The INDEED solution is a cellular-type hybrid line-of-sight link (Figure 16.1, ‘ ).
The downstream light sources are 1550 nm Fabry-Perot cavity semiconductor laser diodes (-FP-LDs). Each cell transmitter contains three diodes and one-diode optical output power is 8 mW.
Their essential characteristics and specifications are:
Several longitudinal modes around 1550 nm and a low degree of coherence to avoid speckle problems at the output of optical multimode fibre divergent adaptors.
The resonance frequency must be higher than 1 GHz to avoid excess relative intensity noise and limits of the modulation frequency.
The emitting area is approximately 1 to 2 ¼ m 2 and its radiation pattern opens up to 40 °.
The photo-detector is a semiconductor PIN-hetero-junction photodiode. The semiconductor is a III-V compound: InGaAs/InP. The active area is 0.2 mm 2 (Figure 16.5).
The OOK modulated signal, coded in NRZ (No Return to Zero), is then amplified and processed to obtain a minimum detectable power (or sensitivity) of ˆ’ 40 dBm (100 nW) at 155 Mbit/s at 10 ˆ’ 9 BER.
Typically, a concentrator [Kahn, 1997; Street, 1997] is used, with a 1550 nm selective filter. The presence of a lens (light energy concentrator) justifies itself especially in directive systems where the concentration bonus is inversely proportional to numerical aperture of receiver (FOV-Rx) [Street, 1997]. As diffuse systems, the gain in power can however reach 3 dB with sources approaching 900 nm [Kahn, 1997].
The INDEED photo-receiver unit is very different from the devices tailored ( custom-designed ) for optical fiber applications:
The photodiode is a "large sensitive area" component to collect the highest number of photons . Therefore, the parasitic "function+metallisation" capacitance is much higher. The structure of the low signal-low noise transimpedance amplifiers need to be revised.
The photodiode is exposed to ambient radiation and must be protected and shielded .
The noise sources are due to:
Shot noise, noise due to random nature of light emission and thus detection, associated with exit current at darkness (photons generation and recombination),
Scintillation noise due to random movements of junction mobile charges,
Detector thermal noise ( Background Limited Infrared Photodetector BLIP, detector very sensitive to IR and the obscurity noise which is essentially due to photons issued from ambient radiation).
Surface effects noise, as the consequence of capture and release of mobile charges.