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October 12, 2015

Satellite radio has been very successful, especially in the US with the Sirius and XM radios, transmitting by dedicated satellites in S-Band. A very interesting alternative would be the usage of Ka-band transmissions enabling global coverage with local high gain spot beams. In this post, we focus on one of the main problems with Ka-band and transmissions for mobile terminals, and this is the problem of blockage. This post presents a blockage protection system based on very long interleaving and long period storage of the received signal, which makes it possible to overcome blockage periods of several seconds. The satellite radio signal is a relatively low-bandwidth signal with very large latency tolerance makes the solution feasible and cost – effective. As for the case of SiriusXM radio, terrestrial augmentation would be needed. This augmentation could not rely on Ka-band transmissions as those provide very short-range coverage for the terrestrial link. In this case a heterogeneous solution, based on auxiliary cellular, WiFi, DVB-T or ASTC would be required. However, one must note that the tendency towards mm-waves for 5G cellular networks, makes a Ka –Band augmentation an equally viable solution.

Blockage Protection Principle

The blockage protection described here is based on a combination of a long interleaver and a low rate, erasure resilient error correcting code. The erasure resiliency of the code is a function of the Line of sight (LOS) Signal to Noise Ratio (SNR) and the code rate. So during fading, the blockage resiliency is lower, but during low blockage probability intervals, the code will be able to endure deep fades. For a given LOS SNR, if a perfect interleaver and a long enough code word are used, the decoding success of a code word depends on the percentage of blockage (erasures) occurred during the interleaving time interval that spanned this specific code word. In order to calculate this success probability, a blockage model needs to be assumed.

Blockage Model

Land mobile channel models frequently use A Markov model. A simple channel model (but still very useful) is the 2 states Markov chain. This model is defined by the state transition probabilities and by a state transition time. the blockage model mathematical algorithm description is beyond the scope of this post.

From Theory to Practice

During late 2008, tens of satellite radio and TV systems with blockage protection were given to “friends and family” in the US. The vehicles were equipped with a small tracking antenna and a car set-top box that delivered 22 video channels + ten of radio channels. Due to the antenna size, a typical LOS SNR was around -7 dB.

The friend and the family campaign was thoroughly analyzed. The GPS data, as well as the modem statuses at each second, was recorded and then analyzed off-line.

A sample histogram of blockage length is shown below:

Analysis of the drive profiles, based on car speed, blockage length, and LOS probability is shown below:

Terrestrial Augmentation

The friend and family campaign showed that while at a typical driving mixture the long interleaver gives excellent results, the core urban areas are still problematic and a terrestrial augmentation may be needed. Using 3D maps of several cities, a ray tracing SW and a driving model, the performance of the combination of a satellite link and a terrestrial link, each with its own interleaver, was evaluated.
The driving model was based on a random path, where the speed between junction, the probability of a stop sign, priority sign, and a stop light, and the waiting time in each case, were randomly chosen according to some pre-defined probabilities. It was assumed that 10MHz around the 2GHz band are available while the transmitting power was a parameter to be optimized.

The Chicago test case

The figure below shows the Chicago street LOS (in green) of Chicago. The street LOS probability is ~ 62% and the video availability on a random drive is ~79% with an 180-second long interleaver.

The figure below shows video availability due to one terrestrial transmitter (with no satellite). The power required to get high video availability (80 dBm) is obviously not practical.

The figure below (left graph) shows the video availability on the same random drive with the combining of the satellite link and the terrestrial link, as a function of the terrestrial transmitter power and its interleaver length. As can be seen, very high video availability is achieved with a reasonable transmit power. The interleaver reduces the required transmitter power by up to ~10 dB. When adding another repeater the required power for the same video availability is reduced by additional 8-10 dBs as seen in the right graph.

The SX-3000 ASIC

The knowledge acquired from the field trials analysis and the simulation described above was exploited in the design of the SX-3000 ASIC. A typical car solution for radio or TV reception based on the SX-3000 ASIC. The SX-3000 enables diversity combining from up to 3 antennas. The TV output can be either in transport stream format or can be transmitted via WiFi to several tablets.

Conclusions

A blockage technology was theoretically analyzed, emulated (3D maps and ray tracing) and field tested. It was shown that with a proper design, very high service availability can be achieved, even in core urban areas, with a small number of terrestrial repeaters. This kind of blockage protection technology is especially relevant to the Ka-band since it handles deep short (tens of seconds) rain fades in the same manner it handles blockages. Finally, the new SX-3000 ASIC that offers an integrated cost-effective implementation of the blockage protection technology was introduced.

unt, and the power supply but those are much time counted as part of the installation, which is the next main target to go after.