Susumu Yoshida

Impact of Shadowing Correlation on Coverage of Multihop Cellular Systems

The impact of spatial correlation of shadowing on the coverage of TDMA multihop cellular systems is investigated in single-cell environments. The introduction of relaying capabilities to cellular systems may enhance the cell coverage as follows. Changing single-hop transmission to multihop transmission reduces per-hop path loss. In addition, since a multihop route is selected among a number of alternatives, the use of mobile stations suffering from severe shadowing can be decreased. In most studies on multihop cellular systems, shadowing is modeled as a location-independent log-normal random variable; however, adjacent shadowing values are spatially correlated because shadowing is caused by terrain configuration or obstacles between the transmitter and receiver. Thus, coverage enhancement due to relaying capabilities may not be achieved as expected. In this paper, first, according to the commonality between multihop transmission and symbol rate control, a similar methodology to formulate the spectral efficiency (SE) and outage probability of rate-adaptive cellular systems is used to estimate these performances of multihop cellular systems. Second, we investigate the impact of decorrelation distance, whose typical value for the urban environment is 20 meters, on the coverage of multihop cellular systems. By using a model for spatially correlated shadowing, numerical results reveal that when the coverage of single-hop cellular systems is ten times larger than the decorrelation distance, the introduction of relaying capability may enhance the coverage without significant degradation due to the spatial correlation of shadowing.

Tradeoff between area spectral efficiency and end-to-end throughput in rate-adaptive multihop radio networks

We investigate the impact of symbol rate control, modulation level control, and the number of hops on the area spectral efficiency of interference-limited multihop radio networks. By controlling symbol rate and modulation level, data rate can be adapted according to received power. In addition, varying the number of hops can control received power. First., we evaluate the achievable end-to-end throughput of multihop transmission assuming symbol rate and modulation level control. Numerical results reveal that by controlling symbol rate or using multihop transmission, the end-to-end communication range can be extended at the cost of end-to-end throughput, and this may result in lower area spectral efficiency. Next, an expression for the area spectral efficiency of multihop radio networks is derived as a function of the number of hops and the end-to-end throughput. Numerical results also reveal that the resulting area spectral efficiency depends on the specific circumstances, which, however, can be increased only by using multihop transmission.

Capacity Improvement of Multihop Inter-Vehicle Communication Networks by STBC Cooperative Relaying

In this paper, we evaluate the effect of space-time coded cooperative relaying technique in multihop inter-vehicle communication (IVC) networks. The IVC systems have an issue that communication links are often blocked by obstacles such as heavy vehicles. The breakage of a radio link in multihop connections may significantly decrease the system throughput in multihop IVC networks. It is demonstrated through system-level evaluations that the cooperative relaying can offer remarkable capacity enhancement by exploiting multi-route diversity and overcoming accidental link breakage resulting from frequent topological changes.

Distributed Power Control for Wireless Ad Hoc Networks: A Game-Theoretic Approach Based on Best-Response Functions

This paper presents a novel framework for distributed power control for ad-hoc wireless networks. We analyze dynamic adaptive power allocation assuming that transmit power is adjusted with respect to experienced interference based on general best-response functions. For this purpose, we develop a general non-cooperative game-theoretic framework in order to characterize optimal equilibrium states and convergence of distributed power control dynamics to such states. Our work provides a more general insight to game-theoretic power control compared to most of recent works in this field. Moreover, our framework is developed in an abstract way without any technical assumption on particular modulation, coding, QoS measure definition or network architecture. To demonstrate an application of our framework, we show that stable linear best-response power control converges exponentially to a unique Nash equilibrium for any initial condition, which we confirm by numerical simulations.

Game-Theoretic Approach to Capacity and Stability Evaluations of Decentralized Adaptive Route Selections in Wireless Ad Hoc Networks

A game-theoretic analysis is applied to the evaluation of capacity and stability of a wireless ad hoc network in which each source node independently chooses a route to the destination node so as to enhance throughput. First, the throughput of individual multihop transmission with rate adaptation is evaluated. Observations from this evaluation indicate that the optimal number of hops in terms of the achievable end-to-end throughput depends on the received signal-to-noise ratio. Next, the decentralized adaptive route selection problem in which each source node competes for resources over arbitrary topologies is defined as a game. Numerical results reveal that in some cases this game has no Nash equilibria; i.e., each rational source node cannot determine a unique route. The occurrence of such cases depends on both the transmit power and spatial arrangement of the nodes. Then, the obtained network throughput under the equilibrium conditions is compared to the capacity under centralized scheduling. Numerical results reveal that when the transmit power is low, decentralized adaptive route selection may attain throughput near the capacity.

Asynchronous Distributed Power and Rate Control in Ad Hoc Networks with Stochastic Channels

This paper analyzes distributed asynchronous power and rate control for wireless ad hoc networks with stochastic channels. In contrast to conventional cellular systems, all network transmitters are assumed to be independent of any management infrastructure and, importantly, to have the freedom to choose their own arbitrary control rules, using as input only the information on local interference and achieved signal-to-interference and noise ratio (SINR). This approach respects links different local network conditions and preferences on quality of service. With the purpose of finding network-wide acceptable equilibria for such an individually defined power/rate allocation dynamics, the authors discuss an entirely general asynchronous and distributed algorithm, whereby stochastic channels are assumed. Moreover, optimum admission scheme for linear/linearized models is given. Numerical simulations show the efficiency of our approach to allocate comparably higher SINRs in random ad hoc networks with changing topologies and user density.

A Power Adapted MAC (PAMAC) Scheme for Energy Saving in Wireless Ad Hoc Networks

Nowadays, numerous Medium Access Control (MAC) contention protocols for ad hoc networks typically use a fixed transmit power level without using any transmit power control. In this paper, we present an enhancement scheme, called Power Adapted Medium Access Control (PAMAC) scheme for achieving energy conservation, which allows a node to vary its own transmit power on a packet basis. The primary objective of this scheme is to use suitable transmit power level for Clear-To-Send (CTS), DATA, and Acknowledgement (ACK) that still allows to achieve a correct reception of a packet despite intervening path loss, noise and interference. The evaluation of the throughput efficiency per node, energy consumption per node and energy per successfully transmitted bit is performed by a computer simulation. Simulation results indicate that the proposed PAMAC scheme can achieve a high reduction of the energy consumption and energy per successfully transmitted bit and also an improvement in the throughput efficiency per node compared to the conventional Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol.

Throughput Evaluation of CSMA Assuming Carrier Detection Probability in Wireless Ad Hoc Networks

In performance evaluations of wireless ad hoc networks, both successful data reception and carrier detection probabilities are assumed to be 1 within a circle with certain radius and 0 outside the circle. In real systems, however, those probabilities are gradually changes from 1 to 0 as the distance between a terminal and access point increases. In this paper, we evaluate the throughput of wireless ad hoc networks assuming successful data reception and carrier detection probabilities to be continuous functions of the distance between a terminal and access point. We also show that new additional factors which decrease the throughput will appear under these assumptions, compared to the case where only probabilities 1 and 0 are assumed

DS-CDMA Non-linear Interference Canceller with Multiple-Beam Reception

In this paper, a multistage parallel interference canceller (MPIC) with multiple-beam reception for a DS-CDMA system is proposed to suppress multiple access interference (MAI) effectively. Its aim is to reduce the computational complexity of the conventional MPIC cascaded with an adaptive array antenna. It employs multiple fixed beams based on phased array and selects suitable beams to demodulate the transmitted signal of each user. Then it suppresses residual interference signals by the MPIC cascaded with multiple-beam receiver. Its bit error rate (BER) performance is evaluated by computer simulations assuming an uplink single-chip-rate multiple-spreading-factor DS-CDMA system over both exponentially decaying 5-path and equal average power 2-path Rayleigh distributed channels. When there are 16 users in an 120°-sectored single cell, the proposed receiver with 6-element array antenna and 2-stage MPIC shows better or comparable BER performance compared with that of the conventional receiver. Moreover, the proposed receiver with 8 beams can reduce the number of complex multiplications to about 40% of that of the complexity-reduced conventional receiver over 5-path channels.

Complexity of Differential Attacks on SHA-0 with Various Message Schedules

The security of SHA-0 with various message schedules is discussed in this letter. SHA-0 employs a primitive polynomial of degree 16 over GF(2) in its message schedule. For each primitive polynomial, a SHA-0 variant can be constructed. The collision resistance and the near-collision resistance of SHA-0 variants to the Chabaud-Joux attack are evaluated. Moreover, the near-collision resistance of a variant to the Biham-Chen attack is evaluated. It is shown that the selection of primitive polynomials highly affects the resistance. However, it is concluded that these SHA-0 variants are not appropriate for making SHA-0 secure.