Chandrasekharan Raman

Scheduling variable rate links via a spectrum server

We consider a centralized spectrum server that coordinates the transmissions of a group of links sharing a common spectrum. Links employ on-off modulation with fixed transmit power when active. In the on state, a link obtains a data rate determined by the signal-to-interference ratio on the link. By knowing the link gains in the network, the spectrum server finds an optimal schedule that maximizes the average sum rate subject to a minimum average rate constraint for each link. Using a graph theoretic model for the network and a linear programming formulation, the resulting schedules are a collection of transmission modes (sets of active links) that are time shared in a fashion that is reminiscent of spatial reuse patterns in cellular networks. In the special case when there is no minimum rate constraint, the optimal schedule results in a fixed dominant mode with highest sum rate being operated all the time. In order to offset the inherent unfairness in the above solution, we introduce a minimum rate constraint and characterize the resulting loss in sum rate when compared to the case when there is no minimum rate constraint. We also investigate alternate fairness criteria by designing scheduling algorithms that achieve max-min fairness and proportional fairness. It is shown that the max-min fair rate allocation maximizes the minimum common rate among the links. Simulation results are presented and future work is described

Fair and Efficient Scheduling of Variable Rate Links via a Spectrum Server

We consider a centralized Spectrum Server that coordinates the transmissions of a group of links sharing a common spectrum. Links employ on-off modulation with fixed transmit power when active. In the on state, a link obtains a data rate determined by the signal-to-interference ratio on the link. With knowledge of the link gains in the network, the spectrum server schedules the on/off periods of the links so as to satisfy constraints on link fairness and efficiency. We express fairness constraints as lower bounds on the average minimum rate for each link. Efficiency constraints are expressed as lower bounds on the ratio of the average rate to the average transmit power for each link. Subject to fairness and efficiency constraints, the spectrum server finds a schedule that maximizes the average sum rate. Using a graph theoretic model for the network and a linear programming formulation, the resulting schedules are a collection of time shared transmission modes (sets of active links). In the special case when there is no minimum rate constraint, varying the efficiency constraint can cause the optimal policy to vary from a fixed dominant mode with highest sum rate being operated all the time to time sharing among singleton modes in which just one link is active. We also address the case of maximum common rate scheduling under efficiency constraints. Simulation results are presented to substantiate our findings.

Evolved multimedia broadcast multicast service in LTE: An assessment of system performance under realistic radio network engineering conditions

With the explosive and sustained growth of data usage on both 3G and 4G mobile broadband wireless networks, new techniques need to be found to deliver data to end users efficiently. One of the key drivers of that data demand is video. Evolved Multimedia Broadcast Multicast Service (eMBMS) is one technique in Long Term Evolution (LTE) that provides a broadcast bearer to deliver video content and file delivery to an unlimited number of users. This bearer makes use of multiple cell sites to build a “single frequency network” (SFN) zone with identical downlink transmission over part of the LTE Orthogonal Frequency Division Multiplexing (OFDM) waveform. The resulting signal is combined at the user equipments antenna in such a way that what is normally a neighboring cell site contributing interference becomes the source of a useful signal, thus improving the overall information signal to interference ratio, as well as spectral efficiency. In this paper, eMBMS technology and architecture are presented along with estimates of achieved performance and the impact on radio network engineering. We conclude that while eMBMS may not be an efficient solution to offer nationwide contiguous services throughout a mobile network, it may be efficiently used across an entire metropolitan area and the surrounding “capacity limited” rural areas when using a low radio band such as 700 MHz or 800 MHz, offering an impressive spectral efficiency of 1.5 b/s/Hz.

Power savings from half-duplex relaying in downlink cellular systems

We evaluate the peak and average power savings due to relay deployments in cellular systems via a simulation study. The peak power savings translate to cost reduction in power amplifiers. The average power savings lead to savings in electricity bills. Half-duplex relays are placed one per sector in a 19-cell, 57-sector cellular network. In the baseline case, the base stations control their transmit powers to achieve a common rate among users. When relays are present in the system, optimal powers are found when the relays get the complete message to be relayed to the user. The codebooks at the relays are chosen such that the users obtain a rate corresponding to the sum of the received powers from the base station and the relay. We observe that when power control is employed, the peak power saving is 2.6 dB and the average total power in the system can be reduced by 3 dB.

Half-Duplex Relaying in Downlink Cellular Systems

We compare the performance of half-duplex relays in downlink cellular system against a baseline system without relays. We simulate the performance of (i) a collaborative power addition scheme, where the relay boosts the received power (P-CPA) at the mobile locations, and (ii) a CPA scheme with power control (PC-CPA) at the base station and relays. Evaluations are done in the context of a 19-cell, 57-sector set-up in which each of the served users must be delivered a message. The user messages are taken to have the same size and 90% of users in the network must be served. Improvements over the baseline due to relay deployments are measured in terms of increase in common rate of users as well as power savings in terms of reduction in peak or average power transmitted by base stations. In the CPA schemes with base stations and relays transmitting at full power, the peak power saving is 1.46 dB, alternately, the throughput improvement over a 1 bit/sec/Hz baseline rate is 21%. In the PC-CPA scheme, the peak power saving is 2.6 dB and the average total power in the system can be reduced by 3 dB.

Cross-Layer Scheduling of End-to-End Flows Using a Spectrum Server

We present a framework for cross-layer scheduling of end-to-end flows in a wireless network with links sharing a common spectrum. Links employ on-off modulation with fixed transmit powers when active. The data rate obtained by an active link is determined by the signal-to-interference ratio on the link. With the knowledge of the link interference gains in the network, the centralized spectrum server schedules the rates on the links and flows on the sessions to maximize a utility function of the source rates. The schedules are a collection of time shared transmission modes (sets of active links).

Relaying in downlink cellular systems

We present a simulation study of two relaying schemes in downlink cellular systems. We simulate the performance of two relaying strategies: (i) a collaborative power addition (CPA) scheme, cooperative scheme where the relay collaborates with the base station, and (ii) a simple orthogonal relaying strategy where base stations and relays transmit in orthogonal time slots. Evaluations are done in the context of a 19-cell, 57-sector set-up in which each of the served users must be delivered a message. The user messages are taken to have the same size and 90% of users in the network must be served. All simultaneous transmissions mutually interfere. Improvements due to relay deployments are measured as throughput improvements in terms of increase in common rate of users as well as power savings in terms of reduction in peak or average power transmitted by base stations. We observe that the simpler orthogonal relaying scheme performs nearly as well as the more complex collaborative relaying in producing throughput gains and power savings.

Random Access for Variable Rate Links

We consider a model where the interfering links employ on-off modulation in each transmission slot. In the on (active) state, a link obtains a data rate determined by the interference from other active links in the network. Based on this model, we compare the throughput regions of centralized scheduling and a probabilistic random access scheme, wherein in each slot, a link is active with a fixed probability chosen independent of other interfering links. We observe that for the case of two interfering links, the probabilistic scheme does not suffer any loss in the rate region relative to the centralized scheme if the interference between the links is sufficiently low. For more than two interfering links, the characterization of throughput rate region for the probabilistic scheme becomes intractable and similar observations are not easily forthcoming. However, we give a distributed algorithm where each link independently updates its transmission probability based on its measured throughput to achieve any desired feasible rate vector in the throughput region of the probabilistic scheme and prove its convergence