Kemal Karakayali

Optimum Zero-forcing Beamforming with Per-antenna Power Constraints

We investigate optimum zero-forcing beamforming in multiple antenna broadcast channels with per-antenna power constraints. We show that standard zero-forcing techniques, such as the Moore-Penrose pseudo-inverse, considered mainly in the context of sum-power constrained systems are suboptimal when there are per-antenna power constraints. We formulate convex optimization problems to find the optimum zero-forcing beamforming vectors. Our results indicate that optimizing the antenna outputs based on the per-antenna constraints may improve the rate considerably when the number of transmit antennas is larger the number of receive antennas. Having more transmit antennas gives rise to additional signal space dimensions that may be exploited effectively to reduce transmit power at particular antennas with limited power budget.

Cross-Layer Optimization for OFDMA-Based Wireless Mesh Backhaul Networks

In this paper, we propose a cross layer optimization framework for multi-hop routing and resource allocation design in an orthogonal frequency division multiple access (OFDMA) based wireless mesh network. The network under consideration is assumed to consist of fixed mesh routers (or base station routers) inter-connected using OFDMA wireless links with some of the mesh routers functioning as gateways to a wired network. The objective of our cross-layer formulation is to allow joint determination of power control, frequency-selective OFDMA scheduling and multi-hop routing in order to maximize the minimum throughput that can be supported to all mesh routers. Results of our investigations under typical cellular deployment, propagation and channel model assumptions show that this approach achieves significant mesh throughput improvements primarily due to the following: (a) frequency selective scheduling with OFDMA which provides improved tone diversity thus allowing more efficient bandwidth utilization relative to single carrier methods; and (b) multi-hop routing which provides improved path diversity relative to single hop transmissions.

Capacity Benefits of Relays with In-Band Backhauling in Cellular Networks

In this paper, the feasibility and capacity benefits of deploying low-cost relay nodes with in-band wireless backhaul capability in cellular networks are studied. Our proposed scheme is similar to a cell-splitting architecture where the base station density is increased, thus reducing the effective cell size. Cell-splitting in an interference-limited deployment provides capacity gains that are proportional to the number of additional base stations deployed, but at significant cost driven primarily by wired backhaul requirements. The use of in-band wireless backhaul may significantly reduce the cost, but sharing bandwidth with the backhaul results in reduced access rates relative to that of cell-splitting with wired backhaul. Further cost savings may be achieved with the use of low power, low complexity relays in lieu of the additional base stations. In this paper, we show that a cellular architecture with base stations supplemented by simple decode-and-forward relays can provide significant capacity improvements despite the overhead associated with in-band backhaul.

Virtual soft handoff enabled dominant interference cancellation for enhanced uplink performance in heterogeneous cellular networks

Heterogeneous cellular networks comprising overlays of macro-cells and small metro-cells are susceptible to excessive interference on the uplink because of the asymmetry between the uplink and downlink signal strengths. While soft handoffs can be used to mitigate this problem, existing methods applied in homogeneous macro-cellular deployments rely on more symmetric coverage in both directions and are not as beneficial in asymmetric coverage scenarios. In this paper, we propose a novel cooperative uplink reception scheme called “Virtual Soft Handoff (VSHO)” that enables cells suffering excessive interference to cancel out-of-cell interference or decode out-of-cell users in close proximity by canceling in-cell interference, leading to significant improvement in spectral efficiency and cell-edge throughput. As an important side benefit, virtual soft handoffs also enable users served by macro-cells to benefit from selection diversity, leading to further improvement in system performance.

An Analysis of Uplink Base Station Cooperation with Practical Constraints

The demand for high-rate wireless services grows unabated as new mobile devices and data-hungry applications continue to emerge. However, despite many advanced physical and link layer techniques that are employed in todays cellular networks, a large number of mobile users, especially at the cell edges, receive inadequate data rates due to high interference and path loss. Several techniques that exploit inter-base station cooperation have been proposed to address the inter-cell interference problem. While the initial studies show promising gains in idealized environments, it is important to analyze these techniques in a more realistic setting. This study provides detailed analyses of the benefits provided by uplink base station cooperation with several practical constraints and issues that would be encountered in a realistic cellular network deployment. Our results indicate that the cooperative signal processing techniques are robust to channel estimation errors given the pilot structure available in contemporary wireless systems. Furthermore, uplink cooperation can substantially improve the interference management capability of the cellular network, providing significant rate improvement over current non-cooperative networks.

Is relayed collaborative communication worth it

We consider a collaborative wireless distributed network, consisting of one source transmitting information to a remote destination in the presence of N relay nodes. We assume that all nodes have similar capabilities, are randomly distributed geographically, immersed in the clutter and close to the ground, each employing a single antenna. Assuming a Decode-and-Forward technique, we propose various collaborative approaches for signaling. Using well-known channel coding techniques, the destination is not able to start decoding its intended message until all nodes have decoded the entire message. Using more sophisticated coding techniques, the destination can start decoding the transmitted message at the start of the communication by the source. Various ways of organizing the relay collaboration are: Generalized Beamforming; Generalized Alamouti; and Simultaneous Casting. We then evaluate the performance of these collaborative approaches by simulations, employing an appropriate fading/path loss channel model. Obtained results will demonstrate the superiority of the sophisticated coding technique used and provide insights on when collaboration in networked multiple antennae systems is useful. Specifically, a gain of the order of 9 dB in the required transmit power is achieved assuming Simultaneous Casting and 0 dB transmit power per node, compared to the case when these sophisticated codes are not used. Finally, we also observe a gain in the system 10th percentile outage rate on the order of 1.4 bit/s/Hz at a 0 dB transmit power and with Generalized Beamforming, compared to the case where relays are not used.

Network-centric cooperation schemes for uplink interference management in cellular networks

The huge growth in demand for wireless data, combined with shortages of spectrum, has led to an urgent need for spectral efficiency and cell-edge performance improvements in cellular networks. Macrocells are shrinking in size, and heterogeneous networks are being deployed where small cells and macrocells now share the same spectrum. With these trends in cellular network evolution, out-of-sector interference is becoming a major impediment. Impairments due to interference are especially severe on the uplink, where near-far effects are more likely to be experienced. This paper provides a holistic view of the network-centric cooperation schemes that have emerged as strong candidates for uplink interference management in evolving cellular networks (e.g., networks based on 3GPP Long Term Evolution based air-interface technologies and beyond). In particular, we introduce three novel approaches: network multiple input multiple output (MIMO), which carries out joint multi-antenna signal processing across sectors; network interference cancellation engine (NICE) which opportunistically cancels dominant interferers that have already been decoded at neighboring sectors and decreases backhaul overhead by one to two orders of magnitude; and a hybrid approach which combines the strengths of both network MIMO and NICE in an attempt to achieve further spectral efficiency without incurring huge backhaul overhead. Several considerations of both theoretical and practical significance (overhead, latency) related to these approaches are discussed, and simpler variants that may apply in the context of heterogeneous networks are also considered. Quantitative investigations of these network-centric cooperation schemes in realistic operating environments show that substantial improvement may be achieved in average and cell-edge spectral efficiency.

A multi-cell space-time-coded transmission scheme for single frequency networks

Single frequency networks (SFN) in combination with a multi-carrier transmission scheme such as OFDM are often proposed as a means to provide continuous coverage of broadcast applications over a large area. While this combination does succeed in providing continuous coverage, it has certain limitations. Specifically, at points where signals from multiple base-stations are received, the overall signal-to-noise ratio can be poor because of relative phase differences. In this paper, we show how, through the use of orthogonal space-time codes, this limitation of SFNs can be overcome to improve their overall performance. This scheme could be viewed as an ldquoopen-looprdquo implementation of multi-cell cooperative transmission.

Uplink performance enhancement in cellular networks via a Generalized Network Interference Cancellation scheme

It is widely believed that cooperation among base stations can be employed to enhance the interference mitigation capability of base station receivers in cellular networks. Among the base station cooperation schemes that have been proposed to address the uplink interference problem, two promising ones are 1) Joint Multi-Cell MMSE Processing also referred to as Network MIMO; and 2) Multi-Cell Successive Interference Cancellation (MC-SIC). While both of these methods can substantially improve the performance of cellular networks, they have complementary strengths and limitations when implemented in a practical setting. In this paper, we propose a hybrid scheme called Generalized Network Interference Cancellation Engine, which combines the strengths of these two approaches while working within the constraints imposed by practical implementations. This view is borne out by the performance results presented in this paper.

Delay-Tolerant Autonomous Transmissions for Short Packet Communications

The cellular network evolution has generally focused on improving system and edge user throughput while reducing latency. Recently, a new class of service involving the communication of short packets has emerged wherein scale, coverage, reliability, and energy efficiency has greater importance than the rate or latency at which the information is delivered. This delay tolerant class of communication may include keep alive application messages from our smart phones or messages for the Internet of Things (IoT) and machine type communications (MTC). In this paper, we propose autonomous transmissions paired with interference cancellation to improve coverage, reliability, scale, and energy/spectral efficiency for delay tolerant short packet communications. Simulation results demonstrate that up to 2x improvement in spectral efficiency can be achieved.