Author Topic: Improved Performance of M-ary PPM in Different Free-Space Optical Channels  (Read 2920 times)

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Improved Performance of M-ary PPM in Different Free-Space Optical Channels due to Reed Solomon Code Using APD
Author : Nazmi A. Mohammed, Mohammed R. Abaza and Moustafa H. Aly
International Journal of Scientific & Engineering Research, IJSER - Volume 2, Issue 4, April-2011
ISSN 2229-5518
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Abstract— Atmospheric turbulence induced fading is one of the main impairments affecting the operation of free-space optical (FSO) communication systems. In this paper, the bit error rate (BER) of M-ary pulse position modulation (M-ary PPM) of direct-detection and avalanche photodiode (APD) based is analyzed. Both log-normal and negative exponential fading channels are evaluated. The investigation discusses how the BER performance is affected by the atmospheric conditions and other parameters such as the forward error correction using Reed Solomon (RS) codes and increasing Modulation level. Results strongly indicate that, RS-coded M-ary PPM are well performing for the FSO links as it reduces the average power required per bit to achieve a BER below 10-9 in both turbulence channels.

Index Terms— Free Space Optics (FSO), M-ary Pulse Position Modulation (M-ary PPM), Reed Solomon (RS) codes, Log Normal Channel, Negative Exponential Channel, Avalanche Photodiode (APD).

Free space optical (FSO) systems have been widely deployed for inter-satellite and deep-space communi-cations. In recent years, however, because of its nu-merous advantages over radio-frequency (RF) technology such as extremely high bandwidth, license-free and interference immunity, FSO has attracted considerable attention for a variety of applications, e.g. last mile connectivity, optical-fiber backup and enterprise connectivity. In such kind of applications, FSO systems basically utilize atmosphere as transmission medium rather than the free space. So, the performance of FSO link is inherently affected by atmospheric conditions. Among these conditions, atmospheric turbu-lence has the most significant effect. It causes random fluctuations at the received signal intensity, i.e., channel fading, which leads to an increase in the bit error rate (BER) of the optical link [1].
Current FSO communication systems employ intensi-ty modulation with direct detection (IM/DD) and use light emitting diodes (LED) or laser diodes as transmitters and PIN photodiode or avalanche photodetectors (APD) as receivers. These devices modulate and detect solely the intensity of the carrier and not its phase. Furthermore, biological safety reasons constrain the average radiated optical power, thereby constraining the average signal amplitude. The most reported modulation technique used for FSO is the on-off keying (OOK) which offers bandwidth efficiency but lacks power efficiency. Binary level signaling though is the simplest and most common modulation scheme for the optical intensity channel and offers low power efficiency and high bandwidth efficiency. Power efficiency as well as the improved system performance can be achieved by adopting pulse position modulation (PPM) schemes. M-ary PPM achieves high power efficiency at the expense of reduced bandwidth efficiency compared with other modulation schemes. The optimal PPM order is high, since a higher order modulation creates the higher peak power needed to overcome the weak average power. M-ary PPM has been previously suggested as a suitable modulation scheme for FSO systems [2]. The IrDA specification for the 4 Mbps short distance wireless infrared links specifies a 4-PPM modulation scheme [3].
Reed Solomon (RS) codes are a class of block codes that operate on symbols rather than bits. So, RS codes can correct both random bit errors and burst symbol errors. Moreover, their hard decoding algorithm can be easily implemented even at a high operation speed. International Telecommunication Union-Telecommunication (ITU-T) standard forward error correction (FEC) scheme based on RS (255,239) codes has been widely used in l0 Gbps practical optical fiber transmission systems [4]. Kiasaleh derived upper bounds on the BER of M-ary PPM over log-normal and exponential distributed channels, when an APD is used [5].
In this paper, Kiasaleh expressions are used to com-pare the effect of the BER of average photons per PPM bit with the former channels without coding and with RS (255,207) coding. The remainder of the paper is organized as follows. The models of FSO channels are presented in Section 2. Based on the theory presented, a numerical analysis of the M-ary PPM is carried out in Section 3. This is followed by the main conclusions in Section 4.

2.1 Log-Normal Channel

The log-normal channel is classified as “weak turbu-lence”, which is characterized by a scintillation index less than 0.75. In general, the scintillation index is a complicated function of the beam parameters, propagation distance, heights of the transmitter and receiver, and the fluctuations in the index of refraction. In fact, the main source of scintillation is due to fluctuations (due to temperature variations) in the index of refraction, which is commonly known as optical turbulence. The log-normal model is also valid for propagation distances less than 100 m [5].

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