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Diversity
Published in Jerry D. Gibson, The Communications Handbook, 2018
In mobile radio environments, signals transmitted on orthogonal polarizations exhibit low fade correlation and, therefore, offer potential for diversity combining. Polarization diversity can be obtained either by explicit or implicit techniques. Note that with polarization, only two diversity branches are available as compared to space diversity, where several branches can be obtained using multiple antennas. In explicit polarization diversity, the signal is transmitted and received in two orthogonal polarizations. For a fixed total transmit power, the power in each branch will be 3 dB lower than if single polarization is used. In the implicit polarization technique, the signal is launched in a single polarization but is received with cross-polarized antennas. The propagation medium couples some energy into the cross-polarization plane. The observed cross-polarization coupling factor lies between 8 to 12 dB in mobile radio[8,1].
Performances of Digital Receivers
Published in Stefan R. Panić, Mihajlo Stefanović, Jelena Anastasov, Petar Spalević, Fading and Interference Mitigation in Wireless Communications, 2013
Stefan R. Panić, Mihajlo Stefanović, Jelena Anastasov, Petar Spalević
Diversity combining represents a concept of increasing system performances at the reception by combining two or more replicas of information bearing signal. The main idea is to utilize the low probability of deep fade concurrence in all signal replicas, in order to decrease the probability of error and outage occurrence. Obtaining multiple replicas of the same information bearing signal could be performed by following various procedures. Some of them are channel coding in combination with limited interleaving—time diversity; transmitting the same narrowband signal at different carrier frequencies, where the carriers are separated by the coherence bandwidth of the channel—frequency diversity (path diversity, i.e., frequency hopping in GSM, multicarrier systems, spread spectrum systems, etc.); using two transmit antennas/two receive antennas with different polarization—polarization diversity [16,17]. However, all of the described techniques have some appliance disadvantages compared to space diversity [1].
Air Interface Enhancements for Multimedia Broadcast/Multicast Service
Published in Borko Furht, Syed Ahson, Handbook of Mobile Broadcasting, 2008
Américo Correia, Nuno Souto, João Carlos Silva, Armando Soares
Soft combining is proposed in 3GPP11 as an enhancement for MBMS PtM radio transmissions. In a PtM MBMS service the transmitted content is expected to be network specific rather than cell specific, that is, the same content is expected to be multicast/broadcast through the entire network or through most of it. Therefore, a natural way of improving the physical layer performance is to take advantage of macrodiversity. Basically, the diversity-combining concept consists of receiving redundantly the same information bearing signal over two or more fading channels, and combining these multiple replicas at the receiver to increase the overall received signal-to-noise ratio (SNR).
Analytical BER Calculation of TAS/MRC-Based Two Hop UWB Communication System Over IEEE 802.15.4a Channel
Published in IETE Technical Review, 2018
Anand Agrawal, Rakhesh Singh Kshetrimayum
We consider the TAS/MRC-based two hop relaying UWB system and it is depicted in Figure 1, where {S, R} terminals have NT number of transmitting antennas and {R, D} terminals have NR number of receiving antennas, respectively. The S and R terminals perform the (NT, 1; NR) TAS/MRC scheme for the source to destination (S − D) and relay to destination (R − D) links. While in the source to relay (S − R) link, S terminal does not perform the TAS scheme and uses the same transmitting antenna which has been selected in the (S − D) link. The basic idea of (NT, 1; NR) TAS/MRC scheme is that among the NT transmitting antennas, one antenna is selected for the transmission such that the received signal to noise ratio (SNR) is maximum. The receiver utilizes the MRC diversity combining scheme to combine all the received signals that arrive at the NR receiving antennas. In order to perform the TAS scheme, we assume that perfect channel state information (CSI) is available at the transmitter through a feed back channel link. Initially, all the transmitting antennas send the pilot signals to the receiver, and the receiver evaluates the instantaneous SNRs. Using the feed back link, the receiver transmits back the decision information (I) of the instantaneous SNRs to the transmitter. The mathematical expression of I is given as [17–19] where hm,n denotes the channel impulse response (CIR) for the mth (m = 1, 2, ..., NT) transmitting to the nth (n = 1, 2, ..., NR) receiving antenna link, respectively. We present a detailed description of hm,n in the next subsection. Under the assumption of the perfect CSI known at the transmitter, the s are arranged as , where i ∈ {m = 1, ..., NT}. On the basis of this sequence, it is assumed that the received SNR is maximum for the ith transmitting antenna and hence, it is selected for transmission. The probability density function (PDF) of ith selected transmitting antenna, is given as [20,21]where and are the PDF and cumulative distribution function (CDF) of .
Analysis of switch and examine combining with post-examining selection in cognitive radio
Published in International Journal of Electronics, 2018
Rupali Agarwal, Neelam Srivastava, Himanshu Katiyar
Generally, the performance of wireless system degrades due to severe multipath fading (Alouini & Goldsmith,1999; Eng & Milstein, 1995; Hu & Beaulieu, 2007; Katiyar & Bhattacharjee, 2009). So the fading compensation is typically required to mitigate the effect of multipath. Diversity combining (Brennan, 1959), which combines multiple replicas of the received signal, is a classical and powerful technique to combat multipath impairment. Among all the diversity combining techniques, maximum ratio combining is the scheme that gives the best performance but at the cost of highest complexity. By using the low-complexity combining techniques, receiver design becomes simpler. Switched diversity and selection diversity are the two most popular less complex diversity combining techniques. In selection combining (SC) (Adinoyi, Yijia Fan, Yanikomeroglu, & Al-Shaalan, 2009), diversity branch with the highest signal to noise ratio (SNR) of individual component is chosen at receiver and co-phasing is not required. This is based on (Theodore, Rappaport Kurt, Kosbar William, & Shanmugan, 2004) the principle of selecting the best signal among all the signals received from different branches at the receiving end. Though SC provides the best performance among all the low-complexity combining techniques, it demands the SNR estimation of all the branches. Whereas in switched combining scheme, the receiver keeps on comparing the SNR of the paths with a predetermined threshold. If the SNR of the used path’s signal is greater than this fixed threshold, no more paths are searched for. But if the SNR of current path gets down, other paths are needed to be searched. In this way, the complexity of receiver reduces as switching from one diversity branch to another is done only on the basis of need. Two strategies can be used as switched diversity combining scheme. One is switch and stay combining (SSC) (H.Katiyar, Bhattacharjee, & Samdarshi, 2010) in which the receiver selects another branch only if SNR of the current branch falls below the required threshold. The other strategy is switch and examine. Classical switch and examine combining (SEC) is used to take the advantages of the multiple diversity paths. In classical SEC, if we take two diversity branches, the results are same as SSC. The switch and examine combining with post-examining selection (SECp; Yang & Alouini, 2006) scheme is an improved version of classical SEC scheme. In classical SEC, if the current path is not of adequate quality then the quality of the next possible path is examined by combiner. This process goes on until an acceptable path is found. And if in case no path is acceptable, i.e. having SNR above a predefined level then the combiner sticks with the last examined path. Whereas in case of SECp, in case of unavailability of acceptable path, the combiner searches for the path having best quality among all the available paths. In this paper, we have analysed the performance of SECp and proved that the detection performance of switched diversity system in Rayleigh fading channel using SECp scheme is better than that of SEC. The contributions of this work are summarised as below: