Communication Infrastructure Corporation on microwave throughput

  This guest blog was provided by Communication Infrastructure Corporation.

We have been discussing microwave and its application in Ultra-Low Latency applications. Let’s discuss the next topic: throughput. How big are those pipes. Microwave transmission is governed by how the signals are modulated. There are many ways to modulate the signals and all of them impact the available throughput of a given channel. Channels of spectrum are regulated by the governments of the each Country. There are some differences in available channels and bandwidth of those channels as we move from Country to Country, but most are similar enough to generalize here. As we increase modulation in a radio, in most cases, we can increase throughput. Each manufacturer has a proprietary approach to their modulation schemes but all must comply to regulatory standards.

As the modulation becomes more complex, it is more difficult for a receiver to demodulate it reliably. It is easier for the receiver to confuse noise with the actual desired signal, therefore, proper operation will require a higher signal to noise ratio (S/N or SNR). The minimum receive signal level (RSL) becomes a higher level with increased modulation, effectively degrading the receive threshold. So, just turn up the transmitter power? It’s not so easy.

All things being equal, the more complex modulation creates an increased spectral emission. The regulatory agencies restrict spectral emission with 2 parameters: bandwidth and spectrum mask. A typical licensed Lower 6 GHz channel in the United States may have a  30 MHz bandwidth, while Europe limits 6 GHz bandwidth to 28 MHz. But there are also restrictions within your licensed channel. You are not allowed to just fill up the spectrum content of your 28 or 30 MHz channel. The regulators have established spectral emission masks which mandate the spectral shape to which the transmitter is limited. So, increasing modulation, by itself, may cause mask violation and require transmitter power to be reduced. Increasing modulation while simultaneously increasing transmitter power is even more likely to cause spectrum mask violation. Therefore, there is a significant variation among manufacturers of the combination of modulation and transmit power. You may be forced to choose between high modulation (BPS throughput) or high system gain (fewer hops).

After RF channel regulatory restrictions, another major factor that impacts the ability of a receiver to deal with S/N has to do with coding, interleave, forward error correction and, perhaps, some other proprietary treatments of the data stream. Generally, these data stream treatments allow for improved data throughput at a given RSL, but they consume processing time and add to latency. On the contrary, improving latency requires less data stream treatment of one sort or the other, leaving only RF solutions to tradeoff throughput versus latency.

One of the most effective measures some OEM’s are employing to reduce latency is to eliminate data stream treatments altogether and use IF Repeaters at some sites. The signal is not demodulated and re-modulated. This can reduce latency through those sites to nanoseconds and keep transmit powers at reasonable levels. It is a reflection upon the quality of the RF equipment (transmitter and receiver) as to how many IF repeaters can be in tandem. The noise of each hop will accumulate over a tandem system and eventually reach a point that the S/N is so bad that the signal cannot be recovered. Note that, even with IF repeaters, the noise accumulates more rapidly at high modulation rates than at lower rates. Again, the tradeoff between S/N and throughput is a crucial one. Finding the right balance and being able to wade through sales hype from manufacturer to manufacturer is important.

Communication Infrastructure Corporation is a silver sponsor of The Trading Show Chicago. Marty Snyder, President and CEO of Communication Infrastructure Corporation, spoke on a panel at the event on the topic “Innovations in network and connectivity – discovering the next generation of fastest and most efficient trading routes.”