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FOCUS: Satellites and WiMAX
by Jorn Christensen, Ph.D, P. Eng.

Improvements in telecommunication infrastructure are usually evolutionary, building upon the existing technology. However, sometimes a new technology that performs the same functions as an existing technology is brought to market and may replace, to a large extent, the existing technology. Many analysts believe that WiMAX has the potential to be a disruptive technology as it may replace wire-based telecommunications services, such as fixed copper telephone lines, or cable coaxial cable. What does this have to do with satellite communications?

First, satellite operators must always be aware of what competing terrestrial technologies are being brought to market as they may compete with satellite offerings. For example, consumer broadband services can be provided via WiMAX and this may be a less expensive offering than a satellite solution. Secondly, WiMAX deployed in the extended C-band causes interference into FSS operations, even when there is a separation in frequency of operation of the two services.

To understand why WiMAX is now being deployed in the C-band, we need to examine where WiMAX fits into the telecommunications landscape.

Development of the WiMAX Family of Standards
The first IEEE 802.16 Standard (WiMAX) specifies the WirelessMAN Air Interface for wireless Metropolitan Area Networks (MAN). The standard focuses on the efficient use of bandwidth between 10 and 66 GHz. This standard supports continuously varying traffic levels at many licensed frequencies (e.g., 10.5, 25, 26, 31, 38 and 39 GHz) for two-way communications. The standard enables interoperability among devices, allowing carriers to use products from multiple vendors. Engineers from the world’s leading operators and vendors created this standard in a two-year, open-consensus process.

Further Development of the IEEE 802.16 Standard
The two main standards now being deployed are the 802.16d (fixed) and 802.16e (mobile) standards. For these standards, WiMAX solutions have been implemented for licensed and unlicensed bands in the 2 to 11 GHz range.

The main mileposts in the development of the WiMAX standards are:
  • December 2001 – 802.16 adopted for wireless Metropolitan Area Network (MAN) in 10-66 GHz frequency range (line-of-sight);
  • January 2003 – 802.16a extension adopted for (2 – 11 GHz, non line-of-sight);
  • June 2004 – 802.16d 2004 Standard adopted for WiMAX (non line-of-sight);
  • December 2005 – 802.16e 2005 Standard adopted for mobile WiMAX.
Fight between 2G/3G and WiMAX
Manufacturers of cellular equipment naturally want their standards recognized to the fullest extent possible. Some years ago, manufacturers, operators, and administrations met under the umbrella of the International Telecommunications Union (ITU) based in Geneva, Switzerland, to try to define one standard for third generation (3G) systems for mobile communications. As it was not possible to agree on one standard, all standards with enough commercial support were included in a family of 3G cellular network standards under the label of IMT-2000. Prior to the inclusion of mobile WiMAX (802.16e) in the IMT-2000 standards, as defined by ITU Rec. M.1457-6, IMT-2000 supported five radio interfaces and three different access technologies, using CDMA, TDMA, and FDMA.

Mobile WiMAX could be a disruptive technology as far as the incumbent cellular network operators are concerned. New entrants not burdened with having to support legacy equipment could establish national networks and potentially offer mobile voice and data services at lower prices.

One way in which the 2G/3G cellular network operators tried to keep mobile WiMAX from offering service was by having spectrum designated in the ITU Table of Frequency Allocations for use by IMT-2000 (3G) technologies, which support legacy networks. WiMAX is a pre-4G standard and does not support circuit switched networks. However, the fight ended with victory of the WiMAX lobby when the mobile version of WiMAX was included into the IMT-2000 family of standards.

IMT-2000 Designation in ITU Table of Frequency Allocations
The increasing demand for more bandwidth for cellular networks had led to the designation at the ITU World Radiocommunication Conferences (WRC) administrations for more and more spectrum for IMT-2000. Prior to WRC-07, over 700 MHz below 3 GHz had been designated for IMT-2000. Based on very optimistic projections for cellular broadband services, there was a drive at WRC-07 for additional spectrum to be designated for IMT-2000. This lead to proposals for spectrum designation in the extended (3.4 to 3.7 Ghz) and standard (3.7 to 4.2 Ghz) C-band. Satellite operators fiercely opposed these proposals.

IMT-2000 and IMT-Advanced Standards
The ITU has clarified the terms IMT, IMT-2000 and IMT-Advanced as follows:

  • The term “IMT-2000” encompasses its enhancements and future developments;
  • The term “IMT-Advanced” is applied to those systems, system components, and related aspects that include new radio interface(s) that support the new capabilities of systems beyond IMT-2000;
  • The term “IMT” is the root name that encompasses both IMT-2000 and IMT-Advanced collectively.
The next cellular network evolution will be from 3G to 4G, although some operators are now deploying what they call 3.5G networks. The main characteristics of 3G and 4G networks are seen in table 1. IMT-2000 technologies are often described as 3G technologies and IMT-Advanced technologies (still to be developed) as 4G technologies. The mobile WiMAX 802.16e Standard is often described as a 4G network as it has all the characteristics of 4G networks, except for the speed. However, the criteria that constitutes a 4G network has not yet been defined. It is more appropriate to speak of mobile WiMAX 802.16e Standard as a pre-4G standard.

Table 1
WRC-07 Results with respect to allocations for IMT in C-band
The best result at WRC-07 for satellite operators would have been no IMT designation within the C-band. While not a complete success, the results were better than expected since WRC-07:
  • Limited IMT designations through opt-in footnotes in the range 3.4 – 3.6 GHz
  • Adopted very stringent protection criteria from IMT interference into FSS operation in neighboring countries

In ITU Region 1 (Europe and Africa), a footnote allocates the band 3.4 to 3.6 GHz to the mobile service on a primary basis and designates the band for IMT in 81 countries.

In ITU Region 2 (the Americas), a footnote allocates the band 3.4 to 3.5 GHz to the mobile service on a primary basis (but without a designation for IMT) in 14 countries.

In ITU Region 3 (Asia, Australia and Oceania), a footnote upgrades the secondary mobile allocation in the band 3.4 to 3.5 GHz to a primary allocation and designates this band for IMT in 7 countries. A footnote also designates the band 3.5 to 3.6 GHz for IMT in 8 countries. (The mobile service is already primary in this band.)

Interference That Satellite Operators Can Expect from WiMAX Operation
At the FSS Earth station antenna, the BWA terrestrial signal is far more powerful than the signal from the satellite. Typically, the power-flux density (pfd) of a C-band satellite signal at the FSS Earth station antenna is about 122 dBW/m2, while the pfd of a 25 watt BWA transmitter at a distance of 500 meters is around -50 dBW/m2. There is difference in power between the two signals of 72 dB. It is difficult to overcome this power difference, either by shielding and/or filtering. At best, the FSS Earth station antenna has a sidelobe/backlobe discrimination of about 30 dB. The interference caused into the FSS Earth station can be divided into three types:

a) Co-frequency Interference
If no shielding is available at the satellite antenna site, then interference can be caused at distances up to about 150 km.

(b) Out-of-band Interference
With the existing out-of-band emission limits for BWA transmitters, interference can be caused at distances up to 2 km. If additional filtering is implemented at the BWA base station and the use of outdoor BWA terminal stations is not allowed, the distance may be shortened to about 0.5 km.

(c) FSS Receiver Saturation Problem
Signals from nearby BWA equipment transmitting in the 3.4 to 3.6 GHz band will cause saturation of FSS receivers with their LNB operating in the 3.7 to 4.2 GHz range. In this case saturation can be caused in satellite receivers located at a distance up to about 1.2 km. Off-the-shelf filters can reduce the interference level by about 10 dB in which case the interference can be caused at distances up to about 0.5 to 0.6 km.

What Satellite Operators Can Expect in Terms of Future Deployment of BWA Technologies Including WiMAX in the 3.5 GHz Band
  • Most of the administrations that decided to be included in the new footnotes in the 3.4 to 3.6 GHz band have tested BWA technologies (usually WiMAX) and have licensed, or are considering licensing, BWA networks
  • In all ITU Regions, the fixed service is primary in all of the C-band (extended and standard), while in ITU Regions 2 and 3 the mobile service is primary in the band 3.5 to 4.2 GHz. Fixed BWA (including WiMAX) can be deployed under the primary fixed allocation and mobile BWA (including WiMAX) can be deployed under the primary mobile allocation. Therefore, in many areas of the world, there is no need for an IMT designation in order to deploy BWA (including WiMAX)
  • Due to the greater attenuation at higher frequencies (say, 2 to 3.5 GHz), cellular networks at these frequencies require more towers for the same coverage area. This means cellular networks at higher frequencies are often used for high capacity, i.e. many smaller cells, whereas cellular networks at lower frequencies are often used for networks having wider coverage. As cellular network operators move to higher capacity networks, they will move up in frequency e.g. into the 3.5 GHz band.
  • The legacy 3G cellular networks typically use channeling bandwidths of 1.25 to 5 MHz. This means the traditional lower frequency cellular frequency bands have been divided into relatively narrow channels. The new generation of pre-4G and 4G networks require wider channels (20 to 30 MHz) to achieve their design efficiency. This will put more pressure on the 3.5 GHz band.
  • The 3.5 GHz band is under pressure on two fronts:
    • Countries with highly developed telecommunications infrastructure need new spectrum for high capacity mobile BWA networks in cities;
    • Countries with large under-served rural areas need new spectrum for fixed BWA networks.

As a result, more countries may decide to deploy BWA networks in the 3.5 GHz band and may join the opt-in IMT footnotes at future WRCs.

About the author
Dr. Jorn Christensen is a satellite communications consultant. He has participated in every satellite World Radiocommunication Conference (WRC) since 1983. He has published over 20 articles and been an invited speaker at over 25 satellite conferences.