Nandu Gopalakrishnan

Evolution of UMTS toward high-speed downlink packet access

An expanded effort is under way to support the evolution of the Universal Mobile Telecommunications System (UMTS) standard to meet the rapidly developing needs associated with wireless data applications. A new, shared channel — the high-speed downlink shared channel (HS-DSCH) — provides support to packet-switched high-speed data users. A number of performance-enhancing technologies are included in the high-speed downlink packet access (HSDPA) system to ensure high peak and average packet data rates while supporting circuit-switched voice and packet data on the same carrier. Lucent Technologies took a pivotal role in specifying many of these techniques, including adaptive modulation and coding (AMC), hybrid automatic repeat request (HARQ), and fat-pipe scheduling. In this paper, we provide system-level simulations results to indicate the achievable performance and capacity with these advanced technologies. We also discuss HSDPA protocol architecture along with the uplink and downlink control channel design and performance. We conclude with a discussion of potential enhancements for the future.

On efficient link error prediction based on convex metrics

A methodology is presented to predict the frame error rate (FER) in wireless links for a wide range of channel conditions and retransmission strategies. This method measures the variations in signal amplitude and noise variance within a frame and applies a convex metric based on Shannons capacity formula to account for Doppler penalties and diversity gains due to multipath combining and retransmission strategies such as hybrid ARQ with Chase combining or with incremental redundancy. This method requires very little information about the received signals and uses a single (AWGN) reference curve for the given channel code. This link error prediction (LEP) method is of great value for system monitoring, scheduling and system simulations.

Rate Selection Algorithms for IR Hybrid ARQ

We obtain throughput expressions for an incremental redundancy (IR) hybrid ARQ (HARQ) system given delayed channel mutual information feedback using renewal process theory. We obtain the optimal rate selection algorithm for conditional throughput under a conditional outage constraint (due to finite horizon operation) in a single user scheduled per slot (non-adaptive) scenario. We compare performance with commonly employed other rate adaptation algorithms. We characterize the HARQ gain (and its components of time diversity and early termination) improving epsiv-outage capacity and study the impact of feedback and opportunistic scheduling at various mobile speeds.

Deprioritization of heavy users in wireless networks

The explosive demand for wireless data services that followed the introduction of application phones continues to create significant challenges for mobile operators. Measurement campaigns indicate that the consumption of wireless network capacity follows a power law where 20 percent of the users consume more than 80 percent of capacity. This creates unfairness among users in terms of the data volume they are allowed to consume and, more important, during congested periods of time it degrades the quality of experience of all the users. As a response, many operators are attempting to control demand by gradually moving away from unlimited data plans with the introduction of volume caps and tiered charging. Many operators throttle the heavy users after they have exceeded their cap, and consider doing the same during times of congestion. This article evaluates the concept of deprioritization of heavy users in wireless networks for congestion management, the difference between deprioritization and throttling, and the enabling technologies to implement the feature in real-world networks.

Self-organizing interference management for LTE

In orthogonal frequency division multiple access (OFDMA) systems such as Long Term Evolution (LTE), it is extremely important to reduce interference between neighboring cells, especially for cell edge users, since the only interference in LTE is inter-cell interference due to the orthogonality of the sub-carriers used in the transmissions. This paper describes a few self-organizing and self-optimizing techniques to manage and reduce this inter-cell interference. These self-optimizing network (SON)-based techniques are inter-cell interference coordination (ICIC) and uplink (UL) interference over thermal (IoT) control. Simulation results are presented showing the improvements that can be obtained with the use of such techniques in interference limited operating scenarios.