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Dive into the research topics where Sundar Subramanian is active.

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Featured researches published by Sundar Subramanian.


IEEE Journal on Selected Areas in Communications | 2013

Vehicular Communications Using DSRC: Challenges, Enhancements, and Evolution

Xinzhou Wu; Sundar Subramanian; Ratul Kumar Guha; Robert G. White; Junyi Li; Kevin W. Lu; Anthony Bucceri; Tao Zhang

Dedicated Short-Range Communications (DSRC) has been designed to support vehicular communications. In the U.S., DSRC operates in the 5.9 GHz licensed spectrum band. Its physical (PHY) and medium access control (MAC) layers, defined in the IEEE 802.11p standard, are based on the IEEE 802.11 family of Wi-Fi standards. Vehicular communication environments differ significantly from the sparse and low-velocity nomadic use cases of a typical Wi-Fi deployment. Thus, there are many challenges to adapt Wi-Fi technologies to support the unique requirements of vehicular communications such as achieving high and reliable performance in highly mobile, often densely populated, and frequently non-line-of-sight environments. The automotive and the communications industries, academia, and governments around the world have been devoting tremendous efforts to address these challenges, and significant achievements have been made. Remaining challenges can be addressed by the future versions of DSRC. In this paper, we investigate the current technologies used by DSRC to support vehicle safety communications, analyze existing and possible DSRC performance enhancements that can be realized in the near term, and provide a few initial thoughts on the DSRC evolution path.


international workshop on vehicular inter-networking | 2012

Congestion control for vehicular safety: synchronous and asynchronous MAC algorithms

Sundar Subramanian; Marc Werner; Shihuan Liu; Jubin Jose; Radu Lupoaie; Xinzhou Wu

The IEEE 802.11p standard specifies the PHY and MAC layer operations for transmitting and receiving periodic broadcast messages for vehicular safety. Many studies have identified issues with the CSMA based IEEE 802.11p MAC at high densities of devices, mainly reflected by low packet reception rate. In this paper, we make an interesting observation that with increasing density, the IEEE 802.11p MAC tends towards an ALOHA-type behavior where concurrent transmissions by close-by devices are not prevented. This behavior can lead to poor packet reception rate even for vehicles in close neighborhood. Many efforts have been made to address the IEEE 802.11p MAC issues to provide better performance for DSRC safety applications, including the introduction of Decentralized Congestion Control (DCC) algorithm to ETSI standards in Europe. In this paper, we evaluate the performance of the proposed DCC algorithm and observe that the nominal parameters in DCC are unsuitable in many scenarios. Using transmit power control as an example, we develop a simple rule within the DCC framework that can significantly improve the safety packet reception performance with increasing densities. The DCC algorithms are fully compatible with the IEEE 802.11p standards and asynchronous in nature. A parallel approach to handle high device densities is a slotted synchronous MAC, where time is slotted based on GPS synchronization and each transmitter contends for a set of recurring time slots (or channels) with periodicity matching the required safety message periodicity. As compared to the per-packet based contention scheme as in CSMA defined in IEEE 802.11, such a scheme is much better suited for periodic safety broadcast. In this paper, we design a standard compliant TDM overlay on top of the MAC layer that can significantly improve the packet reception performance. Combined with a distributed resource selection protocol, the synchronous MAC can discover even more neighboring devices than the improved asynchronous approach, making DSRC safety applications more reliable.


IEEE Journal of Selected Topics in Signal Processing | 2016

Beamforming Tradeoffs for Initial UE Discovery in Millimeter-Wave MIMO Systems

Vasanthan Raghavan; Jürgen Cezanne; Sundar Subramanian; Ashwin Sampath; Ozge H. Koymen

Millimeter-wave (mmW) multi-input multi-output (MIMO) systems have gained increasing traction toward the goal of meeting the high data-rate requirements in next-generation wireless systems. The focus of this work is on low-complexity beamforming approaches for initial user equipment (UE) discovery in such systems. Toward this goal, we first note the structure of the optimal beamformer with per-antenna gain and phase control and establish the structure of good beamformers with per-antenna phase-only control. Learning these right singular vector (RSV)type beamforming structures in mmW systems is fraught with considerable complexities such as the need for a non-broadcast system design, the sensitivity of the beamformer approximants to small path length changes, inefficiencies due to power amplifier backoff, etc. To overcome these issues, we establish a physical interpretation between the RSV-type beamformer structures and the angles of departure/arrival (AoD/AoA) of the dominant path(s) capturing the scattering environment. This physical interpretation provides a theoretical underpinning to the emerging interest on directional beamforming approaches that are less sensitive to small path length changes. While classical approaches for direction learning such as MUltiple SIgnal Classification (MUSIC) have been well-understood, they suffer from many practical difficulties in a mmW context such as a non-broadcast system design and high computational complexity. A simpler broadcast-based solution for mmW systems is the adaptation of limited feedback-type directional codebooks for beamforming at the two ends. We establish fundamental limits for the best beam broadening codebooks and propose a construction motivated by a virtual subarray architecture that is within a couple of dB of the best tradeoff curve at all useful beam broadening factors. We finally provide the received SNR loss-UE discovery latency tradeoff with the proposed beam broadening constructions. Our results show that users with a reasonable link margin can be quickly discovered by the proposed design with a smooth roll-off in performance as the link margin deteriorates. While these designs are poorer in performance than the RSV learning approaches or MUSIC for cell-edge users, their low-complexity that leads to a broadcast system design makes them a useful candidate for practical mmW systems.


Queueing Systems | 2012

On optimizing CSMA for wide area ad hoc networks

François Baccelli; Junyi Li; Thomas Richardson; Sanjay Shakkottai; Sundar Subramanian; Xinzhou Wu

The recent deployment of data-rich smart phones has led to a fresh impetus for understanding the performance of wide area ad hoc networks. The most popular medium access mechanism for such ad hoc networks is CSMA/CA with RTS/CTS. In CSMA-like mechanisms, spatial reuse is achieved by implementing energy-based guard zones. We consider the problem of simultaneously scheduling the maximum number of links that can achieve a given signal to interference ratio (SIR). In this paper, using tools from stochastic geometry, we study and maximize the medium access probability of a typical link. Our contributions are two-fold: (i) We show that a simple modification to the RTS/CTS mechanism, viz., changing the receiver yield decision from an energy-level guard zone to an SIR guard zone, leads to performance gains; and (ii) We show that this combined with a simple modification to the transmit power level—setting it inversely proportional to the square root of the link gain—leads to significant improvements in network throughput. Further, this simple power-level choice is no worse than a factor of two away from optimal over the class of all “local” power level selection strategies for fading channels, and further is optimal in the non-fading case. The analysis relies on an extension of the Matérn hard core point process which allows us to quantify both these SIR guard zones and this power control mechanism.


international conference on computer communications | 2013

A performance analysis of CSMA based broadcast protocol in VANETs

Tien V. Nguyen; François Baccelli; Kai Zhu; Sundar Subramanian; Xinzhou Wu

The broadcast of periodic messages is a key functionality in vehicular ad hoc networks. In the emerging vehicular networks, IEEE 802.11p is the standard of choice to support the PHY and MAC layer functionalities. The broadcast process in IEEE 802.11p is based on the CSMA mechanism where a device transmitting a packet senses the channel for ongoing transmissions and performs a random back-off before accessing the channel. Without RTS/CTS mechanisms, carrier sensing is expected to provide a protection region around the transmitter where no other transmitters are allowed. The point process characterizing the concurrent transmitters is expected to enforce a minimum separation between concurrent transmitters. However, at increasing densities, the CSMA behavior breaks down to an ALOHA-like transmission pattern where concurrent transmitters are distributed as a Poisson point process, indicating the lack of protection around transmitters. In this paper, we model the CSMA mechanism as a slotted system and analytically characterize the critical node/packet arrival density where the CSMA mechanism approaches an ALOHAlike behavior. Further, we use tools from stochastic geometry to establish closed-form expressions for the performance metrics of the broadcast mechanism in the ALOHA regime. Finally, using ns2 (an unslotted asynchronous simulator), we compare the theoretical results with simulations.


modeling and optimization in mobile, ad-hoc and wireless networks | 2011

On optimizing CSMA for wide area ad-hoc networks

Franfois Baccelli; Junyi Li; Thomas Richardson; Sundar Subramanian; Xinzhou Wu; Sanjay Shakkottai

Recent deployments of data-rich smart phones has provided a fresh impetus for designing, deploying and understanding the performance of wide area ad-hoc networks. The most popular medium access mechanism for such ad hoc networks is CSMA/CA with RTS/CTS. In this paper, using tools from stochastic geometry, we study and optimize the throughput performance of such networks. We show that in ad-hoc networks enabled with SIR based scheduling, a simple modification to the transmit power level — setting it to be inversely proportional to the square root of the link gain — leads to large improvements in network throughput. This simple power-level selection is optimal over the class of all ”local” transmit power selection strategies when channels are stationary, and further is at most a factor of two away from optimality in the fading case. Using stochastic geometric techniques, we also provide analytical expressions for the medium access probability in different scenarios.


global communications conference | 2014

Directional Beamforming for Millimeter-Wave MIMO Systems

Vasanthan Raghavan; Sundar Subramanian; Juergen Cezanne; Ashwin Sampath

The focus of this paper is on beamforming in a millimeter-wave (mmW) multi-input multi-output (MIMO) set-up that has gained increasing traction in meeting the high data-rate requirements of next- generation wireless systems. For a given MIMO channel matrix, the optimality of beamforming with the dominant right-singular vector (RSV) at the transmit end and with the matched filter to the RSV at the receive end has been well-understood. When the channel matrix can be accurately captured by a physical (geometric) scattering model across multiple clusters/paths as is the case in mmW MIMO systems, we provide a physical interpretation for this optimal structure: beam steering across the different paths with appropriate power allocation and phase compensation. While such an explicit physical interpretation has not been provided hitherto, practical implementation of such a structure in a mmW system is fraught with considerable difficulties (complexity as well as cost) as it requires the use of per-antenna gain and phase control. This paper characterizes the loss in received SNR with an alternate low-complexity beamforming solution that needs only per-antenna phase control and corresponds to steering the beam to the dominant path at the transmit and receive ends. While the loss in received SNR can be arbitrarily large (theoretically), this loss is minimal in a large fraction of the channel realizations reinforcing the utility of directional beamforming as a good candidate solution for mmW MIMO systems.


global communications conference | 2014

Indoor mm-Wave Channel Measurements: Comparative Study of 2.9 GHz and 29 GHz

Ozge H. Koymen; Andrzej Partyka; Sundar Subramanian; Junyi Li

The millimeter-wave (mm-Wave) frequency band ~30-300 GHz has received significant attention lately as a prospective band for 5G systems. Millimeter-wave frequencies have traditionally been used for backhaul, satellite and other fixed services. While these bands offer substantial amount of bandwidth and opportunity for spatial multiplexing, the propagation characteristics for terrestrial mobile usage need to be fully understood prior to system design. Towards this end, this paper presents preliminary indoor measurement results obtained using a channel sounder equipped with omni- and directional antennas at 2.9 GHz and 29 GHz as a comparative study of the two bands. The measurements are made within a Qualcomm building in Bridgewater, NJ, USA, for two separate floors, each representing a different yet representative type of office plan. We present measurements and estimated parameters for path loss, excess delay, RMS delay and analyze the power profile of received paths. In addition, we present several spherical scans of particular links to illustrate the 3-D angular spread of the received paths. This work represents initial results of an ongoing effort for comprehensive indoor and outdoor channel measurements. The measurements presented here, along with cited references, offer interesting insights into propagation conditions (e.g. loss, delay/angular spread etc.), coverage and robustness for mobile use of millimeter-wave bands. We believe additional extensive measurement campaigns in diverse settings by academia and industry would help facilitate the generation of usable channel models.


allerton conference on communication, control, and computing | 2012

Opportunistic interference alignment in cellular downlink

Jubin Jose; Sundar Subramanian; Xinzhou Wu; Junyi Li

Interference alignment is known to be degrees of freedom optimal for the K-user interference channel. As a result, many alignment-based schemes have been developed in literature. However, most schemes are not easy to implement due to the requirements for a large number of dimensions and an extensive exchange of channel state information. This paper eliminates these requirements by combining interference alignment with the idea of opportunistic scheduling to facilitate alignment in the cellular downlink by opportunistically selecting users and harnessing the multi-user nature of cellular scheduling. Specifically, the opportunistic interference alignment scheme requires no channel state information at the base station transmitters. Simulation results show significant improvement in throughput of cell-edge users without any degradation in performance of other users.


international conference on communications | 2015

Coverage and channel characteristics of millimeter wave band using ray tracing

Zhenliang Zhang; Jung Ryu; Sundar Subramanian; Ashwin Sampath

Utilization of millimeter wave (MMW) bands for supporting very high data rate cellular access has received much attention in recent times. The behavior of the wireless channel in these bands differ significantly from the sub-6 GHz counterparts. Ray tracing is a complementary effort to detailed measurements in providing quick insight into the coarse channel characteristics. In this paper, we model the 28 GHz outdoor channel through ray tracing using the WinProp tool. We simulate the single base station and multi base station scenarios in the downtown Manhattan area and study channel properties such as coverage and path loss, path diversity and delay spreads. We observe that a typical MMW base station provides coverage to a significant fraction of a 100m cell radius with path diversity, i.e. there exist secondary paths that may sustain the link on the failure of the first path. The improvements to coverage and path diversity with multiple base stations are also characterized. The above channel properties are compared with NYUs [4] results from the same region. We observe a reasonable match for many of the above parameters. The delay spreads with omni and beamformed antennas are also compared. The typical RMS delay spread in a beamformed scenario is seen to be in the order of 20-30 ns which is significantly smaller than the measured values of 100ns or more. This phenomenon is likely due to detailed clutter in the environment not being modeled.

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