TD-LTE: an introduction

TD-LTE is an interesting area. Spectrum is cheap and lends itself very well to data traffic. TD-LTE may also present an upgrade path for WiMax, possibly leading to a merger of LTE and WiMax. Touseef Ahmed of Mobile Broadband has been friendly enough to make an intro into the topic, that is presented here as a guest post.

Defining TD-LTE:

LTE (Long Term Evolution) is the fundamental and primary technology for the development of 4G technology, which is an evolution of 3G networks. TDD (Time Division Duplex) version of LTE is called TD-LTE. It was developed by China Mobile in the recent years.

TD-LTE explained:

TD-LTE allows carriers to make use of unpaired spectrum that many of them already own. Compared to the previous standards GSM, EDGE, etc., TD-LTE's commercial release time period is very short, due to its later addition into the standards.

There are some essential similarities and differences between TD-LTE and classic LTE. Basically LTE has the following characteristics:

• Much faster upload and download speeds than the 3G.
• It can reach download speeds of over 150 Mbit/s and upload speeds of over 80 Mbits/s.
• It has a larger cell size where a single LTE cell tower can cover upto 100km. Although its size will be greatly diminished in urban areas, it is still a lot better than 3G.
• It can easily be upgraded as it was developed with the intention of making the implementation of upgrades easier down the line.
• It has a great advantage of being compatible with existing standards.

Differences and similarities between LTE and TD-LTE:

• They run on different bands of wireless spectrum. But the part of spectrum that carries the TD-LTE signal is a lot cheaper and has much less traffic.
• LTE and TD-LTE are so similar that both the networks can be accessed by the same chip, which is easier for handset manufacturers.
• 4G, WiMAX standards are not compatible with LTE, but compatible with TD-LTE.

TD-LTE technical specifications:

• TD-LTE is specified to operate in the frequency range of 1850 to 2620 MHz.
• It uses the same MIMO (Multiple Input, Multiple Output) scenarios.
• There are two frame configurations, each with an overall length of 10 milliseconds and divided into 10 subframes as shown in the figure above.
• That is, the transmitted signal is organized into subframes of 1 millisecond.
• There is only one single carrier frequency and the transmissions (uplink and downlink) in the cell are always separated in time.

TD-LTE frame structures (see figure above):

• The 5ms version has two special subframes when compared to one in the 10ms version which provides greater chances of uplink/downlink flexibility.
• The frame can be dynamically configured to any one of the above depending upon the transmission requirement.
• Each one millisecond downlink subframe contains blocks of data called “Resource Blocks” meant for a number of different users.
• Uplink subframe contains blocks of data from the users to the Base Station.
• The specified latency time is 5 ms or only half a frame for small data packets.
• The current system is made such that the stationary or pedestrian users or the low speed users experience operations done at the highest speed.

Advantages of TD-LTE:

• There is no need to develop new devices for using TD-LTE. It's enough to add TD-LTE support to the existing devices.
• There is a lot of TDD spectrum available and it is cheap in cost.
• The increasing availability allows transition of WiMAX (Worldwide interoperability for Microwave Access) operators to TD-LTE using the same allocated spectrum.
• Industry commitment is great without any limitations.

Future of TD-LTE:

TD-LTE will bring in new challenges to developers and vendors of design. New schemes, new configurations, higher system bandwidths, higher system capacity, lower latency are some of the expected challenges. There is a prediction that there will be 30 to 80 million subscribers and over £70 billion in operator revenues within 5 years.