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Fundamentals of OFDM Transmission
The draft standard IEEE 802.11p is based on OFDM transmission technology. Therefore a higher spectral efficiency, lower sensitivity in synchronization errors and less Inter-Symbol Interference (ISI) could be expected, compared to the other techniques like Time Division Multiple Access (TDMA). In an OFDM signal, a higher data bit rate channel is divided into multiple orthogonal sub-channels in the frequency domain with lower bit rates. By this multiplexing technique, there exist several narrow-band subcarriers instead of a wide band carrier. This conversion is shown clearly in Figure 2.1. In the first part of the figure, three symbols A, B, and C are included in a signal with a specific frequency and separated in time. In the second part of the Figure 2.1 these symbols are extended in time, but separated in carrier’s frequency, so the probability of wasting a symbol due to multipath propagation of the signal is reduced, because in the new situation, the symbols have less overlap on the adjacent symbols. A more complete description of an OFDM signal can be seen in Figure 2.2. As Figure 2.2 shows, an OFDM signal is divided in both time and frequency domain and so increases the capacity of the system in addition to the less interference of the adjacent symbols. The guard interval between the symbols in time domain is shown also in the Figure 2.2. More detail explanation about guard interval in OFDM signal is explained in Section 2.1.2.
IFFT
To create the OFDM symbol in practice, a serial to parallel chip is used in order to convert a signal with N serial symbols to a signal with N parallel symbols.
Each parallel data symbol (orthogonal sub-carrier) is modulated and then the modulated subcarriers are added together. because of practical reasons, this procedure is implemented by an IFFT block. Figure 2.3 shows a simple block diagram of an IFFT system.
Cyclic Prefix Insertion
Wireless communications systems are predisposed to multi-path propagation on the radio channel. Adding a cyclic prefix to the signal reduces the ISI. The cyclic prefix is repetition of the last part of a symbol at the beginning of the same symbol that is illustrated in Figure 2.4. Multi-path propagation causes the original signal to fade, so a guard interval in the symbol will improve the transmission. The cyclic prefix technique is being used as a guard interval in the OFDM signals. In the presence of a cyclic prefix, interference signals do not interfere with the main part of the symbol, [5].
OFDM Spectrum
A number of sub-carriers, k, of an OFDM signal with the same bandwidth is affecting the power level of the side-lobes in the power density spectrum. A faster side-lobe decay is due to a larger amount of the K. Spectrum mask of IEEE 802.11p is described in detail in Section 2.2.1. The spectrum mask in IEEE 802.11p is divided into 64 sub-carriers.
Fundamentals of Frontend Characterization
Emission Power and Spectral Emission
The transmit spectrum mask specifies the power limitation in a specific frequency bandwidth and a certain offsets relative to the maximum carrier power, therefore the unit is in dBr which stands for dB relative. Spectrum masks ensures that multiple WLAN devices do not interfere with each other and adjacent channels also have no interference with each other. The spectrum mask test can be a good indicator of fading presentation in a channel. By transmit power spectrum mask measurements, the in-band (working channel) and out-of-band (adjacent and non-adjacent channels) spurious signals could be specified and measured.
For WLAN, the peak Power Spectral Density (PSD) is used as the reference power in the signal. All offset results are measured according to the peak PSD. In the IEEE 802.11p standard the bandwidth of a channel is 10 MHz and the spectrum mask is defined up to 15 MHz offset from center frequency of each channel.
Transceivers according to the IEEE 802.11p standard, transmit signals in dif-ferent classes (A, B, C, D). A maximum power defined for each class of transceiver and 800 mW (28.8 dBm) is the highest transmit power in class D, [3]. More de-tails about limitations of each class are discussed in section 4.3. In class A, the transmitter has the lowest transmit power, up to 1 mW (0 dBm). For operation in the 5.850 − 5.925 GHz band, according to the IEEE 802.11p standard, the emission power and transmitted spectrum should correspond to Table 2.1 and Table 2.2.
The spectrum masks for all classes are defined for a 10 MHz channel bandwidth. The exact values can be seen in Table 2.2, but in general the higher transmit power the faster spectrum edge decay. So design and fabrication of transceivers in higher values of transmit power could be more difficult because of the nonlinearity of components in transceivers.
In class A through class D the ramp of the main edge in the defined spectrum mask (between 4.5 and 5.5 MHz offset) increases, because of the higher maximum output power in class D. The relative power level of the unwanted signals at an offset of ±15 MHz from the centre frequency reduces from −40 dBr in class A to −65 dBr in class D. Table 2.2 specifies the spectral mask for class A to class D operations.
Error Vector Magnitude (EVM)
The EVM is a parameter used to specify the quality of a transceiver. A signal received by a receiver or sent by a transmitter shall have all constellation points in the I-Q plane like the main signal. Various problems in the implementation of a device (such as carrier leakage, low image rejection ratio, phase noise, etc.) cause deviations of the actual constellation points. Therefore the average distance between ideal constellation points and received points by receiver is called EVM. Figure 2.5 shows the definition of EVM. This parameter could be expressed in dB or percent (%) and is related to the ratio of the power of the error vector to the Root Mean Square (RMS) power of ideal signal.
Possible data rates in IEEE 802.11p and related constellation errors are illus-trated in Table 2.3. Also the related modulation type for each data rate is shown in this table (the data rate changes according to modulation type and code rate). In order to measure the EVM of a signal, the data rate and modulation type has to be specified.
Guard Interval
The guard interval is the separation between two neighbor symbols of telecom-munication signals, in order to prevent the transmission symbols interfere with each other. Therefore signals have more immunity against delay in propagation, when using guard interval. Different delays in multipath propagation are coming from different reflections, diffraction, and transmission of the individual paths. In OFDM, a cyclic prefix technique is implemented as guard interval. At the be-ginning of each symbol, a copy of the last part of the symbol is included, which is called cyclic prefix or guard interval. This is explained in Section 2.1.2. In Figure 2.6 a symbol of an OFDM signal is shown with the definition of the guard interval.
The guard interval duration and symbol duration in the IEEE 802.11p standard are defined as following:
TSY M = TGI + TF F T = 8µs
TGI = 1.6µs
Tsym, TGI , and TF F T is defined in Figure 2.6.
Ramp-up and Ramp-down Time
The ramp-up time is the necessary time for a signal amplitude to rise from 10 % to 90 % of the maximum level of the signal power, and vice versa for the ramp-down time. This factor shows the system delay in order to reach a stable level. It is important to be sure that this delay is not larger than the delay of sending data since the device starts sending data. More details are discussed in Section 4.7. Figure 2.7 shows the definition of ramp-up and ramp-down time.
Third Order Intercept Point (IP3)
The Third Order Intercept Point (IP3) is a factor to measure the nonlinearity of systems and devices. The intercept point is a mathematical concept, and does not correspond to a practically occurring physical power level. The nth order intermodulation products appear at n times the frequency spacing of the input tones. Figure 2.8 shows the intermodulation products of a two tone signal. Third order intermodulation products are more important than other intermodulation products due to the high relative power level and less distance to main tones.
The output power versus the input power, both in logarithmic scale, are shown in Figure 2.9. One curve belongs to the fundamental response of an input signal and is linearly amplified, and the other curve shows the third order intermodu-lation product’s response, which is nonlinearly amplified (with slope 3 ).
The nonlinearity of amplifiers, implemented in transceivers, leads to a 3 dB increase in third order products when the input power is increased only by 1 dB. Therefore a limitation for amplifier’s input power is required in order to prevent disappearing of the main signal. The third order intercept point is defined at the intersection of the linear fundamental response curve with the third order response curve. The third order intercept point is higher than the 1 dB gain compression point, which is defined at the input power, where the saturated fundamental curve is 1 dB below the linear fundamental curve. Both , the third order interception point and the 1 dB compression point are shown in Figure 2.9
Dynamics of the Vehicular Wireless Channel
Fading can be defined generally as the distortion of a modulated telecommuni-cation signal occurred during the signal propagation in the channel. In WLAN communication, fading is caused by multipath propagation. Finding the rate of fading’s changes can help us to program the amplifiers in transmitter and receiver in order to have a clearer signal in receiver. Level Crossing Rate (LCR) is a factor to measure the rapidity of the fading. LCR shows how often the fading crosses a certain threshold in positive direction, [6]. Figure 2.10 shows the concept of LCR.
The dynamic of a wireless channel can be separated in pathloss and fading that is described in the next sections.
Path Loss
Path loss is the attenuation in power of an electromagnetic wave between a trans-mitter and a receiver. Path loss depends on many factors like free space loss, refraction, diffraction, reflection, absorption, ground forms, environment, and propagation medium. In V2V communication, a Line of Sight (LOS) path exists if no blockage is between the transmitter vehicle and receiver vehicle. According to the existence of LOS, different characteristic are defined for path loss. The path loss can be investigated in different scenarios like highway, urban and rural roads, where some factors are different in each scenario, like speed and traffic congestion.
Fading
The variation of the amplitude and/or relative phase in a received signal can be defined as fading. Therefore fading can be described as the variation of the characteristics of the propagation path over time or location. Small scale fading describes the oscillation of the received signal strength over very short time du-ration or a short distance. Rician and Rayleigh fading are often used for small scale fading. Large scale fading is caused by shadowing. The mobile station has to move over a large distance to remove the effects of shadowing. For shadowing the log normal distribution is often used, [8].
Rician Fading
Rician fading is a stochastic model used for radio propagation. When a signal received by receiver from different paths, a part of signal could be canceled by itself due to different path distances. In the case that one of the signals received by receiver be much stronger and be dominant, Rician fading can be considerable. In Rician fading the amplitude of the field strength of the received signal can be described by a Rician distribution.
Table of contents :
1 Background
2 Introduction
2.1 Fundamentals of OFDM Transmission
2.1.1 IFFT
2.1.2 Cyclic Prefix Insertion
2.1.3 OFDM Spectrum
2.2 Fundamentals of Frontend Characterization
2.2.1 Emission Power and Spectral Emission
2.2.2 Error Vector Magnitude (EVM)
2.2.3 Guard Interval
2.2.4 Ramp-up and Ramp-down Time
2.2.5 Third Order Intercept Point (IP3)
2.3 Dynamics of the Vehicular Wireless Channel
2.3.1 Path Loss
2.3.2 Fading
3 Wireless LAN According to IEEE 802.11p
3.1 Definition of Key OFDM Parameters
3.2 Definition of European 5.8GHz Channel Layout
3.2.1 Interference Between ITS and RTTT
3.2.2 Interference Between ITS and FWA
3.2.3 Basic Channel Usage Scenarios
4 Measurements of Transmitter Frontend
4.1 Siemens WLAN Transceiver (DUT)
4.2 Measurement devices
4.3 Emission Power
4.3.1 Measurement Set-Up
4.3.2 Measurement Results
4.4 Spectral Emission
4.4.1 Measurement Set-Up
4.4.2 Measurement Results
4.5 Error Vector Magnitude
4.6 Guard Intervals
4.6.1 Measurement Set-Up
4.6.2 Measurement Results
4.7 Ramp-Up/Ramp-Down Time
4.7.1 Measurement Set-Up
4.7.2 Measurement Result
5 Summary
