Nemmara K. Shankaranarayanan

Radio resource allocation in fixed broadband wireless networks

We consider use of fixed broadband wireless networks to provide packet services for telecommuting and Internet access. Each cell is divided into multiple sectors, each of them served by a sector antenna colocated with the base station (BS), and user terminals also use directional antennas mounted on the rooftops of homes or small offices and pointed to their respective BS antennas. To support a target data rate of 10 Mb/s, a bandwidth of several MHz is required. Since radio spectrum is expensive, the bandwidth needs to be reused very aggressively. Thus, efficient strategies for frequency reuse and managing cochannel interference are critically important. We propose several algorithms for dynamic radio-resource allocation in the fixed wireless networks. In particular, a method to be referred to as the staggered resource allocation (SRA) method uses a distributed scheduling algorithm to avoid major sources of interference while allowing concurrent packet transmission and meeting signal-to-interference objectives. The performance of the method is studied by analytic approximations and detailed simulation. Our results show that the combination of directional antennas plus the SRA method is highly effective in controlling cochannel interference. For reasonable system parameters, the SRA method delivers a throughput in excess of 30% per sector while permitting a given frequency band to be reused in every sector of every cell. It also provides satisfactory probability of successful packet transmission. In addition, a simple control mechanism can be applied in the method to improve performance for harsh radio environments.

Performance of simulcast wireless techniques for personal communication systems

Broadband analog transport facilities using fiber or fiber/coax cable can play a significant role in the evolution of the network infrastructure for personal communications services (PCSs). Low-power PCS systems require a dense grid of radio ports to provide connectivity to the telephone network. Analog transport has a number of important advantages over digital transmission facilities, including the flexibility to support a variety of air interface formats, shared infrastructure cost with other services such as video distribution, and centralized call processing allowing the use of low cost and simple radio ports. A simulcast technique can be used in such systems to permit low rates of handoff (no handoff within each simulcast area) and sharing of hardware resources among multiple radio ports. This paper provides a detailed model and a simulation analysis of the cochannel interference and noise performance as well as the resource sharing benefit of a simulcast PCS system. Several potential PCS air interfaces are considered, including time division multiple access (TDMA) and code division multiple access (CDMA) techniques. Our investigation shows that the impact of multiple antenna noise in a simulcast system is offset by the improved signal-to-interference (SIR) ratio brought about by distributed antennas. Even with distributed antennas, multiple antenna noise places a limit on the maximum number of radio ports that can be assigned to each simulcast group. This limit, however, is shown to have little impact on the achievable resource sharing benefit of simulcasting (i.e., grouping beyond this limit has diminishing returns). A saving of 40% to 60%, in terms of the required central hardware resources, is typical for both TDMA and CDMA systems in suburban environments.

The AT&T Labs broadband fixed wireless field experiment

We describe an ongoing broadband fixed two-way wireless field experiment conducted by AT&T Laboratories-Research in Monmouth County, New Jersey. Our experiment, which is one of the first two-way broadband fixed wireless systems offers an end-to-end broadband packet access service, with telecommuting as the primary application for our employee users. It operates in the 2.6 GHz MMDS spectrum, and is based on cable modem technology. We have developed a Web-based network monitoring/management tool that greatly enhances the ability to manage, diagnose, and optimize the system. The lengthy period of operation has allowed us to make observations about user behavior, weather-related channel impairments, and equipment performance. We have identified several design issues related to the application of cable modem technology to the fixed wireless environment. Also, we have measured a significant path loss effect arising from a combination of rain and foliage.

Is Multipath TCP (MPTCP) Beneficial for Video Streaming over DASH

HTTP-based adaptive protocols dominate todays video streaming over the Internet, and operate using multiple quality levels that video players request one segment at a time. Despite their popularity, studies have shown that performance of video streams still suffers from stalls, quality switches and startup delay. In wireless networks, it is well-known that high variability in network bandwidth affects video streaming. MultiPath TCP (MPTCP) is an emerging paradigm that could offer significant benefits to video streaming by combining bandwidth on multiple network interfaces, in particular for mobile devices that typically support both WiFi and cellular networks. In this paper, we explore whether MPTCP always benefits mobile video streaming. Our experimental study on video streaming using two wireless interfaces yields mixed results. While beneficial to user experience under ample and stable bandwidth, MPTCP may not offer any advantage under some network conditions. We find that when additional bandwidth on the secondary path is not sufficient to sustain an upgrade in video quality, it is generally better not to use MPTCP. We also identify that MPTCP can harm user experience when an unstable secondary path is added to the stable primary path.

Multi-path TCP: Boosting Fairness in Cellular Networks

Cellular providers are rapidly deploying multiple technologies like cell biasing, carrier aggregation, co-ordinated interference control/scheduling to improve capacity and coverage. In this paper, we explore a complementary transport layer approach based on multipath TCP that can concurrently use multiple interfaces to boost throughput of users with poor coverage and improve fairness. Multipath TCP has been recently standardized by IETF and requires no modifications to applications. It has been shown to improve fairness and throughput in wire line environments and individual user throughputs in wireless networks. However, in a wireless multi-user environment, it is not clear that it is always beneficial, as we show in this paper. Therefore, we examine if it is indeed beneficial for a service provider to judiciously decide whether to enable multiple cellular interfaces on a smart phone based on a global centralized view of its network. Alternatively, should a device decide independently based only on a local view? To quantify the network wide impact in a system where users have multiple cellular interfaces, we have developed centralized and distributed heuristic algorithms to evaluate this, particularly in the context of fairness across all the users. Our simulations and numerical models show that there are potential gains in fairness (15-30%) to be realized by judiciously enabling multipath connections at the cell edge. These gains diminish as the number of users in a cell increases or users behave greedily. We also quantify the delicate balance between throughput and fairness. Our analysis provides an intuition on which user(s) in a cellular network stand to benefit the most by enabling multiple interfaces. We also discuss LTE protocol mechanisms to enforce associations of specific interfaces to specific cells.