On-off Keying Schemes Research Articles

Reduction in transmitter power requirement for earth-to-satellite and satellite-to-earth free space optical links with spatial diversity

In the scenario of first laser communication relay satellite being launched into geostationary earth orbit, we evaluate the reduction in transmitter power requirement for earth-to-satellite and satellite-to-earth free space optical links in presence of turbulence and various weather conditions using spatial diversity technique. In channel modeling, Beer Lambert Law incorporates the weather effects. The log-normal probability density function (pdf) models weak turbulence and gamma–gamma pdf moderate to strong turbulence. Using the combined channel state pdf, bit error rate (BER) expressions are derived for on-off keying (OOK), M-ary pulse position modulation (M-PPM) and M-ary differential PPM (M-DPPM) schemes. From the BER plots, we evaluate the minimum average received power required to achieve a desired BER for all three schemes for different channel conditions. Subsequently, minimum transmitter power requirement is evaluated for both uplink and downlink using the range equation. It is observed that presence of moderate, light and thin fog cause additional power requirement. Also, among the three schemes, M-PPM scheme requires the least transmitter power, followed by M-DPPM and OOK schemes. Further, it is seen that the transmitter diversity or multiple input single output technique reduces the uplink minimum transmitter power requirement, whereas for downlink aperture averaging and receiver diversity or single input multiple output techniques can achieve the same. The power requirement for uplink is 8–10 dB more as compared to downlink in presence of turbulence and various weather conditions.

Aperture averaging and receiver diversity for FSO downlink in presence of atmospheric turbulence and weather conditions for OOK, M-PPM and M-DPPM schemes

We evaluate the error performance of a free space optical (FSO) downlink from a geostationary earth orbit satellite to an earth station in presence of turbulence and different weather conditions such as fog, clouds, etc. Combined channel state probability density function (pdf) is derived by using log-normal model for weak turbulence, gamma–gamma model for moderate to strong turbulence and Beer–Lambert Law for weather effects. Bit error rate (BER) expressions are derived using the combined channel state pdf for on-off keying (OOK), M-ary pulse position modulation (M-PPM) and M-ary differential PPM (M-DPPM) schemes. It is observed that the link performance degrades with increase in the strength of ground level turbulence. Presence of weather conditions causes additional degradation with moderate fog causing the worst effect followed by light and thin fog with respect to clear air condition. Dense or thick fog and/or clouds may lead to complete link failure owing to their large attenuation coefficient. Further, enhancement in the link performance by using aperture averaging and receiver diversity techniques is examined and compared for all three schemes. It is seen that performance improves with increase in the receiver diameter and number of multiple independent receivers. Among the three modulation schemes, link with M-PPM scheme gives the best performance in terms of BER followed by M-DPPM and OOK schemes with or without diversity techniques. Thus, an M-PPM based FSO downlink with an array of finite sized receivers i.e., single input multiple output link can be a viable alternative for efficient data transfer in presence of atmospheric turbulence and different weather conditions from a satellite to an earth station.

Multichannel Ultrasonic Data Communications in Air Using Range-Dependent Modulation Schemes.

There are several well-developed technologies of wireless communication such as radio frequency (RF) and infrared (IR), but ultrasonic methods can be a good alternative in some situations. A multichannel airborne ultrasonic data communication system is described in this paper. ON-OFF keying (OOK) and binary phase-shift keying (BPSK) modulation schemes were implemented successfully in the system by using a pair of commercially available capacitive ultrasonic transducers in a relatively low multipath indoor laboratory environment. Six channels were used from 50 to 110 kHz with a channel spacing of 12 kHz, allowing multiple 8-bit data packets to be transmitted simultaneously. The system data transfer rate achieved was up to 60 kb/s and ultrasonic wireless synchronization was implemented instead of using a hard-wired link. A model developed in the work could accurately predict ultrasonic signals through the air channels. Signal root mean square (rms) values and system bit error rates (BERs) were analyzed over different distances. Error-free decoding was achieved over ranges up to 5 m using a multichannel OOK modulation scheme. To obtain the highest data transfer rate and the longest error-free transmission distance, a range-dependent multichannel scheme with variable data rates, channel frequencies, and different modulation schemes, was also studied in the work. Within 2 m, error-free transmission was achieved using a five-channel OOK with a data rate of 63 kb/s. Between 2 and 5 m, six-channel OOK with 60 kb/s data transfer rate was error free. Beyond 5 m, the error-free transmission range could be extended up to 10 m using three-channel BPSK with a reduced data rate of 30 kb/s. The situation when two transducers were misaligned using three-channel OOK and BPSK schemes was also investigated in the work. It was concluded that error-free transmission could still be achieved with a lateral displacement of less than 7% and oblique angles of less than 7°, and three-channel BPSK proved to be more robust than three-channel OOK with transducer misalignment.

Analysis of earth‐to‐satellite free‐space optical link performance in the presence of turbulence, beam‐wander induced pointing error and weather conditions for different intensity modulation schemes

In this study, the authors evaluate the performance of an earth-to-satellite free-space optical link in terms of bit error rate (BER) for three intensity modulation (IM) schemes [on–off keying (OOK), M-ary pulse position modulation (M-PPM) and M-ary differential PPM (M-DPPM)] and direct detection receiver. The performance is analysed in the presence of atmospheric turbulence, beam-wander induced pointing error and weather conditions. Turbulence with beam wander effect is modelled using gamma-gamma distribution and the weather effects are incorporated using the Beer-Lambert law. They obtain closed form BER expressions for the above IM schemes using combined channel state probability density functions. They show that for the same average power, M-PPM offers the best performance, followed by M-DPPM and OOK schemes. Link performance degrades for all IM schemes with increase in the value of ground level refractive index structure parameter and bit rate. Presence of weather conditions like moderate, light and thin fog increase signal attenuation and this increase may be very high in case of thick and dense fog or clouds and can lead to link failure.

Performance analysis of coherent bandwidth-limited time-spreading encoding OCDMA system employing pulse-position modulation

Time-spreading (TS) phase encoding optical code division multiplexing access (OCDMA) systems with pulse-position modulation (PPM) signalling are investigated, the multiple access interference, optical beat noise and bandwidth limitation of receiver are considered. The authors also compare the performance of the PPM system with that of the conventional on-off keying (OOK) system with optimum threshold detection. Numerical results show that there exists an optimum pulse-position multiplicity for the PPM OCDMA system, and the corresponding PPM scheme is superior to OOK scheme in the coherent bandwidth-limited TS OCDMA system under the chip time constraint.