## 4 Results and Discussions
### Dual Data Streaming Design  
An important result emerged from the determination of beam pointing dynamics for Ahmedabad and Guwahati links. It was found that for the Ahmedabad link, at the morning time of 0530 IST, for all of the days in the year, the received power in at least one of the elevated beam sets was either higher or lower than 4 db compared to the power received in the lowest beam set. A similar observation was made on 95% of the days for the evening hours of 1730 IST. For the Guwahati link, the same observation was made on 97% of days for the morning time and 99% of the days for the evening hours. Figure 5 shows the received power, \(P_{r(i)}\), obtained for the lowest beam (\(i=1\)) and the elevated beam (\(i=2\) to 10, whichever delivers the highest power at different times) for both the Ahmedabad and Guwahati links on all the observation days at 0530 IST as well as 1730 IST. The results show that the power received in one of the elevated beams is either higher or lower, but within 4 dB of the power received in the lowest beam at all times. Also, the differences between the yearly averages of the received power in the lowest and elevated beams on both links for morning as well as evening times are within 2 dB. Thus, it can be deduced from the obtained results that at any given time, there are at least two beam sets delivering high powers, with a difference of less than 4 dB between them. Hence, dual data streaming is possible on tropospheric links with dual beams between the transmitter and receiver. Also, as highlighted earlier, the lowest beam set delivers the highest power in almost 80-85% of the time. Thus, one set of transmit and receive beams in a dual-beam design should have fixed elevation angles corresponding to those of the lowest (first) beam set, just to clear their respective radio horizons. The second set of antenna beams should be  
dynamically steered with variable elevation angles corresponding to the elevation angles of beam set 2s to beam set 10. Elevation angles of steerable antenna beams, at any given time, should depend on whichever beam set (amongst beam sets 2 to 10) is giving the maximum received power at that time. Figure 5: Comparison of power received in the lowest beam and elevated beam for the Ahmedabad and Guwahati links. Figure 6 shows the basic design of a tropospheric communication system for dual data streaming using two beams. Solid rays forming the lowest common volume, shown as the shaded region, depict the fixed-beam set. Elevation angles of fixed transmit and receive beams are \(\alpha_{1}\) and \(\beta_{1}\), respectively. Dashed rays forming common volumes shown in the pattern fill depict the dynamically steerable beam set with elevation angles variable from (\(\alpha_{2}\), \(\beta_{2}\)) to (\(\alpha_{n}\), \(\beta_{n}\)). Conventional tropo systems employ quad diversity with two different and physically separated transmit and receive antennas. Each transmit antenna transmits two different frequencies: one vertically polarized and the other horizontally polarized. Similarly, each receiving antenna receives two different frequencies with orthogonal polarization. Quad diversity, i.e., a combination of space and frequency diversity, is necessary to mitigate the effect of fast fading through diversity combining. The dual-beam design should also incorporate quad diversity with two different beams from each of transmit antenna. Different beams from the same antenna may transmit at the same frequency, with individual data streams coded with orthogonal codes to minimize interference. Troposcatter systems conventionally use single transmit and receive antenna beams clearing their respective horizons or highest obstructions in front of the transmit and receive antennas, i.e., the transmit and receive antenna beams with the lowest possible elevation angles, represented by solid rays in Figure 6. The performance of a conventional design using a single beam and a dual data streaming design using two beams needs to be compared in terms of achievable data rates.