IEEE 802.11 basics and evolution toward the IEEE 802.11acamendment

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IEEE 802.11 basics and evolution toward the IEEE 802.11ac amendment

The first IEEE 802.11 [2] standard was launched in 1997, specifying a lot of notions related to wireless transmissions. It defines three PHY layer tech-nologies, namely infrared [18], direct sequence spread spectrum (DSSS) [19] and frequency hopping spread spectrum (FHSS) [20]. These specifications permit to reach data rate ranging from 1 Mbps up to 2 Mbps within a bandwidth of 22 MHz. This release supports mainly both DSSS and FHSS, in contrast to the IF mode that remains a contribution of the standard but has not been implemented. IEEE 802.11-1997 devices operate within the 2.4 GHz frequency band. The original IEEE 802.11 norm can be considered as a beta-specification. The multiple PHY layer options made it challenging for products to be inter-operable indeed. In addition, throughput became quickly a limiting factor as well.
To overcome IEEE 802.11-1997 limitations, new amendments were estab-lished to fix and enhance the network, starting with the IEEE 802.11b [3] amendment that was released in 1999. It maintains all IEEE 802.11-1997 specifications (except for the Frequency Hopping Spread Spectrum (FHSS) PHY layer) while enabling a data rate of 11 Mbps. Nevertheless this stan-dard presented several limitations due to the interference generated by other 2.4 GHz equipments (microwaves, ovens, etc.) which became a real issue for Wi-Fi users.
IEEE 802.11a [4] was thus launched, switched to the 5 GHz frequency band that was a key step to minimise interference issues. It also introduces OFDM that offers a higher user diversity in addition to higher performances compared to IEEE 802.11b [11][10]. Combined with the usage of the 5 GHz channel, it results in a data rate increase from 11 Mbps to 54 Mbps within a 20 MHz bandwidth. A bit later, IEEE 802.11g [5] followed IEEE 802.11a by introducing OFDM in the 2.4 GHz band (like IEEE 802.11b) while maintaining a peak data rate of 54 Mbps.
These standards were followed by IEEE 802.11n [6], also referred to as high throughput (HT) standard, which provides many new techniques that made a substantial change on network performance. IEEE 802.11n’s major improvements are: MIMO, frame aggregation, beamforming and higher spa-tial diversity [21][22][23][24][25]. It also expands the bandwidth from 20 MHz to 40 MHz. All these enhancements enable a ten-fold increase in data rates with regard to 802.11b/a/g, reaching then 600 Mbps. The most significant objectives were to deploy MIMO, as an enhancement of single input single output (SISO), for the same setting as IEEE 802.11a/g and double the band-width to increase the throughput [12]. Likewise, the aggregation mechanism offers a higher transmission efficiency thanks to the reduced overhead. The additional advantage of IEEE 802.11n is its backward compatibility with all previous standards since it works either at 2.4 GHz or 5 GHz. After that, the working group defined IEEE 802.11ac [7][26][27][28], also referred to as very high throughput (VHT) standard, that keeps the specifi-cations of 802.11n while introducing some notable improvements. It allows a larger bandwidth, i.e. up to 160 MHz. It also offers higher constellation orders (up to 256-quadrature amplitude modulation (QAM)) to increase the amount of transferred data. One will also note that, for greater efficiency, IEEE 802.11ac sets the frame aggregation mechanism as mandatory and introduces the multiple users (MU) transmission scheme, namely the MU-MIMO tech-nique. With these improvements, IEEE 802.11ac shows substantial progress, be it in the quality of the network or its throughput [29][30]. Table 1.1 presents some of the enhancements offered by each successive amendment.

Orthogonal frequency division multiplexing (OFDM)

The multicarrier concept has been proposed by Chang in the 1960s [41]. Later in 1971, a time-limited multi-carrier technique has been proposed in [8] by Weinstein and Ebert: it was the birth of OFDM. Because of its high efficiency, this technique has been heavily used with the expanse of applications. For instance, the European digital audio broadcasting (DAB) and digital video broadcasting (DVB) are among the first technologies using OFDM.
OFDM is one of the multiplexing techniques that permits a high data rate transmission while ensuring a better exploitation of the channel compared to the conventional frequency division multiplexing (FDM)[9]. It overcomes the multipath problem by dividing the channel bandwidth into a number of or-thogonal sub-bands that carry one modulation symbol each. It was presented as a new way to transmit signals without inter carrier interference (ICI) or inter symbol interference (ISI). Because of its high efficiency which results in improved data rates, OFDM has been considered as a key improvement when introduced within IEEE 802.11 specifications with IEEE 802.11a. Sev-eral works expose the use of OFDM while showing its performances within wireless networks [11][10]. OFDM splits a large amount of data over available subcarriers, that are then multiplexed to be simultaneously transmitted over the channel. Each subband is orthogonal to all other sub-bands so as to null out ICI. Users can oc-cupy the channel during an OFDM symbol duration and transmit within the corresponding frequency bandwidth. Combined with error correction coding, it allows time and frequency diversity. The subcarriers are spaced by a fre-quency interval denoted by Δf . In order to prevent ISI, a guard interval (GI) is added between two successive OFDM symbols. Practical implementation of OFDM uses inverse fast Fourier transform (IFFT) and FFT at transmitter and receiver respectively. The IFFT/FFT size equals the number of subcarri-ers. Table 1.2 presents different parameters involved in the OFDM technique for the Wi-Fi norm up to IEEE 802.11ac.

Single and multiple user multiple input multiple output (MIMO)

For the purpose of improving the overall network performance through spatial diversity, the IEEE 802.11n [6] included the technique of MIMO. Using multiple antennas at both the transmit and receiver sides enables to improve the spectral efficiency and/or transmission quality without addi-tional bandwidth nor power increment. Thanks to full or partial channel state information (CSI) at the transmitter, optimised precoding of several spatial streams enables to increase the data rate while ensuring transmission quality. IEEE 802.11n supports a deployment of a maximum of 4 exchanged spatial streams (SS) for each single user. This is denoted by single user MIMO (SU-MIMO). Its successor, IEEE 802.11ac [7] extends the SS number from 4 to 8 for each user and also introduces the multiple user (MU) diversity to the Wi-Fi standard. Multiple user MIMO (MU-MIMO) [42] provides a transmission scheme using multiple antennas to simultaneously transmit to multiple users thus increasing the transmission gain and efficiency. However this is done at the expense of an interference increase induced by the multiplicity of either spatial streams or communicating stations. To overcome this issue, beam-forming, which exploits available CSI at the transmitter, can be applied as a solution to limit the interference to the narrow neighbourhood of the target user. It has been normalised in the IEEE 802.11n and revised in the IEEE 802.11ac.

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Modulation and coding scheme

The notion of MCS has been introduced by the IEEE 802.11n to have an index identifying modulation and coding rate couples. Many MCSs have been defined to encode transmitted frames. Each MCS index corresponds to a modulation scheme, a coding rate and a number of SS. These parameters induce a certain data rate depending either on the channel bandwidth or the GI (for amendments under OFDM technique). Either IEEE 802.11a or IEEE 802.11g recommend several modulations (e.g. binary phase shift key-ing (BPSK), quadrature phase shift keying (QPSK), 16-QAM and 64-QAM) under different coding rates (e.g. 1/2, 2/3 or 3/4). It thus allows both amend-ments (i.e. IEEE 802.11a/g) to achieve higher date rates compared to IEEE 802.11b. For IEEE 802.11a/g devices, a data rate of up to 54 Mbps can be reached using 64-QAM modulation and a coding rate of 3/4. A summary of modulation and coding parameters is given in Table 1.3. Table 1.3 presents different supported OFDM-based modulation schemes for amendments IEEE 802.11a/g. With the introduction of additional MCS in IEEE 802.11n, higher data rates (e.g. 65 Mbps versus 54 Mbps for 20 MHz bandwidth and 1 SS) are provided. A bandwidth extension up to 40 MHz is also introduced. The VHT amendment (i.e. IEEE 802.11ac) not only extends the bandwidth compared to the HT amendment (namely including 80 MHz and 160 MHz) but also adds two higher MCSs: 256-QAM with 3/4 and 5/6 coding rates. The highest one (i.e. 256-QAM, coding rate: 5/6) reaches data rate up to 780 Mbps per SS. IEEE 802.11n/ac MCS parameters are reported in Table 1.4.

IEEE 802.11 management operations

IEEE 802.11 supports several architectures as thoroughly exposed in Section 1.2.2. Either under BSS, IBSS or ESS mode, managing the communication is mandatory to ensure the coordination of present devices. This relies on several procedures specified by the Wi-Fi norm [2][16][17] for each node of the wireless networks. These proceedings enable to manage communications from accessing to a WLAN, until leaving it. For instance, each station, mobile or not, could join or leave a network at any moment. Its association or disasso-ciation from the BSS or the IBSS must be handled. The basic management operations within an IEEE 802.11 network are listed below.
(a) Beaconing: It has the function of periodically broadcasting a frame that includes several information such as supported capabilities, oper-ating specifications or management information. It is handled by the AP and the GO for the BSS mode and the IBSS mode respectively.
(b) Scanning: As its name indicates, this function enables to detect existing BSSs. A station can perform two types of scans. The first type is the active scanning where the station actively searches for an appropriate BSS by sending out Probe Request frames. APs (or GOs) can respond using a Probe Response if the station accepts the request. The second scanning type is the passive one. The station waits until it detects a Beacon frame sent by an AP (for infrastructure BSS mode) or an GO (for IBSS mode). An exchange of Probe Request / Response could be used if the station needs some information not available in the Beacon frame.
(c) Authentication: To join a network, stations have to be authenticated under IEEE 802.11 specifications. Two methods are supported to perform this process: open system and shared key authentication. The first one does not require any log-in code to join a BSS in contrast to the second one where stations must have the shared encryption key. Authentication provides a secure network so that the BSS would not be accessed by any user and to prevent ones dropping.
(d) Association/Re-association: Each station wanting to communicate with the BSS must be associated to the AP. To establish this association, a STA first sends an Association Request. If the request is accepted, the AP responds with a positive Association Response frame and thus finalises the association procedure. Information about the network configuration is transmitted during Association Request / Response exchange. Otherwise, in case the station is moving within an ESS, it has to be re-associated to the new BSS so that network mapping is maintained by concerned APs. In addition, a re-association could be performed in case network capabilities were changed.

Table of contents :

Acknowledgement
Résumé
Abstract
List of Figures
List of Tables
Acronyms
Notations
Contents
Résumé étendu en français
Introduction
1 Survey on the IEEE 802.11 norm 
1.1 Introduction
1.2 IEEE 802.11 basics and evolution toward the IEEE 802.11acamendment
1.2.1 Overview of the IEEE 802.11 amendments’ evolution .
1.2.2 IEEE 802.11 architecture
1.2.3 PHY basics and evolution
1.2.4 MAC basics and evolution
1.3 IEEE 802.11ax
1.3.1 IEEE 802.11ax challenges and objectives
1.3.2 PHY layer improvements
1.3.3 MAC layer improvements
1.3.4 Multiple user enhancements
1.4 Summary
2 OFDMA with random access mode 
2.1 Introduction
2.2 State-of-the-art and motivations
2.3 Resource allocation configurations
2.3.1 Configuration (1): Standard configuration
2.3.2 Configuration (2): Scheduled station and RA station .
2.3.3 Configuration (3): Two RA stations
2.4 Analytical representation
2.4.1 Considered assumptions
2.4.2 Allocation of primary STAs
2.4.3 Allocation of secondary STAs
2.5 Analysis and conclusions
2.6 Perspectives
3 Network simulation environment and selected scenarios 
3.1 Introduction
3.2 Network simulation tool: ns-3
3.2.1 Generalities
3.2.2 Organisation and key principles
3.2.3 Architecture of ns-3 Wi-Fi module
3.2.4 ns-3 modules affected by the block acknowledgement policy
3.3 Metrics of interest
3.4 Selected applications
3.4.1 File transfer protocol
3.4.2 Full-buffer application
3.5 Considered IEEE 802.11 scenarios
3.5.1 Common simulation parameters
3.5.2 Scenario (a): Low-density scenario under IEEE 802.11ac 83
3.5.3 Scenario (b): Low-density scenario under IEEE 802.11ax 83
3.5.4 Scenario (c): Medium-density scenario under IEEE 802.11ax
3.6 Conclusion
4 Adaptive negotiation of the BA session 
4.1 Introduction
4.2 IEEE 802.11e block acknowledgement mechanism
4.2.1 Description
4.2.2 Discussion: BA performance analysis
4.3 Proposed adaptive negotiation of the block acknowledgement session
4.3.1 Description of the AN-BA mechanism
4.3.2 AN-BA potential benefits compared to BA
4.3.3 AN-BA backward compatibility
4.4 Analytical model for AN-BA
4.4.1 State-of-the-art
4.4.2 Generic model for IEEE 802.11 standard
4.4.3 Selected buffer size adaptation schemes
4.4.4 Saturation throughput: General expression
4.4.5 Analytical saturation throughput expression for AN-BA mechanism
4.5 Simulation setup
4.5.1 Matlab implementation of the analytical model .
4.5.2 Implementation of the AN-BA in ns-3
4.6 Comparison of analytical model numerical results with ns-3 measured throughput
4.6.1 Results
4.6.2 Discussion
4.7 Comparison of AN-BA with BA reference
4.7.1 Full buffer application
4.7.2 File transfer protocol application
4.7.3 Analysis
4.8 Conclusion
Conclusions and Perspectives
Bibliography

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