Analysis of MIMO Diversity Improvement Using Circular Polarized Antenna. Network Technology Research Institute, China United Network Communications Corporation Ltd., Beijing 1. China. Copyright . This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. MIMO (Multiple Input Multiple Output) technique is one of the important means to enhance the system capacity. Diversity gain could be acquired by using traditional . This problem can be effectively solved by using circular polarized antennas. In this paper, through theory analysis and test, the improvement of MIMO diversity gain using circular polarization antenna is analyzed. Introduction. MIMO technique uses multitransmitting and multireceiving antennas to combat channel fading and increase capacity and spectrum efficiency. MIMO system require low correlation between each pair of transmitting and receiving antenna elements, in order to achieve a good independent channel fading. There are two methods to dispose multi- antenna. The first one is space isolation. It is through the enough space distance to ensure non- correlation between each transmit/receive signals. The other one is polarization isolation. It is through electromagnetic wave orthogonal polarization to ensure non- correlation between each transmit/receive signals. Because the distance requirement is high for space isolation, commonly more than 1. So polarization isolation proves to be an effective and economical method to deploy multiantennas. For polarization isolation, signals go through independent fading in orthogonal polarization direction and hence ensure noncorrelation of two signals. During propagation, polarization direction of electromagnetic wave deflects because of reflection, refraction, and scattering. When signals arrive to the receiver, part of signals in . Because of independent fading in vertical and horizontal direction, polarization diversity gain can be gained. When the power of the signal in vertical direction is equal to that in horizontal direction, maximum diversity gain can be gained . When signals arrive to the receiver, power of signals in . System diversity gain is limited because of influence of minor signal. There is even no diversity gain if two received signals power has great difference . Circular polarization may be referred to as right-handed or left-handed, and clockwise or anti-clockwise, depending on the direction in which the electric field vector rotates. Unfortunately, two opposing historical. Circularly Polarized Microstrip Antenna Marwa Shakeeb A Thesis In The Department of Electrical and Computer Engineering. Circular polarization is. Two signals are transmitted through left- hand circular polarization and right- hand circular polarization wave (as shown in Figure 1). The noncorrelation between two signals is ensured by isolation of two circum- gyrate directions. Figure 1: Left- hand circular polarization (LHCP) wave and right- hand circularly polarized (RHCP) wave. The rest of this paper is organized as follows: MIMO system principle and channel capacity are introduced in Section 2. Section 3 gives the principle analysis of MIMO diversity improvement using circular polarized antenna (Figure 9). The result of MIMO channel capacity improvement is given in Section 5 and conclusions are given in Section 6.
MIMO System and Channel Capacity. MIMO System. Each channel between a pair of transmitting and receiving antennas is considered to be a MIMO subchannel. Assume that there are transmitting antennas and receiving antennas. Hence there are channel matrix which we name as channel matrix . The element of is a subchannel between one pair of transmitting and receiving antennas. When the distance of each pair is large enough, every signal between transmitting and receiving antennas is independent. Then, the rank of the matrix will be large, even full when under ideal circumstances. Vice versa, when near, the rank will be small, because the signals are correlated to each other. From the above, we can conclude that the channel capacity of MIMO is highly related with the matrix . If the channel condition of the transmitting point is unknown, but the index of the matrix and the overall transmission power are fixed, the overall power can be designated to every transmission antenna averagely. Then, the capacity can be calculated as. MIMO Channel Capacity. The model of the channel capacity can be considered as a complex baseband linear system. It has been assumed that there are transmission antennas, receiving antennas, and the overall transmission power . Then, the power of each antenna will be and the receiving power of receiving antenna will be equal to the overall transmission power. If the channel is interfered by AWGN, and the noise power of each antenna is , then SNR of every receiving antenna will be. When the bandwidth of transmitting signal is narrow enough, the frequency response of the channel is flat, and the channel matrix is considered to be matrix , whose element shows the channel fading index between transmission antenna and receiving antenna ; the capacity can be shown as. The MIMO System of “all 1” Channel Matrix . As a result, all signals which come from transmitting antennas can be considered the same. Considering and , the signal coming from antenna can be shown as , , the power of the dedicated antenna is shown as , and SNR of each receiving antenna is ; the overall SNR of receiving point is shown as . This multiantenna system can be seen as a sole- antenna system which hasdiversity gains. The capacity is shown as. If the receiving point adopts noncoherent detection technology, the SNR of each receiving antenna will still be , and the overall SNR will be . This sole- antenna system has an gain compared to the typical sole- antenna system. The capacity is shown as. The MIMO System of Orthogonal Transmitting Channel . Assuming the amount of antennas of transmitting and receiving point is the same (), the matrix can be shown as ( is the unit matrix of ). From (2), we can get the capacity as. The channel capacity gets a gain of compared to the old system due to the coupling of subchannels of each antenna. If the channel index is changed randomly, the capacity of MIMO channel will be a random variable. The average capacity is. Analysis of MIMO Diversity Improvement Using Circular Polarized Antenna. Polarization Matching. Polarization is an important character of antennas. It gives the changing orbit of electric vector and time in certain conditions. Normally, the wave along + can be shown in - axis and - axis as. From the relationship of ’s and ’s amplitude and phase, we can conclude that the electromagnetic wave has three polarizations: linear polarization, circular polarization, and elliptical polarization. If . When or . Electromagnetic amplitude and phase. As shown above, the resultant wave changes along with time, but the orbit is in the line which has a degree over - axis. It is linear polarization wave. When. Electromagnetic amplitude . The resultant wave is not changed with time, but direction changed with time. The vector of electronic field is rotating with angular velocity . It is circular polarization wave. When , . When is subtracted. The degree of the oval’s axis over - axis is. All the vectors are rotating with an oval shape. As a result, this polarization wave is named as elliptical polarization wave. Only when the transmitting and receiving antenna match correctly can the antenna achieve the best receiving effect. When vertical polarized antenna is receiving vertical polarized wave, the transmitting wave will be shown as. The receiving antenna is shown as. Polarization matching index is. The receiving antenna will get the maximum power from the wave and now the transmitting and receiving antenna match perfectly. When circular polarized antenna is receiving linear polarized wave, the transmitting wave is shown as. For the receiving antenna. Polarization matching index is. The antenna will get half the power which means a 3 d. B loss. The same result comes with linear polarized antenna receiving circular polarized wave. When left- hand circular polarized antenna is receiving left- hand circular polarized wave, the wave is shown as. For the receiving antenna. Polarization matching index is. The receiving antenna cannot get power, because they did not match in rotation direction. The same result comes with horizontal/vertical polarized antenna receiving vertical/horizontal polarized wave. When two linear polarized antennas which are in vertical and horizontal direction are receiving circular polarized wave, the wave is shown as. For the receiving antenna. Polarization matching index is. The receiving antennas match transmitting wave in polarization direction perfectly. Cross Polarization Ratio and Polarization Leakage Ratio. Cross polarization ratio (Figure 2), which means the ratio of main polarization electromagnetic wave and orthogonal polarization electromagnetic wave, is the index which shows the dual- polarization antenna’s polarization features. The higher the CRP is, the better performance dual- polarization antenna can achieve and hence the higher diversity gain. Figure 2: Cross- polarization ratio. Due to the multipath effect of reflection and refraction, the electromagnetic wave will deflect to cross polarization from main polarization. If so, the receiving power will not be determined by CPR. That means the CPR of dual- polarization antenna will be 0 d. B. That will also maximize the diversity gain. However, in some typical scenario which has less reflection, refraction, and scattering, the electromagnetic wave in UE side will keep the original polarization direction. The multipath effect cannot put half power of linear polarization wave to the orthogonal polarization direction; that is, CPR does not come to ideal condition 0 d. B. We define an index as polarization leakage ratio. It indicates the ratio of power received by UE antennas in vertical polarization and horizontal polarization. CPR, transmitting channel, and receiving antenna. When transmitting and receiving antenna’s polarization match perfectly and we do not consider the interference of transmitting channel, the relationship betweenand CPR will be.
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