UNIT- 7 Radio wave propagation and propagation models EM waves below 2Mhz tend to travel as ground waves, These wave tend to follow the curvature of the earth and lose strength rapidly as they travel away from the antenna. They do not penetrate through ionospheric layers. Frequencies between 2 to 30Mhz propagate as skywaves, Bouncing off of ionosheric layers, these EM waves may propagate completely around the earth through multiple reflections or hops between the ground and the ionosphere. Frequencies above 30Mhz tend to travel in straight lines they are limited in their propagation by the curvature of the earth. Wave propagation effects at UHF and above Reflection Reflection occurs for EM waves incident upon some types of large surface. For smooth surface the EM waves undergo a specular reflection, means that the angle of incidence equals to the angle of reflection. Scattering scattering occurs when the signal is incident upon a rough surface or obstacles smaller than the wavelength. This case produces diffuse reflection. Diffraction Diffraction causes EM waves to appear to bend around corners. EM wave incident upon a sharp corner causes the generation of a weak point source that can illuminate a shadow or area behind the object. Department of ECE, Canara Engineering College Page 1
Path loss models for various coverage areas Free space propagation model P R = P T G T G R ( λ / 4Πd ) 2 G T G R Are the transmitting and receiving antenna gain d is the distance between transmitting and receiving antenna P R = P o / d 2 P o received signal strength at a distance of 1 meter Other path loss models Department of ECE, Canara Engineering College Page 2
Two-ray model This model assumes a direct LOS signal between the transmitter and the receiver and another signal path that consists of a reflected signal of a flat surface of the earth. The composite signal received at the M.S will consists of EM waves that add either constructively or destructively. P R = P T G T G R ( h T 2 h R 2 / d 2 ) Path loss = 40 log d (10 logg R + 10 log G T + 20 log h T + 20 log h R ) Multipath and Doppler effects The models considered previously do not address real time fluctuation of the received signal strength. Multipath delay spreading leads to both time dispersion of the received signal and frequency selective fading. Error detection and correction coding Error control coding is the term used to denote a technique that codes the transmitted bits in a way that attempts to control the overall bit rate Department of ECE, Canara Engineering College Page 3
Block codes may be used to determine whether an error has occurred during data transmission. Schemes that use block codes to correct errors that might have occurred during data transmission are known as forward error correction. Convolutional and turbo encoders A continuous stream of bits is mapped into output stream that possesses redundancy. The redundancy introduced depends upon incoming bits and several of preceding bits. Turbo encoders are modified form of combined convolutional encoders. Block interleaving It is a technique used by mobile wireless systems to combat the effect of bit errors introduced during transmission of errors. Examples of coding and interleaving The error control code used by the system may be able to correct one bit error out of a block of 8 bits, however it is not able to correct a burst of errors. If the bits of the block can be interleaved with bits from the other blocks then in theory burst of errors can be spread out over other six blocks Department of ECE, Canara Engineering College Page 4
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Digital Modulation Techniques MSK- Continuous phase FSK similar to OQPSK. MSK is encoded with bits alternating between quarternary components with Q component delayed by half symbol period. BPSK encodes 0 s and 1 s as transmitter output signals with either 180 or 0 deg output phases. QPSK encodes information both in phase and amplitude of the transmitted signal. OQPSK uses 4 different values of the phase to transmit.. In QPSK the phase of the signal can jump as much 180 at a time, when the signal is LPF these phase shift results in large amplitude fluctuations. By offsetting the timing of the odd and even bits by one bit period or half symbol period, the Inphase and quadrature components will never change at the same time. i.e it limits the phase shift by no more than 90 deg at a time. This yields much lower amplitude fluctuations than non offset QPSK. Π/4 QPSK uses 2 identical constellations which are rotated by 45 deg with respect to one another. Usually either the even or odd symbols are used to select points from one of the Department of ECE, Canara Engineering College Page 6
constellations and other symbols select points from other constellation. These also reduces the phase shift from a maximum of 180 deg but only to a maximum of 135 deg. OFDM OFDM was chosen as modulation scheme supports high speed packet data transfer. It is the modulation scheme for the IEEe 802.11a wireless standard It is a form of multi carrier, multi symbol, multirate FDM in which user gets to use all the FDM channels together. It possess the property of orthogonality. Spread Spectrum Modulation Techniques Frequency hopping spread spectrum : FHSS consists of a system that changes the centre frequency of transmission on a periodic basis in a pseudorandom sequence. There are usually limited number of different carrier frequencies to hop to and the hopping sequence is designed in such a fashion as to keep the occurrence of various hopping frequencies statistically independent from one another. Direct sequence spread spectrum In DSSS a spreading code is applied to the baseband data stream at the transmitter and the same spreading code is applied to the received signal to perform demodulation. Diversity Techniques Most effective technique that can be used to mitigate the effect of multipath fading is to employ some form of time space or frequency diversity either or both the transmission and reception of the desired signal. Department of ECE, Canara Engineering College Page 7
The basic idea behind these solution s is that fading will not remain same over different signal paths or over different frequencies over the same path. RAKE receiver To combat fading several innovative receiver implementations have been created. Multipath signals will arrive at the receiver over the mobile radio channel, these receivers exploit that fact by isolating the signal paths at the receiver. If the fading of multipath signals is different then the isolation process yield the diverse signals needed to improve receiver performance. Rake receiver is used for the equalization of multipath Department of ECE, Canara Engineering College Page 8
Smart antennas Typical GSM System Hardware Base station controller Department of ECE, Canara Engineering College Page 9
Specific BSC parts Group switch, sub-rate switch exchange/interface circuits, transcoder rate adaptation unit, system control, power supply, and environmental conditioning unit The exchange/ interface unit is basically mux/demux provide interconnections to the MSC, PDN or RBS s The connection from the MSC to the BSC provides 64kbps PCM voice signals. Once voice signals from the PSTN have been transcoded they are multiplexed together and forwarded to the proper RBS over T1/E1/J1 at a much lower data rate. Vocoded speech from the RBS must be transcoded to PCM and multiplexed before being sent to the PSTN. Radio base station RBS is typically self contained unit that contains several subsystems that perform the necessary operations to provide a radio link for the mobile subscriber. Communication link exists between the RBS and BSC to provide transfer of user data and network signaling functions. Radio base station subsystems Department of ECE, Canara Engineering College Page 10
- Distribution switch unit, timing and control, transmitter/receiver units, and combining and distribution units RBS transceiver unit Signal processing and control subsystem, transmitter units, and receiver units Department of ECE, Canara Engineering College Page 11
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Answers to Problems and Questions Chapter 8 Section 8.1 1. The function/purpose of a transmission line is to direct or guide a signal from one point to another in the most efficient manner. 2. The most common techniques used to compensate for noise problems encountered when transmitting digital information over conductor-based transmission lines is to use various encoding schemes and transmission protocols that mitigate these effects. 3. Fiber-optic cables have several important advantages over conductor-based transmission lines. The potential for almost unlimited bandwidth is one advantage and another advantage is the lack of susceptibility to electromagnetic noise. Section 8.2 4. To convert the range of frequencies from ELF to EHF (3 khz to 300 GHz) to a corresponding range of wavelengths, one needs to use the following equation: c f For 3 khz, 8 c 310 f 3 310 5 1 10 100 km For 300 GHz, 8 c 310 f 9 30010 3 1 10 1 mm 5. The three EM wave propagation effects most likely to effect cellular wireless operation are: reflection, scattering, and diffraction. 6. The EM wave propagation effect that can illuminate a shadow behind an object is known as diffraction. Department of ECE, Canara Engineering College Page 13
7. In the context of EM wave propagation, the term multipath refers to the fact that typically for cellular frequencies many signals usually arrive at the receiver simultaneously due to reflections from the environment. 8. Due to the many signals that arrive at the cellular receiver simultaneously due to various propagation effects, use of the free space path loss model is inappropriate for wireless cellular operation. 9. Using the free space path loss model, if the transmitted power is 600 mw at a frequency of 850 MHz, the received signal power at a distance of 5,000 meters is given by: First determine for 850 MHz, 6 30010 6.353 85010 m Then, assuming transmitting and receiving antenna gains of 1.0, 2 2.353 PR PT GT GR 600mW 4d 4 5000 2.353 6 2 mw 4 600mW 600 5.621 10 6.2810 12.631.610 W 18.96 pw This yields a received signal power in dbm of:. Finally, the path loss is given by: Signal Power in dbm 10log 12 9 10 3 10 10 P 1mW 18.9610 10 log 10 log 18.96 10 110 77.22 dbm Path Loss P P T R Converting 600 mw into dbm yields, Department of ECE, Canara Engineering College Page 14
Signal Power in dbm 10log 10 P 1mW 3 60010 10log10 10log 3 10 600 27.78 dbm 110 Path Loss P P 27.78 77.22 105 db and, T R 10. The basic difference between the two-ray path loss model and the free-space path loss model is that the two-ray model assumes an additional signal path for a reflected signal that also reaches the receiver. This yields a different equation (see equation 8.4) for the received signal power. 11. Using the distance-power gradient model with 4 the same conditions as problem #9 yields: First, P 0, must be determined. Then 4.6 7.89610.4738 2 2.353 P0 PT GT GR 600mW 4d 41 mw 4.4738mW PR P0 d.4738mw 5000 12 62510 W or dbm 18.75810 151.2 And, the path loss is given as, Path Loss P P 27.78 151.2 178.98 db T R 12. Using the distance-power gradient model with 3 the same conditions as problem #9 yields: First, P 0, must be determined. Then 4.6 7.89610.4738 2 2.353 P0 PT GT GR 600mW 4d 41 mw Department of ECE, Canara Engineering College Page 15
3.4738mW PR P0 d.4738mw 5000 9 12510 W or dbm 15 3.7910 114.2 And, the path loss is given as, Path Loss P P 27.78 114.2 141.98 db T R Section 8.3 13. The basic operation of an ARQ scheme is as follows: the transmission protocol will cause the system to call for a retransmission of a data packet when necessary (i.e. the bit error rate is too high). 14. The basic purpose of a block code is to determine whether or not an error has occurred during transmission. 15. When a 256-bit digital word is applied to a convolutional encoder with R = 1/3, a total of 3256 768 output bits are produced. 16. The basic process involved in the block interleaving of data bits before transmission is as follows: the bits of one data block are interleaved with the bits of another data block. In theory, this process will provide an important advantage in the transmission of digital data. If a noise burst causes several bit errors to occur, the bits effected will not all belong to the same data block. Hence the errors are spread out and thus easier to correct. Section 8.4 17. The basic advantage that digital modulation offers is bandwidth efficiency. Digital modulation techniques make it easier to transmit more bits per Hz of bandwidth. 18. 8-PSK modulation is a process by which three bits are encoded per transmitted symbol. Each symbol has a different phase angle from a reference phase angle. Department of ECE, Canara Engineering College Page 16
19. An OFDM modulation system uses several to many carriers that are all transmitted simultaneously. Each carrier transmits a sub-symbol that may encode one to many bits of data. The entire transmitted symbol consists of the sum of all the sub-symbols. 20. The overall data rate for an OFDM system that transmits 32 kbps over each carrier and uses 16 carriers is given by: Section 8.5 32 kbps 16 512 kbps 21. FHSS operation is where the carrier frequency is changed on a periodic basis in a pseudorandom sequence. 22. DSSS operation is where a spreading code is applied to the baseband data stream at the transmitter and the same spreading code is applied at the receiver to perform demodulation. Section 8.6 23. Ultra-wideband radio technology uses extremely short or narrow pulses (sub-nanosecond) with a correspondingly ultra-wideband frequency spectra to transmit information (data) over relatively short distances (meters). 24. The typical type of radio pulse used by UWB is a short sub-nanosecond pulse. Section 8.7 25. The basic theory behind the use of diversity in a wireless system is as follows: any fading of the transmitted signal that occurs will not remain the same over time nor will it be the same over different signal paths or be the same for different frequencies. Therefore, if some form of time, space, or frequency diversity is used, the effects of signal fading can be mitigated. Department of ECE, Canara Engineering College Page 17
26. The basic theory behind the operation of a RAKE receiver is as follows: since there are going to be multiple signals arriving at a receiver due to multipath propagation, this fact can be exploited by isolating the signal paths at the receiver. Once the signals have been isolated (they become the diverse signals), they may be used to improve the performance of the receiver. 27. The usual implementation of space diversity for a wireless system is to use multiple transmitting and/or receiving antennas. These antennas must be physically some distance separate or apart from each other. 28. Polarization diversity is implemented for a cellular wireless system by using several antennas with different polarizations. Section 8.8 29. An RF combining unit performs the function of combining several RF signals. Typically, several transmitter outputs may be applied to an RF combiner and the signal sources do not interact with each other. A typical application is to use a combiner to allow several transmitters to share the same antenna. 30. The distribution switch unit in a GSM radio base station serves the function of providing correct timing to the system. User data being carried on a T-1 data link from the BSC needs to be connected to the correct RBS transceiver and the correct timeslot for transmission. 31. The purpose of a cellular duplex filter is to allow for the sharing of an antenna for both transmitting and receiving. 32. Maintenance is usually performed on a modern cellular radio base station through the use of an operations and maintenance terminal (OMT). Through a craft interface port connection, the technician may access the RBS managed objects using OMT software. The OMT software usually allows the technician to isolate the problem to a specific field replaceable Department of ECE, Canara Engineering College Page 18
unit (FRU). Furthermore, the OMT software may be used to upgrade or re-provision the RBS. 33. OMT software is the typical designation of software that can be used to troubleshoot problems with a RBS system. This software can also be used to update the system configuration. Department of ECE, Canara Engineering College Page 19