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TDD: Paving the future of wireless comms

Posted: 28 Jul 2009 ?? ?Print Version ?Bookmark and Share

Keywords:communications wireless? TDD wireless? time division duplex?

Despite some lingering complexity in the market over which wireless standard will next prevail and when, it is very clear that time division duplex (TDD) is one of the most flexible schemes to meet the highly asymmetrical bandwidth demands of today's and tomorrow's networks.

Higher uptake of 3G services and the growing popularity of smart phones over the past two years have caused a surge in the number of subscribers now signing up to 3G networks. The volume of traffic on these wireless networks now arising from increased e-mail, Internet, media downloads etc. is rising considerably. AT&T Mobility stated that on average, one iPhone user generates as much data traffic as 30 basic feature-phone users, and that traffic can only continue to grow as the smart phone becomes the norm.

Traditionally, wireless voice communications have used a spectrum sharing scheme known as frequency division duplex (FDD) to provide bi-directional capacity (downlink or forward channel between base station and mobile, and uplink or reverse channel between mobile and base station). Using paired spectrum, FDD systems use a combination of two frequencies with equal bandwidths (one in a lower and one in a higher band) to allow simultaneous transmissions in both directions. Because FDD operates using a set bandwidth and capacity, it is ideal for balanced communication applications such as voice, in which an equal volume of information is being transmitted in both directions.

TDD, unlike FDD, uses just one frequency band to transmit signals, and switches rapidly between the uplink and downlink streams to make it appear that simultaneous transmissions are being enabled. The capacity of the upstream and downstream links can be dynamically altered in either direction according to demand, allowing for a flexible trade-off between each data link. Because of this flexibility, TDD is able to easily support balanced transmissions such as voice, as well as unbalanced ones such as e-mail and Internet where larger and more diverse volumes of data are typically being sent over the downlink in contrast to what is being sent over the uplink.

Although FDD can also be used for unbalanced transmissions because data traffic is predominately sporadic and varied in terms of volume, it is difficult to accurately and economically allocate the required bandwidth capacity in advance. Ultimately, when used with variable data transmissions, the fixed and equal structure of FDD systems results in the wastage of valuable spectrum.

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