VLC Channel Modeling with IM/DD

1. VLC Channel Basics

VLC (Visible Light Communication) uses light sources (LEDs, LDs) to transmit data. The channel represents how the light travels from the transmitter (LED) to the receiver (photodetector, PD) and how the signal is affected by the environment.

Key points:

• VLC uses intensity modulation: the light’s power/intensity is varied according to the signal.
• At the receiver, the photodetector converts the received light intensity into an electrical current.

2. IM/DD – Intensity Modulation with Direct Detection

IM/DD is the most common technique for VLC.

How it works:

1. Transmitter (IM)
   • The LED’s optical intensity is modulated according to the data signal.
   • Only the power (not the phase or frequency) of light is modulated.
2. Receiver (DD)
   • The PD measures the incident optical power and converts it to an electrical current.
   • This current is proportional to the received light intensity.

Key idea:

VLC with IM/DD is linear in intensity, so the received signal can often be modeled as:

y(t) = R · x(t) · h + n(t)
      

Where:

• y(t) = received electrical signal (current)
• R = responsivity of the PD (A/W)
• x(t) = transmitted optical power (modulated by data)
• h = channel DC gain (depends on distance, angles, Lambertian emission, concentrator gain, etc.)
• n(t) = noise (shot noise + thermal noise)

3. VLC Channel Components in IM/DD

The channel gain (h) in a typical line-of-sight (LOS) VLC channel is given by:

h = ((m+1) * A_det) / (2 * π * D^2) * cos^m(φ) * cos(θ) * G_Con * T_s(θ)
      

Where:

• m = Lambertian order (related to semi-angle at half power)
• A_det = detector area
• D = distance between LED and PD
• φ = angle of irradiance (from LED axis)
• θ = angle of incidence (to PD)
• G_Con = concentrator gain
• T_s(θ) = filter gain (if any)

4. Noise in IM/DD VLC

• Shot noise: due to the quantized nature of light
• Thermal noise: due to electronics in the PD
• These noises are usually modeled as additive Gaussian noise, n(t), in the received signal equation.

5. Summary

• IM/DD VLC uses light intensity modulation at the transmitter and direct detection at the receiver.
• The received signal is modeled as: received current = PD responsivity × optical power × channel gain + noise.
• The channel gain includes effects of distance, angles, Lambertian emission, concentrator gain, and FOV.
• This is exactly what your MATLAB code with H_A1, P_total, and P_rec is computing — it’s a LOS VLC channel model with IM/DD.

Further Reading