Karthikeyan Sundaresan

ATP: a reliable transport protocol for ad hoc networks

Existing works have approached the problem of reliable transport in ad hoc networks by proposing mechanisms to improve TCPs performance over such networks, In this paper, we show through detailed arguments and simulations that several of the design elements in TCP are fundamentally inappropriate for the unique characteristics of ad hoc networks. Given that ad hoc networks are typically stand-alone, we approach the problem of reliable transport from the perspective that it is justifiable to develop an entirely new transport protocol that is not a variant of TCP. Toward this end, we present a new reliable transport layer protocol for ad hoc networks called ATP (ad hoc transport protocol). We show through ns2-based simulations that ATP outperforms default TCP as well as TCP-ELFN and ATCP.

MIDU: enabling MIMO full duplex

Given that full duplex (FD) and MIMO both employ multiple antenna resources, an important question that arises is how to make the choice between MIMO and FD? We show that optimal performance requires a combination of both to be used. Hence, we present the design and implementation of MIDU, the first MIMO full duplex system for wireless networks. MIDU employs antenna cancellation with symmetric placement of transmit and receive antennas as its primary RF cancellation technique. We show that MIDUs design provides large amounts of self-interference cancellation with several key advantages: (i) It allows for two stages of additive antenna cancellation in tandem, to yield as high as 45 dB self-interference suppression; (ii) It can potentially eliminate the need for other forms of analog cancellation, thereby avoiding the need for variable attenuator and delays; (iii) It easily scales to MIMO systems, therefore enabling the coexistence of MIMO and full duplex. We implemented MIDU on the WARP FPGA platform, and evaluated its performance against half duplex (HD)-MIMO. Our results reveal that, with the same number of RF chains, MIDU can potentially double the throughput achieved by half duplex MIMO in a single link; and provide median gains of at least 20% even in single cell scenarios, where full duplex encounters inter-client interference. Based on key insights from our results, we also highlight how to efficiently enable scheduling for a MIDU node.

ATP: a reliable transport protocol for ad-hoc networks

Existing works have approached the problem of reliable transport in ad hoc networks by proposing mechanisms to improve TCPs performance over such networks, In this paper, we show through detailed arguments and simulations that several of the design elements in TCP are fundamentally inappropriate for the unique characteristics of ad hoc networks. Given that ad hoc networks are typically stand-alone, we approach the problem of reliable transport from the perspective that it is justifiable to develop an entirely new transport protocol that is not a variant of TCP. Toward this end, we present a new reliable transport layer protocol for ad hoc networks called ATP (ad hoc transport protocol). We show through ns2-based simulations that ATP outperforms default TCP as well as TCP-ELFN and ATCP.

Medium access control in ad hoc networks with MIMO links: optimization considerations and algorithms

we present a medium access control (MAC) protocol for ad hoc networks with multiple input multiple output (MIMO) links. MIMO links provide extremely high spectral efficiencies in multipath channels by simultaneously transmitting multiple independent data streams in the same channel. MAC protocols have been proposed in related work for ad hoc networks with other classes of smart antennas such as switched beam antennas. However, as we substantiate in the paper, the unique characteristics of MIMO links coupled with several key optimization considerations, necessitate an entirely new MAC protocol. We identify several advantages of MIMO links, and discuss key optimization considerations that can help in realizing an effective MAC protocol for such an environment. We present a centralized algorithm called stream-controlled medium access (SCMA) that has the key optimization considerations incorporated in its design. Finally, we present a distributed SCMA protocol that approximates the centralized algorithm and compare its performance against that of baseline protocols that are CSMA/CA variants.

A fair medium access control protocol for ad-hoc networks with MIMO links

We present a new medium access control (MAC) protocol for ad-hoc networks with multiple input multiple output (MIMO) links. Links that use multiple element arrays (MEAs) at both ends are referred to as MIMO links. MIMO links are known to provide extremely high spectral efficiencies in multipath channels by simultaneously transmitting multiple independent data streams in the same channel. MAC protocols have been proposed in related work for ad-hoc networks with other classes of smart antennas such as switched beam antennas. However, as we substantiate in the paper, the unique characteristics of MIMO links necessitate an entirely new MAC protocol. We identify several advantages of MIMO links, and discuss key optimization considerations that can help in realizing an effective MAC protocol for such an environment. We present a centralized algorithm that has the optimization considerations incorporated in its design. Finally, we present a distributed protocol that approximates the centralized algorithm, and compare its performance against that of baseline protocols that are variants of the CSMA/CA protocol.

Efficient resource management in OFDMA Femto cells

Femto cells are a cost-effective means of providing ubiquitous connectivity in future broadband wireless networks. While their primary purpose has been to improve coverage in current solutions, their decreased cell sizes in turn also provide improved cell capacity through increased spatial reuse. The demand for bandwidth-intensive IP services will soon necessitate the need to tap into this improved capacity. However, the lack of direct coordination between the macro and femto cells, and the completely distributed nature of femto cells make this an extremely challenging task. In this work, we address this challenge by providing efficient resource management solutions for OFDMA-based femto cells along with performance guarantees. In the process, we consider two models that tradeoff performance and overhead. We also propose a novel location-based resource management solution for leveraging maximal spatial reuse from femto cells. Our comprehensive evaluations indicate that in addition to providing improved coverage indoors, with carefully designed resource management solutions that leverage spatial reuse, femto cells have a great potential to increase the system performance by two folds.

FluidNet: a flexible cloud-based radio access network for small cells

Cloud-based radio access networks (C-RAN) have been proposed as a cost-efficient way of deploying small cells. Unlike conventional RANs, a C-RAN decouples the baseband processing unit (BBU) from the remote radio head (RRH), allowing for centralized operation of BBUs and scalable deployment of light-weight RRHs as small cells. In this work, we argue that the intelligent configuration of the front-haul network between the BBUs and RRHs, is essential in delivering the performance and energy benefits to the RAN and the BBU pool, respectively. We propose FluidNet-a scalable, light-weight framework for realizing the full potential of C-RAN. FluidNet deploys a logically re-configurable front-haul to apply appropriate transmission strategies in different parts of the network and hence cater effectively to both heterogeneous user profiles and dynamic traffic load patterns. FluidNets algorithms determine configurations that maximize the traffic demand satisfied on the RAN, while simultaneously optimizing the compute resource usage in the BBU pool. We prototype FluidNet on a 6 BBU, 6 RRH WiMAX C-RAN testbed. Prototype evaluations and large-scale simulations reveal that FluidNets ability to re-configure its front-haul and tailor transmission strategies provides a 50% improvement in satisfying traffic demands, while reducing the compute resource usage in the BBU pool by 50% compared to baseline schemes.

The case for antenna cancellation for scalable full-duplex wireless communications

Recent works have considered the feasibility of full duplex (FD) wireless communications in practice. While the first FD system by Choi et.al. relied on a specific antenna cancellation technique to achieve a significant portion of self-interference cancellation, the various limitations of this technique prompted latter works to move away from antenna cancellation and rely on analog cancellation achieved through channel estimation. However, the latter systems in turn require the use of variable attenuator and delay elements that need to be automatically tuned to compensate for the self-interference channel. This not only adds complexity to the overall system but also makes the performance sensitive to wide-band channels. More importantly, none of the existing FD schemes can be readily scaled to MIMO systems. In this context, we revisit the role of antenna cancellation in FD communications and show that it has more potential in its applicability to FD than previously thought. We advocate a design that overcomes the limitations that have been pointed out in the literature. We then extend this to a two-stage design that allows both transmit and receive versions of antenna cancellation to be jointly leveraged. Finally, we illustrate an extension of our design to MIMO systems, where a combination of both MIMO and FD can be realized in tandem.

The case for re-configurable backhaul in cloud-RAN based small cell networks

Small cells have become an integral component in meeting the increased demand for cellular network capacity. Cloud radio access networks (C-RAN) have been proposed as an effective means to harness the capacity benefits of small cells at reduced capital and operational expenses. With the baseband units (BBUs) separated from the radio access units (RAUs) and moved to the cloud for centralized processing, the backhaul between BBUs and RAUs forms a key component of any C-RAN. In this work, we argue that a one-one mapping of BBUs to RAUs is highly sub-optimal, thereby calling for a functional decoupling of the BBU pool from the RAUs. Further, the backhaul architecture must be made re-configurable to allow the mapping between BBUs and RAUs to be flexible and changed dynamically so as to not just optimize RAN performance but also energy consumption in the BBU pool. Towards this end, we design and implement the first OFDMA-based C-RAN test-bed with a reconfigurable backhaul that allows 4 BBUs to connect flexibly with 4 RAUs using radio-over-fiber technology. We demonstrate the feasibility of our system over a 10 km separation between the BBU pool and RAUs. Further, real world experiments with commercial off-the-shelf WiMAX clients reveal the performance benefits of our reconfigurable backhaul in catering effectively to heterogeneous user (static and mobile clients) and traffic profiles, while also delivering energy benefits in the BBU pool.

Routing in ad-hoc networks with MIMO links

Smart antennas include a broad variety of antenna technologies ranging from the simple switched beams to the sophisticated digital adaptive arrays. While beam-forming antennas are good candidates for use in strong line of sight (LOS) environments, it is the multiple input multiple output (MIMO) technology that is best suited for multipath environments. In fact, the MIMO links exploit the multipath induced rich scattering to provide high spectral efficiencies. The focus of this work is to identify the various characteristics and tradeoffs of MIMO links that can be leveraged by routing layer protocols in rich multipath environments to improve their performance. To this end, we propose a routing protocol called MIR for ad-hoc networks with MIMO links, that leverages the various characteristics of MIMO links in its mechanisms to improve the network performance. We show the effectiveness of the proposed protocol by evaluating its performance through ns2 simulations for a variety of network conditions.