Md Mahfuzur Rahman Bosunia
Department of Information and Communications Engineering (ICE), Hankuk University of Foreign Studies (HUFS) Introduction
To meet up the demand for higher performance of today’s applications e.g., online gaming, streaming audio video and different multimedia contents in wireless local area networks, the institute of Electrical and Electronics Engineers approved several WLAN standards with varying speed and service delivery. The objective of IEEE Task Group is to ensure minimum of 100 megabit-per-sec data delivery. IEEE 802.11 is a set of IEEE standards that provides wireless transmission methods. They are commonly named IEEE 802.11a, IEEE 802.11b, IEEE 802.11g and the newer version IEEE 802.11n. IEEE 802.11n specification is the most significant change in wireless environment. It differs from its predecessors in terms of its modes and configuration that provide maximum data rate. IEEE 802.11n is builds on the previous 802.11 standards by modification to the Physical Layer and also in the Medium Access Control Layer.
IEEE 802.11n Improvements
A number of proposals were made to standardize the 802.11n that all share three common elements: the use of MIMO-OFDM, 20 and 40 MHz channels and packet aggregation techniques. 802.11n Improvements in PHY layer
Improved coding and modulation: 802.11n modify the OFDM method used by 802.11a and offer a maximum data rate of 65Mbps. MIMO Techniques: 802.11n introduces smart radio technology to dramatically increase the quality and data rates by combining multiple antenna strategy with multiple simultaneous data streams. 802.11n systems uses MIMO (multiple input, multiple output) Fig. 1, techniques which can multiple transmit and multiple receive radio using its own antenna that also combine multiple streams of data of the same channel. 802.11n stations can multiply data rates using this spatial streams.
Fig 1: MIMO
Channel Bonding: 802.11n bonds two adjacent 20 MHz channels into a single 40 MHz channel that doubles the burst transmission rate. Spatial Multiplexing: SM like in Fig 2, subdivides an outgoing signal stream into multiple pieces, transmitted through different antennas. Because each transmission propagates along a different path, those pieces – called spatial streams– arrive with different strengths and delays. Provided the individual streams arrive at the receiver with sufficiently distinct spatial signatures, an SM enabled receiver is able to reassemble them back into the original signal stream. Space-Time Block Coding: STBC sends an outgoing signal stream redundantly, using up to four differently-coded spatial streams, each transmitted through a different antenna. By comparing arriving spatial streams, the receiver has a better chance of accurately determining the original signal stream in the presence of RF interference and distortion. That is, STBC like in Fig 2, improves reliability by reducing the error rate experienced at a given Signal to Noise Ratio (SNR). Beam Forming: 802.11n has multiple radios and antennas in each station and also have the beam forming capability by controlling the transmit power and phase of the collection of transmission antennas. It exploits signal reflection and multipath to improve received signal strength and sustain higher data rates.
Fig 2: Signal Processing
More Spectrum: 802.11n uses both 2.4 GHz and 5 GHz bands using the single MAC layer. It is also backward compatible with 802.11b/g stations and channelization.
802.11n Improvements in MAC layer
Frame Aggregation: The 802.11n MAC allows successive frames to be combined up to 64 k bytes with the aggregated MAC Protocol Unit. Fig 3. Represents the frame aggregation process of IEEE 802.11n. There are two aggregation options: MAC Service Data Unit Aggregation (A-MSDU) groups logical link control packets (MSDUs) with the same 802.11e Quality of Service, independent of source or destination. The resulting MAC frame contains...
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