LTE Advanced (LTE+)

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 - Radiocommunications 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.

Key Milestones on the Development of 4G LTE-Advanced
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.

Comparison of LTE-A with other Cellular Technologies
	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	2014 / 15
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.

• 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 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 other there is no mutual interference. The data to be transmitted is split across all the carriers to give resilience against selective fading from multi-path effects..
  Click on the link for an OFDM tutorial
  
  
  
• 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:

  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.
  Click on the link for a MIMO tutorial
  
  
  
  For 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. Read more about Carrier Aggregation, CA
• 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. Read more about Coordinated Multipoint, CoMP
• LTE Relaying:   LTE relaying is a scheme that enables signals to be forwarded by remote stations from a main base station to improve coverage. Read more about LTE Relaying
• 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. Read more about Device to Device communications