DOCOMO Innovations Inc.

Palo Alto, CA, United States

DOCOMO Innovations Inc.

Palo Alto, CA, United States
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Lopez-Perez D.,King's College London | Chu X.,King's College London | Guvenc I.,DOCOMO Innovations Inc.
IEEE Journal on Selected Topics in Signal Processing | Year: 2012

In order to expand the downlink (DL) coverage areas of picocells in the presence of an umbrella macrocell, the concept of range expansion has been recently proposed, in which a positive range expansion bias (REB) is added to the DL received signal strengths (RSSs) of picocell pilot signals at user equipments (UEs). Although range expansion may increase DL footprints of picocells, it also results in severe DL inter-cell interference in picocell expanded regions (ERs), because ER picocell user equipments (PUEs) are not connected to the cells that provide the strongest DL RSSs. In this paper, we derive closed-form formulas to calculate appropriate REBs for two different range expansion strategies, investigate both DL and uplink (UL) inter-cell interference coordination (ICIC) to enhance picocell performance, and propose a new macrocell-picocell cooperative scheduling scheme to mitigate both DL and UL interference caused by macrocells to ER PUEs. Simulation results provide insights on REB selection approaches at picocells, and demonstrate the benefits of the proposed macrocell-picocell cooperative scheduling scheme over alternative approaches. © 2012 IEEE.


Wang C.,DOCOMO Innovations Inc. | Gou T.,Samsung | Jafar S.A.,University of California at Irvine
IEEE Transactions on Information Theory | Year: 2014

We show that the three-user MT ×MR Multiple-Input Multiple-Output (MIMO) interference channel where each transmitter is equipped with MT antennas and each receiver is equipped with MR antennas has d(M, N) δ= min δ M 2?1/? , N 2+1/?degrees of freedom (DoF) normalized by time, frequency, and space dimensions, where M = min(MT , MR), N = max(MT , MR), κ δ = κ M N?M. While the DoF outer bound of d(M, N) is established for every MT , MR value, the achievability of d(M, N) DoF is established in general subject to a normalization with respect to spatial extensions, i.e., the scaling of the number of antennas at all nodes. In particular, we show that qd(M, N) DoF are achievable for the three-user MT ×qMR MIMO interference channel, for some positive integer q, which may be seen as a spatial extension factor. q is the scaling factor needed to make the value qd(M, N) an integer. Given spatial extensions, the achievability relies only on linear beamforming based interference alignment schemes and requires neither channel extensions nor channel variations in time or frequency. In the absence of spatial extensions, it is shown through examples how essentially the same interference alignment scheme may be applied over time-extensions over either constant or time-varying channels. The central new insight to emerge from this paper is the notion of subspace alignment chains as the DoF bottlenecks. The subspace alignment chains are instrumental both in identifying the extra dimensions to be provided by a genie to a receiver for the DoF outer bound, as well as in the construction of the optimal interference alignment schemes. The DoF value d(M, N) is a piecewise linear function of M, N, with either M or N being the bottleneck within each linear segment, whereas the other value contains some redundancy, i.e., it can be reduced without reducing the DoF. The corner points of these piecewise linear segments correspond to two sets, A δ = {1/2, 2/3, 3/4, . . .} and B δ = {1/3, 3/5, 5/7, . . .}. The set A contains all those values of M/N and only those values of M/N for which there is redundancy in both M and N, contains all those values of M/N and only those values of M/N for which there is no redundancy in either M or N, i.e., neither can be reduced without reducing the DoF. Because A and B represent settings with maximum and minimum redundancy, essentially they are the basis for the DoF outer bounds and inner bounds, respectively. Our results settle the question of feasibility of linear interference alignment, introduced previously by Cenk et al., for the three-user MT×MR MIMO interference channel, completely for all values of MT , MR. In particular, we show that the linear interference alignment problem (MT×M R, d)3 (as defined in previous paper by Cenk et al.) is feasible if and only if d ≤ d(M, N). With the exception of the values M/N B, and only with that exception, we show that for every M/N value there are proper systems (as defined by Cenk et al.) that are not feasible. Evidently the redundancy contained in all other values of M/N manifests itself as superfluous variables that are not discounted in the definition of proper systems, thus creating a discrepancy between proper and feasible systems. Our results show that M/N A are the only values for which there is no DoF benefit of joint processing among co-located antennas at the transmitters or receivers. This may also be seen as a consequence of the maximum redundancy in the M/NA settings. © 1963-2012 IEEE.


Huh H.,Samsung | Caire G.,University of Southern California | Papadopoulos H.C.,DOCOMO Innovations Inc. | Ramprashad S.A.,DOCOMO Innovations Inc.
IEEE Transactions on Wireless Communications | Year: 2012

Time-Division Duplexing (TDD) allows to estimate the downlink channels for an arbitrarily large number of base station antennas from a finite number of orthogonal uplink pilot signals, by exploiting channel reciprocity. Based on this observation, a recently proposed "Massive MIMO" scheme was shown to achieve unprecedented spectral efficiency in realistic conditions of distance-dependent pathloss and channel coherence time and bandwidth. The main focus and contribution of this paper is an improved Network-MIMO TDD architecture achieving spectral efficiencies comparable with "Massive MIMO", with one order of magnitude fewer antennas per active user per cell (roughly, from 500 to 50 antennas). The proposed architecture is based on a family of Network-MIMO schemes defined by small clusters of cooperating base stations, zero-forcing multiuser MIMO precoding with suitable inter-cluster interference mitigation constraints, uplink pilot signals allocation and frequency reuse across cells. The key idea consists of partitioning the users into equivalence classes, optimizing the Network-MIMO scheme for each equivalence class, and letting a scheduler allocate the channel time-frequency dimensions to the different classes in order to maximize a suitable network utility function that captures a desired notion of fairness. This results in a mixed-mode Network-MIMO architecture, where different schemes, each of which is optimized for the served user equivalence class, are multiplexed in time-frequency. In order to carry out the performance analysis and the optimization of the proposed architecture in a systematic and computationally efficient way, we consider the large-system regime where the number of users, the number of antennas, and the channel coherence block length go to infinity with fixed ratios. © 2012 IEEE.


Bossen F.,DOCOMO Innovations Inc. | Bross B.,Fraunhofer Institute for Telecommunications, Heinrich-Hertz-Institut | Suhring K.,Fraunhofer Institute for Telecommunications, Heinrich-Hertz-Institut | Flynn D.,Research in Motion Ltd.
IEEE Transactions on Circuits and Systems for Video Technology | Year: 2012

Advances in video compression technology have been driven by ever-increasing processing power available in software and hardware. The emerging High Efficiency Video Coding (HEVC) standard aims to provide a doubling in coding efficiency with respect to the H.264/AVC high profile, delivering the same video quality at half the bit rate. In this paper, complexity-related aspects that were considered in the standardization process are described. Furthermore, profiling of reference software and optimized software gives an indication of where HEVC may be more complex than its predecessors and where it may be simpler. Overall, the complexity of HEVC decoders does not appear to be significantly different from that of H.264/AVC decoders; this makes HEVC decoding in software very practical on current hardware. HEVC encoders are expected to be several times more complex than H.264/AVC encoders and will be a subject of research in years to come. © 1991-2012 IEEE.


Mukherjee S.,DOCOMO Innovations Inc.
IEEE Journal on Selected Areas in Communications | Year: 2012

The Signal to Interference Plus Noise Ratio (SINR) on a wireless link is an important basis for consideration of outage, capacity, and throughput in a cellular network. It is therefore important to understand the SINR distribution within such networks, and in particular heterogeneous cellular networks, since these are expected to dominate future network deployments. Until recently the distribution of SINR in heterogeneous networks was studied almost exclusively via simulation, for selected scenarios representing pre-defined arrangements of users and the elements of the heterogeneous network such as macro-cells, femto-cells, etc. However, the dynamic nature of heterogeneous networks makes it difficult to design a few representative simulation scenarios from which general inferences can be drawn that apply to all deployments. In this paper, we examine the downlink of a heterogeneous cellular network made up of multiple tiers of transmitters (e.g., macro-, micro-, pico-, and femto-cells) and provide a general theoretical analysis of the distribution of the SINR at an arbitrarily-located user. Using physically realistic stochastic models for the locations of the base stations (BSs) in the tiers, we can compute the general SINR distribution in closed form. We illustrate a use of this approach for a three-tier network by calculating the probability of the user being able to camp on a macro-cell or an open-access (OA) femto-cell in the presence of Closed Subscriber Group (CSG) femto-cells. We show that this probability depends only on the relative densities and transmit powers of the macro- and femto-cells, the fraction of femto-cells operating in OA vs. Closed Subscriber Group (CSG) mode, and on the parameters of the wireless channel model. For an operator considering a femto overlay on a macro network, the parameters of the femto deployment can be selected from a set of universal curves. © 2006 IEEE.


Mukherjee S.,DOCOMO Innovations Inc.
IEEE International Conference on Communications | Year: 2012

For an arbitrarily-located user terminal (UE) in a multi-tier heterogeneous cellular wireless network, the joint distribution of the downlink SINR at the UE from the candidate serving base stations (BSs) in the accessible tiers of the network has been derived in closed form for the cases where the candidate serving BS from each accessible tier is chosen as either the one nearest to the UE [1] or the one that is received strongest (equivalently, with maximum SINR) at the UE [2], when the locations of the BSs in the tiers are modeled by independent Poisson Point Processes, and the fading on all links is assumed independent identically distributed (iid) and Rayleigh. The actual serving BS for the UE is chosen as the nearest/strongest/max-SINR candidate serving BS after imposing selection bias across the tiers. The above joint distributions can be used to yield the distribution of the actual SINR at the UE (i.e., when receiving from this serving BS) when no selection bias exists across tiers. However, for the practically important case of selection bias, analytical calculation of the distribution of the actual SINR presents significant challenges. This work derives and summarizes the distribution of actual downlink SINR for all the above criteria for selection of the serving BS accounting for selection bias. We then explore some implications of these results for design and operation of a heterogeneous network. © 2012 IEEE.


Yazici V.,Ozyein University | Kozat U.C.,DOCOMO Innovations Inc. | Sunay M.O.,Ozyein University
IEEE Communications Magazine | Year: 2014

The tremendous growth in wireless Internet use is showing no signs of slowing down. Existing cellular networks are starting to be insufficient in meeting this demand, in part due to their inflexible and expensive equipment as well as complex and non-agile control plane. Software-defined networking is emerging as a natural solution for next generation cellular networks as it enables further network function virtualization opportunities and network programmability. In this article, we advocate an all-SDN network architecture with hierarchical network control capabilities to allow for different grades of performance and complexity in offering core network services and provide service differentiation for 5G systems. As a showcase of this architecture, we introduce a unified approach to mobility, handoff, and routing management and offer connectivity management as a service (CMaaS). CMaaS is offered to application developers and over-the-top service providers to provide a range of options in protecting their flows against subscriber mobility at different price levels. © 2014 IEEE.


Guvenc I.,DOCOMO Innovations Inc.
IEEE Communications Letters | Year: 2011

Range expansion and inter-cell interference coordination (ICIC) can improve the capacity and fairness of heterogeneous network (HetNet) deployments by off-loading macrocell users to low-power nodes. Due to difficulties in analytical treatment, current studies for range expansion and ICIC in HetNets rely mostly on simulations. In this letter, first, off-loading benefits of range expansion in HetNets are captured through cumulative distribution functions (CDFs) of the downlink signal to interference plus noise ratio (SINR) difference between the macrocell and strongest picocell signals. Then, these CDFs are used to investigate the system capacity and fairness as a continuous function of the range expansion bias, and benefits of using ICIC with range expansion are demonstrated through numerical results. © 2006 IEEE.


Mukherjee S.,DOCOMO Innovations Inc.
2011 49th Annual Allerton Conference on Communication, Control, and Computing, Allerton 2011 | Year: 2011

Recent results [1], [2] on the distribution of the downlink SINR in heterogeneous wireless networks assume that the serving base station (BS) for a given user (UE) location is either (a) the BS that is geographically nearest to the UE location [1], or (b) the one that has the highest received power at the UE location [2]. For (a), the distribution of the downlink SINR at an arbitrary UE location can be derived exactly. For (b), the best result for the cumulative distribution function (CDF) of the downlink SINR [2] is exact only for arguments that exceed unity. In this paper, we extend the results in [2] to derive an exact expression for the CDF of the downlink SINR at an arbitrary UE location in a multi-tier heterogeneous network when the serving BS is chosen according to (b). We then explore some interesting implications of the result for coverage probabilities. © 2011 IEEE.


A method and apparatus is disclosed herein for scheduling over ODFM via interference alignment based on multipath intensity profile information. In one embodiment, the method comprises grouping user terminals into groups based on their multipath intensity profiles, where at least one of the groups has two or more terminals; scheduling user terminal groups for MU-MIMO transmission; allocating OFDM resources to the user terminal groups for MIMO transmission; assigning MU-MIMO transmission codes to the user terminal groups; and performing MU-MIMO transmission of the user terminal groups using assigned MU-MIMO transmission codes.

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