Most of the Long-Term Evolution (LTE) systems rolling out across the U.S. and the rest of the world use frequency division (FD) duplexing. Separated but matching spectrum is needed for both the downlink (DL) and uplink (UL) parts of any call. While FD-LTE works great, it takes twice the amount of spectrum than a time division (TD) duplex system like WiMAX. Since TD duplexing is a time-shared method, it is very spectrum efficient—and that’s why it’s becoming an important part of the 4G infrastructure as it’s being built.

China started the TD movement with its 3G TD-SCDMA systems and is now moving aggressively to TD-LTE systems. TD systems conserve precious spectrum. In many instances, they’re simpler and less expensive than FD systems. And luckily, the ITU/3GPP standards now include TD as a formal part of the specifications. It is fast becoming an adopted method in many new systems worldwide, especially in China, India, and other Asian countries. In the U.S., Clearwire is converting its WiMAX system to TD-LTE and will offer wholesale services to Sprint and others. While the LTE infrastructure will continue to be mostly FD-LTE, the TD component could be as much as 25% in the coming years.

While the carriers are getting involved, chip vendors are beginning to meet the needs for handset companies. Qualcomm now makes TD-LTE chipsets. WiMAX chip vendors like Altair and Sequans also now offer TD-LTE chipsets. Others are making the investment, including Intel, to meet what will eventually be a large marketplace. TD-LTE also will find a place in the growing small-cell infrastructure as TD-to-FD and FD-to-TD handover technology is perfected.

According to Nigel Wright, vice president of wireless at Spirent Communications, TD-LTE offers some key technical advantages specifically in the use of MIMO and beamforming. With both DL and UL on the same frequency, these technologies will be simpler and less expensive to implement and inherently more effective. Another interesting advantage is the potential use of FD-LTE UL spectrum with TD-LTE. Most activity on any FD-LTE system is in the downlink, leaving the UL spectrum virtually ignored. Carriers could work out spectrum-sharing methods. The key challenge with TD-LTE is the timing and synchronization required for basestation handoffs. Systems are already in place to deal with that.

Initially, most experts thought LTE would only work with FD. But with recent developments, we can expect two LTE ecosystems to coexist in the future. So, what will happen to WiMAX, the original orthogonal frequency-division multiplexing (OFDM) TD duplexing system?