Kok-Keong Loo

Generalized Pairwise Z-Complementary Codes

An approach to generate generalized pairwise Z-complementary (GPZ) codes, which works in pairs in order to offer a zero correlation zone (ZCZ) in the vicinity of zero phase shift and fit extremely well in power efficient quadrature carrier modems, is introduced in this letter. Each GPZ code has MK sequences, each of length 4NK, where M is the number of Z-complementary mates, K is a factor to perform Walsh-Hadamard expansions, and N is the sequence length of the Z-complementary code. The proposed GPZ codes include the generalized pairwise complementary (GPC) codes as special cases.

Secure route selection in wireless mesh networks

The large scale coverage and multi-hop architecture of wireless mesh networks (WMNs) are such characteristics which are vulnerable to network layer threats. So far many secure routing techniques have been proposed but they are only capable to handle single network layer attack. In this paper, we propose a secure route selection mechanism for WMN, which is robust against a variety of multi-hop threats and performs well over a range of scenarios we tested.

SRPM: Secure Routing Protocol for IEEE 802.11 Infrastructure Based Wireless Mesh Networks

Infrastructure based IEEE 802.11 wireless mesh networks (WMNs) are new paradigm of low cost broadband technology. The large scale city-wide community-based coverage and multi-hop architecture are such characteristics which are vulnerable to network layer threats, and the adversary can exploit them for large scale degradation of the broadband services. So far many secure routing protocols have been proposed for ad-hoc networks, however, due to the different nature and characteristics; they cannot perform well in a WMN environment. In this paper, we discuss the limitations and challenges as well as propose an exclusive secure routing protocol for an infrastructure based wireless mesh (SRPM) network. SRPM is robust against a variety of multi-hop threats and performs well over a range of scenarios we tested.

Analysis of Cyclic Delay Diversity on DVB-H Systems over Spatially Correlated Channel

The objective of this work is to research and analyse the performance of Cyclic Delay Diversity (CDD) with two transmit antenna on DVB-H systems operating in spatially correlated channel. It is shown in this paper that CDD can achieve desirable transmit diversity gain over uncorrelated channel with or without receiver diversity. However, in reality, the respective signal paths between spatially separated antennas and the mobile receiver is likely to be correlated because of insufficient antenna separation at the transmitter and the lack of scattering effect of the channel. Under this spatially correlated channel, it is apparent that CDD cannot achieve the same diversity gain as obtained under the uncorrelated channel. In this paper, a new upper bound on the pairwise error probability (PEP) of the CDD with spatial correlation of two transmit antennas is derived. The upper bound is used to study the CDD theoretical error performance and diversity gain losses over a generalized spatially correlated Rayleigh channel. This theoretical analysis is validated by the simulation of DVB-H systems with two transmit antennas and the CDD scheme. Both the theoretical and simulated results give the valuable insight that the CDD ability to perform well with a certain amount of channel correlation

Analysis of DVB-H Network Coverage With the Application of Transmit Diversity

This paper investigates the effects of the cyclic delay diversity (CDD) transmit diversity scheme on DVB-H networks. Transmit diversity improves reception and quality of service (QoS) in areas of poor coverage such as sparsely populated or obscured locations. The technique not only provides robust reception in mobile environments thus improving QoS, but it also reduces network costs in terms of the transmit power, number of infrastructure elements, antenna height and the frequency reuse factor over indoor and outdoor environments. In this paper, the benefit and effectiveness of CDD transmit diversity is tackled through simulation results for comparison in several scenarios of coverage in DVB-H networks. The channel model used in the simulations is based on COST207 and a basic radio planning technique is used to illustrate the main principles developed in this paper. The work reported in this paper was supported by the European Commission IST project - PLUTO (physical layer DVB transmission optimization).

Wireless mesh network security: A traffic engineering management approach

The wireless mesh network (WMN) is an emerging multihop, heterogeneous, easily scalable and low cost network. The architecture of the WMN is a connectionless-oriented, mobile and dynamic traffic of routed packets. The mesh infrastructure environment easily forms multiple chains of wireless LANs (WLAN) coupled with the simultaneous multihop transmission of data packets from peripherals via mobile gateways to the wireless cloud. WMN operates as an access network to other communication technologies. This exposes the WMN to numerous security challenges not only in the mesh transmission operation security but also in the overall security against foreign attacks. We surveyed and identified the security vulnerabilities in Internet Protocol (IP) broadband networks, the security challenges in the routing layer of the WMN and explored new concepts to solving security challenges in WMN using traffic engineering (TE) security resolution mechanisms. We analyzed the advantages, comparative strengths and weakness in the use of traffic engineering based on simulation results and evaluations.

Multi-Channel Multi-Radio Using 802.11 Based Media Access for Sink Nodes in Wireless Sensor Networks

The next generation surveillance and multimedia systems will become increasingly deployed as wireless sensor networks in order to monitor parks, public places and for business usage. The convergence of data and telecommunication over IP-based networks has paved the way for wireless networks. Functions are becoming more intertwined by the compelling force of innovation and technology. For example, many closed-circuit TV premises surveillance systems now rely on transmitting their images and data over IP networks instead of standalone video circuits. These systems will increase their reliability in the future on wireless networks and on IEEE 802.11 networks. However, due to limited non-overlapping channels, delay, and congestion there will be problems at sink nodes. In this paper we provide necessary conditions to verify the feasibility of round robin technique in these networks at the sink nodes by using a technique to regulate multi-radio multichannel assignment. We demonstrate through simulations that dynamic channel assignment scheme using multi-radio, and multichannel configuration at a single sink node can perform close to optimal on the average while multiple sink node assignment also performs well. The methods proposed in this paper can be a valuable tool for network designers in planning network deployment and for optimizing different performance objectives.

Laboratory Measurement Campaign of DVB-T Signal With Transmit Delay Diversity

The requirements for future DVB-T/H networks demand that broadcasters design and deploy networks that provide ubiquitous reception in challenging indoors and other obstructed situations. It is essential that such networks are designed cost-effectively and with minimized environmental impact. The EC funded project PLUTO has since its start in 2006 explored the use of diversity to improve coverage in these difficult situations. The purpose of this paper is to investigate the performance of Transmit Delay Diversity (DD) with two antennas to improve the reception of DVB-T/H systems operating in different realistic propagation conditions through a series of tests using a SPIRENT SR5500 dual channel emulator. The relationship between correlation coefficient between channels, receiver velocity and diversity gain is investigated. It is shown that transmit delay diversity significantly improves the quality of reception particularly in simulated fast fading mobile broadcasting applications. This paper documents research conducted by Brunel University and Broadreach Systems.

Error-Resilient Performance of Dirac Video Codec Over Packet-Erasure Channel

Video transmission over the wireless or wired network requires error-resilient mechanism since compressed video bitstreams are sensitive to transmission errors because of the use of predictive coding and variable length coding. This paper investigates the performance of a simple and low complexity error-resilient coding scheme which combines source and channel coding to protect compressed bitstream of wavelet-based Dirac video codec in the packet-erasure channel. By partitioning the wavelet transform coefficients of the motion-compensated residual frame into groups and independently processing each group using arithmetic and forward error correction (FEC) coding, Dirac could achieves the robustness to transmission errors by giving the video quality which is gracefully decreasing over a range of packet loss rates up to 30% when compared with conventional FEC only methods. Simulation results also show that the proposed scheme using multiple partitions can achieve up to 10 dB PSNR gain over its existing un-partitioned format. This paper also investigates the error-resilient performance of the proposed scheme in comparison with H.264 over packet-erasure channel.

Multi-Channel Distributed Coordinated Function over Single Radio in Wireless Sensor Networks

Multi-channel assignments are becoming the solution of choice to improve performance in single radio for wireless networks. Multi-channel allows wireless networks to assign different channels to different nodes in real-time transmission. In this paper, we propose a new approach, Multi-channel Distributed Coordinated Function (MC-DCF) which takes advantage of multi-channel assignment. The backoff algorithm of the IEEE 802.11 distributed coordination function (DCF) was modified to invoke channel switching, based on threshold criteria in order to improve the overall throughput for wireless sensor networks (WSNs) over 802.11 networks. We presented simulation experiments in order to investigate the characteristics of multi-channel communication in wireless sensor networks using an NS2 platform. Nodes only use a single radio and perform channel switching only after specified threshold is reached. Single radio can only work on one channel at any given time. All nodes initiate constant bit rate streams towards the receiving nodes. In this work, we studied the impact of non-overlapping channels in the 2.4 frequency band on: constant bit rate (CBR) streams, node density, source nodes sending data directly to sink and signal strength by varying distances between the sensor nodes and operating frequencies of the radios with different data rates. We showed that multi-channel enhancement using our proposed algorithm provides significant improvement in terms of throughput, packet delivery ratio and delay. This technique can be considered for WSNs future use in 802.11 networks especially when the IEEE 802.11n becomes popular thereby may prevent the 802.15.4 network from operating effectively in the 2.4 GHz frequency band.