Two-way wireless operation is in its infancy due to the availability of spectrum, equipment, and the changing FCC regulations. The technology, regulations, and business issues are changing very quickly. A service provider will use a mix of telephone, integrated services digital network (ISDN), and wireless return as necessary to meet customer needs.

Use of ISM Bands

It is possible to use commercially available spread-spectrum modulators in any of the ISM bands. These can be driven from the RS232 output of a cable modem the same way as an external phone modem for telephone return. The advantage is that the spectrum is free and available now. It allows an operator to understand the deployment issues. It can be a viable long-term solution given the cost of spectrum in the traditional frequency bands, and the highly directional 2.4-GHz antennas minimize the probability of interference with other users.

Figure 4. One Way to Use the ISM Band for Wireless Return 900 MHz/2.4 GHz

Use of the QPSK Signal from a Cable Modem
Transverters and upconverters are under development from various vendors to take the 5- to 42-MHz QPSK signal output of a cable-return modem and translate it to the WCS, MDS1, MDS2, and MDS2A bands. A transverter is simply a combined downconverter and upconverter. These units are similar in appearance to downconverters and generate 50 mW or 100 mW of transmitted power. This is enough to reach back to the downstream transmitter site or to the Internet head end if it is closer and a LOS exists. The QPSK signal bitrate can be set to different values, from 256 Kbps to 5.12 Mbps. Lower values allow more users.

Figure 5. Wireless Return Using the QPSK Signal from a Cable Modem

The following are issues facing two-way operators:

• If the transmitter is separated from the Internet head end, a return path must be licensed and purchased similar to the downstream.
• the system cost, including the opportunity cost of spectrum which could be used for downstream signals rather than upstream
• the bandwidth/bit rate used for the upstream; as an example, the 12 MHz of bandwidth in the combined MDS1 and MDS2 bands could be sliced into 200-kHz channels with 256- or 320-Kbps rates. This gives 60 channels supporting around 3000 subscribers with omnidirectional non-sectored antennas. This means that the return-path receive antennas must be sectored as the business grows to achieve the same upstream subscriber capacity as the downstream.
• The QPSK modulator and the transverter upconverter portion determine the frequency stability and spurious signal output of the return-path transmission.
• Transmitting back to the downstream transmitter site allows integrated transmit-and-receive antennas at the customer site. These cost less than separate units and, since they are pointing in the same direction, the transmit antenna is automatically aligned when the receive antenna is installed and aligned.
• Transmitting back to the Internet head-end PoP where possible requires separate antennas with individual alignment. It sidesteps the requirement to get the return signal from the downstream transmitter back to the PoP.