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- 1. RF SIGNAL LOSS AND ITS MINIMIZATION SUNJEEV KUMAR GUPTA RANJIT KUMAR KARNA Kathmandu Engineering College, Kalimati Kathmandu Engineering College, Kalimati ABSTRACT This paper highlights the main issues confronting radio frequency (RF) propagation in non-line-of-sight For VHF, UHF and higher frequency. Attaining good wireless connections. Wireless signals propagating through Line of Sight (LOS) between the sending and receiving the air lose strength while encountering natural and antenna is essential. manmade obstacles. It would be nice if RF signals would propagate without any bounds, but that simply doesn't 3. THE NON-LINE-OF-SIGHT CHALLENGE occur. The problem might be different kinds of losses that are:- free space loss, fading loss, equipments loss etc. It is The nature of a Non-Line-of-Sight link is that seen that there are different mitigation technologies for there are obstacles such as buildings, vehicles, trees and non-line-of-sight solution. For this, some elements of non- hills between the transmitting and the receiving stations, line-of-sight solution are:-High System Gain, Dispersion completely obscuring the line of sight. Even in such Mitigation, Multi-path Compensation and Fading environments, multiple paths do exist between Mitigation. This can be achieved by Space Time Coding, transmitter and receiver via a combination of reflection, Adaptive Modulation, Dynamic Frequency Selection and diffraction and penetration. These “multi-paths” are of Orthogonal Frequency Division Modulation (OFDM), different lengths and have different characteristics. which can deliver the best chance of achieving a reliable, Hence, the signals arrive with varying amplitudes and secure, high-availability wireless connection. The paper disperse over time, causing self-interference. To make mainly focuses on the OFDM in frequency flat and things worse, as the environment changes due to selective fading. movement of obstacles such as trees or vehicles, or even to changes in air pressure or ambient temperature, the 1. INTRODUCTION nature of each path dynamically changes. This fading effect causes the received signal quality to vary Today there has been a high demand for reliable, unpredictably. Fading can reduce a signal’s strength by a secured, high-speed digital wireless communications. factor of up to -40 dB for periods of seconds, minutes or Besides the cellular phone, there are wireless modems, even days in some cases. The remainder of this paper high definition television (HDTV), and digital radios. looks at the techniques and technologies available to Performance of these devices in a wireless environment overcome this significant obstacle. can be severely limited by random fluctuations in amplitude of the received signal called fading. To solve 4. 1. FREE SPACE LOSS these challenges, many innovative techniques have been developed. In this paper, we focus on the combination of As signals spread out from a radiating source, the two of the powerful techniques, Antenna Diversity and energy is spread out over a larger surface area. As this OFDM. occurs, the strength of that signal gets weaker. Free space loss (FSL), measured in dB, and specifies how 2. AN OVERVIEW OF THE LINE-OF-SIGHT much the signal has weakened over a given distance [1]. FSL = 36.6+20 logF+20log D Where F: Frequency in MHz, D: Distance in Km and FSL: Free Space Loss Distance (miles) 2 4 6 10 20 2.4 GHz FSL 110 116 119 124 130 (dB) 5.8 GHz FSL 118 124 127 132 138 (dB) Fig. The overview of line of sight (LOS).
- 2. 4. 2. ENVIRONMENT / ATMOSPHERIC LOSS The radius of the nth Fresnel Zone at its widest point can be calculated by the following formula: It is important to consider any unusual weather conditions that are (un) usual to the site location for the n.d 1d 2 radio link. These conditions can include excessive rn amounts of rain or fog, extreme temperature ranges and (d1 d 2) different adverse situations. They are discussed as: Where d is the link distance in km., is wavelength in 4.2.1RAIN AND FOG meter and r is in meter and n is an integer For example, suppose there is a 2.4 GHz link 5 Attenuation (weakening of the signal strength) due miles (8.35 km) in length. The resulting Fresnel Zone to rain does not require serious consideration for would have a radius of 31.25 feet (9.52 meters). frequencies up to the range of 6 or 8 GHz. When frequencies are at 11 or 12 GHz or above, attenuation due to rain becomes much more of a concern, especially in areas where rainfall is of high density and long duration. If this is the case, shorter paths may be required. In most cases, the effects of fog are considered much the same as rain. However, fog can adversely affect the radio link when it is accompanied by atmospheric conditions such as temperature inversion, or very still air accompanied by stratification. 4.2.2 ATMOSPHERIC ABSORPTION Fig. The path profile which may change over time due to A relatively small effect on the link is from oxygen vegetation, building constructions, etc and water vapor. It is usually significant only on longer paths and particular frequencies. Attenuation in the 2 to 6. FADING 14 GHz frequency ranges, which is approximately 0.01 dB/mile, is not very significant. Fading is the most important cause of distortion that detracts the RF signal being transmitted. Fading occurs 5. FRESNEL ZONE on strong signals and weak signals. Increasing the transmitter power does little to improve the distortion The area that the signal spreads out into space is caused by fading. An analysis of the different causes of called the Fresnel Zone. An obstacle in the Fresnel zone fading is presented this month along with some ideas on diffracts or bends part of the radio signal away from the measures broadcasters and listeners can take to reduce straight-line path. On a point-to-point radio link, this the effects of fading. refraction reduces the amount of RF energy reaching the receiving antenna. A consideration when planning or 6.1. TWO WAYS TO FADE troubleshooting an RF link is the Fresnel Zone. The Fresnel Zone occupies a series of concentric ellipsoid shaped areas around the LOS path, as can be seen in the Fading occurs in two deferent ways: frequency-flat figure. The Fresnel Zone is important to the integrity of fading and frequency-selective fading. Flat fading is seen the RF link because it defines an area around the LOS when the received signal spectrum remains a close that can introduce RF signal interference if blocked. replica of the transmitted signal spectrum except for a Objects in the Fresnel Zone such as trees, hills and change in amplitude. This amplitude change of the signal buildings can diffract or reflect the main signal away spectrum varies over space because of the interference of from the receiver, changing the RF LOS. These same the combined electromagnetic waves. This interference objects can absorb or scatter the main RF signal, causing can be constructive or destructive and, as a result, the degradation or complete signal loss. fade (changes in the received signal magnitude) due to flat fading can be very significant, 30 dB or more. The signal undergoes flat fading if Bs<<Bc and Ts >> στ. Where Ts, Bs are symbol period bandwidth of the transmitted modulation. In addition, Bc, στ are delay spread and coherence bandwidth of the channel. Frequency-selective fading occurs when the delay spread of the channel is more than about 10% of the symbol period, thereby causing the wireless channel to
- 3. alter the received signal spectrum. In the time domain, connections. Elements of a Non-Line-of-Sight solution the received symbols can no longer be identified should include: individually. They interfere with each other since they High System Gain are dispersed in time and overlap one another. This is Fading mitigation known as Inter-Symbol Interference (ISI). In the Dispersion mitigation frequency domain, the channel response can no longer These can be achieved using technologies such as: be considered “flat.” Its amplitude has significant Space Time Coding variation and its phase is not linear with frequency. The Orthogonal Frequency Division Modulation signal undergoes frequency selective fading if Bs>>Bc Adaptive Modulation and Ts << στ. Where Ts, Bs are symbol period bandwidth of the transmitted modulation. In addition, Bc, στ are delay spread and coherence bandwidth of the 6. 3.1. SPACE TIME CODING channel. Space Time Coding (STC) is a method of transmitting multiple data beams on multiple 6.2. SYSTEM OPERATING MARGIN/FADE Transmitters to multiple receivers. Basically, if any one MARGIN path is faded, there is a high probability that the other paths are not, so the signal still gets through. System Operating Margin (SOM) is the difference A simple analogy is if a single coin is tossed, there (measured in dB) between the nominal signal level is a 0.5 chance of a head. If there are four coins, there is received at one end of a radio link and the signal level a 15/16 chance of getting at least one head. required by that radio to assure that a packet of data is For STC to be effective, the paths need to be de- decoded without error. Ideally, the fade margin should correlated (e.g., the signals traveling on those paths need be more than 20 dB. Less SOM can result into unstable to behave differently from each other). This can be done link. using techniques such as spatial separation of the antennas. RX Signal = EIRP – FSL + RX Antenna Gain – Coax Cable Loss 6. 3. 2. ADAPTIVE MODULATION (AMod) Fade margin/SOM =RX Received signal – Receiver In this technique, the radio phase and amplitude Sensitivity modulation are dynamically modified according to the signal level received. Since the channel may vary in intensity. 6.3. COMBATING FADING Adaptive modulation allows the system to transmit the maximum amount of data possible by rapidly optimizing The most commonly used solution to multi-path itself to the channel conditions. The effect is to increase fading is careful site selection to provide a single, the data-rate capability. unobstructed line-of-sight path between the transmitter and receiver either directly. Where this is not feasible, 6. 3. 3. ORTHOGONAL FREQUENCY DIVISION flat fading can be compensated for by a sufficient fade MODULATION margin at the cost of limiting range and coverage. Using Orthogonal Frequency Division Modulation OFDM (Orthogonal Frequency Division Modulation) (OFDM) involves the transmission of data on multiple and can mitigate narrow-band interference. To combat frequencies. By using multiple carriers, communication frequency-selective fading, a wireless system should use is maintained should one or more carriers be affected by a signal-processing technique to remove ISI. ISI occurs either narrow-band or multi-path interference. A key where the channel is dispersive so that the received aspect of OFDM is that the individual carriers overlap to waveform suffers delay spread, causing transmitted improve spectral efficiency. Normally, overlapping symbols to overlap one another. Techniques to overcome signals would interfere with each other. However, ISI are, in general, known as channel-equalization through special signal processing, the carriers in an techniques. Equalization algorithms compensates for ISI OFDM waveform are spaced in such a manner that they created by multipath within the time dispersive channels. effectively do not see each other i.e., they are orthogonal In addition, space diversity by means of multiple to each other so that there is no cross-interference and antennas can help solve the fading problem. With hence no signal loss. adequate antenna separation, when the signal received by The key benefits of OFDM include higher spectral one antenna fades, there is a good probability that the efficiency (throughput/MHz of channel bandwidth) and signal strength at the other antenna is still sufficiently high resistance to multi-path interference and frequency- large. selective fading. A special blend of advanced techniques and OFDM has gained much attention recently. It is technologies is required to overcome fading and other used in European Digital Audio Broadcasting (DAB), interference problems in Non-Line-of-Sight wireless and in still developing IEEE 802.11 wireless LAN standard (5 GHz band). The main idea behind OFDM is
- 4. to split the data stream to be transmitted into N parallel We will give a brief summary of one, and present streams of reduced data rate and to transmit each of them theoretical and experimental results using these on a separate sub carrier orthogonally. Therefore, traditional combining techniques with OFDM in a fading spectral overlapping among subcarriers is allowed, since environment. the orthogonality will ensure that the receiver can separate the OFDM subcarriers, and a better spectral 7. BER DETERMINATION IN FADING efficiency can be achieved than by using simple CHANNELS [2] frequency division multiplexing. In an OFDM For binary PSK and a given received SNR, b, the transmitter, blocks of k incoming bits are encoded into n probability of error is channel bits. Before transmission, an n-point Inverse- FFT operation is performed. When the signals at the I- FFT output are transmitted sequentially, each of the n For a Rayleigh flat fading channel, the received SNR is channel bits appears at a different (sub carrier) frequency. The implementation aspects for OFDM is to transmit serial to parallel encoded nth channel bits. Where is the magnitude of the channel response. Since is Rayleigh distributed, and more Diversity is the powerful communication transceiver importantly, is chi-square distributed for technique used to compensate for fading channel degree freedom. Therefore the pdf of the received SNR impairments and usually implemented by using two or is, more receiving antennas. As with an equalizer, diversity improves the quality of wireless communication link without increasing the transmitted power or bandwidth. Antenna diversity is an effective way to handle Where is the average SNR. Integrating over the multipath fading channels. Its goal is to generate density of b , we obtain multiple independent versions of the same signal by using multiple antennas, usually at the receiver. Thus, even if some of the received versions are deeply faded, it is probable that not all copies are faded. By properly Which can be evaluated as combining or selecting the best diversity branches, performance can be significantly improved. The signals received from the diversity antennas must be combined to form a decision variable. There are Producing, three traditional combining methods – maximal ratio combining (MRC), equal gain combining (EGC), and selection combining (SC). When each branch has the same average SNR, maximal ratio combining is the Optimal combining technique [2]. In general, will depend on the number of diversity branches and the combining methods used. 1 1 However, once is found, the same method can be G1 M applied for any number of diversity branches and any combining method. Likewise, if a different modulation 2 2 Cophase Detector And . G2 method is used, then must be altered accordingly. Sum . For example, if binary DPSK is used instead, then m m Gm and the probability of error for binary DPSK in a flat Antenna fading channel is: Control Adaptive Fig. Block diagram of maximal ration combiner.[3] In the following sections, we will extend these This type of diversity technique takes the signals methods to antenna diversity for OFDM using MRC from all of the m branches are weighted according to combining techniques. We will also use simulation to their individual SNR and then summed. But the obtain performance results. The frequency spectrum at individual signals must be co-phased before being the output of the channel, Y(f)s related to the frequency summed. Hence, this produces an output SNR equal to spectrum at the input, X(f), by the sum of the individual SNRs The general technique for determining the probability of error (P e) which will be used to generate OFDM for MRC in frequency selective fading channels.
- 5. Where H(f) is the channel frequency response and 0.0001 BER. L = 4 produces a 8 dB gain over no No is the noise, The ideal equalizer works by finding diversity at 0.01 BER, and a 21 dB gain at 0.0001 BER. H(f) by dividing Y(f) by X(f) before noise is added in But adding additional diversity branches offers the discrete channel model. The frequency selective incrementally smaller gains, For instance, fading channel is modeled by a FIR filter whose length is determined by the delay spread. . 8. MAXIMAL RATIO COMBINING [2] The maximal ratio combining (MRC) is optimal when the average signal-to-noise ratio (SNR) is the same for all branches. In MRC, the received signals are co- phased, weighted by their respective magnitudes, and then summed. For example, if the transmitted sequence x[n] is sent through L independent flat fading diversity branches, the received sequence for the ith branch is Where is the complex gain for the ith branch of the channel. In the case of flat Rayleigh fading, is a Fig1 Bit Error vs Diversity Gain Rayleigh random variable and θi is uniformly At 0.01 BER, there is a 5.5 dB gain from L = 1 to L distributed. In the maximal ratio combiner, the received = 2, a 2.5 dB gain from L = 2 to L = 4, but only a 1 dB gain from L = 4 to L = 8. signal from the ith branch is multiplied by ,and then the product terms from all L branches are summed. The decision variable U for the nth bit is thus Using MRC, γb is just the sum of the L channel signal-to-noise ratios, γc, which are Rayleigh distributed for a Rayleigh fading channel, i.e. Then, is Fig.2 Diversity Gain vs BER and probability of error is found by integrating There is a closed form solution to the integral is: DISCUSSION AND CONCLUSION In this paper, we examined the performance of antenna diversity for OFDM in both flat and frequency selective --(x) fading channels. More specifically, we examined the diversity combining techniques (MRC). For OFDM, Where . diversity combining can be done on individual Fig.1 shows a comparison between the simulation subcarriers (narrowband combining). results for this system and the analytical results from equation (x) for L = 2. As seen in the figure, the REFERENCES response of the bit error showing the two curves are [1] www.smartbridges.com virtually identical. Thus, we are justified in using the flat [2]W. Jakes. Microwave Mobile Communications. fading expressions for OFDM even on frequency Wiley and Sons, 1974. selective channels. [3] T.S. Rappaport. Wireless communication, 2nd Fig. 2 shows the diversity gain (in terms of Eb/No) edition, PHI. as a function of BER and the diversity order L. From the figure, the diversity gain at high BER is fairly small but, at lower BER, there is a much greater gain from diversity. For example, L = 2 offers a 5.5 dB gain over L = 1 (no diversity) at 0.01 BER, and a 14.8 dB gain at