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Two-way wireless cable in the works

Sun, 11/30/1997 - 7:00pm
Jeffrey Krauss, Wireless Cable Wiring Expert and President of Telecommunications and Technology Policy
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By Jeffrey Krauss, wireless cable wiring expert and President of Telecommunications and Technology Policy
Wireless cable background

Wireless cable operates in the 2500–2690 MHz range. It is comprised of two kinds of radio licenses, Multichannel Multipoint Distribution Service (MMDS) and Instructional Fixed Television Service (ITFS). The ITFS licensees are educational institutions that use the channels for distributing educational coursework and lectures, but they are permitted to lease their "excess capacity" for pay TV distribution by a wireless cable operator. In most cities, the wireless cable service—up to 31 channels of MMDS and ITFS—is operated by the MMDS licensee.

Until now, wireless cable has used a one-way broadcast configuration from a tall tower in order to reach customers as far as 35 miles away. That's efficient for broadcasting, but not for two-way operations.

Last year, the FCC decided to allow wireless cable operators to transmit digital video as well as traditional analog video. This was permitted only after extensive field testing, to show that the digital signals would not cause interference into the traditional analog video transmissions. But even though this was a significant change, the broadcast video nature of wireless cable was retained. That clearly will not be the case if two-way services are permitted.

The two-way proposal

The FCC released a Notice of Proposed Rulemaking in October, in response to a petition submitted by a large group of wireless cable licensees, operators and equipment suppliers, seeking to establish new technical rules that would make it feasible to provide some two-way operations on ITFS/MMDS channels. These are some elements of the proposal:

  • permit use of ITFS/MMDS channels for response (two-way) communications
  • permit cellular reuse of ITFS/MMDS frequencies
  • permit licensees to subchannelize the 6 MHz channels or combine them into wider channels
  • permit use of sector antenna systems in addition to omnidirectional antennas
  • require submission of detailed calculations of potential interference from "response stations."

If these technical proposals are approved, 2.5 GHz wireless cable networks could end up looking like 28 GHz LMDS networks, with numerous small cells instead of a single, large, 35-mile coverage radius. But LMDS will have a single licensee in each city, while wireless cable uses ITFS channels licensed to four or five educational institutions in addition to the MMDS channels. The problem here is going to be maintaining the ITFS and MMDS broadcast video service on some channels, while operating two-way services on other channels.

The initial comments from ITFS licensees on the proposals were mixed. Some would be happy to abandon or decrease the video coursework and use their channels for Internet access and other two-way data services. But a significant number of ITFS licensees want to retain the video distribution nature of the service. They are concerned that a cellularized two-way network may not be suitable for one-way video.

There are serious interference issues. The propagation features of 28 GHz LMDS support cellular reuse of all the spectrum at every cell. In contrast, the 800 MHz cellular mobile telephone service can reuse only about one-seventh of the spectrum at each cell, because of interference from transmission in one cell into receivers in the adjacent cell. It remains to be seen whether similar interference problems will have an impact on two-way wireless cable at 2.5 GHz.

In addition, there are questions about interference between one-way video transmissions and two-way wireless cable. There will be some ITFS licensees that won't want to replace their one-way broadcast transmitters with these cellular two-way systems. So they'll be blasting away at the maximum power needed to send their video signal 35 miles, while on the adjacent channel, there will be systems using much lower power transmissions for cellularized two-way services. It will be quite a challenge to design a two-way wireless cable network that both achieves large frequency reuse and also is robust against interference from high-power ITFS.

The wireless cable folks would like the FCC to re-allocate the spectrum and kick out the ITFS folks, just like the FCC kicked out the private microwave users in order to clear spectrum for PCS. But that isn't in the cards, and everyone knows it. The educational institutions may not have much money, but they have political clout through the Congress, and they've used it in the past to hold onto these frequencies.

In return for allowing the FCC to create the wireless cable service, educational institutions have promoted the idea that the wireless cable operators should pay for new ITFS transmission equipment and should pay for leasing channel capacity from the ITFS licensees. But that took place when everyone had the same goal of broadcasting video programming. It remains to be seen what kind of Christmas present it will take to sweeten the pot this time around for the educational institutions that want to use their ITFS channels for one-way video.

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