Effect of Adaptive Beamforming Upon Varying Interference Level

Effect of Adaptive Beamforming Upon Varying Interference Level

Hina Zahir

Electrical Engineering Department University of Engineering & Technology Peshawar, Pakistan

Bilal Ur Rehman

Electrical Engineering Department University of Engineering & Technology, Peshawar, Pakistan

Abstract In this era the demand for high data rates and all- time connectivity has elevated to a higher level. In order to fulfill these demands a hybrid system that utilizes the already terrestrial networks for the users situated in the urban areas and the satellite networks for the rural area users. The same spectrum could be reused by both the networks. This reusing of spectrum introduces Co-Channel Interference in to the system which could be reduced by adaptive beamforming. A system based on OFDM is simulated using matlab and the number of antenna elements and interference level are varied to observe the difference.

KeywordsBeamforming, Co-Channel Interference Level, Quadrature Phase Shift Keying, Orthognal Frequency Division Multixplixing Etc

1. INTRODUCTION

   In todays period the requirement for wide-reaching connectivity and an elevated system aptitude has amplified to a greater stage [1][2].

   In order to meet the requirements of higher data rates the existing networks are not adequate enough [3]. A hybrid system is the answer to these issues. In a hybrid communication system the users positioned in the rural areas are served by the already deployed satellite networks and the users positioned in the urban areas are provided services by the existing terrestrial networks [4]. This sharing of spectrum introduces Co- Channel interference [5]. In order to reduce this interference adaptive beamforming is applied at both the receiver and sender end [3]. Pre-FFT beamforming based on OFDM is incorporated. The systems performance is checked by passing the data with varying interference levels and number of antenna elements. At the receiver the original data is recovered by applying complex weights to the signal. Least Mean Square algorithm is used for weight updating.

   A system which is also hybrid but that system is supported on (CDMA) and is anticipated by Mobile Satellite Ventures (MSV) [3]. This anticipated MSV design is capable of providing coverage in urban areas by incorporating Ancillary Terrestrial Components (ACTs) and it serves the rural areas by employing satellite links. The drawback associated with this design is that it does not deploy the already existing terrestrial system. For the purpose of precision between these services two satellites are employed and space diversity is used for link margin [3]. This precision commences the CCI in to the system and for its reduction adaptive beamforming is incorporated which is based on CDMA. Similarly the demand for higher data rates has made the researchers to move to the 4th generation terrestrial networks. 3GPP projected Longer

   Time Evolution, which is OFDM, based and as we know that OFDM is bandwidth efficient.

   Utilizing adaptive beamforming in an OFDM based system produces enhanced outcome hence improving the system output. So for the purpose of obtaining high capacity and compatibility with the 4th generation networks, the mobile satellite design must be OFDM based. Usually there are two types of adaptive beamforming algorithms utilized for an OFDM system. The one is Post Fast Fourier Transform (PFFT) and the next is Pre-Fast Fourier Transform (Pre-FFT). Most of the work carried out is on terrestrial networks.

   Frequency Re-use at the Satellite end:-

   The concept of the reuse of the frequency spectrum at the satellite is in research since 1970. The total bandwidth available at a satellite could be increased by reusing the same frequency band in multiple beams towards earth [6].

   Frequency reuse is utilized with the help of multiple spatially isolated beams and dual polarization has been incorporated by a number of commercial communication satellite systems. Several techniques exist to achieve frequency reuse antenna beams [7].

   The frequency reuse in satellite provides for dual polarization and spatial reuse of transmit and receive beams to provide for a twelve-fold increase in the effective bandwidth and number of users that may be supported by the satellite system [8].

   Adaptive Beamforming:-

   Adaptive beamforming is a signal processing technique [9]. Beamforming is a technique of signal processing that is incorporated to organize the route of the signal within an array of transducers either the signal is sent or received [10]. The process of beamforming when integrated would direct majority of the signal in a precise direction transmitted from a group of transducers which could be an audio speaker or an antenna [11].

2. PROPOSED METHOD OFDM SYSTEM MODEL:-

   The representation of an OFDM setup is offered in the figure 1. The beamforming is present at the satellite side. The first part is the source data generation block, random data is produced here. The next block is the modulator; its purpose is to modulate the data in accordance with the data design utilized. The modulation schemes utilized in this scenario are QPSK and BPSK, so they are going to be scrutinized. After

   the modulation insert the pilot process. Pilots are data that are already identified to the receiver; this is carried on for channel estimation. The insertion of pilots could be between data sequence with definite period. We have slot in five pilots in out systems data sequence. When we slot in the pilots into the mapped data, then we represent the output which is in the frequency domain in a multiuser case

   _

   ADAPTIVE Beamforming :-

   Now for the attainment of interference mitigation, the beamformer applies complex weights to the symbols thus processing the output antenna elements. This practice could be shown as

   r = wH V (4)

   it is clear from the block diagram that after applying

   st 1st

   st Inserting

   ~xj

   xj

   Inverting

   xj _

   Xj [k]

   the techniques of beamforming then comes the block of serial

   1 User

   2nd User

   Jth User

   Modulator

   2nd Modulator

   Jth Modulator

   De-Mod

   1 S.P

   2nd S.P

   JTh S.P

   Pilot

   2nd Pilot

   Jth Pilot

   ~r

   ~xj

   ~xj

   Transform

   xj

   2nd Inverting Transform

   xj

   Jth Inverting

   Transform

   r

   Cyclic Prefix

   _

   2nd Cyclic xj

   Prefix

   _

   Jth Cyclic xj

   Prefix

   P.S

   P.S

   P.S

   V

   _

   Xj [k]

   +

   _

   Xj [k]

   To Receiver

   _

   yj [k]

   +

   Noise

   to parallel conversion. When the data is altered from serial to parallel form then it is altered in to the frequency domain by the application of FFT.

   r = FrH (5)

   r is acknowledged as the symbol of OFDM which is obtained and it lies in the domain of frequency . For the purpose of renewing the weight, the beamformer takes both the transmitted and received pilot sequences. Evaluates both of them and then finds out the error vector. Now the adaptive algorithm which is supported on Mean Square Error for the then symbol calculates the subsequent weight by taking into consideration the error vector.

   The error vector is represented below

   Data

   Data

   P.S Fourier Transform

   S.P

   Beam Forming Removing C.P

   +

   Noise

   +

   Noise

   ep = rp – x p (6)

   d

   d

   A terrestrial communication system based on OFDM is first implemented. Then adaptive beamforming is pplied to both

   Fig. 1. Model of OFDM

   Upto this part the signal exists in the frequency domain but when we slot in the pilot, and then the signal is altered to time realm by the application of Inverse Fast Fourier Transform.

   xj = FH (1)

   In the above equation the xj represents the time domain figure of an OFDM system.

   the ends. Interference in the form of interferers is introduced in to the system. The antenna elements in the system are varied along with the interference and the performance is checked.

   F =[

   1 1

   1 ej2(1)(1)/N

   1

   ej2(1)(N1)/N

   ] (2)

   1 ej2(N1)(1)/N ej2(N1)(N1)/N

   F is the matrix

   (.)H is the Hermitian or complex conjugate transpose.

   When the signal is transformed from time domain to frequency domain, then is the phase of adding the cyclic prefix. This extension periodically shifts the symbol. The rationale of including this extension is to prevail over the cause of Inter Symbol Interference (ISI). The cyclic prefix incorporated here is 1/4th of the symbol length. The method of introducing guard interval is

   Fig. 2. Block diagram of the system

   From figure 2 it is clear that after the introduction of noise in to the system, cyclic prefix removed and then apply the process of adaptive beamforming. After that process convert the data to parallel stream and then apply Fourier transforms [12]. Reset again from parallel to serial data stream and demodulate it.

3. SIMULATION RESULTS

   j

   = N,G] H

   [
   [

   N j

   (3)

   Following are the simulation results. The Adaptive beamforming is applied and the output is checked in the form of gain. Interference level along with number of antenna elements is also varied and the output is checked.

   System Parameters:-

   The following are the system parameters utilized

   System Parameters
   Modulation Scheme	QAM, BPSK, QPSK, 8-PSK,16- QAM
   No. Of Subcarriers	32
   Symbols	20,000
   Antenna Elements	2,4
   No Of Users	4
   Interference power	-10dB

   Note that all the modulation methods pointed out in the above table have been implemented single and multiuser OFDM system. But with the beamforming we have compared two modulation schemes that is BPSK and QPSK using 2 and 4 antenna elements.

   Interference level -10dB:-

   In the first scenario the modulation scheme employed is 8- PSK [13]. The desired user is located at 40 and the interferers are located at -8, -60, -30 and 70 degrees respectively. The numbers of antenna elements employed in this scenario are two.

   Fig. 3. Beampattern for 8-PSK for 2 antenna elements

   Fig. 4. Beampattern for QPSK for 2 antenna elements

   From figure 4 manipulate QPSK modulation scheme for the same number of antenna elements and the same level of interference.

   It is clearly observed that the modulation scheme impose an almost negligible effect upon the gain. As we moved from higher order modulation schemes towards lower order modulation schemes and observed that there is not a noticeable difference in the gain.

   Interference level -5dB:-

   Fig. 5. Beampattern for 8-PSK for 4 antenna elements

   Figure 5 represents a scenario with an increased number of antenna elements from two to four. The interference level is also increased from -10 to -5. But it is clearly observed that even with the increase in the interference level the gain has still increased. The reason is the increased number of antenna elements.

   Fig. 6. Beampattern for QPSK for 4 antenna elements

   In figure 6 the same scenario is with 4 antenna elements is checked with QPSK modulation scheme and it can be noticed that the gain has increased.

   It is again observed that the modulation scheme has an almost negligible effect upon the gain.

   Fig. 7. Beampattern for QPSK for 6 antenna elements

   In order to check the effect of antenna elements more effectively, figure 7 represents a scenario with 6 antenna elements and 8-PSK modulation scheme. In this scenario the desired user is at maximum gain and the interferers except the one at -8 are at complete nulls.

4. CONCLUSION

In a hybrid network the CCI caused by the mobile user to the satellite end is reduced by the incorporation of Adaptive beamforming. The number of antenna elements has a great effect upon the gain. As the number of antenna elements increases so do the gain and the interference decreases. But by increasing the antenna elements the weight of the satellite increases and so does its power requirement, so there is a limit upon the number of antenna elements employed.

REFERENCES

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   P.D. Karabinis, S. Dutta, and W. W. Chapman, Interference potential to MSS due to terrestrial reuse of satellite band frequencies. [online]. Available: www.msvlp.com

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