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 confrontingradio frequency (RF) propagation in non-line-of-sight For VHF, UHF and higher frequency. Attaining goodwireless connections. Wireless signals propagating through Line of Sight (LOS) between the sending and receivingthe air lose strength while encountering natural and antenna is essential.manmade obstacles. It would be nice if RF signals wouldpropagate without any bounds, but that simply doesnt 3. THE NON-LINE-OF-SIGHT CHALLENGEoccur. The problem might be different kinds of losses thatare:- free space loss, fading loss, equipments loss etc. It is The nature of a Non-Line-of-Sight link is thatseen that there are different mitigation technologies for there are obstacles such as buildings, vehicles, trees andnon-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 suchMitigation, Multi-path Compensation and Fading environments, multiple paths do exist betweenMitigation. 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 ofOrthogonal 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 andsecure, high-availability wireless connection. The paper disperse over time, causing self-interference. To makemainly focuses on the OFDM in frequency flat and things worse, as the environment changes due toselective 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 asecured, high-speed digital wireless communications. factor of up to -40 dB for periods of seconds, minutes orBesides the cellular phone, there are wireless modems, even days in some cases. The remainder of this paperhigh definition television (HDTV), and digital radios. looks at the techniques and technologies available toPerformance of these devices in a wireless environment overcome this significant obstacle.can be severely limited by random fluctuations inamplitude of the received signal called fading. To solve 4. 1. FREE SPACE LOSSthese challenges, many innovative techniques have beendeveloped. In this paper, we focus on the combination of As signals spread out from a radiating source, thetwo of the powerful techniques, Antenna Diversity and energy is spread out over a larger surface area. As thisOFDM. 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 . 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).
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 weatherconditions that are (un) usual to the site location for the n.d 1d 2radio 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 in4.2.1RAIN 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 Zoneto 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. Whenfrequencies are at 11 or 12 GHz or above, attenuationdue to rain becomes much more of a concern, especiallyin areas where rainfall is of high density and longduration. If this is the case, shorter paths may berequired. In most cases, the effects of fog are consideredmuch the same as rain. However, fog can adverselyaffect the radio link when it is accompanied byatmospheric conditions such as temperature inversion, orvery 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, etcand water vapor. It is usually significant only on longerpaths and particular frequencies. Attenuation in the 2 to 6. FADING14 GHz frequency ranges, which is approximately 0.01dB/mile, is not very significant. Fading is the most important cause of distortion that detracts the RF signal being transmitted. Fading occurs5. 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 ofcalled the Fresnel Zone. An obstacle in the Fresnel zone fading is presented this month along with some ideas ondiffracts or bends part of the radio signal away from the measures broadcasters and listeners can take to reducestraight-line path. On a point-to-point radio link, this the effects of fading.refraction reduces the amount of RF energy reaching thereceiving antenna. A consideration when planning or 6.1. TWO WAYS TO FADEtroubleshooting an RF link is the Fresnel Zone. TheFresnel Zone occupies a series of concentric ellipsoidshaped areas around the LOS path, as can be seen in the Fading occurs in two deferent ways: frequency-flatfigure. The Fresnel Zone is important to the integrity of fading and frequency-selective fading. Flat fading is seenthe RF link because it defines an area around the LOS when the received signal spectrum remains a closethat can introduce RF signal interference if blocked. replica of the transmitted signal spectrum except for aObjects in the Fresnel Zone such as trees, hills and change in amplitude. This amplitude change of the signalbuildings can diffract or reflect the main signal away spectrum varies over space because of the interference offrom the receiver, changing the RF LOS. These same the combined electromagnetic waves. This interferenceobjects can absorb or scatter the main RF signal, causing can be constructive or destructive and, as a result, thedegradation 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
alter the received signal spectrum. In the time domain, connections. Elements of a Non-Line-of-Sight solutionthe received symbols can no longer be identified should include:individually. They interfere with each other since they High System Gainare dispersed in time and overlap one another. This is Fading mitigationknown as Inter-Symbol Interference (ISI). In the Dispersion mitigationfrequency domain, the channel response can no longer These can be achieved using technologies such as:be considered “flat.” Its amplitude has significant Space Time Codingvariation and its phase is not linear with frequency. The Orthogonal Frequency Division Modulationsignal undergoes frequency selective fading if Bs>>Bc Adaptive Modulationand Ts << στ. Where Ts, Bs are symbol periodbandwidth of the transmitted modulation. In addition,Bc, στ are delay spread and coherence bandwidth of the 6. 3.1. SPACE TIME CODINGchannel. Space Time Coding (STC) is a method of transmitting multiple data beams on multiple6.2. SYSTEM OPERATING MARGIN/FADE Transmitters to multiple receivers. Basically, if any oneMARGIN 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 isreceived 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 needbe more than 20 dB. Less SOM can result into unstable to behave differently from each other). This can be donelink. using techniques such as spatial separation of the antennas.RX Signal = EIRP – FSL + RX Antenna Gain – CoaxCable Loss 6. 3. 2. ADAPTIVE MODULATION (AMod)Fade margin/SOM =RX Received signal – Receiver In this technique, the radio phase and amplitudeSensitivity 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 increasefading is careful site selection to provide a single, the data-rate capability.unobstructed line-of-sight path between the transmitterand receiver either directly. Where this is not feasible, 6. 3. 3. ORTHOGONAL FREQUENCY DIVISIONflat fading can be compensated for by a sufficient fade MODULATIONmargin at the cost of limiting range and coverage. Using Orthogonal Frequency Division ModulationOFDM (Orthogonal Frequency Division Modulation) (OFDM) involves the transmission of data on multipleand can mitigate narrow-band interference. To combat frequencies. By using multiple carriers, communicationfrequency-selective fading, a wireless system should use is maintained should one or more carriers be affected bya signal-processing technique to remove ISI. ISI occurs either narrow-band or multi-path interference. A keywhere the channel is dispersive so that the received aspect of OFDM is that the individual carriers overlap towaveform suffers delay spread, causing transmitted improve spectral efficiency. Normally, overlappingsymbols 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 antechniques. Equalization algorithms compensates for ISI OFDM waveform are spaced in such a manner that theycreated 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 andantennas 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 spectralone antenna fades, there is a good probability that the efficiency (throughput/MHz of channel bandwidth) andsignal 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 istechnologies 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
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  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 . 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 flatAntenna fading channel is: Control Adaptive Fig. Block diagram of maximal ration combiner. 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.
Where H(f) is the channel frequency response and 0.0001 BER. L = 4 produces a 8 dB gain over noNo 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 offersthe discrete channel model. The frequency selective incrementally smaller gains, For instance,fading channel is modeled by a FIR filter whose length isdetermined by the delay spread. .8. MAXIMAL RATIO COMBINING  The maximal ratio combining (MRC) is optimalwhen the average signal-to-noise ratio (SNR) is the samefor all branches. In MRC, the received signals are co-phased, weighted by their respective magnitudes, andthen summed. For example, if the transmitted sequencex[n] is sent through L independent flat fading diversitybranches, the received sequence for the ith branch is Where is the complex gain for the ith branchof the channel. In the case of flat Rayleigh fading, is a Fig1 Bit Error vs Diversity GainRayleigh random variable and θi is uniformly At 0.01 BER, there is a 5.5 dB gain from L = 1 to Ldistributed. 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 ,andthen 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 channelsignal-to-noise ratios, γc, which are Rayleigh distributedfor 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 fromequation (x) for L = 2. As seen in the figure, the REFERENCESresponse of the bit error showing the two curves are  www.smartbridges.comvirtually identical. Thus, we are justified in using the flat W. Jakes. Microwave Mobile Communications.fading expressions for OFDM even on frequency Wiley and Sons, 1974.selective channels.  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 thefigure, the diversity gain at high BER is fairly small but,at lower BER, there is a much greater gain fromdiversity. 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