Mariam Kaynia

Improving the Performance of Wireless Ad Hoc Networks Through MAC Layer Design

In this paper, the performance of the ALOHA and CSMA MAC protocols are analyzed in spatially distributed wireless networks. The main system objective is correct reception of packets, and thus the analysis is performed in terms of outage probability. In our network model, packets belonging to specific transmitters arrive randomly in space and time according to a 3-D Poisson point process, and are then transmitted to their intended destinations using a fully-distributed MAC protocol. A packet transmission is considered successful if the received SINR is above a predefined threshold for the duration of the packet. Accurate bounds on the outage probabilities are derived as a function of the transmitter density, the number of backoffs and retransmissions, and in the case of CSMA, also the sensing threshold. The analytical expressions are validated with simulation results. For continuous-time transmissions, CSMA with receiver sensing (which involves adding a feedback channel to the conventional CSMA protocol) is shown to yield the best performance. Moreover, the sensing threshold of CSMA is optimized. It is shown that introducing sensing for lower densities (i.e., in sparse networks) is not beneficial, while for higher densities (i.e., in dense networks), using an optimized sensing threshold provides significant gain.

Performance of ALOHA and CSMA in Spatially Distributed Wireless Networks

In this paper the performance of unslotted ALOHA and CSMA are analyzed in spatially distributed wireless networks. Users/packets arrive randomly in space and time according to a Poisson process, and are thereby transmitted to their intended destinations using a fully-distributed MAC protocol (either ALOHA or CSMA). An SINR-based model is considered, and a packet transmission is successful if the received SINR is above a threshold value for the duration of the packet. Accurate bounds to the probability of outage, which is a function of the density of transmissions, are developed for both MAC protocols. These bounds are used to evaluate the performances of ALOHA and CSMA, and to gain insight into the design of general MAC protocols for ad hoc networks. Moreover, CSMA with receiver- sensing is proposed to improve the performance of CSMA.

Optimal Transmission Capacity of Ad Hoc Networks with Packet Retransmissions

In this paper we investigate the transmission capacity of wireless networks when packet retransmissions are allowed. We consider networks modeled as a homogeneous Poisson point process operating under different medium access control schemes, namely unslotted and slotted ALOHA, and CSMA with carrier sensing at the transmitter and with carrier sensing at the receiver. For these scenarios, we derive analytical expressions to compute the maximum number of retransmissions attempts that leads to the optimal transmission capacity. Numerical results based on our formulation show that CSMA with carrier sensing at the receiver (asynchronous transmissions) reaches the highest maximum transmission capacity when traffic intensity is low, while slotted ALOHA (synchronous transmissions) is the best choice when traffic intensity is high.

Distributed power control and beamforming on MIMO interference channels

This paper considers the so-called Multiple-Input-Multiple-Output interference channel (MIMO-IC). We address the design of precoding (i.e. beamforming) vectors and power control at each data stream with the aim of striking a compromise between beamforming gain at the intended receiver (Egoism) and the mitigation of interference created towards other receivers (Altruism). Combining egoistic and altruistic beamforming has been shown previously to be instrumental to optimizing the rates in a Multiple-Input-Single-Output (MISO) interference channel [1], [2] and MIMO-IC [3], [4]. Here we extend these concepts to multi-stream scenarios and further improve the rate performance by allowing power control which is not addressed in previous interference alignment related works. The key idea behind power control in interference coordination schemes is that it can help restore feasibility conditions in the high SNR regime, thus avoiding a saturation of the sum rate. Our analysis and simulations attest improvement in terms of complexity and performance.

Impact of fading on the performance of ALOHA and CSMA

This paper considers the performance of the ALOHA and CSMA MAC protocols in wireless ad hoc networks in the presence of fading. Increasing the rate of successful reception of packets is our objective, and thus, outage probability is used as the performance evaluation metric. In our network model, packets belonging to specific transmitters arrive randomly in space and time according to a 3-D Poisson point process, and are then transmitted to their intended destinations using a fully-distributed MAC protocol. A packet transmission is considered successful if the received SINR is above a predefined threshold for the duration of the packet. Approximate expressions are derived for the outage probability of ALOHA and the different flavors of CSMA, namely CSMA with transmitter sensing, receiver sensing, and joint transmitter-receiver sensing. The introduction of fading adds to the hidden and exposed node problems of CSMA, resulting in an up to 75% increase in the outage probability. Interestingly, however, the relative difference between the protocols remains unchanged.

Comparative performance evaluation of MAC protocols in ad hoc networks with bandwidth partitioning

This paper considers the performance of the MAC protocols ALOHA and CSMA in wireless ad hoc networks, where the total system bandwidth may be divided into smaller subbands. In the network model used, the arrival of users/packets follows a Poisson point process, communication between nodes is continuous in time, selection of a subband to transmit across is made randomly at each transmitter, and the outage assessments made in the network are based on SINR measurements. Accurate bounds on the probability of outage for the MAC protocols are derived, and evaluated with respect to the number of subbands. It is observed that there exists an optimal number of subbands for each protocol, for which the probability of outage is minimized. For ALOHA, we obtain an analytical expression for this optimal value, while in CSMA, the optimal value is observed through simulations. Furthermore, we improve the performance of CSMA by introducing channel sensing across all subbands, in order to decrease the probability that a packet is in outage upon arrival. The obtained results are used to compare the performance of the two MAC protocols. Finally, we also evaluate the performance of our network in terms of sum capacity.

Analytical study of the outage probability of ALOHA and CSMA in bounded ad hoc networks

The performance of the ALOHA and CSMA protocols is evaluated within a bounded ad hoc network. Our network model comprises packets being distributed in finite space and time according to a 3-D PPP. Upon the formation of each packet, it is transmitted to its intended destination located a fixed distance away, using a fully-distributed MAC protocol. If the SINR of the received packet falls below a certain threshold any time during its transmission, the packet is received in outage. The evaluation metric of our network is outage probability, which is directly related to throughput. Approximate analytical expressions are derived for the outage probability of the different MAC protocols as a function of the spatial density of transmissions and the physical size of the network, and validated by simulations. Our bounded network is shown to yield a lower outage probability than infinite domains, due to the lower level of interference resulted from edge effects. Furthermore, the performance of the different MAC protocols are compared, and CSMA with receiver sensing [1] is shown to yield the lowest outage probability of all the unslotted protocols.

Analytical Assessment of the Effect of Backoffs and Retransmissions on the Performance of ALOHA and CSMA in Manets

This paper considers the impact of backoffs and retransmissions on the performance of the ALOHA and CSMA protocols in wireless mobile ad hoc networks. In our network model, packets belonging to specific transmitters arrive randomly in space and time according to a 3-D PPP, and are then transmitted to their intended destinations using a fully-distributed MAC protocol. A packet transmission is considered successful if it is initiated and the received SINR is above a predefined threshold for the duration of the packet. Approximate expressions are derived for the outage probability of ALOHA (slotted and unslotted) and CSMA (with transmitter sensing and with receiver sensing), as a function of the transmission density and the number of backoff and retransmission attempts. Simulations verify our analytical expressions, and it is shown that the outage probability is a decreasing function of the maximum allowable number of backoffs and retransmissions. By allowing for high number of backoffs, CSMA with receiver sensing can even outperform slotted ALOHA. For high number of retransmissions, however, ALOHA may outperform CSMA. Finally, the average transmission delay of packets is also evaluated through simulations.

Efficiency of the ALOHA protocol in multi-hop networks

This paper presents the evaluation of the multi-hop aggregate information efficiency of the slotted and unslotted ALOHA protocols. We consider a multi-hop wireless network where the nodes are spatially characterized by a Poisson point process and the traffic generation also follows a Poisson distribution. By applying the properties of stochastic geometry, we derive a closed-form lower bound on the outage probability as a function of the required communication rate, the single-hop distance, the number of hops and the maximum number of retransmissions. The results indicate that slotted ALOHA always outperforms its unslotted version, demonstrating the importance of synchronization in distributed networks. In addition, we show that it is always possible to optimize the network efficiency by properly setting the required rate for a given packet density. Finally, in the scenario considered, the use of retransmissions and multiple hops never achieves the best performance if compared to the option of single-hop links without retransmissions.

The Impact of Edge Effects on the Performance of MAC Protocols in Ad Hoc Networks with Fading

In this paper, the impact of edge effects on the performance of MAC protocols is evaluated. We consider a network where packets are distributed in finite space according to a 2-D PPP, while packets arrive in time according to a 1-D PPP. Each transmitter communicates with its own receiver a fixed distance away using either the ALOHA or CSMA protocol. The metric used for analysis is outage probability, which is the probability that the measured SINR falls below a predefined threshold for the duration of a packet. Approximate analytical expressions are derived for the outage probability of the different MAC protocols assuming a fading channel. Compared to unbounded regions, ALOHA and CSMA are shown to yield lower outage probabilities in finite regions, since the boundedness can be regarded as spatial filtering. Furthermore, the benefit seems to be greater for small outage probabilities, i.e., in the region of practical interest.