By: Frank Ohrtman

Note: This is the first of two articles on backhaul to support WiMAX access. This article will deal with preliminary technical considerations in choosing a backhaul solution. A second article will deal with considerations for architecting the backhaul solution.

The next generation of residential and business services will require great deals of IP bandwidth.  HDTV, for example, commands 19 Mbps per TV set.  Multiply that by 2 or 3 TV sets per household in the US, for example, and we begin to see the heretofore unimagined bandwidth demand necessary to support a triple or quadruple play of competitive telecommunications services.  Service providers planning nation wide rollouts are currently shopping for and testing wireless backhaul solutions.

This article will explore service provider options in deploying gigabit Ethernet backhaul to support high bandwidth WiMAX services.  Figure 1 below provides a high overview of what is necessary to support a WiMAX network to an appropriate capacity.
 

Figure 1: A successful WiMAX deployment will usually require a wireless backhaul infrastructure

Demand for business broadband is soaring.  Where a T1 (1.54 Mbps) was once thought to be a business luxury, multiple megabits, if not gigabits, for internet connectivity are now mandatory in even the least technical of businesses.  WiMAX service for any business district will require gigabit speeds in order to support 40 Mbps WiMAX radios (think multiple radios per base station scaling to meet demand in the coverage zone).

Wireless Backhaul Solutions
There are many wireless backhaul technologies on the market.  For supporting WiMAX operations in the enterprise and urban markets, the best solutions are millimeter-wave and E-Band. The options for wireless backhaul, simply put, are 60 GHz and 80 GHz radios.  The short range (a few kilometers) of these products is, relative to other wireless backhaul options, the strength of the solution.  Why the short range?  Oxygen molecules absorb electromagnetic energy at 60 GHz.  Ergo, the electromagnetic waves diffuse at ranges beyond a few kilometers thus diminishing their ability to interfere with other broadcasters on the same frequency or to be intercepted and exploited by hackers.

60 GHz: Millimeter-wave
The 57-64 GHz band (best known as 60 GHz) is located in the millimeter-wave portion of the electromagnetic spectrum.  Advantages of using this band include interference mitigation, strong security, traffic prioritization (QoS), frequency re-use and rain fade mitigation.  In the US, the 60 GHz band is unlicensed.
 

80 GHz: E-Band
E-Band refers to 10 GHz of licensed-band spectrum allocated by the US FCC, split between 71-76 GHz and 81-86 GHz.  With more spectrally efficient modulations, full duplex data rates of 10 Gbps (OC-192 or 10GigE) can be reached.  Diffraction in the 80 GHz band is not as severe as it is at 60 GHz thus allowing greater range at 80 GHz over 60 GHz. E-Band products are similar to millimeter-wave except that in the US, for example, E-Band frequencies are licensed by the FCC in a streamlined licensing process.  Licensed spectrum products are allowed more power than unlicensed spectrum products; further enhancing range and throughput for the 80 GHz products.

Very Narrow Beamwidth
The advantages of the 60 and 80 GHz Bands is that they over come the fallibilities of lower frequency backhaul options.  Antenna directivity (beamwidth) is limited by the physical principle of diffraction wherein the beamwidth is inversely proportional to the operating frequency. So, at 60 GHz, for example, the beamwidth is far narrower than at lower frequencies (5.8 GHz, for example).  This very narrow beam gives the backhaul solution at least 4 advantages:

(1) Avoid interference from other emitters in the same band as the beam is so narrow the potential for interfering with another such beam on the same frequency is very remote.

(2) Offer superior security: the beam is so narrow, it is difficult to intercept or other wise exploit

(3) Offers a high rate for frequency reuse in the backhaul network

(4) Power to overcome rain fades
 

Table 1: Comparisons of beamwidths of wireless backhaul solutions

 

Backhaul Requirements
When engineering a backhaul network to support a WiMAX network, the service provider should consider the following elements in backhaul planning:

1. Range and throughput demands of the market
2. Security
3. Quality of service (QoS)
4. Interference mitigation
5. Frequency reuse
6. Rain fade
7. Reliability/availability
8. Exclusive/near-exclusive frequency use

The selection of backhaul solutions, like the WiMAX platforms they support, should be driven by the business plan.  That is, selecting the appropriate platform the market.  This article assumes an enterprise/urban market.

1. Range and Throughput
At first look, a backhaul solution offering a range of a couple kilometers would not seem to fit the notion of backhaul.  There are wireless backhaul solutions on the market that offer ranges up to 160 Kms. For enterprise or urban markets, this isn't necessary.  Millimeter-wave and E-Band backhaul solutions are best suited for the enterprise and urban market.  As illustrated in Figure 2 below, Manhattan south of Central Park is approximately 3 Kms wide and 8 Kms long.  The 60 and 80 GHz wireless backhaul solutions are an ideal fit for the demands of markets like this.
 

Table 2: Range and throughput parameters for 60 and 80 GHz backhaul solutions

An old saw in the telecom world goes "Bandwidth is the answer, now what was the questions?" The 60 and 80 GHz products are also known as Gig-E radios indicating their throughput.  A WiMAX service provider in an enterprise/urban market must plan for gigabit speeds through their base stations.  These speeds are not available through other wireless backhaul solutions.

Figure 2: Long-range backhaul is not required in most enterprise or dense urban markets. Example: Manhattan south of Central Park

 

2. Security
Given that a backhaul solution will need to support base station that might have dozens of WiMAX radios servicing thousands of enterprise subscribers, the security of a wireless backhaul solution should be of paramount concern to the WiMAX service provider. In the service provider market, casual hackers are or less of a concern than wholesale theft of service via rogue base stations.  Figure 4 below illustrates the narrows beamwidths of 60 and 80 GHz solutions.  Figure 3 demonstrates the difficulty in intercepting a point-to-point backhaul signal.


Figure 3: Beamwidth comparisons for wireless backhaul solutions

A very narrow beamwidth is not enough to ensure good security on any wireless network. Most backhaul solutions can be engineered for rigorous authentication processes followed by equally demanding encryption programs for the data stream.

 

3. Quality of Service (QoS)
One of the major selling points of WiMAX is its ability to prioritize traffic to deliver the best possible quality of service (QoS) relative to the traffic (example: VoIP and video can be assigned top priority).  It will do the WiMAX service provider little good to have a high quality "last mile" solution such as WiMAX if the backhaul solution doesn't offer the same flexibility in prioritizing traffic on the backhaul portion of the network.  Most 60 and 80 GHz products offer traffic prioritization schemes to ensure VoIP and video over the backhaul link.  In addition, most of these products also offer sophisticated modulation schemes (up to QAM-256 in some products) to ensure good QoS with up to 99.999% availability.  Forward Error Correction (FEC) and Frequency Division Duplexing are features offered on some products to ensure QoS.

Potentially, the number one detractor to QoS in wireless links is latency.  Millimeter wave and E-Band products keep latency over their respective wireless links at single digit milliseconds (< 10 ms). This ensures good QoS for time sensitive applications such as VoIP and video.

 

4. Interference mitigation
Excessive interference can take down a backhaul link.  It is imperative that the backhaul solution be engineered to mitigate interference as much as possible.


Figure 4: The narrow beamwidth of 60 and 80 GHz solutions mitigates interference while enhancing security.

As illustrated in Figure 4 above, not only does the narrow beamwidth of 60 and 80 GHz products alleviate the possibilities of interference, but also focuses the power of the beam making for a strong link budget over its short transmission range which further mitigates interference.

 

5. Frequency Reuse
One advantage of using the 60 and 80 GHz bands is that its use does not take up precious access spectrum (2.5 or 3.5 GHz, for example). Another advantage is, given its short range, the ability to reuse backhaul frequencies across the backhaul network.


Figure 5 The 60 and 80 GHz bands offer robust frequency reuse

 

6. Rain fades
Like excessive interference, service providers must engineer their backhaul solutions for local meteorological conditions that can degrade the performance of their networks.  Data shows that currently available commercial equipment can achieve gigabit speeds at 99.999% availability (uptime) with links of approximately1 mile regardless of rain.  For a lower 99.9% availability, distances approaching 3 miles are routine.

 

7. Reliability/availability
Most 60 and 80 GHz products can deliver 99.999% availability over their wireless links. The platforms offer mean time between failure (MTBF) figures in the 30 and 40 year range. Given the low cost of these radios, redundancy can be economically built into the network.

 

8. Ease of Licensing: E-Band in the US
An advantage to E-Band solutions is the relative ease in obtaining licensed spectrum.  Given its short range and narrow beamwidth, its propensity to interfere with other broadcasters is minimal and, in the US, the FCC provides a streamlined licensing process for E-Band spectrum.  The price for a nation-wide, non-exclusive, one-time license is about $1,500.  Additional links there after are about $250.

E-Band link registration is normally completed using an automated on-line registration database system.  The system checks for possible interference between a proposed E-Band installation and all existing registered E-Band links according to GPS coordinates of E-Band radio locations and the operating parameters of the proposed radio.  Registrants have 12 months to complete construction of the link, and the license is valid for 10 years, at which time it can be renewed.

 

Conclusion
The 60 and 80 GHz wireless backhaul solutions are ideal for supporting WiMAX networks - no other wireless solution can approach these platforms in terms of range and throughput.  These solutions also have advantages over fiber optic cable in terms of cost and time to market.  In answering the usual objections to wireless products, these solutions shine in terms of range, throughput, security, interference mitigation, QoS, frequency reuse and rain fade.  No WiMAX network should be planned without giving 60 and 80 GHz backhaul solutions serious consideration.  Expect to see WiMAX service providers with national footprints deploying these solutions to support their WiMAX networks.


References:
Koh, Chris, The Benefits of 60 GHz Unlicensed Wireless Communications, YDI Wireless
E-Band Link Registration - A Lightweight Approach to Spectrum Licensing, Bridgewave
Wells, Jonathan, White Paper on WiMAX Backhaul at 70/80 GHz, Gigabeam Corporation, October 2006;
Phone conversation with Randy Montoya, Bridgewave, 26 December 2006