What are FD LTE and TD LTE?
Frequency division (FD) LTE and Time division (TD) LTE are two different standards of 4G LTE technology.
The FD LTE standard is a migration from the path of the 3G network while TD LTE has its roots in China’s TD SCDMA.
The frequency division and time division labels refer to the different duplex schemes each standard uses. Duplex schemes are necessary for radio communications systems to communicate in both directions. By using a duplex scheme the transmitter and receiver are organised so that they can transmit and receive.
Frequency division duplexing and time division duplexing are the two most popular methods in use.
What are the trade-offs between FD LTE and TD LTE?
The main differences between FD LTE and TD LTE come in their spectrum type. FD LTE requires paired spectrum with different uplink and downlink channels. TD LTE uses unpaired spectrum, transmitting uplink and downlink assignments on the same channel.
According to a report from Goldman Sachs, this means that TD LTE is more efficient in its spectrum usage. However, using the same channel means that a guard interval is required between the TD LTE uplink and downlinks to separate them.
Over short distances the interval is acceptable says Goldman Sachs, but it becomes more of an issue over longer distances. In addition the interval causes a minor degree of latency as data cannot be immediately routed and transmitted after being generated.
The two LTE technologies differ quite significantly in terms of capacity regulation. FD LTE capacity is determined by frequency allocation by a country’s regulatory authorities and is therefore difficult to radically alter. TD LTE allows capacity to be altered according to requirements, through changes to the uplink and downlink capacity ratio.
In terms of coverage, FD LTE is thought to be superior to TD LTE due to its power efficiency. A report by Qualcomm claims that FD LTE covers an area 80% larger than TD LTE for 2:1 downlink/uplink allocation on 2.6GHz frequency.
Can mobile devices support both LTE types?
According to Monica Paolini, founder and president of Senza Fili Consulting, FD LTE and TD LTE are very similar and devices should be able to support both interfaces through a single chipset. Rather than developing separate devices for the different technologies, manufacturers will be able to create devices compatible with both LTE types.
Qualcomm and ST-Ericsson are already developing mobile processors with integrated modems supporting both FD LTE and TD LTE. According to Goldman Sachs these are likely to become commercially available in early or mid 2012.
Where is FD LTE being implemented and where will TD LTE be adopted?
FD LTE is the technology of choice in the US and for the bulk of providers across the globe. The world’s largest LTE provider, Verizon Wireless, uses FD LTE as does its competitor AT&T. In Europe, companies such as TeliaSonera and Deutsche Telekom have also begun adopting the FD LTE standard.
Although TD LTE services are not yet available, major carriers are expected to launch limited TD LTE services in China, India and Japan between late 2012 and early 2013. Goldman Sachs believes that India may well be the first country to commercially launch TD LTE with Reliance Industries, Bharti airtel and Tikona expected to launch TD LTE services by mid-2012.
TD LTE technology is not expected to face the restrictions of its TD SCDMA predecessor which was limited to the Chinese market due to a lack of global support. Goldman Sachs estimates that 12 carriers have committed to deploying TD LTE so far and according to Qualcomm there are 17 TD LTE trials taking place in the world today.
FD LTE has a head start with its deployment in the trailblazing US market, but there is what could be described as a clash of superpowers in China. The Chinese government has refused to grant FD LTE licences in the country until TD LTE is ready for launch. Goldman Sachs expects China Mobile to be one of the key players in TD LTE.
Will TD LTE replace WiMAX?
The decline of WiMAX, particularly in the US where Verizon’s large scale FT LTE rollout has dominated the market, favours TD LTE technology. Berge Ayvazian, senior consultant at Heavy Reading, says that there is a growing industry consensus that many WiMAX operators are planning on migrating their networks to TD LTE over the next few years.
In March 2010 a group of vendors, including Sprint and Clearwire, petitioned the 3GPP standards body to allow TD LTE to be deployed in the US using the 2.6 GHz band of spectrum allocated to WiMAX. The proposal was accepted and Clearwire is planning to overlay TD LTE.