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PHY Layer Characteristics
The physical layer of IEEE 802.11n is the interface between the wireless medium and the MAC layer. As described above, it can operate on both 2.4 GHz and 5 GHz frequency bands with a high varying data rate from 65 Mbps up to 600 Mbps due to the use of MIMO and OFDM. A summary of the PHY layer specifications is presented in Table 1.1
Orthogonal Frequency Division Multiplexing (OFDM)
OFDM is a parallel transmission scheme, where a high-rate serial data stream is split up into a set of N low-rate substreams, each of which are modulated on a separate subcarrier. The subcarrier spacing is denoted by f. To obtain high spectral efficiency, adjacent subcarriers are modulated by selecting orthogonal subcarrier frequencies, i.e., f = 1=Tu, where Tu is the duration of the useful part of the OFDM symbol, as shown in Figure 1.2. This modulation technique is a robust solution to counter the adverse effects of mul-tipath propagation and Inter-Symbol Interference (ISI), because the bandwidth of the subcarriers is short compared to the coherence bandwidth of the channel; that is the individual subcarriers experience flat fading. This implies that the symbol period of the substreams is long compared to the delay spread of the time-dispersive radio channel. Besides, a guard interval (GI) is added to the useful part of each OFDM symbol in order to prevent ISI [Ano13, Pra04]. IEEE 802.11n on 20 MHz channel uses an FFT (Fast Fourier Transform) of 64, of which: 56 OFDM subcarriers, 52 are for data and 4 are pilot tones with a carrier sep-aration of f = 312:5 kHz (20MHz64 ) (3:2 s) or 114 subcarriers on a 40 MHz channel. Besides, each of these data subcarriers can be modulated by BPSK, QPSK, 16-QAM or 64-QAM modulations, with a total symbol duration of 3.6 s or 4 s, which includes a guard interval of 0.4 s or 0.8 s respectively (see Table 1.1). In any case, the subcar-rier spacing is always fixed to f = 312:5 kHz. Note that, as will be seen in chapter 3, this parameter is a key parameter which plays a crucial role in the performance of communication when electromagnetic interferences are present. Then, depending on the characteristics of the PHY layer, the IEEE 802.11n stan-dard supports multiple transmission rates corresponding to different Modulation Coding Scheme (MCS) index values.
Modulation Coding Scheme (MCS) index
The MCS is also a key parameter for the interpretation of communication perfor-mances in the presence of interferences. Modulation Coding Scheme index is a value that determines the modulation type (e.g., BPSK, QPSK, 16-QAM, 64-QAM), forward error correction (FEC) coding rate (e.g., 1/2, 2/3, 3/4, 5/6) and number of spatial channels, as shown in Table 1.2. Each MCS index has an associated data rate and the values are given for both 20 MHz and 40MHz channel bandwidths and for both 800 ns and 400 ns GI. 800 ns GI is the legacy mode as well as the default mode for 802.11n devices, and 400 ns GI is an optional mode for 802.11n devices [Lit12].
The table 1.2 gives rate-dependent parameters for MCSs with indices 0 to 76. MCSs with indices 0 to 7 and 32 have a single spatial stream; MCSs with indices 8 to 31 have multiple spatial streams using equal modulation (EQM) on all the streams; MCSs with indices 33 to 76 have multiple spatial streams using unequal modulation (UEQM) on the spatial streams. MCSs indices 77 to 127 are reserved [Sta12, p.1688].
Distributed Coordination Function (DCF)
The Distributed Coordination Function (DCF) is a distributed channel access mech-anism. The DCF is designed for asynchronous data transport. It allows medium sharing between compatible WLAN nodes through the use of CSMA/CA. In CSMA/CA when a station wants to transmit data, it previously senses the chan-nel by the Carrier Sense (CS) mechanism over a fixed time duration to determine if another station is transmitting. If the medium is found to be idle, the station may initiate its transmission, otherwise the transmission is deferred, and the station waits for the medium to be idle. Once the medium remains idle during a Distributed Inter Frame Space (DIFS) pe-riod, the station has to perform a backoff procedure with a backoff timer and then it can start to send the frames to the receiving (Rx) station. If the receiving station correctly receives the frame, it sends back an acknowledgement (ACK) frame to the transmitting (Tx) station within the Short Interframe space (SIFS) period. The ACK frame indicates a successful reception. In the case that the ACK frame is not received due to frame transmission errors or an ACK frame transmission error, the Tx station assumes that the frame transmission failed and it defers its own transmission for an Extended Interframe Space (EIFS) period and schedules a retransmission of the same frame after the backoff procedure [PS13].
The backoff procedure
The backoff procedure is invoked by a station in two cases. It can be to transfer a frame once the medium is idle during a DIFS period and following a busy detection by the CS mechanisms. Or, it can be when the transmitting station infers a failed trans-mission. The random backoff procedure is very useful in a loaded network in order to avoid collision. Indeed, when several stations are waiting an idle medium to initiate their frame transmissions, each one will select a random time, and as a result, it is unlikely that two or more stations start transmission at the same time.
The backoff time is calculated using the following equation: Backof f T ime = Random() SlotT ime (1.1) where Slot Time (ST): is a system parameter that depends on the characteristics of the PHY layer. It is defined as the minimum duration to determine the channel state (CCATime), plus the round-trip time, the propagation time and the processing time of the MAC layer.
Random: refers to the number of slots and it is an integer in the range of [0, CW], where CW is the Contention Window and can vary 3 between CWmin and CWmax (Table 1.3)[Cos10, Sta12, TWT+13, MAE07].
Table of contents :
List of figures
List of tables
List of acronyms and variables
Introduction
1 IEEE 802.11n standard
1.1 IEEE 802.
1.2 IEEE 802.11n standard
1.3 PHY Layer Characteristics
1.3.1 Orthogonal Frequency Division Multiplexing (OFDM)
1.3.2 Modulation Coding Scheme (MCS) index
1.3.3 Rate adaptation algorithms
1.4 MAC Layer Characteristics
1.4.1 Distributed Coordination Function (DCF)
1.4.2 The backoff procedure
1.4.3 Inter Frame Space (IFS)
1.4.4 CSMA/CA with RTS/CTS
1.5 Carrier sense (CS) mechanisms
1.5.1 Physical carrier sense
1.6 Analysis of the frame types used by IEEE 802.11
1.7 802.11 MAC Frame Format
1.7.1 Data frame
1.7.2 Control frame
1.7.3 Management frames
1.8 IEEE 802.11n communication
1.9 Conclusion
2 Electromagnetic interference
2.1 Unintentional electromagnetic interference
2.1.1 Introduction to the different kind of emissions in the transport system
2.1.2 Characteristics of the EM interferences produced by contact losses between the catenary and the pantograph
2.1.3 Mathematical model of EM interference produced by contact losses between the catenary and the pantograph
2.2 Intentional electromagnetic interference
2.2.1 Classification of jamming
2.2.2 Analysis of conventional jammers
2.2.3 Mathematical model of the frequency sweeping jamming
3 Measurements and interpretations
3.1 Experimental approach
3.1.1 Testing tools
3.1.1.1 Iperf
3.1.1.2 Wireshark
3.1.2 Measurement equipment
3.1.2.1 Tektronix AWG7102 Signal Generator
3.1.2.2 LeCroy WaveMaster 813Zi Oscilloscope
3.1.2.3 J7211A Attenuation Control Unit
3.1.2.4 GRF5060 RF Power Amplifier
3.1.2.5 Antennas
3.1.3 Experimental setup
3.1.3.1 Equipment setup
3.1.3.2 Measurement Setup
3.2 Frequency sweeping jamming
3.2.1 Signal Parameters
3.2.1.1 Interference to signal power ratio (ISR)
3.2.1.2 Sweep Period (SP)
3.2.2 Experimental setup
3.2.3 Measurement results
3.2.4 Interpretation of the measurements
3.2.4.1 Observations based on the spectrum
3.2.4.2 Observations based on the CCA
3.2.5 Conclusions on frequency sweeping jamming
3.3 Transient EM interferences
3.3.1 Signal Parameters
3.3.1.1 Repetition period (T)
3.3.1.2 Interference to signal power ratio (ISR)
3.3.2 Experimental setup
3.3.3 Measurement results
3.3.4 Interpretation of the measurements
3.3.5 Conclusion on the transient EM interferences
General conclusion and perspectives
Bibliography
