Raul Palacios

Performance Optimisation of Soft and Hard Spectrum Sensing Schemes in Cognitive Radio

Cognitive radios represent a promising technique to address the problem of efficient spectrum utilisation. However, their performance is limited by their capability to sense the spectrum. This letter proposes a fair comparison between soft and hard spectrum sensing schemes, based on the practical assumption of limited time resources, which was never explored in previous studies. By considering the achieved detection probability as the performance metric and the false alarm probability as constraint, this work analytically proves that the hard scheme can represent a favourable solution, especially in short sensing times and/or large numbers of users. The sufficient condition by which the soft scheme outperforms the hard scheme is also introduced. This condition is represented by the number of collected samples from the channel in a closed-form expression, which depends on the sensing time, the Signal-to-Noise-Ratio (SNR), the number of available users and the predefined false alarm probability threshold. All these factors are independently evaluated throughout this work. The analysis is carried out through mathematical equations and confirmed by simulation results.

Energy-efficient spectrum sensing in Cognitive Radio Networks by coordinated reduction of the sensing users

One of the main challenges in Cognitive Radio Networks (CRN) is the high energy consumption during the spectrum sensing stage, especially employing a cooperative approach. The algorithm proposed in this paper aims to reduce the energy consumption while maintaining the probability of detection and false alarm probability to the desired thresholds. The algorithm is based on decreasing the number of sensing users using a simple and practical approach. The performance of our approach is then compared in terms of energy efficiency with the different data fusion rules available in the literature. As a result, more than 95% energy saving can be achieved, as shown through mathematical equations and confirmed by simulation results.

Analysis of an energy-efficient MAC protocol based on polling for IEEE 802.11 WLANs

This paper analyzes the performance of a duty-cycled polling-based access mechanism that exploits the Transmission Opportunity Power Save Mode (TXOP PSM) defined in the IEEE 802.11ac to improve the energy efficiency of Wireless Local Area Networks (WLANs) based on the IEEE 802.11. The basic idea behind the proposed approach, named GreenPoll, is to enable contention free periods, based on polling with beacons, during which wireless stations can save energy by turning off their radio transceivers after exchanging data with the access point. The closed expression of energy efficiency of GreenPoll is formulated in this paper and is used to evaluate the performance of GreenPoll considering important parameters like the traffic load, packet length, data rate, and number of stations in the network. Both analytical and simulation results show the high energy efficiency of GreenPoll with gains of up to 330% and 110% when compared to the legacy Distributed Coordination Function (DCF) and the Point Coordination Function (PCF) defined in the IEEE 802.11, respectively.

An energy-efficient point coordination function using bidirectional transmissions of fixed duration for infrastructure IEEE 802.11 WLANs

The Point Coordination Function (PCF) of the IEEE 802.11 standard represents a well-known Medium Access Control (MAC) protocol providing Quality-of Service guarantees in Wireless Local Area Networks (WLANs). However, with the currently employed polling mechanism WLANs consume a significant amount of the energy resources from battery-powered user devices. To provide energy saving, an improved MAC protocol is presented in this paper, where bidirectional transmissions of fixed duration are incorporated into PCF in order to enable dynamic scheduling of real-time traffic. Based on this new strategy, wireless access points (APs) can estimate the proper duration of the Contention Free Period (CFP), in order to allow mobile stations to acknowledge any received data packet with a data packet equal to the received packet in size. Having this information, a mobile station, following the data exchange with the AP, can determine its wake-up timer and activate the sleep mode for the rest of the CFP interval. Comprehensive computer-based simulations demonstrate the feasibility of the proposed MAC improvements to achieve energy efficiency with negligible impact on packet delivery delay.

Analysis of a network coding-aware MAC protocol for IEEE 802.11 wireless networks with Reverse Direction transmissions

The implementation of Network Coding (NC) in IEEE 802.11-based wireless networks presents the important challenge of providing additional transmission priority for the relay nodes responsible for coding. These nodes are able to convey more information in each transmission than those that forward single packets, by combining several received packets in a single coded packet. To transmit data, the nodes execute the IEEE 802.11 Medium Access Control (MAC) protocol, called the Distributed Coordination Function (DCF). Thus, they compete for the access to the wireless channel and get equal transmission opportunities under high congestion. As a result, congested relay nodes will severely limit the performance of the network. In this paper, we investigate a backwards-compatible mechanism, called Reverse Direction DCF (RD-DCF), that allows relay nodes to transmit data upon successful reception of data. We analyze the performance limits of the proposed protocol with and without NC in terms of throughput and energy efficiency. The performance evaluation considers different traffic loads, packet lengths, and data rates. The results of this work show that the proposed RD-DCF+NC protocol can improve throughput and energy efficiency up to 335% when compared to legacy DCF.

Reverse direction transmissions and network coding for energy-efficient Wi-Fi networks

This paper proposes a combination of the Reverse Direction Protocol (RDP) defined in the IEEE 802.11n and the Network Coding (NC) technique to improve the energy efficiency of wireless networks based on the IEEE 802.11. The aim of the combined approach, named BidCode, is to allow intermediate nodes to combine several received packets into coded packets and immediately forward them upon successful reception of data. The energy efficiency of BidCode is analyzed in this paper and compared to those of the Distributed Coordination Function (DCF) and an important NC protocol based on IEEE 802.11, called COPE. Both analytical and simulation results show the high energy efficiency of BidCode with gains of up to 350% and 130% when compared to DCF and COPE, respectively.

An energy efficient distributed coordination function using bidirectional transmissions and sleep periods for IEEE 802.11 WLANs

The IEEE 802.11 Distributed Coordination Function (DCF) is the fundamental access method providing asynchronous best-effort services in Wireless Local Area Networks (WLAN). In this standard, the currently employed Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) and the Binary Exponential Backoff (BEB) mechanism represent major sources of energy consumption at both the access point and mobile stations of a WLAN. To improve energy efficiency in WLANs, this paper introduces an enhanced DCF protocol incorporating bidirectional transmissions in combination with sleep periods, called Bidirectional Sleep DCF (BDSL-DCF). By following this new scheme, every successfully established connection between a sender and its intended destination can be used to exchange data, hence reducing control overhead and channel contention. Furthermore, this functionality allows those mobile stations not participating in data transmission to activate the sleep mode to conserve energy. Simulation results show that BDSL-DCF can outperform DCF in terms of energy efficiency and throughput, with negligible impact on packet transmission delay.

Energy efficiency of an enhanced DCF access method using bidirectional communications for infrastructure-based IEEE 802.11 WLANs

The Distributed Coordination Function (DCF) is the fundamental access method defined in the IEEE 802.11 Standard for Wireless Local Area Networks (WLANs). With this standard, the Access Point (AP) and the mobile stations consume a significant amount of energy to contend for access to the shared wireless channel. In order to improve the energy efficiency of WLANs, we investigate in this paper a simple and backwards compatible mechanism, called Bi-Directional DCF (BD-DCF), that enables bidirectional communications between wireless stations with a single channel access invocation. The key idea is to let the AP or any mobile station transmit a data packet together with the acknowledgement upon the successful reception of a data packet. This approach reduces the communication overhead, the channel contention, and better balances uplink and downlink transmission opportunities. We evaluate the performance of the proposed BD-DCF by means of computer-based simulation considering different traffic loads, degrees of traffic symmetry, data packet sizes, and data rates. The results presented in this paper show that the BD-DCF protocol can improve the energy efficiency of DCF up to 50%.

Coding-aware MAC: Providing channel access priority for network coding with reverse direction DCF in IEEE 802.11-based wireless networks

An important challenge for the implementation of network coding in IEEE 802.11-based wireless networks is to give additional priority for channel access to the relay stations responsible for coding. These relay stations are able to provide more information in a single transmission than those that forward single packets, hence improving throughput and energy efficiency. The Distributed Coordination Function (DCF) of the IEEE 802.11 standard is a contention-based Medium Access Control (MAC) protocol that provides an equal distribution of channel access opportunities for all competing stations. However, the relay station represents a congestion point and additional transmission slots should be assigned to it to increase the overall network performance. To address this issue we investigate a coding-aware MAC protocol, called Reverse Direction DCF (RD-DCF), which enables bidirectional communications between the relay station and another station with a single channel access invocation. This simple and backwards compatible mechanism allows the relay station to transmit a coded packet together with the acknowledgement immediately after receiving a data packet. The simulation results show a gain of up to 130% in terms of both throughput and energy efficiency for RD-DCF with network coding when compared to DCF.

Experimental evaluation of reverse direction transmissions in WLAN using the WARP platform

This paper describes an experimental implementation of a variation of the Reverse Direction (RD) Medium Access Control (MAC) Protocol (RDP) defined in the IEEE 802.11n using the Wireless Open-Access Research Platform (WARP). The proposed approach, named Bidirectional MAC (BidMAC), allows the receiver of a valid data sequence to perform an RD transmission to the transmitter without contending for tAhe channel. Whereas in RDP the RD transmission must be initiated by the transmitter, in BidMAC it can be dynamically initiated by the receiver according to its traffic requirements. Previous results based on mathematical analyses and computer-based simulations have shown that BidMAC can better balance downlink and uplink transmission opportunities in a Wireless Local Area Network (WLAN) where the Access Point (AP) handles bidirectional data flows for some of its wireless stations (STAs). This paper aims at going one step further and demonstrating that such superior performance can be attained in real environments. Towards this end, an implementation of BidMAC has been carried out in a reference design of WARP compatible with the IEEE 802.11a/g and tested in a proof-of-concept network formed by an AP and two STAs. Experimental results confirm the superior performance of BidMAC when compared to the legacy Distributed Coordination Function (DCF) of the IEEE 802.11 versus the traffic load, packet length, and data rate, yielding gains of up to 60%*.