Microwave Backhaul

This February at the Mobile World Congress, Intel invited me to experience their WiMAX network that they had set up in the city during the event. Of course I had lots of technical questions and one of them was on the backhaul equipment they used to connect the base stations to the network. I was told that they were using 50 Mbit/s microwave links with equipment from Dragonwave. Impressive I thought at the time and was thus happy to see that Dragonwave attended a recent WiMAX event in Munich. An excellent opportunity to get more details. Turns out that the 50 MBit/s, hyperfast compared to the 2 MBit/s E1 links used today in most 3G networks, is just the entry level speed. According to the data sheets, the system is capable of doing about 800 MBit/s with dual polarization and 256QAM modulation over a 56MHz channel and 1.6 GBit/s when using 2 channels. The range of the system at this speed is several kilometers. That's much faster than what you need even for LTE and WiMAX in the short and medium term per base station. However, base stations are usually daisy chained in a ring configuration so if you chain five base stations together that each require 100 MBit/s (e.g. three sectors), that's already 500 MBit/s.

The longest distance they have bridged with their kit so far at 50 MBit/s hiltop to hiltop was 75km. Another impressing number.

Equally impressing is the power output required. I always thought microwave equipment operates at very high power. Looks like I was wrong, the output power of the system is a mere half watt. Of course the gain of the antenna is very high but still…

As the system transports Ethernet/IP frames natively, most LTE and WiMAX base station can use the system right away. Current 3.5G base stations, however, are still using TDM links. For these, a pseudo wire box can be used to tunnel such connections over Ethernet.

I also learnt a lot about Microwave frequency license costs. Looks like the cost differs widely. In the U.S., I was told, a basic license for a channel (can't remember the exact bandwidth anymore) is around 1800 dollars for ten years. Compare that to France where the cost for the same license is around a thousand Euros  for a single year. Channel sizes that can be licensed start around 6 MHz and go up to 28 MHz. Again depending on the country, the microwave spectrum is anywhere between 11and 38 GHz.

Like all wireless systems, microwave links don't like things such as rain and snowfall. E1 based microwave links therefore need a security margin in order not to fail under such conditions. In the world of Ethernet, however, this margin can be translated into higher speeds when conditions are fine and the automatic link adaptation during  rain and snowfall by automatically lowering the data rate. By prioritizing VoIP packets, it can be ensured that voice calls are not impacted by this while background priority traffic such as web browsing gets slower during such times. I know, easier said than done in practice, IMS definitely needed for this. But that's another topic.

When asked on the price point of the system I was quoted a mid four digit number for an end to end link, depending of course on the quantity purchased. Compared to the price of a single 2 MBit/s E1 link, which easily reaches several hundred Euros per month, that's very competitive. Comforting to know that backhaul prices scale well with rising consumer demand for wireless access.

Thanks again to Dragonwave for the interesting interview, I learnt a lot!