Wahidah Hashim

Empirical conversion of rainfall rate distribution for various integration times in Malaysia

The study of rainfall rate integration time is important to designers of satellite communication systems. In most part of the world, long-term rain statistics are typically available for integration time of very long durations; even perhaps of one hour or more. The method and appropriate conversion of rainfall rate distribution from variable integration times to the equivalent 1-minute are indeed vital. This is due to the fact that established prediction methods for estimating the rain induced attenuation on satellite-Earth links utilize the 1-minute rainfall rate data.

Performance comparison of differential space-time signalling schemes for OFDM systems

Differential transmit diversity is an attractive alternative to its coherent counterpart, especially for multiple antenna systems where channel estimation is more difficult to attain compared to that of single antenna systems. In this paper we compare two different types of differential transmit diversity techniques for OFDM based transmissions. The first technique uses differential space-time block codes (DSTBC) from orthogonal designs and the second uses the differential cyclic delay diversity (DCDD). The results compare the bit error performance for several transmit antenna configurations. The results show that DCDD offers a very close performance to that of DSTBC, with the advantage of a simplified receiver structure.

A new hop-count and node-degree based routing protocol for Vehicular Ad-hoc Networks (VANETs)

This research propose a new routing protocol that is based on the hop-count and node-degree metrics to determine route strength. The hop-count is derived by identifying the next-hop node that has the minimum distance to the sink node. Meanwhile, the node-degree is obtained through the vehicle ID of each neighbouring nodes in its radius. All the nodes or vehicles in VANETs context were assumed to be equipped with an OnBoard Unit (OBU) that will periodically send beacon messages to other nodes around the transmission range area. From the simulation results, it is found that the HDbR protocol is suitable to be used in Vehicular Ad-hoc Networks (VANETs) as it produces high network throughput and minimum end-to-end delay. Thus it is comparable to existing protocol such as AODV when it was evaluated in various speeds scenario.

Cognitive Selection Mechanism for Indoor Propagation

The evolution of communication device demands for faster connection and higher data rates. This results in proliferation of new broadband technologies that are being introduced into the market for each interval of every 2 to 3 years. However, these technologies are still facing the challenge of unsatisfied users who request not only faster connection but also reliable connectivity. This paper proposes two cognitive selection mechanisms termed as successive and comparative selection which are applied in multiple broadband user terminals in order to maintain good connection rate. By employing such mechanisms, we can observe their performances in terms of optimized speed and switching delay. Cognitive radio aspects are also introduced within the selection system. These include sensing network condition capability, performing analysis and decision making for selecting the most optimum broadband network. The empirical study was conducted within the indoor environment of a residential area. The location was selected to vary the evaluation of the performance of these selection mechanisms. It was found that successive selection had shown a better optimized speed performance up to 37% improvement as compared to comparative selection. However, comparative selection shows a low switch delay performance in contrast to successive selection.

An optimal circular 16-QAM modulation technique for wavelet and fourier based OFDM

A new approach of an optimal 16-QAM modulation has been introduced to be used for both Fourier and wavelet based OFDM. The comparison between circular and square 16-QAM is included. It is shown that the power consumption of circular is better than the square by 0.5359 W or −2.71 dB for the same minimum distance 2d. Simulation works for Fourier and wavelet based OFDM have been performed. In Fourier base OFDM, the circular has slightly out-performed the square when the number of subcarriers is varied from 32 to 256 at different BER values. When considering wavelet based OFDM, all wavelet families except Biorthogonal and Reverse-Biorthogonal have performed similar result for both modulation schemes. Of all Biorthogonal families, bior5.5 is the most out-performed one when using circular and square 16-QAM respectively.

Cluster-Based Spectrum Sensing Scheme in Heterogeneous Network

In OFDM based Heterogeneous Networks (HetNet) deploying femtocells (HeNodeBs) causes’ inter-cell interference in uplink as well as downlink. Spectrum allocation is one of the technique as many researcher identified. However, using frequency reuse/cognitive system for dynamic spectrum reallocation is also a challenging issues. Therefore, this paper focuses on unused spectrum sensing (SS) through proposing cluster-based spectrum sensing for OFDM based two-tier HetNets. The proposed SS scheme adopted the basic model of energy detector and compared with cooperative spectrum sensing scheme. In the proposed scheme, the energy detection model is extended into multiple channel detection model. To detect spectrums from nearby macrocell (macro-eNodeB) HeNodeBs are grouped into cluster. A cluster formation algorithm is also proposed in where, cluster head (CH) and signaling node (SN) will detect implies the multi-channel SS technique. The simulation results demonstrate that the proposed scheme can improve the spectrum detection with minimum error. The performance of the proposed scheme is evaluated in Monte Carlo simulation validate the scheme performance.

Performance Analysis of Spectrum Sensing Methods: A Numerical Approach

Recent enhancement of Long Term Evolution / Advanced (LTE/LTE-A) is aimed to increase the throughput of the Orthogonal Frequency Division Multiplexing (OFDM) system. This is quite obvious where both frequency and time division are employed in order to augment the system capacity. In LTE/LTE-A heterogeneous Networks (Het Nets), femto cells (He Node Bs) are the optimal choice to extend the coverage within indoor environment. Nonetheless, the deployment of such base stations, somewhat to certain extent, does create cross-tier and co-tier interferences with the macro cell users (UEmacro-eNB) that ostensibly can become a critical challenge. It is implied that the unutilized / idle spectrums reallocation might be one of the solutions to mitigate such challenge. However, the errorless spectrum sensing does pose another issue. This paper outlines the investigation findings where spectrum sensing techniques is deployed to maximize the spectrum detection capacity with minimum error in OFDM based Het Nets. In addition, in the attempt to increase the efficiency of spectrum resources, this paper proposes a sensing technique which is imposed over advanced energy detection technique to detect the idle spectrums. The result of the proposed scheme is evaluated using Monte Carlo simulation.

Cognitive selection mechanism performance in IEEE 802.11 WLAN

Recent growth in Wireless Local Area Network (WLAN) usage has generated considerable interest in the establishment of the IEEE 802.11 WLAN standards. WLAN has been developed with the capability to handle tremendous growth of data traffic and therefore have been proliferating worldwide. The influencing factors and the parameters involved prior choosing which access point (AP) to connect with are to be carefully evaluated and dealt with in the process of WLAN deployment. Properly designed selection mechanisms that able to provide not only radio coverage but also a good internet connection speed are in turn expected to be able to improve the Quality of Service (QoS). In this paper, a new approach for the AP selection in IEEE 802.11 is proposed. The new technique takes into account internet connection speed in contrast to the classic approach that solely based on the Received Signal Strength (RSS). A heuristic selection model was developed namely Evaluative AP Selection which used LINUX Bash script programming whilst the internet connection speed test was ran by means of PHP script. The results for the selection were evaluated respectively to see the variation in selection pattern when compared to the default selection algorithm. The time interval during the scanning and selection process was also analysed in the investigation to further identify the performance of the model. The enhancement reaped from the developed selection algorithm is expected to bring benefit to different user at various places with diverse requirements.

Comparison of differential space-time block codes and differential cyclic delay diversity for a multi-carrier wireless system

Differential transmit diversity is an attractive alternative to its coherent counterpart, especially for multiple antenna systems where channel estimation is more difficult to attain compared to that of single antenna systems. In this paper we compare two different types of differential transmit diversity techniques for a multi-carrier based transmissions in particular OFDM. The first technique uses differential space-time block codes from orthogonal designs and the second uses differential cyclic delay diversity. The results present the bit error performance for several transmit antenna configurations. The results show that DCDD offers close performance to that of DSTBC with the advantage of simplified receiver.

Performance comparison of differential multiple transmit diversity schemes for OFDM systems

Differential transmit diversity is an attractive alternative to its coherent counterpart, especially for multiple antenna systems where channel estimation is more difficult to attain compared to that of single antenna systems. In this paper we compare two different types of differential transmit diversity techniques for OFDM based transmissions. The first technique uses Differential Space-Time Block Codes (DSTBC) from orthogonal designs and the second uses the Differential Cyclic Delay Diversity (DCDD). The results compare the bit error performance for several transmit antenna configurations. The results show that DCDD offers a very close performance to that of DSTBC, with the advantage of a simplified receiver structure.