Channel capacity and error exponents of variable rate adaptive channel coding for Rayleigh fading channels

We have evaluated the information theoretical performance of variable rate adaptive channel coding for Rayleigh fading channels. The channel states are detected at the receiver and fed back to the transmitter by means of a noiseless feedback link. Based on the channel state informations, the transmitter can adjust the channel coding scheme accordingly. Coherent channel and arbitrary channel symbols with a fixed average transmitted power constraint are assumed. The channel capacity and the error exponent are evaluated and the optimal rate control rules are found for Rayleigh fading channels with feedback of channel states. It is shown that the variable rate scheme can only increase the channel error exponent. The effects of additional practical constraints and finite feedback delays are also considered. Finally, we compare the performance of the variable rate adaptive channel coding in high bandwidth-expansion systems (CDMA) and high bandwidth-efficiency systems (TDMA).published_or_final_versio

Performance of variable rate bit interleaved coding for high bandwidth efficiency

We propose a bandwidth efficient error correction scheme, namely the variable rate adaptive bit-interleaved coded modulation (ABICM), for wireless mobile channel. The code rate and modulation level are varied according to the current channel state to exploit the timevarying nature of the wireless channel. Design challenges to achieve symbol-by-symbol adaptation and component codes design are addressed. A multi-level puncturing scheme is proposed to solve the problem of symbol-by-symbol puncturing and interleaving. The optimal adaptation thresholds are derived. It is found that there are significant gains relative to the fixed rate coding in terms of SNR and throughput. It is also found that the ABICM scheme is essentially not degraded in small interleaving depths. This makes the ABICM very suitable for real time applications.published_or_final_versio

Design of adaptive bit interleaved TCM for Rayleigh fading channels

In this paper, we propose a bandwidth efficient error correction scheme, namely the variable rate adaptive bit-interleaved trellis coded modulation (ABICM), for wireless mobile channels. The code rate and modulation level are varied according to the current channel state to exploit the time-varying nature of the wireless channel. Design challenges to achieve symbol-by-symbol adaptation, component codes design, puncturing and interleaving design, adaptation thresholds determination are addressed. A multi-level puncturing scheme is proposed to tackle the problem of symbol-by-symbol adaptive puncturing and interleaving. We introduced the concept of equivalent distance spectrum for designing component codes of the ABICM system. Two operation modes, namely the constant throughput and the constant BER controls, are introduced. The design is illustrated with an example and it is found that there are significant gains relative to the fixed rate coding in terms of SNR and throughput. It is also found that the ABICM scheme is essentially not degraded in small interleaving depths. This makes the ABICM very suitable for real time applications.published_or_final_versio

Joint design of adaptive channel coding and multiple access control for integrated voice and data services in a cellular wireless network - with contention queue

A new MAC protocol employing a joint design of the multiple access control (MAC) layer and the physical layer is proposed for cellular wireless systems with integrated voice and data services. This is a novel concept and is shown to achieve very significant gains (in capacity, packet loss and delay) compared to existing protocols through the exploitation of the synergy between the two protocol layers.published_or_final_versio

Variable rate adaptive trellis coded QAM for high bandwidthefficiency applications in Rayleigh fading channels

A high bandwidth efficiency variable rate adaptive channel coding scheme, ATCQAM, is proposed. Known pilot symbols are transmitted periodically to aid demodulation. Past channel states are fed back to the transmitter with delay. Current channel state is then predicted at the transmitter to decide on the appropriate modulation mode for the current symbol. At good channel states, high level modulation is used to boost up the average throughput. At bad channel states, low level modulation is used to increase error protection. By matching the variable modulation level with a variable rate channel coder, the physical bandwidth is maintained constant. Design issues for the ATCQAM are considered. The effects of finite feedback delay, finite interleaving depth and mobile speed are investigated.published_or_final_versio

Variable rate adaptive modulation for DS-CDMA

An adaptive coding scheme is introduced for a discrete sequence code-division multiple-access system. The system uses noncoherent M-ary orthogonal modulation with RAKE receiver and power control. Both a fast fading channel and a combined fast fading, shadowing and power control channel are considered. Analytical bounds and simulations are done to evaluate the performance of the system. It is found that there is significant improvement in the average throughput and the bit-error-rate performance in the adaptive coding scheme. The amount of improvement drops with the increase of diversity branches used. More importantly, it is found that adaptive coding scheme is relatively robust to shadowing, while fix-rate codes are ineffective in the shadowing environment. Finally, adaptive coding scheme is found to be robust to mobile speed, feedback delay, and finite interleaving depth.published_or_final_versio

Channel adaptive fair queueing for scheduling integrated voice and data services in multicode CDMA systems

CDMA (code division multiple access) systems are critical building blocks of future high performance wireless and mobile computing systems. While CDMA systems are very mature for voice services, their potentials in delivering high quality data services are yet to be investigated. One of the most crucial component in an advanced wideband CDMA system is the judicious allocation of bandwidth resources to both voice and high data rate services so as to maximize utilization while satisfying the respective quality of service requirements. Specifically, in a multicode CDMA system, the problem is to intelligently allocate codes to the users' requests. While previous work in the literature has addressed this problem from a capacity point of view, the fairness aspect, which is also important from the users' point of view, is largely ignored. In this paper, we propose a new code allocation approach that is channel adaptive and can guarantee fairness with respect to the users' channel conditions. Simulation results show that out approach is more effective than the proportional fair approach.published_or_final_versio

BGCA: bandwidth guarded channel adaptive routing for ad hoc networks

To support truly peer-to-peer applications in ad hoc wireless networks, a judicious and efficient ad hoc routing protocol is needed. Much research has been done on designing ad hoc routing protocols and some well known protocols are also being implemented in practical situations. However, one major drawback in existing state-of-the-art protocols, such as the AODV (ad hoc on demand distance vector) routing protocol, is that the time-varying nature of the wireless channels among the mobile terminals is ignored, let alone exploited. In this paper, by using a previously proposed adaptive channel coding and modulation scheme which allows a mobile terminal to dynamically adjust the data throughput via changing the amount of error protection incorporated, we devise a new ad hoc routing algorithm that dynamically changes the routes according to the channel conditions. Extensive simulation results indicate that our proposed protocol is more efficient in that shorter delays and higher rates are achieved.published_or_final_versio

Power control approach for IEEE 802.11 ad hoc networks

In packet radio networks, especially an ad hoc wireless network using IEEE 802.11 as the MAC (media access control) protocol, power control is a crucial issue. By using a judicious power control mechanism, co-channel interference can be significantly reduced, thus improving the channel spatial reuse and network capacity. However, efficient power control in an IEEE 802.11 system is very challenging because according to the standard, fixed power is used for transmitting packets, and there is only one channel. In this paper, we propose an enhancement to the standard IEEE 802.11 MAC protocol by improving the handshaking mechanisms and adding one separate power control channel. With the control channel, the receiver notifies its neighbors its noise tolerance. Thus, the neighbors can adjust their transmission power levels to avoid packet collisions at the receiver. Through extensive simulations using NS-2, our proposed power control mechanism is found to be effective in that network throughput can be increased by about 10%, and the battery utilization can also be improved at the same time.published_or_final_versio

X-knife stereotactic radiosurgery for cerebral AVM: Queen Mary Hospital experience

Meeting Theme: Degenerative Lumbar SpineOral-Poster Presentation 1INTRODUCTION: This is a retrospective review of the effectiveness, safety, complications of LINAC based X-knife stereotactic radiosurgery for the treatment of cerebral AVM in Queen Mary Hospital, Hong Kong. METHODS: Retrospective search through medical records of a single institution. From 2003-2013, all patients who received X-knife stereotactic radiosurgery for cerebral AVM were included. Demographics, presenting symptoms, size of AVM, Spetzler-Martin grading, dosages, complications, follow-up …published_or_final_versio