Device-to-device (D2D) communication enables simultaneous data transmissions by cellular users (CU) and D2D user pairs, but at the expense of additional interference between them. The literature on resource allocation in D2D systems often assumes that the base station (BS) has complete channel state information (CSI) about all the links between all the users in a cell. However, acquiring the CSI of cross links between the CUs and the D2D receivers is a critical bottleneck because the number of cross links is the product of the number of CUs and D2D pairs. We study a novel partial CSI model in which the overhead of feeding back the CSI of the cross links is much lower. For a cell with one D2D pair and multiple CUs, we propose a novel throughput-optimal joint mode selection, user scheduling, and rate adaptation policy that exploits information about the statistics of the cross links and incorporates inter-cell interference. We derive closed-form expressions for the feedback-conditioned goodput for the underlay mode, which drives this optimal policy. We also present extensions that incorporate user fairness, quantized CSI, and multiple D2D pairs and multiple subchannels.

Cloud Radio Access Networks (Cloud RAN) is an emerging architectural paradigm that attempts to exploit operational efficiencies through centralization of baseband functions, pooling efficiencies for RAN baseband processing, and air-interface performance gains by fast-time-scale multi-cell coordination. In this paper, we aim to derive insights underlying key technologies and tradeoffs that drive Cloud RAN. We highlight principles that explain the impact of Cloud RAN architectures and functional splits on multi-cell coordination gains. We show that the ability to extract pooling gains can differ significantly between RAN functions, depending on whether they are per-user or per-cell functions. We propose a method for elastic scaling of RAN functions that takes into account both real-time and non-real-time needs. We conclude with a proposal for a methodology for operators to evaluate the overall tradeoffs they will face in deciding whether to adopt Cloud RAN.

Centralized RAN and baseband pooling architectures are increasingly being seen as the way cellular technologies will be deployed in the future due to a number of advantages they provide to the operator. However, LTE and LTE-Advanced were designed with a distributed architecture in mind. Therefore, there are certain inherent technical issues when the link connecting the baseband and RRU (Remote Radio Unit) has a non-negligible latency. In this paper, we study the impact of this non-ideal fronthaul on the downlink and uplink performance of LTE-Advanced networks. We show that, while the impact on the peak throughput is quite severe, it is more modest when we have a larger number of active UEs. The degradation in performance is larger in the uplink than in the downlink due to the synchronous HARQ mechanism used in LTE uplink. In the downlink, we compare the performance of centralized RAN with that of distributed RAN in case of non-ideal fronthaul (FH) and backhaul (BH), respectively, and determine whether it is better to incur backhaul or fronthaul latency from a performance perspective. In the uplink, we analyze the loss in performance due to fronthaul latency. We have developed a novel look-ahead scheduling scheme that results in significant improvement in performance, especially when the number of active UEs is small.

In this paper, we study the performance of Dynamic Point Selection (DPS) Cooperative Multipoint (CoMP) techniques in heterogeneous networks (Hetnets). We show that to obtain good performance with spectral efficiency (SE) based DPS, the transmission point (TP) switching metric should be based on weighted average of the Almost Blank Subframes (ABS) and non-ABS SE values, where the weights are the fraction of ABS and non-ABS subframes. We compare the performance of SE-based DPS with intra-site CoMP cluster and with "liquid cooperation cluster." Our results show that the system performance in the two cases are similar when the Cell Range Extension (CRE) is set to the optimal value. We also enhance the load-based DPS scheme, the preferred scheme with finite buffer traffic, and show that the TP switching metric requires a proper interference mitigation factor to discourage switches that make a large compromise in SE. This is especially important in Hetnets where there is usually a large load difference between the macro and small cells. Finally, we show how the DPS scheme can work in the presence of non-ideal backhaul between cooperating neighboring cells and quantify the performance gains obtained in such scenarios.

In this paper, we propose solutions to the transmission mode selection and resource allocation problems for D2D unicast communications. We first formulate a joint mode selection and resource allocation problem, which we further decompose into separate mode selection and resource allocation problems, which are run at different timescales. Our mode selection algorithm determines the optimal transmission mode for all D2D transmitter-receiver user equipment (UE) pairs. Our proposed algorithm involves computing a switching metric that takes into account the spectral efficiency in the two modes, possibility of resource reuse, and the traffic load in the two modes. We allow a D2D UE pair to switch the transmission mode only if it improves the overall system performance. Our resource allocation scheme schedules the cellular and D2D users based on the selected transmission mode such that it maximizes the sum proportional fairness (PF) metric across all users. Because an exhaustive search over all UEs will be prohibitive when there are a large number of active UEs, we develop a greedy heuristic that incrementally adds UEs such that co-scheduling of every additional UE results in improvement of the sum PF metric. Our numerical results show that we are able to achieve near- optimal solution with our proposed greedy heuristic. We also show numerical results that quantify the gains from our mode selection and resource allocation algorithms for different scenarios.

In this paper, we focus on the design of an optimal power allocation algorithm and an efficient user selection scheme for the downlink of a non-orthogonal multiple access (NOMA) system with power-domain user multiplexing at the transmitter side and successive interference cancellation (SIC) on the receiver side. We have derived an optimal power allocation solution in closed-form with the objective of maximizing the weighted sum rate subject to a maximum power constraint. The optimization problem is solved using the Karush-Kuhn-Tucker (KKT) conditions. A greedy user selection algorithm is used to schedule the best set of multiplexed users satisfying the criterion of maximizing sum proportional fairness (PF) metric. Systemlevel simulation results are provided to assess and compare the performance gains of the proposed algorithm in terms of important key system performance indicators.

Multi-Cell coordination or Coordinated Multi-Point (CoMP) schemes are seen as the next evolution of LTE-Advanced to yield a further improvement in overall system performance. In this paper, we study in detail the performance of one such CoMP scheme, namely Multi-cell coordinated scheduling (MuCCS). In our previous work, we have shown that the performance of MuCCS using a distributed "liquid cooperation cluster," where each cell coordinates with its nearest neighbors, out-performs a centralized scheme where the coordination is done over a small set of non-overlapping cells (either three cell or nine cells). In this work, we answer the question of whether there are any additional gains to be obtained by a centralized implementation, if the cooperation cluster is allowed to be arbitrarily large. We also quantify the additional performance gain obtained from allowing intermediate transmit power values besides just the binary on/off power values. Finally, we evaluate the performance of MuCCS when the eNodeBs are connected by a non-ideal backhaul, and devise a distributed MuCCS scheme that retains its advantage over a centralized coordination scheme even in the presence of a non-ideal backhaul. Overall, our results validate that additional high-complexity extensions of coordinated scheduling does not yield large gains over the distributed MuCCS scheme with "liquid cooperation clusters" and on/off power values. In this paper, we use the geometric mean of UE throughputs as the performance metric, and we motivate the rationale behind use of this as a single metric for performance comparison.

DPS is a Cooperative Multi-Point (CoMP) transmission scheme, where the serving Transmission Point (TP) can be switched on a fast time-scale. In this paper, we refine and improve our previously proposed greedy Instantaneous Load based DPS scheme (IL-DPS) to work well under realistic bursty traffic conditions and load imbalanced scenarios. We introduce three additional, simple, multiplicative correction factors: 1) A geometric-mean correction factor to identify a UE switch that maximizes overall system utility as opposed to a single-UE's throughput. 2) A multi-user diversity correction factor that accounts for the multi-user diversity gains when determining the throughput improvement of the candidate UE to be switched. 3) An interference correction factor that accounts for the net change in the inter-cell interference on the other cell UEs' throughputs due to the candidate UE's TP being switched. The main advantage of these simple proposed schemes is that they provide improved system performance gains without requiring any additional scheduling information exchange.

Managing inter-cell interference is one of the main challenges in current and next generation wireless systems that aggressively reuse the frequency. Cooperation between interfering cells has been sought to mitigate interference. In this paper, we address the problem of jointly optimizing the transmit powers, user scheduling, and user association in a cellular network to maximize the weighted sum rate (WSR). To this end, we develop a distributed interference penalty algorithm in which the cells update their transmit powers and user schedule to maximize its utility minus an interference cost. The proposed algorithm involves only limited exchange of information via backhaul and has convergence guarantees. Furthermore, we propose a sub-optimal algorithm with lower computational and backhaul overhead. In it, the users are first associated to the base stations (BSs) based on their signal-to-interference-plus-noise-ratios (SINRs). It is then followed by joint optimization of BS transmit powers and user scheduling, for which we develop an interference penalty algorithm as well. We show that the proposed algorithms outperform the computationally complex weighted minimum mean squared error (WMMSE) algorithm.

We compare the algorithms for centralized and decentralized multi-cell joint optimization of coordinated scheduling with muting. For conventional non-overlapping clusters of cooperating cells, we show that for the same latency of communication, an appropriate decentralized algorithm can equivalently achieve the same jointly optimal solution achieved by a centralized approach. In this decentralized algorithm, cells do not exchange CSIs of all users with other cells, but instead exchange per-cell benefit metrics. In non-overlapping clusters, border cells in a cluster do not coordinate with strong interferers outside the cluster. We introduce a new cluster design called Liquid Cluster which allows a cell to flexibly cooperate with all its relevant interferers. The decentralized algorithm easily extends to Liquid Clusters, performing better than conventional non-overlapping 9-cell clusters.

Heterogeneous networks (Hetnets) introduce an imbalance between downlink and uplink because the downlink transmit powers of the macro and pico eNodeBs can differ by as much as 16 dB. However, the user equipment (UE)'s maximum uplink transmit power is the same regardless of whether the UE is connected to the macro or the pico eNodeB. The uplink throughput performance depends both on the SINR that the UEs see and the number of PRBs that each UE can get allocated. Due to the difference in the transmit powers of the eNodeBs, the number of UEs that attach to the pico eNodeB is typically smaller than the number that attach to the macro eNodeB. In this work, we consider these impacts and optimize the power control settings of the macro and pico UEs so that the cell-edge and average UE throughput are improved. We study the performance under two different power control schemes: Fractional Power Control (FPC) and Interference Penalty Algorithm (IPA). Using simulations, we study the impact of various parameter settings on the performance of IPA. Based on our studies, we can conclude that the macro UE's transmit power needs to be larger than that of the pico UE to overcome the larger number of UEs attaching to the macro eNodeB. We also study the impact of imperfect knowledge of neighbor cell path losses on the uplink performance of IPA.

Dynamic Point selection (DPS) is a key Downlink (DL) Coordinated Multipoint (CoMP) technique that switches the serving data Transmission Point (TP) of a User Equipment (UE) dynamically among the UE's cooperating set of TPs without requiring a cell handover. It provides performance improvement due to TP selection-diversity gains and dynamic UE load balancing benefit. In this paper, we propose two simple DPS schemes that take into account the UE's current channel conditions and the cell loading conditions to make the UE's TP switching decisions. We show that these schemes improve the system performance under different practical and realistic settings, such as, cell handover margin, TP switching periods, bursty traffic conditions, and cooperation cell cluster sizes.

Multi-User MIMO (MU-MIMO) is a useful technique to enhance uplink capacity in LTE because the base stations employ multiple receive antennas whereas the user equipments transmit with only a single antenna. In this paper we investigate several UE pairing schemes that maximize the total system utility while adhering to the LTE Release-8 constraint of allocating identical contiguous resource blocks (RBs) to the paired UEs. We design pairing decisions using either the wideband or sub- band channel cross-correlation values of the UEs. These mechanisms offer varying trade-offs between performance and complexity.We show that with appropriate modifications, these schemes work well even for finite queue traffic.

In this paper, we develop a novel inter-cell interference co-ordination scheme that takes into account the interference cost on neighboring cells. We formulate a multi-cell utility maximization problem and subsequently decouple it into single-cell optimization problems by including an interference penalty. By solving this decoupled problem, we derive policies for user selection, resource allocation, and power-control. Since the coupling between cells is indirectly taken into account by means of the user's channel gain to the neighboring cells, our simulation results show that this distributed solution has no degradation in performance while little or no inter-cell co-ordination is required. We present simulation results that show that the Interference Penalty Algorithm (IPA) provides significant improvement in sector and cell-edge throughputs.

Femtocells are low-cost, miniature base-stations intended to improve indoor coverage in 3G networks and beyond. One of the main issues in adopting femtocells en masse is the surge in interference to the mobile users served by the macrocell arising from unplanned networks and private access. Therefore, distributed power control mechanisms for femtocells are essential to shield the existing users of the macrocell as well as to enable scalable femtocell deployments. This paper studies several such power control schemes that strike an effective balance between the throughput of the femto users and the degradation in macrocell performance.

We study several channel quality reporting strategies for OFDMA systems that maximize the sum-rate while constrained by feedback and signaling overhead. These reporting schemes are based on the grouping idea, which essentially limits the number of users accessing a sub-channel or physical resource block. This in turn reduces the overhead needed to convey channel quality indication (CQI). On the other hand, grouping user and/or sub-channels reduces the multi-user diversity. We investigate this trade-off for various grouping schemes. Numerical experiments show that even for reasonably high overheads, the grouping schemes achieve a substantial gain in sum-rate as compared to reporting on all sub-channels. Reporting fine- granular channel quality information on the user's best subchannels, is found to be the optimal strategy.

In this paper, we consider the single frequency network (SFN) operation of broadcast/multicast services, and develop resource allocation schemes to minimize the amount of resource over-provisioning. Over-provisioning is required when SFN areas are overlapping because all the cells in the SFN area should use the same resource to transmit a given MBMS service. We first develop an optimal resource allocation scheme that minimizes the over-provisioning. Due to the computational complexity of this scheme, we develop sub-optimal schemes whose complexity is quadratic in the number of services. In addition, we develop an incremental resource allocation scheme for newly arriving services that does not alter the resources allocated to existing services. We present numerical results showing the benefits of our proposed schemes. Motivated by these numerical results, we design a hierarchically overlapping SFN areas scheme that strikes the right balance between complexity and resource over-provisioning.

Scheduling of uplink data by the network typically requires that the buffer status information from each User Equipment (UE) is available at the network to ensure that the air interface resources are not wasted. The presence of delay constrained flows or radio bearers at the UE may impose a constraint that the delay information be available at the network in addition to the size of buffer data. In this paper, we discuss the different uplink buffer status reporting schemes considered in 3GPP Long Term Evolution (LTE) and the current agreement. We propose modified limited feedback buffer status reporting schemes for delay-constrained radio bearers. Using simulation results, we illustrate that delay information at the network is important for scheduling real-time flows without dropping packets.

Orthogonal Frequency Division Multiple Access (OFDMA) is the preferred multiple access scheme for future broadband cellular systems as it provides high spectral efficiency. The choice of OFDMA scheduler and the amount of channel quality feedback from users together determine the achievable throughput and fairness. Channel quality reporting by all active users across the entire carrier bandwidth may result in significant uplink overhead. This paper discusses the performance of two possible OFDMA schedulers based on the class and channel condition weighted proportionally fair scheduler. The impact of different scheduler parameters are investigated. The paper then analyzes the performance of the top-Mscheme, which is a candidate channel quality overhead reduction proposal being discussed in 3GPP Long Term Evolution. It is shown that the top-Mscheme, with significantly lower uplink overhead, provides similar performance as the case where the network has complete channel quality information.

One of the main encoding techniques employed in robust header compression (ROHC) is the window-based least significant bits (W-LSB) encoding technique. In this paper, we use a combination of analytical, simulation, and numerical techniques to study the impact of different channel impairments and configurable parameters on the performance of W-LSB. Based on our investigation, we recommend suitable values for the configurable parameters to achieve enhanced performance. We show that for smaller packet loss rates, smaller window widths are optimal, and larger packet loss rates need a larger window width. In addition, we show that, for links that cause packet reordering, some changes in the default ROHC parameters can provide much better performance.

As 3G wireless systems evolve towards 4G, various wireless network technology organizations looking at network architectures for 4G are considering a redesign of the network away from the traditional centralized, hierarchical design towards a more distributed operation of network functions. In this paper, we present some mechanisms for distributed operation of paging in 4G systems. Our focus is on highlighting the tradeoffs that drive the design choice of whether to centralize or distribute the paging operation. One such tradeoff arises from considerations of service reliability and availability. We present a distributed redundancy scheme for improving the reliability and availability for distributed paging. We analyze the fraction of time that mobiles remain reachable through paging (i.e. the paging service availability) in idle mode in the presence of base station and backhaul failures.

With the proliferation of air interface technologies, there are increasingly multi-mode user equipment devices (UEs) that can operate in more than one technology, although not at the same time. To save battery life, the UEs will camp in only one technology at any point in time even when there is coverage of multiple technologies. One key requirement is to limit the uplink signaling when UEs capable of operating in multiple radio access technologies perform inter-technology handovers in idle mode. In this paper, we propose a scheme that reduces this signaling by sending an inter-technology update to the network only when the UE loses coverage of the previously used technology. We also analyze the performance benefits of this scheme with that of the approach of camping on the preferred technology whenever coverage of that technology is available.

In this paper, we investigate the distance-based location update approach of CDMA 1X EV-DO's route update protocol. We develop an analytical model to compare its performance with that of the static location update approach of earlier cellular technologies. We show that, for the same paging delay and cost, the distance-based approach reduces the overhead traffic due to location update messages. In addition, this reduced loading is equally shared among all cells in the distance-based approach, while it is confined to the border cells in the static approach. We also introduce the non-uniform neighbor choice approach to model mobility more accurately and study its impact on the performance of the two algorithms. Using simulations we study a different mobility model, and the simulation results confirm the benefits of the distance-based approach of 1X EV-DO

We study an approach for sharing channels to improve network utilization in packet-switched cellular networks. This scheme exploits unused resources in neighboring cells without the need for global coordination. We formulate a minimax approach to optimizing the allocation of channels in this sharing scheme. We develop a distributed algorithm to achieve this objective and study its convergence. We illustrate, via simulation results, that the distributed channel sharing scheme performs better than the fixed channel scheme over a wide variety of traffic conditions.

The problem of handoffs in cellular networks is compounded in a LEO (low Earth orbit) satellite-based cellular network due to the relative motion of the satellites themselves with respect to a stationary observer on Earth. Typically, the velocity of motion of mobile telephones can be ignored when compared to the very high velocity of the footprints of satellites. We exploit this property of the LEO satellite systems and propose a handoff scheme that results in a significant decrease in handoff dropping. For the same handoff dropping probability, our scheme has a significantly lower new call blocking probability than the conventional reservation scheme. We present an analytical approximation that is in very good accord with simulation results.