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4G LTE (Long Term Evolution) Advanced

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March 2, 2011 by Isybel Harto

Preface

With the standards definitions now available for LTE, the Long Term Evolution of the 3G services, eyes are now turning towards the next development, that of the truly 4G technology named IMT Advanced. The new technology being developed under the auspices of 3GPP to meet these requirements is often termed LTE Advanced.

In order that the cellular telecommunications technology is able to keep pace with technologies that may compete, it is necessary to ensure that new cellular technologies are being formulated and developed. This is the reasoning behind starting the development of the new LTE Advanced systems, proving the technology and developing the LTE Advanced standards.

In order that the correct solution is adopted for the 4G system, the ITU-R (International Telecommunications Union – Radio communications sector) has started its evaluation process to develop the recommendations for the terrestrial components of the IMT Advanced radio interface. One of the main competitors for this is the LTE Advanced solution.

One of the key milestones is October 2010 when the ITU-R decides the framework and key characteristics for the IMT Advanced standard. Before this, the ITU-R will undertake the evaluation of the various proposed radio interface technologies of which LTE Advanced is a major contender.

Key milestones for ITU-R IMT Advanced evaluation

The ITU-R has set a number of milestones to ensure that the evaluation of IMT Advanced technologies occurs in a timely fashion. A summary of the main milestones is given below and this defines many of the overall timescales for the development of IMT Advanced and in this case LTE Advanced as one of the main technologies to be evaluated.

Milestone Date
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

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 development of LTE Advanced / IMT Advanced can be seen to follow and evolution from the 3G services that were developed using UMTS / W-CDMA technology.

WCDMA

(UMTS)

HSPA 

HSDPA / HSUPA

HSPA+ LTE

LTE Advanced

(IMT Advanced)

Max downlink speed 

bps

384 k 14 M 28 M 100M 1G
Max uplink speed 

bps

128 k 5.7 M 11 M 50 M 500 M
Latency 

round trip time

approx

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
Access methodology CDMA CDMA CDMA OFDMA / SC-FDMA

OFDMA / SC-FDMA

LTE Advanced is not the only candidate technology. WiMAX is also there, offering very high data rates and high levels of mobility. However it now seems less likely that WiMAX will be adopted as the 4G technology, with LTE Advanced appearing to be better positioned.

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:

1.Peak data rates: downlink – 1 Gbps; uplink – 500 Mbps.

2.Spectrum efficiency: 3 times greater than LTE.

3.Peak spectrum efficiency: downlink – 30 bps/Hz; uplink – 15 bps/Hz.

4.Spectrum use: the ability to support scalable bandwidth use and  spectrum aggregation where non-contiguous spectrum needs to be used.

5.Latency: from Idle to Connected in less than 50 ms and then shorter than 5 ms one way for individual packet transmission.

6.Cell edge user throughput to be twice that of LTE.

7.Average user throughput to be 3 times that of LTE.

8.Mobility: Same as that in LTE

9.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.

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 transmission that uses a large number of close spaced carriers that are modulated with low rate data. Normally these signals would be expected to interfere with each other, but by making the signals orthogonal to each another there is no mutual interference. This is achieved by having the carrier spacing equal to the reciprocal of the symbol period. This means that when the signals are demodulated they will have a whole number of cycles in the symbol period and their contribution will sum to zero – in other words there is no interference contribution. The data to be transmitted is split across all the carriers and this means that by using error correction techniques, if some of the carriers are lost due to multi-path effects, then the data can be reconstructed. Additionally having data carried at a low rate across all the carriers means that the effects of reflections and inter-symbol interference can be overcome. It also means that single frequency networks, where all transmitters can transmit on the same channel can be implemented.

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:

Two major limitations in communications channels can be multipath interference, and the data throughput limitations as a result of Shannon’s Law. MIMO provides a way of utilising the multiple signal paths that exist between a transmitter and receiver to significantly improve the data throughput available on a given channel with its defined bandwidth. By using multiple antennas at the transmitter and receiver along with some complex digital signal processing, MIMO technology enables the system to set up multiple data streams on the same channel, thereby increasing the data capacity of a channel.

For LTE Advanced, the use of MIMO is likely to involve further and more advanced techniques with additional antennas in the matrix to enable additional paths to be sued, 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.

With data rates rising well above what was previously available, it will be necessary to ensure that the core network is updated to meet the increasing requirements. It is therefore necessary to further improve the system architecture.

These and other technologies will be used with LTE Advanced to provide the very high data rates that are being sought along wit the other performance characteristics that are needed.

LTE Advanced Summary

The full specification for LTE Advanced the new 4G technology also referred to as IMT Advanced is still some while away. However many of the features and technologies have been trialed and are ready to be incorporated into the standard. Yet despite this it will take many months after the finalization of the standard for LTE Advanced before equipment is available and networks start to be deployed.

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