2,000 Reasons to Choose RDMS™

How Much Does a dB Cost?

Hint: It’s probably less than you think.

Quasonix delivers equipment from end to end on telemetry links, including transmitters, antennas, receivers (with integrated combiner, demodulators, and decoders), and stream combiners. We are ideally positioned to help our customers find the best possible equipment lineup to ensure flawless telemetry data with optimized cost and schedule.
 
The fundamental figure of merit in designing a complete TM link is link margin, which quantifies whether the link has enough signal-to-noise ratio (SNR) to ensure error-free data transmission. The legacy approaches to increasing link margin have included increasing the transmitter power and increasing the gain of the antenna that receives the transmitter power.  Both approaches are technically effective but carry impacts in size, weight, power consumption, cost, and delivery schedule.
 
Every dB of link margin is as valuable as every other dB, but some are very expensive.  So, we have to ask the question, how much does a dB of link margin need to cost?
 
For the transmitter, RF power and cost are related, but not in a “linear” fashion. For example, a 10-Watt transmitter is not twice as expensive as a 5-Watt; in fact, a “typical” 10-watt transmitter might only cost 10% to 15% more than a comparable 5-watt transmitter. However, there are only so many dB available in the transmitter aisle of the link margin store. 20 to 25 watts is generally regarded as the practical “high end” for transmitter power. Trying to develop more than that exacts a high price in power consumption, heat, size, and weight.
 
On the receiving antenna side of the link equation, there are a lot of dBs available from increasing the antenna size. However, doubling the area of a receiving antenna (same link margin gain as doubling the transmit power) can be very expensive.  Taking into account the infrastructure necessary to utilize a larger antenna, increasing link margin through larger receive antennas can become cost prohibitive very quickly.
 
In short, buying link margin through transmit power is moderately cost effective, but limited in reach. Larger receive antennas can develop enormous gains in link margin but can quickly run to seven figures in cost and take months or years to deploy. What to do?
 
In 2011, Quasonix introduced Low Density Parity Check (LDPC) coding to the telemetry market. The math defining LDPC codes dates back to 1963, when it was introduced by Robert Gallager at MIT. While the performance of the LDPC codes was spectacular, there was no practical hardware implementation with the silicon of that era. However, the math began to take a hardware form in the late 1990s, and in 2015, the Range Commanders Council adopted the LDPC codes as a standard in IRIG 106-15.
 
The link margin is directly related to the “coding gain” afforded by the codes, as shown in the figure below.

The coding gain essentially lowers the SNR required to reach the desired bit error rate. In the figure above, the target BER is 1e-6, and the gain for the code highlighted with the black arrow is about 9.8 dB. This is 9.8 dB that adds directly to the link margin. That addition can be used to increase range, decrease transmitter power, reduce the receive antenna size, or some combination of the three.
 
Using the LDPC codes requires the transmitter and receiver to be compatible, (which they need to be anyway) and is simply achieved by enabling the coding in both the TX and RX equipment. Adding all six of the IRIG-standard LDPC code variants to a typical 10-watt Quasonix transmitter costs less than $3,000, offering a huge return on investment. It’s worth noting, however, that achieving these results is not a given; it depends on the receiver you are using. The LDPC decoding algorithm is a computationally demanding task, and some decoders in the TM market fall short of the measured performance, shown in the figure above, that is achieved by Quasonix RDMS receivers.
 
Conclusion: The LDPC codes in the Quasonix transmitters and receivers can add over 10 dB to the link margin. No other technology can create so much link margin at such low cost.