Aamir Hasan

The Guard Zone in Wireless Ad hoc Networks

In ad hoc networks, it may be helpful to suppress transmissions by nodes around the desired receiver in order to increase the likelihood of successful communication. This paper introduces the concept of a guard zone, defined as the region around each receiver where interfering transmissions are inhibited. Using stochastic geometry, the guard zone size that maximizes the transmission capacity for spread spectrum ad hoc networks is derived - narrowband transmission (spreading gain of unity) is a special case. A large guard zone naturally decreases the interference, but at the cost of inefficient spatial reuse. The derived results provide insight into the design of contention resolution algorithms by quantifying the optimal tradeoff between interference and spatial reuse in terms of the system parameters. A capacity increase relative to random access (ALOHA) in the range of 2 - 100 fold is demonstrated through an optimal guard zone; the capacity increase depending primarily on the required outage probability, as higher required QoS increasingly rewards scheduling. Compared to the ubiquitous carrier sense multiple access (CSMA) which essentially implements a guard zone around the transmitter rather than the receiver - we observe a capacity increase on the order of 30 - 100%

Clustered CDMA ad hoc networks without closed-loop power control

This paper develops a system design for clustered wireless ad hoc networks, combining CSMA and CDMA to enable spatial reuse and simultaneous transmissions. Typically, CDMA networks require fine-tuned power control, but here that requirement is eliminated through a combination of open loop power control, user ordering, and successive interference cancellation (SIC). A network topology with high network awareness via a broadcast CSMA channel is developed. The resulting system increases network throughput and overcomes existing problems with IEEE 802.11.

The critical radius in CDMA ad hoc networks

In ad hoc networks, it is necessary to suppress transmissions by nodes around the desired receiver in order to achieve successful communication. This minimum separation, the critical radius, has important implications on carrier sensing and other MAC-level protocols. Previously, the critical radius has not been well understood. The critical radius is investigated in CDMA ad hoc networks, with non-spread spectrum ad hoc networks being a special case where the spreading gain is unity. It is shown that the size of this exclusion zone has a large impact on the transmission capacity of ad hoc networks, and an optimal critical radius is found using stochastic geometry.

Pseudo Random Number Based authentication to counter denial of service attacks on 802.11

Wireless networks have gained unmatched popularity as compared to the wired network access technologies due to expedient deployment, flexibility and low cost. However, the security aspects in wireless local area networks (WLANs) is an active research area. The inherent security weaknesses of the wireless medium pose a much more stern threat as compared to the wired networks. Even with the ratification of 802.11i, WLANs based on 802.11 Standard remain vulnerable to denial of service (DoS) attacks due to unprotected and unauthenticated Management and Control Frames. Different types of defense techniques and protocols have been proposed to counter these threats. These either possess certain deficiencies or have implementation complexities. Moreover, no solution encompassing all DoS attacks based on Management and Control Frames has yet been proposed. The paper proposes a robust solution to effectively counter all Management and Control Frame based DoS attacks by using Pseudo Random Number Based authentication. The mechanism involves replacement of Cyclic Redundancy Checksum 32 (CRC32) in the frame check sequence field (FCS) with CRC16 and using the spared 16 bits for authentication. The proposed scheme is easily deployable without any implementation complexities and does not require any additional infrastructure or hardware for its employment.

Improved Nonce Construction Scheme for AES CCMP to Evade Initial Counter Prediction

IEEE 802.11i standard offers arguably uncompromised confidentiality and integrity services by utilizing advance encryption standard in counter with cipher block chaining message authentication code protocol (AES CCMP). However the Nonce construction mechanism employed in the standard is weak, leading to Initial Counter prediction. Resultantly, the effective Key Length used for encryption is reduced from 128 to 85 bits and Time Memory Trade Off (TMTO) attack becomes a possibility. In this paper, an improved Nonce construction scheme is proposed for the AES CCMP to effectively prevent Initial Counter Prediction and the possibility of a subsequent TMTO attack. The proposed technique involves randomization of the Nonce value to make it unpredictable. The devised technique can be easily employed as a software upgrade in the existing 802.11i based Wireless Local Area Network (WLAN) devices, without any requirement of hardware up gradation.

Scheduling Using Near-optimal Guard Zones for CDMA Ad Hoc Networks

Scheduling algorithms in ad hoc networks allow nodes to share the wireless channel so that concurrent transmissions can be decoded successfully. On one hand, the scheduling needs to be efficient to maximize the spatial reuse and minimize retransmissions due to collisions. But on the other hand, due to the very nature of uncentralized wireless networks, the scheduling algorithm needs to be easily implementable in a distributed fashion with little, if any, coordination with other nodes in the network. The goal of this paper is to propose and evaluate a simple scheduling technique based on receiver guard zones. In particular, using stochastic geometry, we show that a near-optimal guard zone can easily be realized in a distributed manner, and that this has about a 2 - 100x increase in capacity as compared to an ALOHA network; the capacity increase depending primarily on the required outage probability E, as lower E tolerances increasingly reward scheduling. By implementing guard zone-based scheduling, we show that the attained performance is about 70 - 80% of a well-known near-optimal (and practically infeasible) centralized scheme.

Cancellation error statistics in a power-controlled CDMA system using successive interference cancellation

Successive interference cancellation (SIC) is a practical scheme for substantially increasing the capacity of CDMA systems. In order to cancel the received signals of interfering users, estimation of their channel parameters is crucial. To the extent that the channel estimates are inaccurate, residual cancellation interference exists, which is the principal capacity limiting issue in SIC systems. The statistics and probability density function of the residual cancellation error are derived in the presence of fast closed-loop power control. This distribution, which depends directly on the statistics of the power controlled channel, can be accurately modelled with an appropriate Gaussian approximation. The presented results can be directly used in the modelling of future CDMA systems with interference cancellation, in both academia and industry.

An intelligent hybrid spread spectrum MAC for interference management in mobile ad hoc networks

This paper presents and analyses a fully distributive intelligent hybrid spread spectrum MAC for ad hoc networks. The IHSS MAC scheme has been developed with the aim to mitigate far field interference with the use of a robust DSSS physical layer, while managing near field interference with the use of intelligent slow frequency hopping. The IHSS design ensures a minimum required SINR threshold at active receivers, under low outage probability constraints. IHSS does not inhibit any nodes in space neither thins them out in time. A lower bound on transmission capacity for the case where the size of the frequency hopping zone is variable is derived in this paper. The mathematical model is validated through simulation. The simulation is based on a hopping sequence selection methodology that randomizes the available hopping sequences within a frequency hopping zone around each active receiver. The implementation utilises the RTS/CTS concept of the CSMA MAC, with a slight modification. The performance criterion used for analysis is transmission capacity normalized by the required bandwidth. It is observed that implementation of a suitable sized frequency hopping zone using the proposed IHSS MAC, shows superior performance over ALOHA and guard zone based MACs.

An intelligent hybrid spread spectrum MAC protocol for increasing the transmission capacity of wireless ad-hoc networks

Unmanaged interference is a threat to reliable communication in ad hoc networks. Currently, ad hoc networks suppress or inhibit interferers around an active receiver to meet a minimum required signal-to-interference-plus-noise ratio threshold. Techniques like carrier sensing, random back-offs, spread spectrum, guard zone based inhibition, etc. are being used for interference management. We have developed a MAC protocol that works on the principle of not inhibiting any interferer near an active receiver. The proposed scheme uses Direct Sequence Spread Spectrum network-wide and intelligent Frequency Hopping within a zone around the receiver. Mathematical model based on stochastic geometry is validated through simulation. It is observed that proposed MAC protocol shows performance improvement over other MACs, in terms of Transmission Capacity normalized by the used spectrum.

Interference cancelation vs. interference suppression in ad hoc networks

DS-CDMA physical layer with MAC scheduling is an effective technique to combat interference in ad hoc networks. The reduction in SINR requirement due to spreading gain provides attractive robustness against aggregate interference of more distant interferers. However, nearby interferers that can still cause an outage are inhibited through MAC scheduling. An alternative to scheduling is to employ interference-aware receivers that exploit information in the interfering signal with the goal of negating its effect on the desired transmission. In this paper interference supersession using MAC scheduling is compared with interference cancelation by employing successive interference cancelation (SIC). The results show that under strict outage constraint, inhibiting nearby nodes through scheduling is a better option than to employ SIC with imperfect channel estimation.