SNR Analysis for Visible Light Communication Systems

DOI : 10.17577/IJERTV3IS100435

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SNR Analysis for Visible Light Communication Systems

Aastha Agarwal1

Department of Electronics and Communication Engineering,

    1. Student, NITTTR Chandigarh.1

      Garima Saini2

      Department of Electronics and Communication Engineering,

      Assistant Proffessor,NITTTR,Chandigarh.2

      Abstract: White LEDs offers various advantages such as high illuminance, long lifetime and low power consumption. An Indoor optical wireless communication system utilizing white LEDs offers a huge potential to provide high speed data transmission for various indoor applications such as flight entertainment, video transmission and other multimedia services. These systems use white LEDs as the source which can transmit and receive data by flashing light at a speed which is undetectable to the human eyes. In this paper we discuss about the usage of multiple LED arrays i.e. MIMO techniques as compared to single LED array i.e. SISO technique for indoor communication so as to maximize Signal to Noise Ratio (SNR) received.

      1. INTRODUCTION

        The demand for wireless broadband communications has been growing steadily for last several years. The congestion and limitations on bandwidths of the radio spectrum have inhibited unrestricted growth of the radio wireless systems. Wireless optical, however, holds the promise of delivering data at much higher rates[1].

        The advancements in the Solid state Lightning (SSL) have triggered research in the direction to use Light Emitting diodes for illumination as well as for communication in a cost effective, power efficient way in order to give high data rates for indoor communication. An optical wireless (OW) communication system relies on optical radiations to convey information in free space, with wavelengths ranging from infrared (IR) to ultraviolet (UV) including the visible light spectrum. The transmitter/source converts the electrical signal to an optical signal, and the receiver/detector converts the optical power into electrical

        This paper is organized as follows. In section II, LED as a source for data transmission is discussed. In section III, the features of proposed system are shown. The effect of number of transmitters on SNR is discussed in section IV and the effect of FOV field of view) on SNR is also discussed. Finally, conclusions are given in section V.

      2. LED AS A SOURCE FOR DATA TRANSMISSION

        Ideally, a LED source is a Lambertian emitter, i.e., irradiance distribution or illuminance is a cosine function of the viewing angle. A practical approximation of the irradiance distribution following the illustration in Fig.1 is given as

        , = () (1)

        where is the viewing angle and E0(d) is the irradiance (W/m2), also given in luminous flux (lm) on the axis at a distance d from the LED. The number n is given by the half power angle, 1/2(an angle provided by the manufacturer) defined as the view angle when irradiance is half of the value at 0°. The relation between 1/2 and n can be expressed in (2). The LED emitter is modeled using a

        = (2)

        (/)

        generalized Lambertian radiation pattern Ro(). Assuming that Pt is the transmitted power, the radiation intensity is given by:

        current [2].

        As the demand for spectrum is rapidly increasing day by day so in order to fulfill this, need of the hour is Optical

        =

        +

        ();

        ,

        (3)

        Wireless Communication (OWC). There are certain advantages of OWC over radio frequency communication which makes it viable option for indoor communication. The OWC utilizes the unregulated, unlicensed part of the electromagnetic spectrum and offers huge bandwidth thus supports very high data rates for a small region [3]. Disorders in immune system and other health issues associated with radio frequency are nonexistent in OWC systems. Also, these systems are highly secure as their signals cannot penetrate through the walls. Moreover these systems have less installation cost and circuitry required as compared to RF systems [3].

        The power emitted by the LED is PLED, and and are

        the irradiance and incidence angles. The transmitted power is Ptx = PLED * Ro().

        Fig.1 Lambertian emission pattern for mode n. [4].

        In this paper we consider an optical wireless MIMO transmission system employing intensity modulation (IM) and direct detection (DD) of the optical carrier using incoherent light sources, e.g. LEDs. The system is equipped with Nt transmitters and Nr photo-detectors at the receiver side. The received signal vector is

        Y = Hs + N (4)

        where N is the sum of ambient shot light noise and thermal noise. It is independent of the transmitted signals and the main noise impairment as commonly assumed in OWC [5].

        Consequently, N is real valued additive white Gaussian noise (AWGN) with zero mean and a variance 2 = 2shot

        +2 thermal, where 2shot is the shot noise variance and 2thermal is the thermal noise variance . Thus, the noise power is given by 2 = N0*B, where N0 is the noise power spectral density and B is the bandwidth. [5] .The transmitted signal vector is denoted by s = [s1 . . . sNt ]T , with []T being the transpose operator. The elements of s indicate which signal is emitted by each optical transmitter,

        i.e. sNt denotes the signal emitted by transmitter Nt. The Nr

        ×Nt channel matrix H is given by

        = (5)

        where represents the transfer factor of the wireless link between transmitter Nt and receiver Nr.

      3. SYSTEM MODEL

A visible-light indoor optical wireless system utilizing one array, two arrays and four arrays of LED lamps is shown in Fig.2(a), 2(b) and 2(c).

Fig 2. Top view of the position of Transmitters on the ceiling.(a) Single Transmitter,(b) Two Transmitters, (c) Four Transmitters

A room of 5mx5mx3m dimensions is considered for the purpose of analysis. The room contains of a receiver photodiode which is placed on a table of height 0.85m from the floor of the room.

The light signal transmitted from the LED arrays are received by a photodiode. The signals received depend on basic ray optics theory, room geometry, angle of incidence and reflection and field of view of the receiver. The LED emits radiation with intensity R0() at an irradiance angle of . The signal is received by a photo detector of area Arx, at a distance d. The photo detector can receive a signal which lies in its field of view. The radiated signal passes though an optical filter and concentrator to ensure that maximum light falls within the field of view of the receiver. Table 1 shows the various parameters considered for simulations in this paper.

Room size

5m x5m 3m

Desk height from the

ceiling

0.85m

Amplifier Bandwidth

50 MHz

Single LED power PLED

20mW

Semi-angle at half power

30

No. of LEDs per array

3600

Ceiling reflectivity

0.7

Wall reflectivity

0.8

Detector physical area of

PD

1cm2

Transmission coefficient

of optical filter

1

Refractive index of lens

at PD

1.5

Table1. System Model Parameters

Photodiode responsivity

0.4

Noise-bandwidth factor

0.562

Absolute temperature

298K

Fixed Capacitance

112pF/cm2

Transconductance

30mS

In this paper we have analyzed the SNR at three location scenarios as shown in Fig.2. Light beams propagate from the LED to the receiver via two main channels: light of sight (LOS) and diffuse channels.

The LOS Channel transfer function is expressed as

The received diffused power Pdiff with the receiving area Arx is Pdiff = Arx*I.

At the receiver, light passes through the optical filter and concentrator, so the received power is

= + (12)

where Tf () is the transmission coefficient of the optical filter, and g() is the concentrator gain.

The photodiode is used to convert the received optical power into the electrical current, and the output current is:

=

>

(6)

= (13)

where R is the photodiode responsivity (A/W).

The SNR is given by:

()

where Arx is the detector area, d is the distance between the transmitter and the receiver, Ro() is the transmitter radiant

intensity and given by (3), is the angle of incidence, c is the FOV of the photodiode.

=

total

where 2

(14)

is total noise variance and it is given by:

= + + (15)

shot

The total power of n LEDs in the directed path is

where the shot-noise variance 2shot is given by 2 =

=

=

()

(7)

2*q*R (Prx + Pn) *Bn (16)

where Bn = I2Rb, where Rb is data rate and I2 is the noise- bandwidth factor [6].

where (0) the LED channel DC gain.

The amplifier noise variance is given by:

2

= i

2

amplifier

amplifier Ba

(17)

In order to calculate the diffuse channel response an integrating sphere model for the optical wireless diffuse signal was introduced in [7] and this is used here. Here,

where Ba is the amplifier bandwidth. The thermal noise variance is given by

= +

only first reflections from the walls, ceiling and surface are considered. In a room of surface area the first diffuse

(18)

reflection of a wide-beam optical source emits a intensity I1 and is given by

where the two terms represent feedback resistor noise, and

channel noise respectively. Here K is the Boltzmanns

=

(8)

constant G is the open loop voltage gain, Tk is the absolute

temperature, n is the capacitance of photo detector per unit area, gm is the transconductance [8].

where 1 is the reflectivity of the surface and PtotalLED is the total power transmitted by all the LEDs .

The average reflectivity inside the room is defined as

IV SNR PERFORMANCE

An SNR can express the quality of communication. The

=

(9)

analysis of signal to noise ratio is done in the absence of multipath fading effects. In our channel model, the

where the individual reflectivities of walls, ceiling, floor and other objects in the room are weighted by their individual areas Ai. Therefore, the total intensity is

information carrier is a light wave whose dimensions are in the order of thousands of wavelengths, leading to spatial diversity, which prevents multipath fading. For the above reasons multipath fading can be neglected[8]. The

=

=

where the index j is the number of reflections.

(10)

modulation technique used is OOK . The noise added due to the transmission through the channel is shot noise and thermal noise and during detection of signal an additional amplifier noise is also added. Here the analysis is done for one transmitter, 2 transmitters and four transmitters located at positions as shown in Fig 2.

Fig. 3 SNR distributions for single transmitter inside a room.

Fig 3 shows the SNR obtained is case of single transmitter. The maximum value of SNR is 47.8dB with a minimum of 17.9dB and average of 32.6dB. The simulations are done at a data rate of 1Mbps. Fig.4 shows the simulation for SNR in the presence of two transmitting arrays. It is observed that the maximum SNR obtained is 48.62dB with a minimum of 25.975dB and an average of 39.0514dB. Further, Fig.5 shows the SNR distribution for four transmitters. The maximum value is 49.59dB, minimum is 36.40dB and an average of 45.73dB is achieved.

Fig 4 SNR distributions for two transmitters inside a room.

Figure 5 SNR distributions for four transmitters inside a room.

Table 2 shows the comparison of average SNR at different data rates for the above three configurations. It is seen that the best SNR performance is given by 4×4 system which can be used to transmit data up to 1Gbps.

Table2 SNR for SISO, 2×2 MIMO and 4×4 MIMO.

Data rate

SNR (1 Tx)

SNR(2 Tx)

SNR(4 Tx)

10bps

33.54dB

39.92dB

46.60dB

1kbps

33.54dB

29.92dB

46.60dB

100kbps

33.45dB

39.83dB

46.51dB

10Mbps

27.27dB

33.65dB

40.33dB

1Gbps

-7.03dB

-0.650dB

6.033dB

100Gbps

-46.95dB

-40.56dB

-33.88dB

Fig 6 Average SNR vs. BER for SISO, 2×2 MIMO and 4×4 MIMO systems.

Fig 6 shows the graph for comparison of average SNR for various bit rates. The value for SNR decreases as bit rate

increases. Thus, using visible light communication a data rate transfer up to 1Gbps can be done. It is seen from the table that SNR is negative for bit rate above 1Gbps for all the three systems.

The negative value of SNR represents that the signal power is less than the noise power, i.e. noise signal is the dominating signal, such region is known as blind region.

In visible light communication the inter symbol interference depends on the FOV of the receiver and the transmitted data rate. Fig 7 shows the dependence of SNR on the field of view of the signal and data rate for a four transmitter system. It is seen that as field of view increases the SNR decreases continuously. Also, for each angle the performance of 4×4 MIMO is better than 2×2 MIMO which is further better than SISO systems. Fig 7 shows the dependence of SNR on FOV at a bit rate of 10Mbps.

Fig 7 Average SNR vs. FOV at bit rate of 10Mbps.

V CONCLUSION

In this paper we proposed the use of multiple input multiple output system for indoor optical wireless communication systems as compared to single input single output systems. The simulations are done for SISO, 2×2 MIMO and 4×4 MIMO systems. It is seen that 4×4 MIMO gives better results as compared to the other two systems at higher data rates and large field of view angles. The 4×4 system also provides better illuminance as compared to the other two systems.

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