毫米波通信技術應用介紹（二）

Campus & Enterprise Facility Networks

Millimetre Wave Wireless Networks are very suited to both long term and short term solutions where organisations need to connect existing or new buildings to a high bandwidth broadband network. Installing leased lines between two buildings can be a long and expensive process but with the ability to install and set up the wireless links within hours, short term, immediate Millimetre Wave Wireless Networking applications are possible. With the high throughput and distribution density capabilities, long term Millimetre Wave applications are also very well suited.

Connection Redundancy and Failure Recovery

Many businesses and locations rely on fibre optic cable connections to deliver high bandwidth internet access and it has been known that incidents can occur, damaging these cables and completely removing all connectivity. This results in leaving these businesses in a critical condition in which they cannot function for a long period of time until the fibre optic cable is repaired. It is therefore very desirable to implement a back up, failover system to stop this from happening, reducing downtime.

Millimetre Wave Point-to-Point links can be installed alongside fibre optic connections to ensure that whenever a problem is detected with the optic cable, internet and network access is transferred directly to the wireless link minimising the effect on connection performance and availability.

Cellular Network Backhaul

With mobile users putting ever increasing demands on the cellular network, with respect to utilising the new bandwidth intensive applications, infrastructure technology has to improve to be able to cater for this. Currently used T1/E1 leased lines will not be able to handle this demand soon and Millimetre Wave Wireless Networking links will have to be implemented between the 4G Access Points and the network's core.

Low cost licensing and high bandwidth capabilities make Millimetre Wave Point-to-Point Ethernet Bridges the technology of choice for the immediate and foreseeable future, in comparison to laying a fibre line from the network core to the tower, with the ability to upscale deployment and bandwidth easily in confined spaces.

DAS (Distributed Antenna Systems)

Distributed Antenna Systems are used to provide even coverage of connectivity for networks across wide areas where one central antenna could not. Millimetre Wave Point-to-Point links can be installed to couple these remote antennas to the main base station to provide high throughput data transfer where laying fibre cables would not be practical. These remote antenna and Millimetre Wave link pairings can also be used throughout large enterprise buildings where a large subscriber base will be utilising all of the bandwidth capability making it more efficient to use multiple links rather than bottle necking a single line. Millimetre Wave links are the ideal solution for transferring DAS signals wirelessly.

Some other solution areas where Millimetre Wave applications can be used are:

• Disaster Recovery

• Financial/Banking

• HDTV Broadcast

• Healthcare

• Retail

• Transportation

• Video Surveillance

Performance

Bandwidth & Scalable Capacity

The main benefit that Millimetre Wave technology has over lower RF frequencies is the spectral bandwidth of 5GHz being available in each of the E-Band ranges, resulting in current speeds of 1.25Gbps Full Duplex with potential throughput speeds of up to 10Gbps Full Duplex being made possible. Once market demand increases and better modulation techniques are implemented, spectral efficiency of the equipment will improve allowing the equipment to meet the higher capacity demands of prospective future networks.

Whereas low frequency, microwave signals have a wide beamwidth angle which reduces the reuse of transmission of the same signal within the local geographic area, Millimetre Wave signals transmit in very narrow, focused beams which allows for multiple deployments in tight proximity whilst using the same frequency ranges. This allows a density of around 15 times more when comparing a 70GHz signal to a 20GHz example making Millimetre Wave ideal for Point-to-Point Mesh, Ring and dense Hub & Spoke network topologies where lower frequency signals would not be able to cope before cross signal interference would become a significant limiting factor.

Propagation & Signal Attenuation

In general, Millimetre Wave links can range in anywhere up to 10km depending on factors such as equipment specifications and environmental conditions. The propagation properties of Millimetre Waves are much like those of the other popular wireless networking frequencies in that they are most significantly affected by air moisture levels; atmospheric Oxygen is also a large factor in the 60GHz band but almost negligible in the other ranges, under 0.2 dB per km.

Water vapour affects the signal at between 0 and 3dB/km at high humidity levels and the propagation due to clouds and fog acts in a very similar way depending on the density and amount of droplets in the air. These losses are relatively low and only play a major factor when considering links at 5km+.

At the 70 to 80GHz bands, water, in the form of rain, plays the most significant role in signal attenuation as it does with lower frequency signals too. The rate of rainfall, measured in mm/hour, is the depending factor in signal loss meaning that the harder it is raining, the lower the signal strength will be. Signal Propagation loss is also directly proportional to distance, so if the distance between transmitter and receiver is doubled, the loss in dB will be twice as much. Millimetre Wave performance is quite heavily dependent on rainfall and strongly affects Availability (discussed below), however, successful links can even be set up in areas of occasional heavy downpours.

Availability

The reliability of a Millimetre Wave Wireless Network relies on the same principles as any other, in particular, the distance of operation, the radio's link margin (being factors of transmit power, receiver sensitivity and beam divergence) and others such as redundancy paths. A link may be heavily affected by a period of intense rainfall but if it has a large enough margin, it will not suffer an outage.

The reliability of a network is called the availability and is measured as a percentage of time that the network will be functioning, for example, an availability of 99.999% over a year will equate to just over 5 hours of downtime. Much research by the ITU (International Telecommunication Union) has gone into collecting rainfall date from metropolitan areas around the world and how it will affect Millimetre Wave transmissions. You can see below an example of the expected availability of a widely available Millimetre Wave link for a few global cities and their respective availability for a 2km link.

Security is also an issue when dealing with wireless transmissions but due to Millimetre Wave's inherently low beam widths ("pencil beams") at about 0.36° radius with a 2ft. antenna along with, generally, lower peak transmit powers relative to lower frequencies the technology has a low probability of intercept and detection which is vital for the transference of confidential material.