Reduction of Channel Correlation for MultiuserMIMO System in Visible Light Communications Ajit Kumar
S.K.Ghorai
Dept. of Electronics & comm. Engineering Birla Institute of Technology, Mesra Ranchi, India Email:
[email protected]
Dept. of Electronics & comm. Engineering Birla Institute of Technology, Mesra Ranchi, India Email:
[email protected]
Abstract— The present paper proposes a method in which a multiuser system with different receivers having different Field of view (FOV) has been used to reduce channel correlation in visible light communication (VLC), which will be helpful for signal separation at the user terminal. FOV of receivers would be different at different room location based on RMS delay of reflected light from walls. In the simulation, both LOS and NONLOS components of light have been considered. Simulation results show that as delay decreases bit rate increases. Therefore receiver with narrower FOV is placed at the center of the room as the bit rate is maximum at that location whereas FOV with wider FOV is placed at the corner.
P.Fahamuel et al. [1] have used a receiver with angular diversity receiver in order to improve capacity and reduce channel correlation. A.Nuwanpriya et al. [2] have introduced novel angle diversity receiver for mobile users. P.Butala et al. [3] have studied the performance of optical spatial modulation and spatial multiplexing with imaging receiver. A.Burton et al. [4] have discussed the design and angular-segmented VLC receiver. Cuiwei He et al. [5] have used two FOV receiver systems having a different field of view, one with narrow and another with wide FOV. This leads to invertible channel matrix even if PDs are spaced close to each other. However, in all these research works it has been considered that all receivers belong to the same user terminal which in the practical scenario is not true. Yang-hong et al. [6] have described multiuser terminals where each terminal is having numbers of receivers. Also, they have used different FOV for each receiver in order to weaken correlation between receivers. But this research lacks to show the different FOV requirements according to the position of the receiver on receiving plane. In the present work, a new approach of multiuser (MU) system having receivers of different FOVs according to the position has been proposed depending upon RMS delay due to the reflection of light by walls. Hence NON-LOS component of light including LOS component has been considered in our research. Through simulation, it has been shown that as delay decreases bit rate increases. Therefore that position where RMS delays are less has been placed with narrow FOV receiver because this helps in reducing correlation of channel gain from different LEDs. This approach solves two problems. First one is that the channel gain between LED and two receivers of the same user having different FOVs will be different. Hence channel matrix of MIMO system will be well conditioned and invertible. Due to this, it would become easy for signals separation at receiver. The second advantage is that signals from reflected light is not wasted but is captured through wide FOV receiver. Simulation has been done to find the RMS delay due to reflection from all the four walls in reaching the receiver plane. Corresponding to this delay, bit rate available at different receiver location has been calculated.
Keywords—VLC, MIMO, RMS delay, Field of view(FOV), Multiuser
I.
INTRODUCTION
LED-based indoor visible light communication has been in great demand in recent years due to various advantages associated with it. Some of the advantages are like license free, electromagnetic interference free, low cost, the dual purpose of lightening as well as secure communication, long life etc. However, there is drawback associated with LED that is limited modulation bandwidth. To counteract this problem many techniques like OFDM and MIMO have been implemented. Also for indoor visible light communication, many LEDs are mounted on the ceiling for sufficient illumination. This feature helps in exploiting the MIMO technique in practical indoor VLC. Another problem is that MIMO-VLC technique uses intensity modulation and direct detection (IM/DD) hence optical channel gains are highly correlated. It results in a limited multiplexing gain. Many authors have suggested their ideas in reducing channel correlation. Light propagating from LEDs to PDs is made up of two components. First one is LOS which directly reaches PDs and hence its strength is good. The second component is NON-LOS which reaches PDs via reflection from different surfaces like a wall, table and other sorts of materials and hence its strength is not as good as LOS component. That is why maximum researcher excludes NON-LOS component in their research work.
This research work is sponsored by University Grants commission (UGC) fellowship.
978-1-5090-2684-5/16/$31.00 ©2016 IEEE
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III.
RECEIVED POWER FROM LEDS
Received power from LEDs is the resultant power from direct LOS path and from reflected path. A. Received power from LOS path In optical wireless communication, the DC channel gain between LED and PD is given by [8, 9]
(m+1)A m cos (φ)Ts (ψ)g(ψ) cos(ψ),ψc ≤ψ ≤ 0 (4) H(0) = 2ΠDd2 0,ψ ≥ψc
Fig. 1 Indoor Model
where A is the photodetector area, Dd is the distance between transmitter and receiver, is the angle of irradiance, is the angle of incidence, c is the FOV of the receiver, Ts( ) is the gain of optical filter, g( ) is the gain of optical concentrator given by n2 (5) g (ψ ) = sin 2 (ψ c ) where n is the refractive index of the optical concentrator. The received optical power is given in terms of transmitted power Pt (6) Pd = Pt .H (0)
This paper is organized into three sections. Section II describes the indoor model. In section III received power from LOS and NON-LOS has been calculated. In Section IV results and discussion have been presented. And finally, section V covers conclusion. II.
INDOOR MODEL
The schematic diagram of indoor VLC model is illustrated in Fig.1. The above diagram shows the room whose dimension is 5m×5m×3m with receiver plane at height of 0.85m from the floor. There are Nt numbers of LEDs mounted on the roof whereas Nr numbers of PDs with different FOVs are placed on receiver's plane. The light rays reach the receiver plane through LOS path as well as NLOS path. The light ray strikes the wall at a various point which again acts as Lambertian source and emits light to reach photodetector. In this paper, only single reflected light ray has been considered. Although receiver can be placed anywhere on the receiver plane but in the present paper; it has been shown that how depending upon RMS delay different position can be located where PDs having different values of FOVs can be placed. In the present work, LEDs having Lambertian pattern of radiation have been considered. The irradiance of LED on concentrator is given by [7]
I s (l , φ ) =
Pt R o l2
B. Received power from reflected light
(1) Fig. 2 Reflection of light from wall
where is the angle of emission relative to the optical axis of LED, l is the distance between LED and optical concentrator, Pt is transmitted power, Ro( ) is a Lambertian pattern of LED which is given by
R o (φ ) =
m +1 cos m (φ ) 2Π
The DC channel gain of the first reflection of the light from the wall is given by [8]
(m + 1) A ρdAWall cosm (Φ) cos(α ) 2 2 2ΠD1 D2
(2)
H Re f (0) = cos(β )Ts (ψ ) g (ψ ) cos(ψ ),ψ c ≤ ψ ≤ 0 (7)
here m is the order of Lambertian radiation given by
m=
− ln 2 ln(cos Φ 1 / 2 )
0,ψ ≥ ψ c
(3)
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where is reflection coefficient of the walls, D1 is the distance between LED and reflective point, D2 is the distance between reflective point and receiver, is the angle of irradiance to the reflective point, is the angle of irradiance to the receiver, dAwall is the area of reflective point, is the angle of incidence. The received power is the sum of reflected light power and direct light power given by
Pref =
LED
Pt Hd (0) + Pt dHref (0)
(8)
Wall
IV.
RESULTS AND DISCUSSION
The total received power for the multipath is given by
PrT =
N i =0
Pd , i +
M j =0
Pref , j
(9)
Fig. 3 RMS Delay at different location in room
Fig.3 shows a simulation of RMS delay inside the room of size 5m×5m×3m. For this result, LED chip has been kept at the center of the room. Hence the delay is more at the corner and least at the center of the room which is apparent from the
where N and M are the total numbers of direct paths from the transmitter to receiver and reflection path paths to the same receiver respectively. Pd,i is the received optical power from ith path and Pref,j is the received optical power from jth reflected path. The mean excess delay is given by [10] N
µ=
i =0
Pd ,it d ,i +
M j =0
Pref , j tref , j (10)
PrT
The RMS delay spread is given as
Drms = µ 2 − ( µ ) 2
(11)
where N
µ2 =
i=0
Pd , i t d2 , i +
M j=0
2 Pref , j t ref ,j
PrT
(12)
Fig. 4 Mean delay spread at different location in the room
above figure. Fig.4 shows a simulation of mean delay spread inside the room.
The maximum bit rate is decided by the RMS delay of the light. The maximum bit rate that can be transmitted without channel equalizer is given by [11]
1 Rb ≤ 10 × Drms
Fig.5 shows the simulation result of bit rate inside the room. From (13) maximum bit rate available throughout the room has been calculated. The aim of this paper is to find the required FOV at a different location of the room according to the RMS delay of the reflected light. So simulation has been done to find the relation between RMS delay and bit rate by varying FOV of the receiver. Fig.6 shows the simulation result. The simulation was done by taking the different value of FOV as 20 degrees, 30 degrees, 40 degrees, 50 degrees, 60 degrees and 70 degrees.
(13)
The table1 shows the values of various parameters that were used for simulation. TABLE I.
SIMULATION PARAMETERS
Simulation parameters Power of LED Reflection coefficient of all walls Room dimension Optical filter gains Ts Physical area of PD
Values 1W 0.8 5m×5m×3m 1 1cm2
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V.
CONCLUSION
In this paper, a multi-user MIMO system has been presented where each user having multiple receivers with different FOV can be placed inside the room. Based upon the RMS delay of the reflected light from the walls, the maximum bit rate was calculated for different field of view at different receiver plane. Through this analysis, it has been seen that the receivers with different FOV can be placed on the receiver plane to weaken the channel gain correlation. Thus well-conditioned and invertible channel matrix can be obtained in MIMO VLC system. Also, signals from reflected light are not wasted but are captured through wide FOV receivers. The method will help in getting uncorrelated channel gain which reduces the problem of signal separation.
Fig. 5 Bit rate at different location in room
ACKNOWLEDGEMENT This Research work is supported by fellowship granted from University Grants commission (UGC). REFERENCES [1]
P. Fahamuel, J. Thompson and H. Haas, “Improved indoor VLC MIMO channel capacity using mobile receiver with angular diversity detectors,” in Proc. IEEE Globecom, 2014, pp. 2060– 2065. [2] Nuwanpriya, S.-W. Ho and C. S. Chen, “Indoor MIMO visible light communications: novel angle diversity receivers for mobile users,” IEEE J. Sel. Areas Commun., vol. 33, no. 9, pp. 1780– 1792, 2015. [3] P. Butala, H. Elgala, and T. Little, “Performance of optical spatial modulation and spatial multiplexing with imaging receiver,” in Proc. IEEE WCNC, 2014, pp. 394–399. [4] A. Burton, Z. Ghassemlooy, S. Rajbhandari, and S.-K. Liaw, “Design and analysis of an angular-segmented full-mobility visible light communications receiver,” Trans. on Emerging Telecommun. Tech., Mar. 2014. [5] Cuiwei He, “Performance of Optical Receiver Using Photodetectors with Different Fields of View in a MIMO ACOOFDM System,” Journal of Lightwave Technology, vol.33, pp. 4957-4967, 2015. [6] Yang hong, Jian Chen, Zixiong Wang, Changyuan Yu, “Performance of a precoding MIMO system for Decentralized Multiuser Indoor Visible Light Communications,” IEEE Photonics journal, Vol.5, no.4, Aug.2013. [7] Thomas Q. Wang, Y. Ahmet Sekercioglu, “Analysis of an Optical Wireless Receiver Using a Hemispherical Lens With Application in MIMO Visible Light Communications,” IEEE journal on Lightwave technology, vol. 31, no. 11, June 1, 2013,pp.1744-1754. [8] J. R. Barry, “Wireless Infrared Communications,” Kluwer Academic Press, Boston, MA, 1994. [9] F. R. Gfeller and U. Bapst, “Wireless in-house data communication via diffuse infrared radiation,” Proc. IEEE, vol. 67, no. 11, pp. 1474-1486, 1979. [10] Z. Ghassemlooy, W. popoola, S. Rajbhandari, “Optical Wireless Communications: System and Channel Modeling with MATLAB,” CRC press, 2012. [11] F Xu, A Khalighi, P Caussé and S Bourennane, Channel coding and time-diversity for optical wireless links, Optics Express, 17, 872–887, 2009
Fig. 6 RMS Delay vs. bit rate
Fig. 7 Placement of receivers having different FOV according to RMS delay
The result shows that for wide FOV i.e 70 degree, the hyperbolic relation between RMS delay and bit rate holds true. But as FOV decreases it follows the relation of a straight line. Also as RMS delay decreases, bit rate increases. The bit rate is maximum at the center of the room and it decreases towards the corner of the room. Hence receiver with 20 degrees FOV is placed at the center of the room. In this way, other receivers have been placed inside the room as shown in Fig. 7.
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