivalent by employing the equalāpower allocation strategy and by employing the minimumāpower allocation strategy. Then we further study the typical interference relationship between a link and its surrounding links. To characterize the extent of the interference between a link and its surrounding links, a new metric, the interference coefficient, is given, and its property is explored in detail by means of analysis and simulation. B...
Corr, Aug 7, 2008
This paper considers the design of the optimal locationupdate area (LA) of the distance-based sch... more This paper considers the design of the optimal locationupdate area (LA) of the distance-based scheme for personal communication service (PCS) networks. We focus on the optimization of two design parameters associated with the LA: 1) initial position upon LA update; 2) distance threshold for triggering of LA update. Based on the popular continuous-time random walk (CTRW) mobility model, we propose a novel analytical framework that uses a diffusion equation to minimize the location management cost. In this framework, a number of measurable physical parameters, such as length of road section, angle between road sections, and road section crossing time, can be integrated into the system design. This framework allows us to easily evaluate the total cost under general call arrival distributions and LA of different shapes. For the particular case of circular LA and small Poisson call-arrival rate, we prove the following: (1) When the drift is weak, the optimal initial position approaches the center of the LA; when the drift is strong, it approaches the boundary of the LA. (2) Comparing the optimal initial-position and center-initial-position solutions (which is assumed in most prior work), when the drift is weak, the optimal distance threshold and the minimum total cost are roughly equal; when the drift is strong, the optimal distance threshold in the later is about 3 2 (ā1.260) times that in the former, and the minimum total cost in the later is about 3 4 (ā1.587) times that in the former. That is, optimizing on initial position, which previous work did not consider, has the potential of reducing the cost measure by 37%.
Transactions on Emerging Telecommunications Technologies, 2016
In this paper, we revisit the forward, backward and bidirectional Bahl-Cocke-Jelinek-Raviv (BCJR)... more In this paper, we revisit the forward, backward and bidirectional Bahl-Cocke-Jelinek-Raviv (BCJR) soft-input soft-output (SISO) maximum a posteriori probability (MAP) decoding process of rate-1 binary convolutional codes. From this we establish some interesting explicit relationships between encoding and decoding of rate-1 convolutional codes. We observe that the forward and backward BCJR SISO MAP decoders can be simply represented by their dual SISO channel encoders using shift registers in the complex number field. Similarly, the bidirectional MAP decoding can be implemented by linearly combining the shift register contents of the dual SISO encoders of the respective forward and backward decoders. The dual encoder structures for various recursive and non-recursive rate-1 convolutional codes are derived.
2008 IEEE International Conference on Communications, 2008
This paper concerns the problem of finding the minimum-length TDMA frame of a power-controlled wi... more This paper concerns the problem of finding the minimum-length TDMA frame of a power-controlled wireless network subject to traffic demands and SINR (signalto-interference-plus-noise ratio) constraints. We formulate the general joint link scheduling and power control problem as an integer linear programming (ILP) problem. The linear relaxation of the ILP problem has been claimed to be NP-hard in the literature. We present a computationally efficient heuristic algorithm, called the Increasing Demand Greedy Scheduling (IDGS) algorithm, to solve the general ILP problem. In addition, we propose using a column generation (CG) method as an augmentation to IDGS to further improve its performance. Simulation results show that integration of IDGS and CG can achieve superior performance in terms of both algorithm run time and solution optimality.
2004 IEEE Wireless Communications and Networking Conference (IEEE Cat. No.04TH8733), 2004
The rapid growth of wireless LAN (WLAN) deployments will bring about many novel mobile applicatio... more The rapid growth of wireless LAN (WLAN) deployments will bring about many novel mobile applications. Among them will be real-time multimedia streaming applications running on UDP, which may interfere with current data applications running on TCP. This paper is a first attempt to investigate how to ensure the performance of these two groups of applications when they co-exist over a WLAN. Toward this end, we have designed and implemented a UDP rate adaptation scheme called Adaptive-Buffer Rate Control (ABRC) for multimedia streaming over WLAN. ABRC has two distinguishing features compared with other schemes: 1) It can achieve arbitrary bandwidth allocations between UDP and TCP in the WLAN, as opposed to previously proposed "TCP friendly" schemes, which can only achieve uniform bandwidth allocations; 2) The majority of previously proposed flexible bandwidth-allocation schemes achieve arbitrary bandwidth allocations by prioritizing and scheduling packet transmissions within network equipment (i.e., within routers, base stations, etc.). In contrast, ABRC is an end-to-end application-layer solution that does not require changes to current WLAN products, making it more readily deployable over existing networks.
2011 IEEE 36th Conference on Local Computer Networks, 2011
This paper investigates the problem of link scheduling to meet traffic demands with minimum airti... more This paper investigates the problem of link scheduling to meet traffic demands with minimum airtime in a multitransmit-receive (MTR) wireless network. MTR networks are a new class of networks, in which each node can simultaneously transmit to a number of other nodes, or simultaneously receive from a number of other nodes. The MTR capability can be enabled by the use of multiple directional antennas or multiple channels. Potentially, MTR can boost the network capacity significantly. However, link scheduling that makes full use of the MTR capability must be in place before this can happen. We show that optimal link scheduling can be formulated as a linear program (LP). However, the problem is NP-hard because we need to find all the maximal independent sets in a graph first before the LP can be set up. We propose two computationally efficient algorithms, called Heavy-Weight-First (HWF) and Max-Degree-First (MDF) to solve this problem. Simulation results show that both HWF and MDF can achieve superior performance in terms of runtime and optimality. Specifically, we have conducted 1,000 simulation experiments with different network topologies and traffic demands. On average, the HWF and MDF solutions are within 90% of the optimal solutions.
IEEE GLOBECOM 2008 - 2008 IEEE Global Telecommunications Conference, 2008
To date, most analysis of WLANs has been focused on their operation under saturation condition. T... more To date, most analysis of WLANs has been focused on their operation under saturation condition. This work is an attempt to understand the fundamental performance of WLANs under unsaturated condition. In particular, we are interested in the delay performance when collisions of packets are resolved by an exponential backoff mechanism. Using a multiple-vacation queueing model, we derive an explicit expression for packet delay distribution, from which necessary conditions for finite mean delay and delay jitter are established. It is found that under some circumstances, mean delay and delay jitter may approach infinity even when the traffic load is way below the saturation throughput. Saturation throughput is therefore not a sound measure of WLAN capacity when the underlying applications are delay sensitive. To bridge the gap, we define safe-bounded-mean-delay (SBMD) throughput and safe-bounded-delay-jitter (SBDJ) throughput that reflect the actual network capacity users can enjoy when they require bounded mean delay and delay jitter, respectively. The analytical model in this paper is general enough to cover both single-packet reception (SPR) and multipacket reception (MPR) WLANs, as well as carrier-sensing and non-carrier-sensing networks. We show that the SBMD and SBDJ throughputs scale super-linearly with the MPR capability of a network. Together with our earlier work that proves super-linear throughput scaling under saturation condition, our results here complete the demonstration of MPR as a powerful capacity-enhancement technique for both delay-sensitive and delay-tolerant applications.
2014 IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP), 2014
This study designs space-time codes in analog network coding for asynchronous two-way relay netwo... more This study designs space-time codes in analog network coding for asynchronous two-way relay networks, where asynchronous transmissions can cause diversity loss. We propose a novel code, called zero-padded interleave reversal Alamouti code (ZP-IR AC) to achieve full diversity with fast maximum likelihood decoding. Specifically, to combat symbol misalignment caused by asynchronous transmissions, the two terminals insert zero padding when transmitting to relays. Thereafter, the second relay performs an interleave reversal procedure to retain full diversity at each terminal. A salient feature of ZP-IR AC is that it can be decoupled into several independent parts that facilitate fast maximum likelihood decoding. Simulations of ZP-IR AC show full diversity gain. The bit error rate performance of ZP-IR AC is comparable to that of the synchronized Alamouti code and outperforms those of some recent schemes.
GLOBECOM '03. IEEE Global Telecommunications Conference (IEEE Cat. No.03CH37489)
In the traditional IEEE 802.11 wireless LAN using infrastructure mode, all users share the same c... more In the traditional IEEE 802.11 wireless LAN using infrastructure mode, all users share the same channel and all packets are forwarded by an access point (AP). As a result, as the number of users in the cell increases, the throughput for each user degrades substantially. If there are users communicating with each other within the cell (as in conferencing or file exchange applications), such throughput degradation could be relieved by making these users communicate through ad hoc connections without going through the AP. The advantages are multi-fold. First, the traffic load at the AP is reduced, hence relieving the contention. Second, ad hoc connections are single-hop, hence improving the channel efficiency. Moreover, ad hoc connections could use different channels, hence multiplying the system bandwidth. In this paper, we propose to integrate the infrastructure mode and the ad hoc mode in a wireless network so as to achieve these advantages. We present a framework for such mixed-mode wireless LAN (termed M 2 -WLAN). In such a network, a node can dynamically switch between the infrastructure mode and the ad hoc mode according to the instruction of the AP, and hence the switching is transparent to the users. Using simulations, we show that M 2 -WLAN can indeed improve system throughput substantially without user's manual configuration.
IEEE INFOCOM, 1990
The switch modules are studied because they are the key building blocks in large multistage switc... more The switch modules are studied because they are the key building blocks in large multistage switch architectures. The switch module considered has n inputs and m outputs. A packet destined for a particular output address (out of g) needs to access only one of the r available physical output ports: m =gr. Input-buffered, output-buffered, and unbuffered switch modules are studied.
2013 Proceedings IEEE INFOCOM, 2013
Spectrum sensing, the task of discovering spectrum usage at a given location, is a fundamental pr... more Spectrum sensing, the task of discovering spectrum usage at a given location, is a fundamental problem in dynamic spectrum access networks. While sensing in narrow spectrum bands is well studied in previous work, wideband spectrum sensing is challenging since a wideband radio is generally too expensive and power consuming for mobile devices. Sequential scan, on the other hand, can be very slow if the wide spectrum band contains many narrow channels. In this paper, we propose an analog-filter based spectrum sensing technique, which is much faster than sequential scan and much cheaper than using a wideband radio. The key insight is that, if the sum of energy on a contiguous band is low, we can conclude that all channels in this band are clear with just one measurement. Based on this insight, we design an intelligent search algorithm to minimize the number of total measurements. We prove that the algorithm has the same asymptotic complexity as compressed sensing while our design is much simpler and easily implementable in the real hardware. We show the availability of our technique using hardware devices that include analog filters and analog energy detectors. Our extensive evaluation using real TV "white space" signals shows the effectiveness of our technique.
Real-life experiments of Multi-Radio Multi-Channel wireless mesh networks: 802.11n is not any better than 802.11a!
ICICS 2011 - 8th International Conference on Information, Communications and Signal Processing, 2011
Abstract Wireless Mesh Network (WMN) has become a popular access network architecture in the comm... more Abstract Wireless Mesh Network (WMN) has become a popular access network architecture in the community due to its low cost and readily deployable nature. However, it is well known that multi-hop transmission in WMN is vulnerable to bandwidth degradation, primarily due to contention and radio interference. A straightforward solution to this problem is to use mesh nodes with multiple radios and channels. In this paper, we demonstrate through real-world experiments that the use of multiple radios and channels solely cannot solve the multi-hop ...
Linearly-coupled fountain codes for network-coded multiple access
2014 IEEE Information Theory Workshop (ITW 2014), 2014
We propose a low-complexity digital fountain approach for network-coded multiple access (NCMA), w... more We propose a low-complexity digital fountain approach for network-coded multiple access (NCMA), where each source node encodes its input packets using a fountain code. In NCMA, both physical-layer network coding and multiuser decoding are employed in the physical layer of the sink node, so that the output of the physical layer is the coupling of the fountain codes employed at the source nodes. We demonstrate that a belief propagation (BP) decoding algorithm can effectively decode the coupled fountain codes to recover the input packets of all source nodes. Our approach significantly reduces the decoding complexity compared with the previous NCMA schemes based on Reed-Solomon codes and random linear codes, and hence has the potential to increase throughput and decrease delay in computation-limited NCMA systems.
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Papers by Soung-chang Liew