Krishna Balachandran

Channel quality estimation and rate adaptation for cellular mobile radio

We propose a technique to measure channel quality in terms of signal-to-interference plus noise ratio (SINR) for the transmission of signals over fading channels. The Euclidean distance (ED) metric, associated with the decoded information sequence or a suitable modification thereof, is used as a channel quality measure. Simulations show that the filtered or averaged metric is a reliable channel quality measure which remains consistent across different coded modulation schemes and at different mobile speeds. The average scaled ED metric can be mapped to the SINR per symbol. We propose the use of this SINR estimate for data rate adaptation, in addition to mobile assisted handoff (MAHO) and power control. We particularly focus on data rate adaptation and propose a set of coded modulation schemes which utilize the SINR estimate to adapt between modulations, thus improving the data throughput. Simulation results show that the proposed metric works well across the entire range of Dopplers to provide near-optimal rate adaptation to average SINR. This method of adaptation averages out short-term variations due to Rayleigh fading and adapts to the long-term effects such as shadowing. At low Dopplers, the metric can track Rayleigh fading and match the rate to a short-term average of the SINR, thus further increasing throughput.

Mobile responder communication networks for public safety

This article proposes a paradigm shift from the prevailing public safety model of disparate, agency-owned and -operated Land Mobile Radio networks to Mobile Responder Communication Networks (MRCNs) that are created by unifying communications resources and are shared across cooperating public safety agencies to provide local, regional, or national service. MRCNs use a common IP-based core network employing service-intelligent session control to bridge networks based on LMR and commercial wireless access technologies, thus allowing the support of emerging IP-based multimedia services, high data rate access, and mission-critical tactical group voice and interoperable communications during emergency responses.

Delay analysis of selective-repeat ARQ with applications to link adaptation in wireless packet data systems

Radio link control (RLC) protocols are typically employed for reliable in-sequence delivery of service data units (SDUs) in wireless packet data systems. The RLC layer segments packets obtained from the upper layer (referred to as SDUs) into smaller RLC transmission units (or blocks) and uses selective-repeat automatic repeat request (SR-ARQ) for error recovery of RLC blocks. In earlier work, SR-ARQ performance is typically characterized in terms of the long-term throughput or in-sequence delivery delay of RLC blocks. The SDU delivery delay which is a more meaningful measure of RLC performance (in terms of the service provided to a higher layer, e.g., transmission control protocol) has not been quantified. In this paper, we analyze the SDU delivery delay of SR-ARQ as a function of the SDU size and the channel coding scheme employed. Closed-form delay expressions as well as approximations are provided. The analysis is verified through enhanced general packet radio service RLC simulations. Based on the analysis, we propose that link adaptation be backlog dependent in order to reduce the SDU delivery delay at the RLC layer.

Design and analysis of an IEEE 802.16e-based OFDMA communication system

IEEE 802.16e has emerged as a strong candidate standard for future wireless systems primarily because it offers the potential for high spectral efficiency, flexible spectrum options (e.g., 2–6 GHz), scalable carrier bandwidth options (e.g., from 1.25 MHz to 20 MHz), multiple duplexing options (time and frequency division duplex), various subchannelization options, and, unlike its IEEE 802.16 predecessors, mobility. Because of the recent emergence of IEEE 802.16e and the complexity it poses in system analysis, there is little published work in the literature regarding the actual system capacity/throughput performance of IEEE 802.16e for high data rate services. In this paper, we investigate the link and system level performance on the downlink of an IEEE 802.16e-based orthogonal frequency division multiple access (OFDMA) communication system and provide recommendations on high performance IEEE 802.16e system design and deployment configurations. We also propose dynamic resource allocation methods that may be used in OFDMA systems and investigate their performance.

GPRS-136: high-rate packet data service for North American TDMA digital cellular systems

This article provides an overview of the flexible, high-performance packet data channel that has been designed for high-rate packet data services over IS-136 TDMA channels. To achieve the highest data rates in the limited 30 kHz channel bandwidth, the packet data channel is designed for adaptive modulation and, in addition to a fixed coding mode, permits operation using an incremental redundancy mode.

Cross-Layer Optimization for OFDMA-Based Wireless Mesh Backhaul Networks

In this paper, we propose a cross layer optimization framework for multi-hop routing and resource allocation design in an orthogonal frequency division multiple access (OFDMA) based wireless mesh network. The network under consideration is assumed to consist of fixed mesh routers (or base station routers) inter-connected using OFDMA wireless links with some of the mesh routers functioning as gateways to a wired network. The objective of our cross-layer formulation is to allow joint determination of power control, frequency-selective OFDMA scheduling and multi-hop routing in order to maximize the minimum throughput that can be supported to all mesh routers. Results of our investigations under typical cellular deployment, propagation and channel model assumptions show that this approach achieves significant mesh throughput improvements primarily due to the following: (a) frequency selective scheduling with OFDMA which provides improved tone diversity thus allowing more efficient bandwidth utilization relative to single carrier methods; and (b) multi-hop routing which provides improved path diversity relative to single hop transmissions.

Neighbor Discovery With Dynamic Spectrum Access In Adhoc Networks

In this paper, different neighbor discovery methods are considered for adhoc networks without fixed spectrum allocations where discovery of neighboring nodes may need to occur over a large frequency range (e.g. tens of MHz to a few GHz). In such systems, co-existence policies or dynamic spectrum access protocols may also introduce temporal variability in the availability of frequencies for neighbor discovery thus making it challenging to discover neighbors in a short period of time and with low energy consumption. In this paper, we analyze the performance of a baseline scheme (representative of current techniques) where nodes perform discovery by transmitting and receiving beacons randomly across a large frequency range. We also propose and analyze a family of fast, energy-efficient neighbor discovery techniques that use pseudo-random frequency hopping sequences for beacon transmission and reception. For the cases considered, these techniques provide substantial improvements in discovery time and energy consumption relative to the baseline

Efficient transmission of ARQ feedback for EGPRS radio link control

ARQ feedback in enhanced general packet radio service (EGPRS) is provided through the use of bitmaps that indicate the receipt status of individual radio link control (RLC) blocks. The use of a robust coding and modulation scheme for the transmission of ARQ feedback limits the length of the bitmap that can be accommodated in a single feedback message to a fraction of the receiver window. Depending on the feedback frequency, round trip delay and the rate of transmission of RLC blocks, incomplete feedback may cause considerable protocol stalling. In this paper, the loss of throughput due to partial feedback is characterized. We also propose various techniques that minimize protocol stalling through the efficient transmission of multiple non-overlapping bitmaps that cover the receiver window. Simulation results show the relative benefits of the proposed techniques.

Capacity Benefits of Relays with In-Band Backhauling in Cellular Networks

In this paper, the feasibility and capacity benefits of deploying low-cost relay nodes with in-band wireless backhaul capability in cellular networks are studied. Our proposed scheme is similar to a cell-splitting architecture where the base station density is increased, thus reducing the effective cell size. Cell-splitting in an interference-limited deployment provides capacity gains that are proportional to the number of additional base stations deployed, but at significant cost driven primarily by wired backhaul requirements. The use of in-band wireless backhaul may significantly reduce the cost, but sharing bandwidth with the backhaul results in reduced access rates relative to that of cell-splitting with wired backhaul. Further cost savings may be achieved with the use of low power, low complexity relays in lieu of the additional base stations. In this paper, we show that a cellular architecture with base stations supplemented by simple decode-and-forward relays can provide significant capacity improvements despite the overhead associated with in-band backhaul.

Adaptive sleeping and awakening protocol (ASAP) for energy efficient adhoc sensor networks

An adaptive sleeping and awakening protocol (ASAP) is proposed for nodes in a synchronous adhoc sensor network. In order to increase energy efficiency, nodes within the network enter into a sleep mode and awaken at pre-determined time slot(s) to listen for transmissions from its immediate neighbors. Implicit knowledge of awakening slots for neighboring nodes is used to schedule transmissions within the neighborhood. Finally, nodes adapt their sleeping cycles based on neighbor topology and remaining battery life in order to maximize the network lifetime while satisfying the latency requirements of the underlying sensor application. Simulation results show that with delay constrained routing, ASAP can achieve twice the energy efficiency (or battery life) of a synchronous awakening approach.