Diversity techniques for wireless communication


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Diversity techniques for wireless communication

  1. 1. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME144DIVERSITY TECHNIQUES FOR WIRELESS COMMUNICATIONPravin W. Raut1, Dr. S.L. Badjate2L 1Research Scholar, HOD ET Dept, Shri Datta Meghe Polytechnic, NagpurL 2Vice Principal & Professor, S. B. Jain Institute of Technology, Management. & Research,NagpurABSTRACTThe users of the wireless communication demands for higher data rates, good voicequality and higher network capacity restricted due to limited availability of radio frequencyspectrum, Bandwidth, Channel Capacity, physical areas and transmission problems caused byvarious factors like fading and multipath distortion.The wireless communication channel suffers from much impairment which leadsdegradation of the overall system performance. There are many performance degradationfactors in wireless communication channels but FADING problem is the major impairmentproblem.This paper addresses the Diversity techniques which are the useful methods to reducefading problem in wireless communications. In diversity technique, the receiver is suppliedmultiple replicas of transmitting signals instead of one signal that have passed over differentfading channels. As a result, the probability that all replicas of signals will fadesimultaneously is reduced considerably. The uncorrelated faded signals are collected fromdiversity branches and combine in such manner that can improve the performance ofcommunication systems, called as diversity combining method to increase overall receivedSNR. The best diversity techniques can be selected by analyzing and comparing the differenttypes of diversity techniques. Moreover, different diversity techniques can be combined andused in wireless communication systems to get the best result to mitigate fading problems.Keywords: Signal to Noise Ratio (SNR), Transmitter, Receiver, Fading, Diversity, multipleAntenna, MIMO, Channel estimation.INTERNATIONAL JOURNAL OF ADVANCED RESEARCH INENGINEERING AND TECHNOLOGY (IJARET)ISSN 0976 - 6480 (Print)ISSN 0976 - 6499 (Online)Volume 4, Issue 2 March – April 2013, pp. 144-160© IAEME: www.iaeme.com/ijaret.aspJournal Impact Factor (2013): 5.8376 (Calculated by GISI)www.jifactor.comIJARET© I A E M E
  2. 2. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME145I. INTRODUCTIONThere are two types of communication systems such as wired communication andwireless communication. The wired communication almost out of market and / or limiteduse due to its various limitations. Wireless communication is divided into mobilecommunications and fixed wireless communications. Each type of communication hashuge demand according to customers need in the market. Wireless data transmissiongives us every opportunity to get all feasible necessary access to the world wherever weare and wherever we need from. The exceptional growth of the telecommunicationindustry in recent years fueled by the widespread popularity of mobile phones, wirelesscomputer networking and continues to expand everyday with new technology invention.This growth of wireless communication is restricted due to the limitations ofavailable frequency resources, bandwidth, channel capacity, complexity, reliability,transmission data rate, physical areas and communication channel between transmitterand receiver.The transmission data rates can be increase by increasing the transmissionbandwidth or using higher transmitter power. But, from a practical point of view,increasing the transmission bandwidth or the transmitter power is not always feasible dueto high system deployment costs. Moreover, increasing the transmit power results in anincreased interference to other transmissions and also reduces the battery life-time ofmobile transmitters.The uncertainty or randomness is the main characteristic of wirelesscommunication which appears in user’s transmission channel and user’s location.Wireless communication channels suffer from various factors but FADING problem isthe major impairment problem which leads the degradation of overall systemperformance.Fading means the loss of propagation experienced by the radio signal on forwardand reverse links. The signals which is received by mobile terminals come from severalpropagation paths those are called multi-path propagation.To improve the performance of those fading channels, diversity techniques areused. In diversity technique, communication channel is supplied with multipleTransmitting and Receiving antennas. The signal is transmitted and received throughmultiple paths. As a result, the probability that all replicas of signals will fadesimultaneously is reduced considerably.For getting full benefit, uncorrelated faded signals are collected from diversitybranches and combine in such manner that can improve the performance ofcommunication systems. This is called diversity combining method and also used tooptimize received signal power or signal-to-noise ratio (SNR). This combining methodcan be used in receiver mainly.The diversity techniques are classified under three domains namely, Temporaldiversity, frequency diversity and spatial diversity. Among these, due to its efficiency interms of system resource usage (no extra power and bandwidth utilization necessary)spatial diversity with multiple transmitting and receiving antennas are most popular.Spatial diversity provides a more robust and reliable communication, and by introducingadditional degrees of freedom to the system, it provides a higher system capacity withoutrequiring any additional power or bandwidth.
  3. 3. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME146II. PERFORMANCE DEGRADATION FACTORS IN WIRELESS COMMUNICATIONFollowing are the major performance degradation factors of wireless communication.A. Characteristics of Wireless Communication ChannelThe main characteristic of wireless communication is Uncertainty or randomnesswhich is of two types: randomness in user’s transmission channels and randomness in user’sgeographical locations causes random signal attenuation.The behavior of the wireless communication channel is a function of the electromagnetic(Radio) waves propagation effect in an environment. The information suffers attenuationeffects by several reasons which are called fading of radio waves. These attenuation effectscan vary with time and variations depends on user mobility, which makes wireless channel achallenging medium of communication.Also it is impossible to get proper Line Of Sight (LOS) or proper communicating nodesbecause of some natural or constructive obstacles to establish a wireless communication link.To overcome this problem, a transmission path is modeled randomly which has varyingpropagation path in such an environment in which signal propagation over multipath made usneed to model wireless channels as wireless multi-path channels.B. Factors Affecting the Wireless Communication ChannelWireless communication system sends the signal information through radiopropagation environment. The different copies of signal undergo different attenuation,distortion, phase shift and delays during transmission.The following impairments are responsible to the suffering of radio wave propagation andhence the overall performance of wireless communication system.Path loss Shadowing loss NoiseInterference Channel spreading Channel fading(a) Path loss: The loss of power on the way of radio wave propagation in space is called asPath Loss which attenuates the signal and depends on the distance between thecommunicating nodes of the system and hence limits the coverage of a transmitter. In anyreal channel, signals attenuate as they propagate. The path loss is given bywhere ‘λλλλ’ is the wavelength of the signal and ‘d’ is the distance between the source andthe receiver (Communicating nodes). The power of the signal decays as the square of thedistance.In land mobile wireless communication environments, similar situations are observed.The mean power of a signal decays as the nthpower of the distance : L = c dnwhere ‘c’ is a constant and the exponent ‘n’ typically ranges from 2 to 5. The exactvalues of c and n depend on the particular environment.
  4. 4. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME147(b) Shadowing loss: The loss due to the presence of large-scale obstacles in the propagationpath of the radio signal is called as Shadowing loss. Due to the relatively large obstacles,movements of the mobile units do not affect the short term characteristics of the shadowingeffect. Instead, the nature of the terrain surrounding the base station and the mobile units aswell as the antenna heights determines the shadowing behavior.Usually, shadowing is modeled as a slowly time-varying multiplicative random process.Neglecting all other channel impairments, the received signal r(t) ) is given by:r(t)= g(t) s(t)where s(t) is the transmitted signal and g(t) is the random process which models theshadowing effect.For a given observation interval, we assume g(t) is a constant g, which is usuallymodeled as a lognormal random variable whose density function is given byNotice that ln g is a Gaussian random variable with mean µ and variance σ2. Thistranslates to the physical interpretation that µ and σ2are the mean and variance of the lossmeasured in decibels due to shadowing.(c) Noise : In radio wave propagation there are two types of noise, Natural noise and man-made noise. The Main source of natural noise is the ignition systems of vehicles. However,natural noise source such as galactic noise, solar, atmospheric noise, has less effect in land-mobile communication systems but lots in radio channel. The noise generated in thecomponents in the communication systems also corrupt the received signals besides thenatural and man-made noise.The characteristics of different noise sources are simply different but the noise processmost commonly and frequently modeled with Additive White Gaussian Noise (AWGN).(d) Interference:In radio system the source of interference can be originated at the system itself or locatedat external source. The self-centered interference can be divided into two types: inter-cell andintra-cell interface. The inter-cell interference comes from other mobile stations and itnormally approximately 60% of total interference in a radio system. In the uplink case, theintra-cell interface comes from other mobile station cells.There are two types of interference, such as: Inter-symbol-interference (ISI) and Co-channel-interference (CCI)Inter-Symbol-Interference (ISI):When transmit, a small part of symbol overlap on the next symbol. It spreads more thanits normal extension and smeared into the next symbol and make the noise called as Intersymbol interference (ISI) which is an unavoidable consequence introduced by the delayspread in a propagating channel as shown in fig-1 (a) and 1 (b).The original time of pulse is called is called Symbol time shown in fig 1-(a). ISI can becontrolled by slowing down the transmit data and also by using a maximum likelihoodsequence detector.
  5. 5. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME148Fig.1 (a)- The Transmitter symbolsFig.1 (b)- The Received symbolsThe next information pulse should send when the received signal damps down. The timeit takes to damp is called delay spread. To suppressed ISI, one can select a space-time filterthat equalizes the channel.Co-Channel Interference (CCI):When the neighboring cells operate at the same frequencies then CCI occurs. Thisscenario is shown in following fig 2. Some other factors responsible for CCI are insufficientcross-polarization isolation, non-linearity of power amplifier, poor radiation from antennaside lobs, faulty filtering etc.The CCI can be suppressed by using an orthogonal space time filter to the interferencechannels. Another method to solve this problem is the estimation of Time of Arrival (TOA)which uses the synchronized time difference between original signal and the CCI signal. Anun-synchronized base station is used to suppressed the CCI signal and the signal without CCIis received which has good voice quality and data quality.Fig.2: Co-channel Interference scenario for neighboring base station
  6. 6. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME149(e) Channel Spreading:When information carrying signal energy spreads in space or concerning time orfrequency axis then such spreading of the signal is called as Channel Spreading.The reason of spreading of signal energy in the space dimension, time or in the frequencyaxis is due to Propagation of a radio wave from a user or to a user in multipath channel. Thedesign of receiver is affected by the characteristics of channel spreading.There are two types of channel spreading such as : Doppler spread and Delay spreadDoppler Spread:Due to movement of the communicating device or other components in a multipathpropagation environment the information carrying radio signal experience a shift infrequency domain. The shift in frequency domain is also called Doppler shift. The amplitudeof the Doppler shift is depends on the path direction of the signal arrival. Doppler shift ismuch smaller than the carrier frequency and it is bounded to positive and negative amplitude.For example, it can be seen that component waves arriving from ahead of the vehicleexperience a positive Doppler shift, while those arriving from behind the vehicle have anegative Doppler shift .The result of Doppler spread in the channel characteristics is that it change the channelcharacteristics sharply in time, it gives rise to the so called time selectivity. The fadingchannel can be considered as constant during the coherent time and the coherent time isinversely proportional to the Doppler spread.Delay Spread:Sometimes, multipath propagation is characterized by different version of transmittedsignal arriving at the receiver attenuation factors and delays. When spreading in time domainthen it called delay spread and this is responsible for the selectivity of the channel infrequency domain. The coherence bandwidth is inversely proportional to the delay spread.The significant delay spread causes strong inter-symbol interference.(f) Channel Fading :Fading in a channel is the propagation losses by radio signal on both forward and reverselinks. This impairment is a major problem of wireless communication channel. Fadingintroduce for the combined effect of multiple propagation paths, high speed of mobile unitsand reflectors.In a typical wireless communication environment, multiple propagation paths often existfrom a transmitter to a receiver due to scattering by different objects. Signal copies followingdifferent paths can undergo different attenuation, distortions, delays and phase shifts.Constructive and destructive interference can occur at the receiver. When destructiveinterference occurs, the signal power can be significantly diminished. This phenomenon iscalled fading. The performance of a system (in terms of probability of error) can be severelydegraded by fading. The result is a time-varying fading channel. Communication throughthese channels can be difficult. Special techniques may be required to achieve satisfactoryperformance.III. PARAMETERS OF FADING CHANNELThe performance analysis of general time varying channel for wirelesscommunication channel is too complex to understand. Many practical wireless channels can
  7. 7. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME150be adequately approximated by the Wide-Sense Stationary Uncorrelated scattering (WSSUS)model. In this model, the time varying fading is seems to be wide-sense stationary randomprocess and the signal copies from the scattering by different objects are assumed to beindependent. The following parameters are often used to characterize a WSSUS fadingchannel.1.Multi-path Spread, TmThis finds the maximum delay between paths of significant power in the channel. Whenwe send a very narrow pulse in a fading channel, the received power can be measured as afunction of time delay (fig 3.1).The average received power P(r) is called multi-path intensityprofile or delay power spectrum when is a function of excess time delay. Sometimes, P(r)is essentially non-zero over a range of values of a function of excess time delay . Then it’scalled multipath spread of the channel, Tm.Fig 3.1: Multipath delay profile2. Coherent Bandwidth, (∆f)CCoherent bandwidth in channel gives us an idea how far signals should be separated infrequency. In a fading channel, signals with different frequencies can undergo differentdegrees of fading. If two frequency signals are separated by more than coherent bandwidththen the signals may undergo different degrees of fading. The relationship between (∆f)C andTm is3. Coherent Time (∆T)CThe coherent time identify the time duration over which the channel impulse is invariantor highly correlated which is vary with time in a time varying channel. If the time duration issmaller than the coherent time then the channel considerably invariant during the reception ofsymbol.4 Doppler Spread BdA signal propagating in a channel may undergo frequency shift or Doppler shift due to itstime varying nature. If a bunch of frequency is transmitted through a channel, the receivedpower spectrum can be plotted against the Doppler shift according to figure3.2. called asDoppler power spectrum. The Doppler spread is the range of the values where the Dopplerpower spectrum is non-zero. Coherent bandwidth and Doppler spread is related by thefollowing equation:
  8. 8. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME151Fig 3.2 : Doppler Power Spectrum5. Mean path lossThe mean path loss describes the attenuation of a radio wave in a free space propagationenvironment due to isotropic power spreading which is given by the inverse square law,Where Pr and Pt are received and transmitted powers, λ is wavelength of the radio wave,d is the range, Gt.Gr are gains of transmitter and receiver respectively.The main path is accompanied sometimes by a surface reflected path which maydestructively interfere with the primary path. There is a model called path loss model isdeveloped to handle this effect.Where ht and hr are the effective heights of transmitter and receiver respectively. Thispath loss model built in according to an inverse fourth power law and the path loss exponentmay vary from 2.5 to 5 depending on the environment.IV. CLASSIFICATION OF FADING CHANNELBased on the parameters of the channels and the characteristics of the signals to betransmitted, time varying fading channels can be classified as:Frequency non-selective versus frequency selectiveWhile on comparing with Coherence Bandwidth (∆ƒ)c, all frequency components of thesignal would undergo fading of almost same amount if it is found that the transmittedbandwidth of the signal is small. The channel is then classified as frequency non-selectivewhich is also called flat fading.On the other hand, if the transmitted bandwidth of the signal is large in comparison toCoherence Bandwidth (∆ƒ)c, then different frequency components of the signal (that differby more than Coherence Bandwidth (∆ƒ)c) would undergo fading of different degrees. Thechannel is then classified as frequency selective.Slow fading versus fast fadingIf the symbol duration is small compared with Coherence Time (∆t)c, then the channel isclassified as slow fading. Slow fading channels are very often modeled as time-invariantchannels over a number of symbol intervals. The channel parameter of these varying, may beestimated with different estimation techniques.On the other hand, if (∆t)c is close to or smaller than the symbol duration, the channel isconsidered to be fast fading (also known as time selective fading). In general, it is difficult to
  9. 9. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME152estimate the channel parameters in a fast fading channel. The fig4.1 and 4.2 shows thecomparison of slow and fast fading channel.The above classification of a fading channel depends on the properties of the transmittedsignal. The above two ways of classification give rise to following four different types offading channel:Frequency non-selective slow fadingFrequency selective slow fadingFrequency non-selective fast fadingFrequency selective fast fadingFig 4.1: Slow fading Vs fast fadingFig 4.2: Slow fading Vs fast fadingV. DIVERSITY TECHNIQUESDiversity technique is used to decreased the fading effect and improve systemperformance in fading channels. Instead of transmitting and receiving the desired signalthrough one channel, we obtain L copies of the desired signal through M different channels.The idea is that while some copies may undergo deep fades, others may not. We might stillbe able to obtain enough energy to make the correct decision on the transmitted symbol.Following are the types of diversity techniques.
  10. 10. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME153Multipath/frequency diversityTemporal/time diversitySpatial/space diversityPolarization diversityAngle diversityAntenna diversityFrequency / Multipath diversity:The diversity can be achieved by modulating information signal through L differentcarrier frequencies (refer fig 5.1). Each carrier should be separated from the others by at leastthe coherence bandwidth (∆f)c so that different copies of the signal undergo independentfading.This L independently faded copies are “optimally” combined at the receiver to construct theoriginal signal. The optimal combiner is the maximum ratio combiner, which will beintroduced later. Frequency diversity can be used to combat frequency selective fading.Fig.5.1 : frequency DiversityTime / Temporal diversity:Another approach to achieve diversity is to transmit the desired signal in L differentperiods of time, i.e., each symbol is transmitted L times (refer fig 5.2). The intervals betweentransmission of the same symbol should be at least the coherence time (∆T)c. so that differentcopies of the same symbol undergo independent fading. Optimal combining can also beobtained with the maximum ratio combiner. Notice that sending the same symbol L times isapplying the (L,1) repetition code. Actually, non-trivial coding can also be used. Error controlcoding, together with interleaving, can be an effective way to combat time selective (fast)fading.Fig.5.2 : Time Diversity
  11. 11. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME154Spatial / Space diversity:This diversity techniques uses multiple antennas at the transmitting and Reception side(refer fig 5.3). So L antennas to receive L copies of the transmitted signal. The antennasshould be spaced far enough apart so that different received copies of the signal undergoindependent fading. This is Different from frequency diversity and temporal diversity, noadditional work is required on the transmission end and no additional bandwidth ortransmission time is required. However, physical constraints may limit its applications.Spatial diversity can be employed to combat both frequency selective fading and timeselective fading. The spatial diversity increased the SNR.Fig.5.3: Spatial / Space DiversityPolarization Diversity:In polarization diversity, the electric and magnetic fields of the signal carrying theinformation are modified and many such signals are used to send the same information. Thusorthogonal type of Polarization is obtained.Angle Diversity/ Pattern Diversity / Direction Diversity:This diversity system needs a number of directional antennas those respondsindependently to wave propagation. An antenna response to a wave propagates at a specificangle and receives a faded signal which is uncorrelated with other signals.The procedure to obtain angle diversity is to fix antennas with narrow beam widthsdifferent sector in the system. Then the arriving multi-paths from the different beamdirections are resolved and combined advantageously. This procedure not only createsdiversity but also increases the antenna gain and reduces interference by providing angulardiscrimination.Antenna Diversity:Antenna diversity is a popular and extensively used technique to improve performance inwireless communication systems. The technique reduces fast fading and inter-channelinterference effects in the wireless network system. In an antenna diversity system, two ormore antennas are used and fixed in positions which will provide uncorrelated signals withthe same power level. Then the signals are combined and created an improved signal. Thebasic method of antenna diversity is that the antennas experiences different kind of signalsbecause of individual channel conditions and the signals are correlated partially. Then we canexpect that if one signal from one antenna is highly faded, other signals from other antennasare not faded such way and these signals are our expected quality signals.
  12. 12. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME155VI. DIVERSITY COMBINING TECHNIQUESIt is important to combine the uncorrelated faded signals which were obtained fromthe diversity branches to get proper diversity benefit. The combing system should be in sucha manner that improves the performance of the communication system like the signal-to-noise ratio (SNR) or the power of received signal. Mainly, the combining should be appliedin reception; however it is also possible to apply in transmission. Following are the variousdiversity combining methods available, out of these MRC,EGC and SC are mainly used.Maximal ratio combining (MRC)Equal gain combining (EGC)Selection combining (SC)Switched Combining (SWC)Periodic Switching MethodPhase Sweeping MethodMaximal Ratio Combining (MRC):Fig 6.1 : Maximal-Ratio Combining (MRC)In the MRC combining technique needs summing circuits, weighting and co-phasing.The signals from different diversity branches are co-phased and weighted before summing orcombining. The weights have to be chosen as proportional to the respective signals level formaximizing the combined carrier-to-noise ratio (CNR). The applied weighting to thediversity branches has to be adjusted according to the SNR. For maximizing the SNR andminimizing the probability of error at the output combiner, signal of dthdiversity branch isweighted before making sum with others by a factor, cd*/σ2n,d . Here σ2n,d is noise variance ofdiversity branch dthand cd*complex conjugate of channel gain.As a result the phase-shifts are compensated in the diversity channels and the signalscoming from strong diversity branches which has low level noise are weighted morecomparing to the signals from the weak branches with high level of noise. The term σ2n,d inweighting can be neglected conditioning that σ2n,d has equal value for all d. Then therealization of the combiner needs the estimation of gains in complex channel and it does notneed any estimation of the power of noise.This is a very useful combining process to combat channel fading. This is the bestcombining process which achieves the best performance improvement comparing to othermethods. The MRC is a commonly used combining method to improve performance in anoise limited communication systems where the AWGN and the fading are independentamongst the diversity branches.
  13. 13. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME156Equal-gain Combining (EGC):The EGC is similar to MRC with an exception to omit the weighting circuits. Theperformance improvement is little bit lower in EGC than MRC because there is a chance tocombine the signals with interference and noise, with the signals in high quality which areinterference and noise free. EGC’s normal procedure is coherently combined the individualsignal branch but it non-coherently combine some noise components according to followingfigure 6.2.MRC is the most ideal diversity combining but the scheme requires very expensivedesign at receiver circuit to adjust the gain in every branch. It needs an appropriate trackingfor the complex fading, which very difficult to achieve practically. However, by using asimple phase lock summing circuit, it is very easy to implement an equal gain combining.The EGC can employ in the reception of diversity with coherent modulation. The envelopegains of diversity channels are neglected in EGC and the diversity branches are combinedhere with equal weights but conjugate phase. The structure of equal-gain combining (EGC) isas following since there is no envelope gain estimation of the channel.Fig,6.2 Equal Gain Combining
  14. 14. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME157Selection Combining (SC):Fig,6.3- Selection CombiningThe techniques MRC and EGC are not suitable for very high frequency (VHF), ultrahigh frequency (UHF) or mobile radio applications because realization of a co-phasing circuitwith precise and stable tracking performance is not easy in a frequently changing, multipathfading and random-phase environment. SC uses simple implementation procedure and ismore suitable comparing to MRC and EGC in mobile radio application.In SC, the diversity branch which has the highest signal level has to be selected.Therefore, the main algorithm of this method is on the base of principle to select the signalamongst the all signals at the receiver end. Refer the fig 6.3, the general form of selectioncombining is to monitor all the diversity branches and select the best one (the one which hasthe highest SNR) for detection. Therefore we can say that SC is not a combining method but aselection procedure at the available diversity. However, measuring SNR is quite difficultbecause the system has to select it in a very short time. But selecting the branch with thehighest SNR is similar to select the branch with highest received power when average powerof noise is the same on each branch. Therefore, it is practical to select the branch which hasthe largest signal composition, noise and interference.It is experimentally proved that the performance improvement achieved by theselection combining is just little lower than performance improved achieved by an ideal MRCrefer to fig 6.4,As a result the SC is the most used diversity technique in wirelesscommunication. If there is an availability of feedback information about the channel state ofthe diversity branch the selection combining also can be used in transmission.Fig 6.4 : SC and MRC comparison
  15. 15. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME158Switched Combining (SWC):It is impractical to monitor the all diversity branches in selection combining. Inaddition, if we want to monitor the signals continuously then we need the same number ofreceivers and branches. Therefore, the form of switched combining is used to implementselection combining. According to the figure 6.5 (a), switching from branch to branch occurswhen the signal level falls under threshold.The value of threshold is fixed under a small area but the value is not the bestnecessarily over the total service area. As a result the threshold needs to be set frequentlyaccording to the movement of vehicle fig 6.5 (b). It is very important to determine theoptimal switching threshold in SWC. If the value of threshold is very high, then the rate ofundesirable switching transient increases. However, if the threshold is very low then thediversity gain is also very low. The switching of switch combining can be performedperiodically in the case of frequency hopping systems.Fig. 6.5(a): Switching combining methods with fixed threshold (a) and variable threshold (b)Periodic Switching Method:In a simple switching method, the diversity branches are selected periodically by aconventional, free-running oscillator. This procedure is useful in comparably largedeviational and low-speed frequency modulation systems which includes phase transientscreates by switching can be diminished. The only selectable parameter switching rate can bechosen as twice the height of the bit rate of signal. As a result the signal of the better branchcan select per signaling period. However, performance improvement may reduce in adjust-channel area because this channel spectrum may be folded into desired channel band byperiodic switching in the pre-detection radio frequency stage (refer fig 6.6). So we can see anoverlap here which can be solved by rising selectivity of the adjust-channel at the receiver.
  16. 16. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME159Fig. 6.6: Periodic switching methodPhase Sweeping Method:Phase sweeping method is another version of switching method which uses a singlereceiver. In phase sweeping method sweeping rate is more than twice the highest frequencyof modulation signal. But we can gain the same diversity improvement which we achieve bythe periodic switching method. The phase sweeping method is as like mode-averagingmethod where spaced antennas are used with electrically scanned directional patterns (referfig 6.7)..Fig. 6.7: Phase Sweeping methodVI. CONCLUSIONThe diversity techniques is used to provide the receiver with several replicas of thetransmitted signal, used to overcome the fading problem and to improve the performance ofthe radio channel without increasing the transmitted power and improves the SNR. Amongthe various diversity techniques spatial diversity is best suitable for the wirelesscommunication. Multi-input-multi-output (MIMO) wireless communication uses spatialdiversity techniques.Among the various Diversity Combining methods: MRC outperforms the SelectionCombining; Equal gain combining (EGC) performs very close to the MRC. Unlike the MRC,the estimate of the channel gain is not required in EGC. Among different combiningtechniques MRC has the best performance and the highest complexity, SC has the lowestperformance and the least complexity. Alamouti suggested new transmit diversity techniquesto provide the same diversity order as that of MRC by using two transmit antenna and onereceive antenna.Thus by employing multiple transmit and receive antenna the diversity can beachieved to improve the performance of radio wireless communication channel.
  17. 17. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME160REFERENCES[1] Siavash M. Alamouti. A Simple Transmit Diversity Technique for Wireless Communications.IEEE Journal on Select Areas in Communcations, 16(8):1451–1458,October 1998.[2] MIMO: The next revolution in wireless data communications By Babak Daneshrad[3] Bit Error Rate Performance of MIMO Channels for various Modulation Schemes using MaximumLikelihood Detection Technique, IP Multimedia Communications A Special Issue from IJCA[4] Maximum-Likelihood Equalisation for STBC-MIMO on Test-Bed ,Ashu Taneja AssistantProfessor,Department of Electronics & Communication Engineering Chitkara University, Baddi.International Journal of Computer App (0975 – 8887) Volume 46– No.22, May 2012[5] G. J. Foschini and M. J. Gans, “On limits of wireless communications in a fading environmentwhen using multiple antennas,” Wireless Pers. Commun., vol. 6, pp. 311–335, Mar. 1998.[6] G. J. Foschini, “Layered space–time architecture for wireless communication in a fadingenvironment when using multielement antennas,” Bell Labs Tech. J., pp. 41–59, Autumn 1996.[7] D. Gesbert, M. Shafi, D.-S. Smith, and Naguib, From theory to practice: an overview of MIMOspace-time coded wireless systems,” IEEE Sel. Areas in Comm.Journal, vol. 21, 3, pp.281–302,Apr.2003.[8] Diversity in Spread Spectrum-Tan F.Wong: Spread Spectrum cdma[9] Transmit Diversity vs. Spatial Multiplexing in Modern MIMO Systems -Angel Lozano, NiharJindal, IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 9, NO. 1, JANUARY2010[10] An Overview of Diversity Techniques in Wireless Communication Systems, Hafeth Hourani,Helsinki Univ. of Techn. Communications Lab[11] Physical Diversity and Virtual Diversity for Wireless Comm. Networks-A ReviewAli and SalehA. Alshebieli,(PSATRI), Saudi Arabia,[12] C. N. Chuah, Tse, J M.Kahn, and R.Valenzuela, “Capacity scaling in MIMO wireless systemsunder correlated fading,” IEEE Trans. Inform. Theory, vol. 48, pp. 637–650, Mar. 2002.[13] L. Hanlen and M. Fu, “Multiple antenna wireless communication systems: Capacity limits forsparse scattering,” in Proc. 3rd Australian Communication Theory Workshop, Aus CTW 2002,[14] Jack Winters "Optimum Combining in Digital Mobile Radio with Cochannel Interference,"Special Issue on Mobile Radio Communications IEEE Journal on Selected Areas in Communications,July 1984,[15] Raleigh, G. G. and Jones, V. K. "Multivariate modulation and coding for wirelesscommunication", IEEE J. Selected Areas in Communication, vol. 17, no. 5, pp. 851-866, May 1999[16] Gerard. J. Foschini "Layered Space-Time Architecture for Wireless Communication in a FadingEnvironment When Using Multi-Element Antennas". Bell Laboratories Technical Journal, pp: 41-59.October 1996[17] L. Zheng and D. N. C. Tse (May 2003). "Diversity and multiplexing: A fundamental tradeoff inmultiple-antenna channels". IEEE Trans. Inf. Th.49 (5): 1073–1096.doi:10.1109/TIT.2003.810646.[18] Sudha.P.N and Dr U.Eranna, “Source and Adaptive Channel Coding Techniques for WirelessCommunication”, International Journal of Electronics and Communication Engineering &Technology (IJECET), Volume 3, Issue 3, 2012, pp. 314 - 323, ISSN Print: 0976- 6464, ISSN Online:0976 –6472.[19] T.Regu and Dr.G.Kalivarathan, “Prediction of Wireless Communication Systems in the Contextof Modeling”, International Journal of Electronics and Communication Engineering & Technology(IJECET), Volume 4, Issue 1, 2013, pp. 11 - 17, ISSN Print: 0976- 6464, ISSN Online: 0976 –6472.[19] Haritha.Thotakura, Dr. Sri Gowri .Sajja and Dr. Elizabeth Rani.D, “Performance of CoherentOFDM Systems Against Frequency offset Estimation under Different Fading Channels”, InternationalJournal of Electronics and Communication Engineering & Technology (IJECET), Volume 3, Issue 1,2012, pp. 244 - 251, ISSN Print: 0976- 6464, ISSN Online: 0976 –6472.