Tal Philosof

On the Loss of Single-Letter Characterization: The Dirty Multiple Access Channel

For general memoryless systems, the existing information-theoretic solutions have a ldquosingle-letterrdquo form. This reflects the fact that optimum performance can be approached by a random code (or a random binning scheme), generated using independent and identically distributed copies of some scalar distribution. Is that the form of the solution of any (information-theoretic) problem? In fact, some counter examples are known. The most famous one is the ldquotwo help onerdquo problem: Korner and Marton showed that if we want to decode the modulo-two sum of two correlated binary sources from their independent encodings, then linear coding is better than random coding. In this paper we provide another counter example, the ldquodoubly-dirtyrdquo multiple-access channel (MAC). Like the Korner-Marton problem, this is a multiterminal scenario where side information is distributed among several terminals; each transmitter knows part of the channel interference while the receiver only observes the channel output. We give an explicit solution for the capacity region of the binary doubly-dirty MAC, demonstrate how this region can be approached using a linear coding scheme, and prove that the ldquobest known single-letter regionrdquo is strictly contained in it. We also state a conjecture regarding the capacity loss of single-letter characterization in the Gaussian case.

The rate loss of single letter characterization for the “dirty” multiple access channel

For general memoryless systems, the typical information theoretic solution, when exists, has a ldquosingle-letterrdquo form. This reflects the fact that optimum performance can be approached by a random code (or a random binning scheme), generated using independent and identically distributed copies of some single-letter distribution. Is that the form of the solution of any (information theoretic) problem? In fact, some counter examples are known, perhaps the most famous being the Korner-Marton ldquotwo help onerdquo problem, where the modulo-two sum of two binary sources is to be decoded from their independent encodings. In this paper we provide another counter example, the ldquodoubly-dirtyrdquo multiple access channel (MAC). Like the Korner-Marton problem, this example is associated with a multiterminal scenario where side information is distributed among several terminals; each transmitter knows part of the channel interference but the receiver is not aware of any part of it. We give an explicit solution for the capacity region of a binary version of the doubly-dirty MAC, demonstrate how this capacity region can be approached using a linear coding scheme, and prove that the ldquobest known single-letter regionrdquo is strictly contained in it. We also state a conjecture regarding a similar rate loss of single letter characterization in the Gaussian case.

Signal Processing Challenges in Cellular-Assisted Vehicular Communications: Efforts and developments within 3GPP LTE and beyond

Vehicular communications is an important enabler for enhancing the safety on roads by supporting mutual awareness of vehicles as well as for improving the efficiency of transportation through smart traffic management by intelligent transport systems (ITSs). Governments around the world have set ambitious goals for road fatality reduction in the near future; e.g., the European Union targets a 50% reduction of road fatalities by 2020 as compared to the year 2010. Furthermore, traffic telematic systems aim to minimize the environmental impact of transportation and maximize the utilization of available road infrastructure by adaptive traffic management. To realize these challenging targets, autonomous wireless information exchange among vehicles-vehicle to vehicle (V2V)-and with roadside infrastructure-vehicle to infrastructure (V2I)-are central ingredients. In addition to traffic efficiency and safetyrelated issues, vehicular communications is increasingly recognized as an important revenue driver by car manufacturing companies since it enables wirelessly connected in-vehicle entertainment systems that support on-demand video streaming and online Internet access for passengers. Also, in the future, machine-type communication is expected to play a major role in vehicular environments, with more sensors that monitor the internal state of vehicles and autonomously exchange service and maintenance information with cloud servers of manufacturers. Depending on the considered use-case, distinct quality of service (QoS) requirements come into play [1]: infotainment applications for in-car users require high bandwidth and network capacity, active road safety relies on delay- and outage-critical data transmission, whereas information exchange for road traffic efficiency management typically comes without strict QoS requirements and exhibits graceful degradation of performance with increasing latency.

Leakage-based multicast transmit beamforming

In this paper, we investigate downlink physical layer multicast transmit beamforming in wireless cellular networks, considering interference between multiple independent multicast transmitters (base stations). Transmit beamforming can exploit multiple antennas at the transmitter to direct the multicast signal towards the intended users, while minimizing the interference leakage caused to other users of the network. We propose a multicast beamformer optimization problem that maximizes the achievable multicast transmission rate while restricting the interference leakage caused to other users, by applying a semidefinite relaxation to approximate this NP-hard problem with a convex optimization problem that can be solved efficiently. Furthermore, we consider multiple receive antennas at the users and propose an antenna combiner that maximizes the achievable user rate. Finally, we combine the proposed beamforming and receive antenna combining methods via alternating optimization and evaluate the performance using Monte-Carlo simulations.

Convex-optimization based geometric beamforming for FD-MIMO arrays

With the persistent densification of cellular networks, especially in urban environments, three-dimensional beam-forming utilizing full dimension MIMO antenna arrays becomes of interest for mobile communications, since it allows to mitigate the inter-cell interference and enables high order spatial multiplexing of a multitude of users. In this paper, we consider beamformer optimization with the goal of maximizing the signal power within a certain angular area of interest, while controlling the interference leakage caused outside this area. We relax this non-convex optimization problem to a closely related convex problem and numerically evaluate the performance of this feasible solution.

In-vehicle hybrid electrical architecture

One of the major challenges of todays in-vehicle electrical architecture is the rapidly increasing complexity of interconnecting controllers, sensors, actuators and other on-board electronic components. Traditionally, this task is accomplished using standard wired bus technologies such as Controller Area Network (CAN). In this paper we introduce the concept of hybrid electrical architecture as one that combines wireless and wired communication in a tight fashion. Namely, communication over the wired backbone is aware and in accordance with the communication on the wireless media and vice versa. The reasons for adopting hybrid architecture for intra-vehicle communication are increased robustness, reliability, lower latency, while preserving both wireless and wired resources. In this paper we propose a hybrid communication protocol tailored to a CAN based backbone. Our protocol increases end-to-end hybrid communication links reliability while preserving wired CAN bus load, minimizes latency, and achieves all that with very low complexity.

Wireless controller area network for in-vehicle communication

One of the major challenges of todays in-vehicle electrical architecture is the rapidly increasing complexity of interconnecting controllers, sensors, actuators and other on-board electronic components. Traditionally, this task is accomplished using standard wired bus technologies such as Controller Area Network (CAN). In this paper we introduce the Vehicular wIreless CAN (ViCAN) - a CAN based hybrid communication architecture that is tailored to the in-vehicle environment and that aims at reducing wiring complexity by combining wireless and traditional wired CAN physical layers in a cost effective and seamless fashion. We show by analysis that such a hybrid architecture can potentially support highly reliable end-to-end communication with very low complexity and no/small degradation in data-rate.

The capacity region of the binary dirty MAC

The general two-user memoryless multiple-access channel with partial or full channel state information among the encoders has no single-letter solution which explicitly characterizes its capacity region. In this paper a binary dirty multiple-access channel with interference known at one/both encoders is considered. Specifically, we derive formulas for the capacity region of the doubly-dirty multiple-access channel where each encoder knows one of two independent interference sequences and for the binary dirty multiple-access channel with a single informed user where the interference is known to only one encoder.

Binary dirty MAC with common state information

The general two-user memoryless multiple-access channel, with common channel state information known to the encoders, has no single-letter solution which explicitly characterizes its capacity region. In this paper a binary “dirty” multiple-access channel (MAC) with “common interference”, when the interference sequence is known to both encoders, is considered. We determine its sum-capacity, which equals to the capacity when full-cooperation between transmitters is allowed, contrary to the Gaussian case. We further derive an achievable rate region for this channel, by adopting the “onion-peeling” strategies which achieve the capacity region of the “clean” binary MAC. We show that the gap between the capacity region of the clean MAC and the achievable rate region of dirty MAC stems from the loss of the point-to-point binary dirty channel relative to the corresponding clean channel.

Gaussian Modeling of Spatially Correlated LOS/NLOS Maps for Mobile Communications

The wireless channel behaves markedly different depending on whether a line-of-sight (LOS) between transmitter and receiver exists or not. State-of-the-art wireless channel models for mobile communications, such as, IST WINNER-II and the 3GPP 3D channel model specified in TR 36.873, define distinct macro- and microscopic fading parameters for LOS and non-LOS (NLOS) situations. Whether a user is in LOS/NLOS is randomly determined by a distance-dependent LOS probability; commonly, for each position in the network, the random realization of LOS/NLOS propagation is independently determined from this distance-dependent LOS probability. Since in this case random draws of neighboring positions are statistically independent, base station assignment regions in system-level simulations become highly irregular. To mitigate this problem, we propose an efficient method to generate spatially correlated LOS/NLOS channel maps that follow predefined distance-dependent LOS probability and additionally enable spatial clustering of LOS positions.