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Jubin Jose

Bio: Jubin Jose is an academic researcher from Qualcomm. The author has contributed to research in topics: Throughput & Network packet. The author has an hindex of 16, co-authored 96 publications receiving 3613 citations. Previous affiliations of Jubin Jose include University of Texas System & Indian Institute of Technology Madras.


Papers
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Journal ArticleDOI
TL;DR: The paper shows that the precoding matrix used by the base station in one cell becomes corrupted by the channel between that base station and the users in other cells in an undesirable manner and develops a new multi-cell MMSE-based precoding method that mitigates this problem.
Abstract: This paper considers a multi-cell multiple antenna system with precoding used at the base stations for downlink transmission. Channel state information (CSI) is essential for precoding at the base stations. An effective technique for obtaining this CSI is time-division duplex (TDD) operation where uplink training in conjunction with reciprocity simultaneously provides the base stations with downlink as well as uplink channel estimates. This paper mathematically characterizes the impact that uplink training has on the performance of such multi-cell multiple antenna systems. When non-orthogonal training sequences are used for uplink training, the paper shows that the precoding matrix used by the base station in one cell becomes corrupted by the channel between that base station and the users in other cells in an undesirable manner. This paper analyzes this fundamental problem of pilot contamination in multi-cell systems. Furthermore, it develops a new multi-cell MMSE-based precoding method that mitigates this problem. In addition to being linear, this precoding method has a simple closed-form expression that results from an intuitive optimization. Numerical results show significant performance gains compared to certain popular single-cell precoding methods.

1,306 citations

Posted Content
TL;DR: In this paper, a multi-cell multiple antenna system with precoding used at the base stations for downlink transmission is considered, where the precoding matrix used by the base station in one cell becomes corrupted by the channel between that base station and the users in other cells in an undesirable manner.
Abstract: This paper considers a multi-cell multiple antenna system with precoding used at the base stations for downlink transmission. For precoding at the base stations, channel state information (CSI) is essential at the base stations. A popular technique for obtaining this CSI in time division duplex (TDD) systems is uplink training by utilizing the reciprocity of the wireless medium. This paper mathematically characterizes the impact that uplink training has on the performance of such multi-cell multiple antenna systems. When non-orthogonal training sequences are used for uplink training, the paper shows that the precoding matrix used by the base station in one cell becomes corrupted by the channel between that base station and the users in other cells in an undesirable manner. This paper analyzes this fundamental problem of pilot contamination in multi-cell systems. Furthermore, it develops a new multi-cell MMSE-based precoding method that mitigate this problem. In addition to being a linear precoding method, this precoding method has a simple closed-form expression that results from an intuitive optimization problem formulation. Numerical results show significant performance gains compared to certain popular single-cell precoding methods.

1,040 citations

PatentDOI
03 Jun 2011
TL;DR: In this paper, the authors proposed a method, wireless device and computer program product for expanding the coverage of a cellular network through the use of multi-hopping, where a wireless device may request permission to communicate with the base station over a non-cellular interface via hopping off another wireless device when its signal strength is below a threshold.
Abstract: A method, wireless device and computer program product for expanding the coverage of a cellular network. A wireless device (e.g., cellular telephone) is able to communicate with a base station in a cell of the cellular network over a non-cellular interface via another wireless device in a cell through the use of multi-hopping. A wireless device may request permission to communicate with the base station over a non-cellular interface via hopping off another wireless device when its signal strength is below a threshold. Alternatively, a wireless device may receive a request to communicate with the base station over a non-cellular interface via hopping off the wireless device that sent the request when that wireless device has excess capacity in its bandwidth with the base station. By enabling wireless devices to communicate with a base station in such a manner, the effective capacity of the cellular network is expanded and the effective capacity of the cellular network is improved.

219 citations

Proceedings ArticleDOI
28 Jun 2009
TL;DR: A multi-cell MMSE-based precoding is proposed that, when combined with frequency/time/pilot reuse techniques, mitigate this problem of pilot contamination.
Abstract: This paper considers a multi-cell multiple antenna system with precoding at the base stations for downlink transmission. To enable precoding, channel state information (CSI) is obtained via uplink training. This paper mathematically characterizes the impact that uplink training has on the performance of multi-cell multiple antenna systems. When non-orthogonal training sequences are used for uplink training, it is shown that the precoding matrix used by the base station in one cell becomes corrupted by the channel between that base station and the users in other cells. This problem of pilot contamination is analyzed in this paper. A multi-cell MMSE-based precoding is proposed that, when combined with frequency/time/pilot reuse techniques, mitigate this problem.

213 citations

Proceedings ArticleDOI
25 Jun 2012
TL;DR: A standard compliant TDM overlay on top of the MAC layer that can significantly improve the packet reception performance and combined with a distributed resource selection protocol can discover even more neighboring devices than the improved asynchronous approach, making DSRC safety applications more reliable.
Abstract: 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.

102 citations


Cited by
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Journal ArticleDOI
Thomas L. Marzetta1
TL;DR: A cellular base station serves a multiplicity of single-antenna terminals over the same time-frequency interval and a complete multi-cellular analysis yields a number of mathematically exact conclusions and points to a desirable direction towards which cellular wireless could evolve.
Abstract: A cellular base station serves a multiplicity of single-antenna terminals over the same time-frequency interval. Time-division duplex operation combined with reverse-link pilots enables the base station to estimate the reciprocal forward- and reverse-link channels. The conjugate-transpose of the channel estimates are used as a linear precoder and combiner respectively on the forward and reverse links. Propagation, unknown to both terminals and base station, comprises fast fading, log-normal shadow fading, and geometric attenuation. In the limit of an infinite number of antennas a complete multi-cellular analysis, which accounts for inter-cellular interference and the overhead and errors associated with channel-state information, yields a number of mathematically exact conclusions and points to a desirable direction towards which cellular wireless could evolve. In particular the effects of uncorrelated noise and fast fading vanish, throughput and the number of terminals are independent of the size of the cells, spectral efficiency is independent of bandwidth, and the required transmitted energy per bit vanishes. The only remaining impairment is inter-cellular interference caused by re-use of the pilot sequences in other cells (pilot contamination) which does not vanish with unlimited number of antennas.

6,248 citations

Journal ArticleDOI
TL;DR: In this paper, the tradeoff between the energy efficiency and spectral efficiency of a single-antenna system is quantified for a channel model that includes small-scale fading but not large scale fading, and it is shown that the use of moderately large antenna arrays can improve the spectral and energy efficiency with orders of magnitude compared to a single antenna system.
Abstract: A multiplicity of autonomous terminals simultaneously transmits data streams to a compact array of antennas. The array uses imperfect channel-state information derived from transmitted pilots to extract the individual data streams. The power radiated by the terminals can be made inversely proportional to the square-root of the number of base station antennas with no reduction in performance. In contrast if perfect channel-state information were available the power could be made inversely proportional to the number of antennas. Lower capacity bounds for maximum-ratio combining (MRC), zero-forcing (ZF) and minimum mean-square error (MMSE) detection are derived. An MRC receiver normally performs worse than ZF and MMSE. However as power levels are reduced, the cross-talk introduced by the inferior maximum-ratio receiver eventually falls below the noise level and this simple receiver becomes a viable option. The tradeoff between the energy efficiency (as measured in bits/J) and spectral efficiency (as measured in bits/channel use/terminal) is quantified for a channel model that includes small-scale fading but not large-scale fading. It is shown that the use of moderately large antenna arrays can improve the spectral and energy efficiency with orders of magnitude compared to a single-antenna system.

2,770 citations

Journal ArticleDOI
TL;DR: How many antennas per UT are needed to achieve η% of the ultimate performance limit with infinitely many antennas and how many more antennas are needed with MF and BF to achieve the performance of minimum mean-square error (MMSE) detection and regularized zero-forcing (RZF), respectively are derived.
Abstract: We consider the uplink (UL) and downlink (DL) of non-cooperative multi-cellular time-division duplexing (TDD) systems, assuming that the number N of antennas per base station (BS) and the number K of user terminals (UTs) per cell are large. Our system model accounts for channel estimation, pilot contamination, and an arbitrary path loss and antenna correlation for each link. We derive approximations of achievable rates with several linear precoders and detectors which are proven to be asymptotically tight, but accurate for realistic system dimensions, as shown by simulations. It is known from previous work assuming uncorrelated channels, that as N→∞ while K is fixed, the system performance is limited by pilot contamination, the simplest precoders/detectors, i.e., eigenbeamforming (BF) and matched filter (MF), are optimal, and the transmit power can be made arbitrarily small. We analyze to which extent these conclusions hold in the more realistic setting where N is not extremely large compared to K. In particular, we derive how many antennas per UT are needed to achieve η% of the ultimate performance limit with infinitely many antennas and how many more antennas are needed with MF and BF to achieve the performance of minimum mean-square error (MMSE) detection and regularized zero-forcing (RZF), respectively.

2,433 citations

Posted Content
TL;DR: It is shown that the use of moderately large antenna arrays can improve the spectral and energy efficiency with orders of magnitude compared to a single-antenna system.
Abstract: A multiplicity of autonomous terminals simultaneously transmits data streams to a compact array of antennas. The array uses imperfect channel-state information derived from transmitted pilots to extract the individual data streams. The power radiated by the terminals can be made inversely proportional to the square-root of the number of base station antennas with no reduction in performance. In contrast if perfect channel-state information were available the power could be made inversely proportional to the number of antennas. Lower capacity bounds for maximum-ratio combining (MRC), zero-forcing (ZF) and minimum mean-square error (MMSE) detection are derived. A MRC receiver normally performs worse than ZF and MMSE. However as power levels are reduced, the cross-talk introduced by the inferior maximum-ratio receiver eventually falls below the noise level and this simple receiver becomes a viable option. The tradeoff between the energy efficiency (as measured in bits/J) and spectral efficiency (as measured in bits/channel use/terminal) is quantified. It is shown that the use of moderately large antenna arrays can improve the spectral and energy efficiency with orders of magnitude compared to a single-antenna system.

2,421 citations

Journal ArticleDOI
TL;DR: This paper addresses the potential impact of pilot contamination caused by the use of non-orthogonal pilot sequences by users in adjacent cells, and analyzes the energy efficiency and degrees of freedom provided by massive MIMO systems to enable efficient single-carrier transmission.
Abstract: Massive multiple-input multiple-output (MIMO) wireless communications refers to the idea equipping cellular base stations (BSs) with a very large number of antennas, and has been shown to potentially allow for orders of magnitude improvement in spectral and energy efficiency using relatively simple (linear) processing. In this paper, we present a comprehensive overview of state-of-the-art research on the topic, which has recently attracted considerable attention. We begin with an information theoretic analysis to illustrate the conjectured advantages of massive MIMO, and then we address implementation issues related to channel estimation, detection and precoding schemes. We particularly focus on the potential impact of pilot contamination caused by the use of non-orthogonal pilot sequences by users in adjacent cells. We also analyze the energy efficiency achieved by massive MIMO systems, and demonstrate how the degrees of freedom provided by massive MIMO systems enable efficient single-carrier transmission. Finally, the challenges and opportunities associated with implementing massive MIMO in future wireless communications systems are discussed.

2,046 citations