Technology Provides Dynamic Bandwidth In Wireless Broadband Access Systems

September 1999
By Michelle L. Hankins

Air link protocol makes waves with effective spectrum use.

An evolving technology promises efficient spectrum use to enable bandwidth on demand in wireless broadband access systems. The technology is being implemented in point-to-multipoint systems operating across the millimeter wave region to provide wireless communications transmissions.

Industry is exploring its benefits, and one Virginia academic institution is using it in research about deploying efficient wireless systems in rural areas, where cost-effective communications are needed. The technology addresses the challenge of bringing voice, data and video directly to the user location from high-speed fiber networks to what is typically referred to as the last mile.

Known as time division duplexing (TDD), this technology uses a single frequency for both transmission and reception. Some experts argue that this concept could replace frequency division duplexing (FDD), which requires separate transmit and receive carriers to pass signals. FDD has been used in the commercial marketplace at the microwave frequency since the 1970s, and it is widely used today in North American cellular telephones, radios, microwave point-to-point radios and satellite systems.

A drawback to using FDD technology is the need for hardware to provide transmit and receive isolation. Because FDD requires two separate carriers per link, a costly frequency duplexer circuit is necessary to prevent the transmitter from damaging the receiver.

In contrast, TDD does not require a duplexer because the technology uses time instead of frequency to separate communications traffic. To do this, TDD uses a repeating signaling frame structure. In this structure, the signal direction alternates on a single channel between transmit and receive. Instead of a duplexer, the technology employs a two-way monolithic switch. TDD developers maintain that this reduces the complexity of the system.

TDD requires a minimum of one channel of available spectrum. FDD systems require tracking of matched pairs and cannot offer complete coverage with a single radio. The need for radio matching decreases flexibility in large-scale efforts such as local multipoint distribution service. In addition, TDD allows the use of contiguous or noncontiguous spectral blocks—an advantage over FDD systems.

Proponents often herald the technology’s ability to offer bandwidth on demand. With TDD, there is no need to overallocate bandwidth. Because it uses spectrum in terms of time, all spectral capacity—in either forward or reverse direction—is dynamically exploitable. This allows maximum use of the frequency.

Also, TDD enables symmetrical and asymmetrical communications. In a symmetrical format, uplinks and downlinks are equal, while asymmetrical transmissions have varying uplink and downlink traffic, which can be enhanced by dynamically allocated bandwidth to support needs at any given time.

Wavtrace Incorporated is one company whose founding was based on the TDD technology. Established in 1996, the company is headquartered in Bellevue, Washington. It uses TDD in its point-to-multipoint wireless broadband access systems. Director of Marketing Kimberly Tassin summarizes, “The company was built from the ground up based on TDD technology.”

The company’s original goals were to tap into the growing market that had been stimulated by increasing demand for bandwidth in the local loop. To do this, Wavtrace developed its point-to-multipoint broadband access systems based on the air link protocol of TDD versus FDD.

Tassin maintains that systems based on TDD have many benefits, including spectral efficiency and simpler radio architecture. She points to its use in last-mile service and notes that the technology is especially good for asymmetrical communications such as handling data traffic.

The Wavtrace product based on TDD technology is called PTM 1000. It is a wireless broadband access system that addresses the need for both high-quality voice and high-speed data transmissions.

The product has a hub and remote indoor and outdoor units, air link technology and network management software. The hub indoor unit modem cards combine with the outdoor equipment to form one radio frequency carrier. These beams are created by the hub indoor unit, which has up to eight modem cards, and the outdoor unit transceivers. The beams work to allow up to 11 virtual tributaries of symmetrical 10Base T and/or DS1 traffic at 1.5 megabits per second to go up to four remote units at once.

A remote unit distributes both transmit and receive traffic to individual subscribers. The system’s processor connects to a network operations center through an Ethernet port. This processor serves as the master timing source. The PTM 1000 uses OC-3, 10/100Base-T and DS3 network interfaces.

The hub outdoor unit has one or more transceivers mounted on a rooftop. It requires horn antennas, distribution electronics and mounting frames. Signals from each transceiver are multiplexed onto transmit and receive coaxial cables. These cables run up to 1,000 feet to the hub indoor unit.

The TDD technology that is employed in the PTM 1000 allows the system to transmit and receive on the same 8.33-megahertz frequency based on units of time. This is in contrast to FDD technology, which requires transmit and receive separation.

Transmit and receive are electronically controlled in TDD systems and are accomplished so fast that users do not notice, Wavtrace Product Manager Mike Schy says. “We slice it up in time,” he adds, noting that shared use of a single channel increases efficient use of the spectrum. TDD offers better use of network resources by allowing dynamic bandwidth allocation. Also, operators can oversubscribe one channel to meet needs and maximize use.

Unlike in FDD, frequencies can bump up against each other in TDD. “We don’t need any guard band,” Schy says about the flexibility offered by TDD design. TDD does not require a duplexer. “Our radios do not need this expensive piece of hardware,” he says.

While TDD has existed for some time, commercial use of the technology in the millimeter waveband is a growing possibility. Schy suggests that FDD may become a legacy technology if TDD takes off. This could occur if the efficiency of operating in the time domain increases the capability to transmit voice and data traffic beyond that of FDD systems.

One test of TDD is in progress at Virginia Polytechnic Institute and State University, also known as Virginia Tech, Blacksburg, Virginia. The school is deploying high-bandwidth wireless technology to form a local multipoint distribution service network for two-way high-speed data, voice and video traffic. The university began installation of the equipment in May.

The school plans to test the TDD technology used in the PMT 1000 and conduct research that might enhance its services in the area.

Virginia Tech’s local multipoint distribution service project director, Cortney Martin, says the university views the technology as both a research and economic development opportunity that could serve as a model for use of the technology by private industry.

The project is part of a licensing agreement that covers 16,507 square miles in Virginia, North Carolina and Tennessee. Right now, the university is testing the technology with students, faculty and staff. Martin says the school is “interested in the testbed research aspects to get things off the ground.” University officials anticipate that private companies will later want to embrace the technology of this university-fostered project.

“This is really just the beginning of a research and development-based relationship,” Martin says about the university’s work with Wavtrace. Virginia Tech enjoys working with multiple vendors to find cost-efficient technology solutions, she adds.

Citing Virginia Tech’s commitment to helping the Commonwealth of Virginia develop information technology, Martin says the school’s efforts are geared toward providing services for higher education, government agencies and commercial users. Education, economic development and distance learning top Virginia Tech’s agenda for the project, which is being implemented in phases. First, a hub and two remote sites were installed. This summer, a third remote site joins the system, and two more remote units are expected by the end of this fall, for a total of five sites. Some of these sites are as far away from the hub as 1.5 miles.

The school is using the equipment mainly for data and Internet traffic but will also be testing voice and video traffic. Right now, the technology works on a fixed time slot, but Martin offers that the dynamic time slots may be put in place in the fall.

Virginia Tech’s communications, networks and services, which provides telephone, cable and Internet services to the university, is in charge of the operational aspects of the project. The Center for Wireless Telecommunications is conducting much of the research.

This type of technology could prove extremely useful in taking services typical to densely populated urban areas out to less densely populated rural regions. While Martin sees room for the continued use of both FDD and TDD technologies for multiple applications, she welcomes TDD as an opportunity for the university to do research in a fairly new area to see if TDD could benefit rural communities.

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