4G LTE Advanced Tutorial

This 4G LTE Advanced technology tutorial is split into several pages:
[1] 4G LTE Advanced Tutorial
[2] 4G LTE Advanced Carrier Aggregation
[2] 4G LTE Advanced CoMP
[4] 4G LTE Advanced Relay
See also: 3G 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.

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.

Click on the link for an OFDM tutorial

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 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 offers considerably higher data rates than even the initial releases of LTE. While the spectrum usage efficiency has been improved, this alone cannot provide the required data rates that are being headlined for 4G LTE Advanced.

To achieve these very high data rates it is necessary to increase the transmission bandwidths over that used by the first releases of LTE. The method being proposed is termed carrier aggregation or sometimes channel aggregation. Using LTE Advanced carrier aggregation, it is possible to utilise several carriers and in this way increase the overall transmission bandwidth.

LTE carrier aggregation basics

In order to provide the required data bandwidth, several carriers may be used in a process called carrier aggregation. Using this processes several carriers are aggregated on the physical layer to provide the required bandwidth.

To an LTE terminal, each component carrier appears as an LTE carrier, while an LTE-Advanced terminal can exploit the total aggregated bandwidth.

LTE Advanced non-contiguous carrier aggregation

LTE may not have access to sufficient levels of contiguous spectrum to carry the required data using contiguous LTE carrier aggregation techniques.

Some new bands were identified for use by IMT / IMT Advanced technologies at the World Radio Conference in 2007. Possible bands included:

• 450-470 MHz
• 698-862 MHz
• 790-862 MHz
• 2.3-2.4 GHz
• 3.4-3.6 GHz

These allocations are not yet confirmed and they may not be available on a worldwide basis. Additionally the LTE bands may not be large enough in many countries to enable contiguous spectrum use. It is for this reason that non-contiguous LTE carrier aggregation work is on-going.

LTE Advanced carrier aggregation using non-contiguous spectrum, while possible is challenging in terms of its implementation. This means that even though LTE carrier aggregation using non-contiguous spectrum will be supported, it will be constrained and limited to a number of specific scenarios, and only supported by the most advanced terminals.

LTE CoMP or Coordinated Multipoint is a technology that is being developed for LTE Advanced.

LTE Coordinated Multipoint is a method of transmitting to or receiving from a user equipment using several base stations. This has a number of advantages in terms of data throughput.

Essentially, LTE CoMP turns the inter-cell interference into useful signal, especially at the cell borders where performance may be degraded.

LTE CoMP basics

One of the key parameters for LTE as a whole, and in particular 4G LTE Advanced is the high data rates that are achievable. These data rates are relatively easy to maintain close to the base station, but as distances increase they become more difficult to maintain.

Obviously the cell edges are the most challenging. Not only is the signal lower in strength because of the distance from the base station (eNB), but also interference levels from neighbouring eNBs are likely to be higher as the UE will be closer to them.

4G LTE CoMP, Coordinated Multipoint requires close coordination between a number of geographically separated eNBs. They dynamically coordinate to provide joint scheduling and transmissions as well as proving joint processing of the received signals. In this way a UE at the edge of a cell is able to be served by two or more eNBs to improve signals reception / transmission and increase throughput particularly under cell edge conditions.

In essence, 4G LTE CoMP, Coordinated Multipoint allows two modes of operation:

• Joint simultaneous transmission of user data from multiple eNBs to a single UE
• Dynamic cell selection with data transmission from one eNB

To achieve either of these modes, highly detailed feedback is required on the channel properties in a fast manner so that the changes can be made. The other requirement is for very close coordination between the eNBs to facilitate the combination of data or fast switching of the cells.

Downlink LTE CoMP

LTE CoMP generates the main issues within the downlink where a number of scenarios may arise:

• UE responds as for single point transmission: Using this approach, the terminals are not aware of the fact that transmissions are emanating from several geographically different points, i.e. different eNBs. The UE receiver processing and reporting is the same as for a transmission emanating from a single eNB. The network determines which eNBs can be sued to transmit to a given UE dependent upon path loss indications - these indications are gained using the reference signals from the UE that are available even in the earlier releases of LTE.
  
  This system provides diversity gains similar to those found in broadcast single frequency networks. As a result, RF power utilisation within the network is improved and this has the overall advantage of reducing interference.
• Terminals provide channel status information: The second alternative for downlink 4G LTE CoMP, coordinated multipoint is for the UE to provide channel status for all the downlink channels it can see, while retaining the processing as for a single point transmission.
  
  For the network, all the processing is accomplished by a single node to enable sufficiently fast processing to occur as well as coordination of the transmissions at the different points.
• Terminals have details of eNB transmissions: using this approach for 4G LTE CoMP, Coordinated Multipoint, the terminals or UEs are provided with details of the transmissions from the different ENBs. These details include from which eNBs and also information about the transmission details - from which eNBs, channel and transmission weights, etc.. This can be used to improve the signal processing, although it comes at the price of increased signal processing.

Uplink LTE CoMP

For the uplink, LTE CoMP, coordinated multipoint requires the application of the relevant signal processing within the receiver. It is very similar in concept and application to the macro-diversity schemes used in previous cellular systems. The concepts of this are well established.

Relaying is one of the features being proposed for the 4G LTE Advanced system. The aim of LTE relaying is to enhance both coverage and capacity.

The idea of relays is not new, but LTE relays and LTE relaying is being considered to ensure that the optimum performance is achieved to enable the expectations of the users to be met while still keeping OPEX within the budgeted bounds.

Need for LTE relay technology

One of the main drivers for the use of LTE is the high data rates that can be achieved. However all technologies suffer from reduced data rates at the cell edge where signal levels are lower and interference levels are typically higher.

The use of technologies such as MIMO, OFDM and advanced error correction techniques improve throughput under many conditions, but do not fully mitigate the problems experienced at the cell edge.

As cell edge performance is becoming more critical, with some of the technologies being pushed towards their limits, it is necessary to look at solutions that will enhance performance at the cell edge for a comparatively low cost. One solution that is being investigated and proposed is that of the use of LTE relays.

LTE relay basics

LTE relaying is different to the use of a repeater which re-broadcasts the signal. A relay will actually receives, demodulates and decodes the data, apply any error correction, etc to it and then re-transmitting a new signal. In this way, the signal quality is enhanced with an LTE relay, rather than suffering degradation from a reduced signal to noise ratio when using a repeater.

For an LTE relay, the UEs communicate with the relay node, which in turn communicates with a donor eNB.

Relay nodes can optionally support higher layer functionality, for example decode user data from the donor eNB and re-encode the data before transmission to the UE.

The LTE relay is a fixed relay - infrastructure without a wired backhaul connection, that relays messages between the base station (BS) and mobile stations (MSs) through multihop communication

LTE relay types

There are two types of LTE relay being proposed:

• Type 1 LTE relay nodes: These LTE relays control their cells with their own identity including the transmission of their own synchronisation channels and reference symbols. Type 1 relays appear as if they are a Release 8 eNB to Release 8 UEs. This ensures backwards compatibility.
• Type 2 LTE relay nodes: These LTE relaying nodes do not have their own cell identity and look just like the main cell. Any UE in range is not able to distinguish a relay from the main eNB within the cell. Control information can be transmitted from the eNB and user data from the LTE relay.

There is still much work to be undertaken on LTE relaying. The exact manner of LTE relays is to be included in Release 10 of the 3GPP standards and specifications.

参考：http://www.radio-electronics.com/info/cellulartelecomms/lte-long-term-evolution/3gpp-4g-imt-lte-advanced-tutorial.php