Since the beginning of wireless there has been a hard and fast trade-off between capacity and range. The further a wireless connection can go, the less data it can support and vice versa. Today’s applications and user demands are pushing networks limits by demanding more and more bits/second and at the same time requiring these bits travel miles.
Taking Gigabits Further
mmWave connections fight a different battle. mmWave links deliver easily fiber capacities of 1Gbps up to 10Gbps full duplex. And the only trade-off is availability versus rain events. Links up to 1 to 2 miles achieve high availability in the 4 to 5 9’s; however customers want these links to stretch to four, five, six miles or more. Unfortunately, this flies in the face of availability versus rain. But there is a solution: with a well thought out approach multi-gigabit per second links can meet these range and availability requirements.
mmWave and Range
In the 70/80GHz mmWave spectrum, the E-band, range is determined by a combination of system gain or link budget and availability targets taking into account the statistical rain patterns in the area. Availability is measured in how many seconds or minutes per year the link is up and transporting traffic. For example, a “5 9’s” availability means the link will be connected 99.999% of the time, or all but 6 minutes per year. Statistical rain patterns are gathered by international organizations such as the International Radio Telecommunications Union (ITU-R) and organized in prediction models for the calculations of their effect on radio links.
The needed availability is determined by any organization and their application or use case for the link. 99.999% used to be the Telco standard in the days of E911 POTS (plain old telephone service) and their networks being the carrier of last resort. Values between 99.99% are much more common when redundant facilities are also easily implemented. This means that out of all the seconds, minutes and hours contained in a year the mmWave link will be up and operational for all but 53 mins. This equates to a little more than four minute per month, and is almost always experienced in disruptions of a few seconds at a time, when the rain is at the heaviest.
With mmWave systems, a 10Gbps FD link may reach a full mile with a 99.99% availability in Las Vegas. But what if the requirement is to reach 6 miles?
Extending mmWave Range
When the same 10Gbps FD link above, same system gain, same location, has the availability requirement reduced to 99.9% the link can go as far as 4 miles. However, this means the connection is down an aggregate of 600 minutes per year, and now perhaps a few minutes at a time, which is clearly not allowed.
Fortunately, there are a number of other radio bands in which a lower capacity back up link can be deployed in conjunction with the mmWave link, and which will support a 4-mile connection with 99.99% or above availability, and with enough capacity to carry the critical traffic. The concept here is not hard, but delivering the dual link solution in a way that minimizes installation work, tower space, administration and cost is the challenge. Key components of a viable dual band solution include:
- A single antenna if possible. This means deploying a dual band antenna, E-Band + 5GHz, E-Band + 11GHz, E-Band + 18GHz etc. A single antenna would reduce installation time and most importantly the tower rental costs.
- A single cable run limiting the installation and wiring efforts. The mmWave radio should have a built in high speed switch to connect to the backup link as well as supply a power to the secondary radio link, ideally with PoE.
- Because the secondary link is lower capacity, even though it may only be in use for a few seconds at a time, it is vital that the system recognizes traffic priority or classes of services (CoS). The system should identify high priority packets such as voice or control traffic, and apply appropriate QoS to ensure timely delivery while delaying other less delay sensitive data packets.
- Advanced software that configures and monitors the main and the backup links and determines based on rf quality and throughput when to automatically switch to the backup link, and as important when to switch back to the full capacity mmWave system.
- A combination of proper hardware design and software implementation can perform the switching between radio links in sub-50 milli seconds, and do so with zero packet loss.
If the mmWave solution supports the five key features listed above, installation, cost and operation can be reduced to an absolute minimum. The result is a multi-gigabit mmWave solution that goes the distance and does so at a cost that makes sense.