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The basic LTE, long term evolution cellular services were launched around 2010 with some advance deployments well before this. It was never envisaged that this initial form of LTE would provide the full performance intended. This required some additional elements that were in what was termed LTE Advanced.
LTE Advanced, LTE-A incorporated a number of new techniques that enabled the system to provide very much higher data rates, and also much better performance, particularly at cell edges and other areas where performance would not normally have been so good.
LTE Advanced took a few more years to fully develop and roll out across the networks, but when introduced it enabled its many advanced features to provide significant improvements over basic LTE.
LTE Advanced development history
With 3G technology established, it was obvious that the rate of development of cellular technology should not slow. As a result initial ideas for the development of a new 4G system started to be investigated. In one early investigation which took place on 25 December 2006 with information released to the press on 9 February 2007, NTT DoCoMo detailed information about trials in which they were able to send data at speeds up to approximately 5 Gbit/s in the downlink within a 100MHz bandwidth to a mobile station moving at 10km/h. The scheme used several technologies to achieve this including variable spreading factor spread orthogonal frequency division multiplex, MIMO, multiple input multiple output, and maximum likelihood detection. Details of these new 4G trials were passed to 3GPP for their consideration
In 2008 3GPP held two workshops on IMT Advanced, where the "Requirements for Further Advancements for E-UTRA" were gathered. The resulting Technical Report 36.913 was then published in June 2008 and submitted to the ITU-R defining the LTE-Advanced system as their proposal for IMT-Advanced.
The ITU-R also set a number of milestones to ensure that the development of LTE Advanced too place in a timely fashion.
|Key Milestones on the Development of 4G LTE-Advanced|
|Issue invitation to propose Radio Interface Technologies.||March 2008|
|ITU date for cut-off for submission of proposed Radio Interface Technologies.||October 2009|
|Cutoff date for evaluation report to ITU.||June 2010|
|Decision on framework of key characteristics of IMT Advanced Radio Interface Technologies.||October 2010|
|Completion of development of radio interface specification recommendations.||February 2011|
Comparison of LTE-A against other technologies
To see how LTE Advanced, LTE-A provided a significant improvement in performance, it is sometimes interesting to check its capability against other cellular services.
|Comparison of LTE-A with other Cellular Technologies|
HSDPA / HSUPA
|Max downlink speed|
|384 k||14 M||28 M||100M||1G|
|Max uplink speed|
|128 k||5.7 M||11 M||50 M||500 M|
round trip time
|150 ms||100 ms||50ms (max)||~10 ms||less than 5 ms|
|3GPP releases||Rel 99/4||Rel 5 / 6||Rel 7||Rel 8||Rel 10|
|Approx years of initial roll out||2003 / 4||2005 / 6 HSDPA|
2007 / 8 HSUPA
|2008 / 9||2009 / 10||2014 / 15|
|Access methodology||CDMA||CDMA||CDMA||OFDMA / SC-FDMA||OFDMA / SC-FDMA|
LTE Advanced key features
With work starting on LTE Advanced, a number of key requirements and key features are coming to light. Although not fixed yet in the specifications, there are many high level aims for the new LTE Advanced specification. These will need to be verified and much work remains to be undertaken in the specifications before these are all fixed. Currently some of the main headline aims for LTE Advanced can be seen below:
- Peak data rates: downlink - 1 Gbps; uplink - 500 Mbps.
- Spectrum efficiency: 3 times greater than LTE.
- Peak spectrum efficiency: downlink - 30 bps/Hz; uplink - 15 bps/Hz.
- Spectrum use: the ability to support scalable bandwidth use and spectrum aggregation where non-contiguous spectrum needs to be used.
- Latency: from Idle to Connected in less than 50 ms and then shorter than 5 ms one way for individual packet transmission.
- Cell edge user throughput to be twice that of LTE.
- Average user throughput to be 3 times that of LTE.
- Mobility: Same as that in LTE
- Compatibility: LTE Advanced shall be capable of interworking with LTE and 3GPP legacy systems.
These are many of the development aims for LTE Advanced. Their actual figures and the actual implementation of them will need to be worked out during the specification stage of the system.
LTE Advanced technologies
There are a number of key technologies that will enable LTE Advanced to achieve the high data throughput rates that are required. MIMO and OFDM are two of the base technologies that will be enablers. Along with these there are a number of other techniques and technologies that will be employed.
- Orthogonal Frequency Division Multiplex, OFDM OFDM forms the basis of the radio bearer. Along with it there is OFDMA (Orthogonal Frequency Division Multiple Access) along with SC-FDMA (Single Channel Orthogonal Frequency Division Multiple Access). These will be used in a hybrid format. However the basis for all of these access schemes is OFDM.
Note on OFDM:
Orthogonal Frequency Division Multiplex, OFDM is a form of signal format that uses a large number of close spaced carriers that are each modulated with low rate data stream. The close spaced signals would normally be expected to interfere with each other, but by making the signals orthogonal to each other there is no mutual interference. The data to be transmitted is shared across all the carriers and this provides resilience against selective fading from multi-path effects.
Read more about OFDM, Orthogonal Frequency Division Multiplexing.
- Multiple Input Multiple Output, MIMO: One of the other key enablers for LTE Advanced that is common to LTE is MIMO. This scheme is also used by many other technologies including WiMAX and Wi-Fi - 802.11n. MIMO - Multiple Input Multiple Output enables the data rates achieved to be increased beyond what the basic radio bearer would normally allow.
Note on MIMO:
MIMO is a form of antenna technology that uses multiple antennas to enable signals travelling via different paths as a result of reflections, etc., to be separated and their capability used to improve the data throughput and / or the signal to noise ratio, thereby improving system performance.
Read more about MIMO technologyFor LTE Advanced, the use of MIMO is likely to involve further and more advanced techniques including the use of additional antennas in the matrix to enable additional paths to be used, although as the number of antennas increases, the overhead increases and the return per additional path is less.
In additional to the numbers of antennas increasing, it is likely that techniques such as beamforming may be used to enable the antenna coverage to be focused where it is needed.
- Carrier Aggregation, CA: As many operators do not have sufficient contiguous spectrum to provide the required bandwidths for the very high data rates, a scheme known as carrier aggregation has been developed. Using this technology operators are able to utilise multiple channels either in the same bands or different areas of the spectrum to provide the required bandwidth.
- Coordinated Multipoint : One of the key issues with many cellular systems is that of poor performance at the cell edges. Interference from adjacent cells along with poor signal quality lead to a reduction in data rates. For LTE-Advanced a scheme known as coordinated multipoint has been introduced.
- LTE Relaying: LTE relaying is a scheme that enables signals to be forwarded by remote stations from a main base station to improve coverage.
- Device to Device, D2D: LTE D2D is a facility that has been requested by a number of users, in particular the emergency services. It enables fast swift access via direct communication - a facility that is essential for the emergency services when they may be on the scene of an incident.
LTE-Advanced has been able to provide some significant improvements in performance. Not only has the radio access network seen upgrades and improvements, but so too has the core network.
The result of all the upgrades is that users see significant performance improvements with LTE Advanced. Also operators see greater returns. The cost per bit is reduced, and with the faster speeds, users tend to consume more data, thereby raising revenues. Accordingly LTE-Advanced has provided improvements to both users and operators, as well as those providing additional services.
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