Yeongyoon Choi

Capacity-optimized power allocation scheme in an integrated voice and data DS-CDMA system

In this paper, a power allocation scheme is proposed to enhance the system capacity, defined as the sum of throughput for data users and Erlang capacity for voice users in a mixed voice and data DS-CDMA system. The proposed scheme increases the supportable data traffic load which is limited to satisfy the pre-determined QoSs of voice users such as the required BER, by allocating the transmission power of the data users to be less than that of voice users. Through a numerical example, this scheme shows the enhancement of system capacity. Especially, we can get double system capacity by allocating the transmission power of data users to be half of that of voice users, given the voice traffic load of 0.1.

Analysis of three-phase multiple access with continual contention resolution (TPMA-CCR) for wireless multi-hop ad hoc networks

In this paper, a new medium access control (MAC) protocol entitled three-phase multiple access with continual contention resolution (TPMA-CCR) is proposed for wireless multi-hop ad hoc networks. This work is motivated by the previously known threephase multiple access (TPMA) scheme of Hou and Tsai [2] which is the suitable MAC protocol for clustering multi-hop ad hoc net- works owing to its beneficial attributes such as easy collision detectible, anonymous acknowledgment (ACK), and simple signaling format for the broadcast-natured networks. The new TPMA-CCR is designed to let all contending nodes participate in contentions for a medium access more aggressively than the original TPMA and with continual resolving procedures as well. Through the systematical performance analysis of the suggested protocol, it is also shown that the maximum throughput of the new protocol is not only superior to the original TPMA, but also improves on the conventional slotted carrier sense multiple access (CSMA) under certain circumstances. Thus, in terms of performance, TPMA-CCR can provide an attractive alternative to other contention-based MAC protocols for multi-hop ad hoc networks.

An analysis of mobile radio ad hoc networks using clustered architectures

In this paper we present a novel approach for evaluating the routing efficiency of clustered architectures, as compared to flat ones in mobile scenarios. For multihop ad hoc networks there have been much research done on clustered architectures due to their advantages, such as the spatial reuse of resources, the easier update of hierarchical topology and the smaller overheads on routing procedures. However, the efficiency of using a clustered architecture depends on many factors such as the network size, how the clusters are formed, type of traffic, connection data rates, network connectivity and topology changes due to in particular node mobility. Our results show that the clustered architecture needs to be very carefully designed in order to be more efficient than a flat architecture for small to medium sized networks of up to 400 nodes. That is, a clusterhead needs to have good knowledge of the links in its cluster such a good route through the cluster can be selected.

Practical subchannel power controlled OFDM system under time-varying multipath channel

We evaluate the BER performance of the subchannel power controlled OFDM system considering more practical cases including the limitation in word length in subchannel power control tap-weights and the time-varying multipath channel. According to simulation results, the total required number of bits is 5 for the optimal subchannel power control and the main source of the performance degradation of the subchannel power controlled OFDM system is the time-varying multipath channel.

Effects of correlated shadowing and multipath fading on OFDM systems with postdetection-combining-based macrodiversity

The performance of orthogonal frequency division multiplexing (OFDM) using postdetection-combining-based macrodiversity is analyzed on a shadowed multipath channel, and the effects of correlated shadowing are investigated. We show that the postdetection-combining-based macrodiversity provides larger performance gain than the selection-based one, especially for the channel characterized by random multipath parameters. It is also shown that the gain obtained by using the postdetection-combining-based macrodiversity is reduced more and more as the correlation among the macrodiversity ports increases. Even for the fully correlated shadowing situation, however, the postdetection-combining-based macrodiversity still provides a considerable gain compared with no diversity case.

A dynamic resource allocation scheme to maximize throughput in a multimedia CDMA system

In this paper, a resource allocation scheme is proposed to maximize the throughput of a multimedia CDMA system. When we let the throughput be a function of various data rates for multimedia traffic, the scheme is to find the optimum set of data rates for concurrent user groups which makes the system throughput maximize. It is shown that the optimum data rate set includes one non-trivial rate while all others keep the minimum required data rate, where the non-trivial rate is assigned dynamically to one group out of the set of VBR groups according to various QoS requirements and the number of users.

MAC prioritization scheme resolving unfairness problem

IEEE 802.11e is an extension of IEEE 802.11 for the Quality of Service (QoS) enhancement for wireless LAN application using MAC layer priority scheme. The IEEE 802.11e standard divides the network traffic into four different categories by the traffic type. Delay-sensitive applications need a high media access priority, which can be achieved by using different MAC parameters. The priority scheme is also used for resolving the unfairness and bottleneck problems of multihop wireless networks, where nodes operate as host or relay nodes. It is well-known that these problems occur since there may exist overloaded nodes that the packets from various source nodes converge on. Fine control of access opportunity at MAC layer can deal with this unfairness situation. In this paper, we provide an analysis of the priority scheme based on CWmin which can induce a required throughput ratio, and verify it with simulation results. Also, the simulation results are given to verify that the unfairness and bottleneck problems in multihop networks are solved by using the proposed priority scheme.

A New Adaptive Transmission Scheme for Wireless Communication SystemsUtilizing Frequency Diversity

In this paper, we propose a new adaptive transmission scheme for wirelesscommunication systems utilizing frequency diversity combining technique. Thecapacity of Rayleigh fading chanels under the proposed adaptive transmissionscheme is analytically evaluated and compared with the capacity under theconventional adaptive transmission scheme. The results show that the channelcapacity under the new adaptive scheme exceeds that under the conventionalone, and this capacity increment becomes more apparent especially for loweraverage carrier-to-noise ratio (CNR) and larger number of diversity branches.Also, the capacity with the new scheme isshown to exceed even the capacity ofAWGN channels for some small values of CNR and large number of diversitybranches.