Impact of increasing threshold level on higher bit rate in free space optical communications

Abstract

The biggest challenge facing free space optical deployment is optical signal propagation in different atmospheric conditions such as fog, low clouds, rain, snow, dust, haze and various combinations of each. A transmission and detection technique for free space optical communications is proposed where it employs two beams, one modulated with the data, the other with the inverted version of the same data. A differential detection technique is used at the receiver whereby the inverted data are used as the reference to perform the decision making, as opposed to the fixed threshold used in the conventional technique. The probability of error under threshold instability in the proposed differential technique and Intensity Modulation/Direct Detection (IM/DD) technique are compared. A simulation under heavy rainfall condition of 8.33 × 10 −4 cm/sec at bit rate of 155 Mbps to 10 Gbps with 0 dBm of launch power and 1.5 km distance shows that this technique achieves an improvement compared to the conventional IM/DD. The differential detection can support a higher bit rate (up to 9 Gbps) because of its noise reduction capabilities due to higher threshold level implemented in the receiver. The IM/DD in contrast, can support bit rates of only 2.5 Gbps. This analysis focuses on weather conditions in Malaysia.

FSO Detection Using Differential Signaling in Outdoor Correlated-Channels Condition

“…This is similar to the work in [2] with the difference that the authors did not take into account the variance of the detection threshold in (8b), which results from AT. However, for ߩ ଵ,ଶ = 1 (i.e., the highly correlated channels), we have Var(ܸ ௧௦ ) = 2ߪ ଶ .…”

“…HE received signal in free space optical (FSO) communication systems is highly sensitive to the atmospheric effects such as fog, smoke, low clouds, snow, rain and the atmospheric turbulence (AT) [1][2][3][4] that may result in severe power loss and channel fading. In non-return-to-zero on-and-off keying (NRZ-OOK) intensity-modulation/directdetection (IM/DD) systems, an optimal detection threshold level (DTL) at the receiver can be used to distinguish the received '0' and '1' bits.…”

“…Since these optical power fades are often very deep, simply increasing the transmit power ܲ ௧ and using a fixed optimal DTL in this case would not be effective [2].…”

“…Also the same idea was adopted in [2] that used a pre-fixed optimal DTL for various atmospheric channel conditions (rain, AT, etc.). The detection technique did not relied on CSI (with increased computational load at the receiver) and PS or a training sequence [2]. However, their simulation based investigation only considered narrow collimated beams without overlapping and with no experimental verification.…”

Impact of Link Parameters and Channel Correlation on the Performance of FSO Systems With the Differential Signaling Technique

“…he received signal in a free space optical (FSO) communication system is highly sensitive to the deterministic and random factors associated with the atmospheric channel, such as fog, smoke, low clouds, snow, rain, and turbulence [1][2][3]. Additionally, the pointing errors due to the building sway vibration and thermal expansion can further deteriorate the FSO link performance [4][5][6].…”