Effects of Data Frame Size Distribution on Wireless Lans

The continuous need to replace cables and deploy mobile devices in the communications industry has led to very active research on the utilization of wireless networks. IEEE 802.11 WLAN is known to achieve relatively small throughput performance compared to the underlying physical layer’s transmission rate and this is as a result of large overhead information composed of medium access control header, physical layer preamble information back-off duration control frames (ACK) transmissions and even inter-frame spaces. This paper provides an overview of frame size distribution using the dual fixed frame size as a case in point and subsequently a performance evaluation in a multi-user transmission channel condition is carried out. The distribution of frame size and its effect on the throughput is also investigated. This is done through comparison of various scenarios such as fixed frame size distribution and dual-fixed frame size distribution. The dual- fixed frame size distribution case has both equality and inequality cases and both are investigated. The case of equal frames was found to have improved marginal throughput compared to the other cases. The analysis and measurement results from OPNET simulation has shown that an equal fixed frame size distribution can be efficient in optimizing useful data

Effect of power randomization on saturation throughput of IEEE 802.11 WLAN

In this paper, we evaluate the saturation throughput for an IEEE 802.11 based wireless network considering capture effect at the receiver, while nodes transmit with random powers. In this respect, we consider a scenario consisting of a specific number of wireless nodes. Then, we derive the transmission as well as collision probabilities with respect to the perfect capture effect. In order to maximize the saturation throughput we set up an optimization problem and obtain how to compute optimum values for the probabilities corresponding to different power levels. By providing the numerical results, we deduce that power randomization may lead to a significant improvement in saturation throughput

Feasibility study of multiantenna transmitter baseband processing on customized processor core in wireless local area devices

The world of wireless communications is governed by a wide variety of the standards, each tailored to its specific applications and targets. The IEEE802.11 family is one of those standards which is specifically created and maintained by IEEE committee to im-plement the Wireless Local Area Network (WLAN) communication. By notably rapid growth of devices which exploit the WLAN technology and increasing demand for rich multimedia functionalities and broad Internet access, the WLAN technology should be necessarily enhanced to support the required specifications. In this regard, IEEE802.11ac, the latest amendment of the WLAN technology, was released which is taking advantage of the previous draft versions while benefiting from certain changes especially to the PHY layer to satisfy the promised requirements. This thesis evaluates the feasibility of software-based implementation for the MIMO transmitter baseband processing conforming to the IEEE802.11ac standard on a DSP core with vector extensions. The transmitter is implemented in four different transmis-sion scenarios which include 2x2 and 4x4 MIMO configurations, yielding beyond 1Gbps transmit bit rate. The implementation is done for the frequency-domain pro-cessing and real-time operation has been achieved when running at a clock fre-quency of 500MHz. The developed software solution is evaluated by profiling and analysing the imple-mentation using the tools provided by the vendor. We have presented the results with regards to number of clock cycles, power and energy consumption, and memory usage. The performance analysis shows that the SDR based implementation provides improved flexibility and reduced design effort compared to conventional approaches while main-taining power consumption close to fixed-function hardware solutions

A Hardware Testbed for Measuring IEEE 802.11g DCF Performance

The Distributed Coordination Function (DCF) is the oldest and most widely-used IEEE 802.11 contention-based channel access control protocol. DCF adds a significant amount of overhead in the form of preambles, frame headers, randomised binary exponential back-off and inter-frame spaces. Having accurate and verified performance models for DCF is thus integral to understanding the performance of IEEE 802.11 as a whole. In this document DCF performance is measured subject to two different workload models using an IEEE 802.11g test bed. Bianchi proposed the first accurate analytic model for measuring the performance of DCF. The model calculates normalised aggregate throughput as a function of the number of stations contending for channel access. The model also makes a number of assumptions about the system, including saturation conditions (all stations have a fixed-length packet to send at all times), full-connectivity between stations, constant collision probability and perfect channel conditions. Many authors have extended Bianchi's machine model to correct certain inconsistencies with the standard, while very few have considered alternative workload models. Owing to the complexities associated with prototyping, most models are verified against simulations and not experimentally using a test bed. In addition to a saturation model we considered a more realistic workload model representing wireless Internet traffic. Producing a stochastic model for such a workload was a challenging task, as usage patterns change significantly between users and over time. We implemented and compared two Markov Arrival Processes (MAPs) for packet arrivals at each client - a Discrete-time Batch Markovian Arrival Process (D-BMAP) and a modified Hierarchical Markov Modulated Poisson Process (H-MMPP). Both models had parameters drawn from the same wireless trace data. It was found that, while the latter model exhibits better Long Range Dependency at the network level, the former represented traces more accurately at the client-level, which made it more appropriate for the test bed experiments. A nine station IEEE 802.11 test bed was constructed to measure the real world performance of the DCF protocol experimentally. The stations used IEEE 802.11g cards based on the Atheros AR5212 chipset and ran a custom Linux distribution. The test bed was moved to a remote location where there was no measured risk of interference from neighbouring radio transmitters in the same band. The DCF machine model was fixed and normalised aggregate throughput was measured for one through to eight contending stations, subject to (i) saturation with fixed packet length equal to 1000 bytes, and (ii) the D-BMAP workload model for wireless Internet traffic. Control messages were forwarded on a separate wired backbone network so that they did not interfere with the experiments. Analytic solver software was written to calculate numerical solutions for thee popular analytic models for DCF and compared the solutions to the saturation test bed experiments. Although the normalised aggregate throughput trends were the same, it was found that as the number of contending stations increases, so the measured aggregate DCF performance diverged from all three analytic model's predictions; for every station added to the network normalised aggregate throughput was measured lower than analytically predicted. We conclude that some property of the test bed was not captured by the simulation software used to verify the analytic models. The D-BMAP experiments yielded a significantly lower normalised aggregate throughput than the saturation experiments, which is a clear result of channel underutilisation. Although this is a simple result, it highlights the importance of the traffic model on network performance. Normalised aggregate throughput appeared to scale more linearly when compared to the RTS/CTS access mechanism, but no firm conclusion could be drawn at 95% confidence. We conclude further that, although normalised aggregate throughput is appropriate for describing overall channel utilisation in the steady state, jitter, response time and error rate are more important performance metrics in the case of bursty traffic

Cooperative communication in wireless local area networks

The concept of cooperative communication has been proposed to improve link capacity, transmission reliability and network coverage in multiuser wireless communication networks. Different from conventional point-to-point and point-to-multipoint communications, cooperative communication allows multiple users or stations in a wireless network to coordinate their packet transmissions and share each other’s resources, thus achieving high performance gain and better service coverage. According to the IEEE 802.11 standards, Wireless Local Area Networks (WLANs) can support multiple transmission data rates, depending on the instantaneous channel condition between a source station and an Access Point (AP). In such a multi-rate WLAN, those low data-rate stations will occupy the shared communication channel for a longer period for transmitting a fixed-size packet to the AP, thus reducing the channel efficiency and overall system performance. This thesis addresses this challenging problem in multi-rate WLANs by proposing two cooperative Medium Access Control (MAC) protocols, namely Busy Tone based Cooperative MAC (BTAC) protocol and Cooperative Access with Relay’s Data (CARD) protocol. Under BTAC, a low data-rate sending station tries to identify and use a close-by intermediate station as its relay to forward its data packets at higher data-rate to the AP through a two-hop path. In this way, BTAC can achieve cooperative diversity gain in multi-rate WLANs. Furthermore, the proposed CARD protocol enables a relay station to transmit its own data packets to the AP immediately after forwarding its neighbour’s packets, thus minimising the handshake procedure and overheads for sensing and reserving the common channel. In doing so, CARD can achieve both cooperative diversity gain and cooperative multiplexing gain. Both BTAC and CARD protocols are backward compatible with the existing IEEE 802.11 standards. New cross-layer mathematical models have been developed in this thesis to study the performance of BTAC and CARD under different channel conditions and for saturated and unsaturated traffic loads. Detailed simulation platforms were developed and are discussed in this thesis. Extensive simulation results validate the mathematical models developed and show that BTAC and CARD protocols can significantly improve system throughput, service delay, and energy efficiency for WLANs operating under realistic communication scenarios