Introducing LTE-Advanced (Part 2 of 4)

Contributed by Agilent Technologies, Inc

ITU Requirements for IMT-Advanced (4G)

The third generation of cellular radio technology was defined by the ITU-R through the International Mobile Telecommunications 2000 project (IMT-2000). The requirements for IMT-2000, defined in 1997, were expressed only in terms of peak user data rates:

• 2048 kbps for indoor office
  
• 384 kbps for outdoor to indoor and pedestrian
  
• 144 kbps for vehicular
  
• 9.6 kbps for satellite

Of significance is that there was no requirement defined for spectral efficiency in 3G. The situation is quite different for IMT-Advanced.

The ITU’s high level requirements for IMT-Advanced include the following [5]:

• A high degree of common functionality worldwide while retaining the flexibility to support a wide range of local services and applications in a costefficient manner
  
• Compatibility of services within IMT and with fixed networks
  
• Capability for interworking with other radio systems
  
• High quality mobile services
  
• User equipment suitable for worldwide use
  
• User-friendly applications, services, and equipment
  
• Worldwide roaming capability
  
• Enhanced peak data rates to support advanced mobile services and applications (in the downlink, 100 Mbps for high mobility and 1 Gbps for low mobility)

For the most part these are general purpose requirements that any good standard would attempt to achieve. The key requirement that sets 4G apart from previous standards is reflected in the last item, which gives the expectations for peak data rates that reach as high 1 Gbps for low mobility applications and 100 Mbps for high mobility. This is a huge increase from 3G, which specified a peak rate of 2 Mbps for indoor low mobility applications and 144 kbps vehicular. The peak rates targeted for 4G will have fundamental repercussions on system design.

To date, 14 industry groups have registered with the ITU to evaluate whether or not the technology proposals submitted as candidates for 4G meet the requirements.

In addition to the general requirements above there are specific requirements for spectral efficiency summarized later in Table 3.

3GPP Requirements for LTE-Advanced

The work by 3GPP to define a 4G candidate radio interface technology started in Release 9 with the study phase for LTE-Advanced. The requirements for LTE-Advanced are defined in 3GPP Technical Report (TR) 36.913, “Requirements for Further Advancements for E-UTRA (LTE-Advanced) [6].” These requirements are based on the ITU requirements for 4G and on 3GPP operators’ own requirements for advancing LTE. Major technical considerations include the following:

• Continual improvement to the LTE radio technology and architecture
  
• Scenarios and performance requirements for interworking with legacy radio access technologies
  
• Backward compatibility of LTE-Advanced with LTE. An LTE terminal should be able to work in an LTE-Advanced network and vice versa. Any exceptions will be considered by 3GPP.
  
• Account taken of recent World Radiocommunication Conference (WRC-07) decisions regarding new IMT spectrum as well as existing frequency bands to ensure that LTE-Advanced geographically accommodates available spectrum for channel allocations above 20 MHz. Also, requirements must recognize those parts of the world in which wideband channels are not available.

3GPP cites the fact that IMT-conformant systems will be candidates for any new spectrum bands identified by WRC-07 as one reason to align LTE-Advanced with IMT-Advanced [7]. In addition, it is significant that the ITU has renamed its IMT-2000 spectrum as “IMT” spectrum with the intention that all spectrum previously identified for IMT-2000 (3G) is also applicable for IMT-Advanced (4G). This is significant because it means there is no such thing as 3G spectrum or 4G spectrum; there is just one pool of IMT spectrum. What then drives deployment of specific technologies in specific bands will depend on local circumstances. It could be argued this ITU decision frees up the industry to make appropriate local decisions but it also has the effect of increasing the likely fragmentation of markets. The frequency band choices for early 2G and 3G systems were far simpler and focused the industry on one or two key bands (900 MHz for GSM and 2.1 GHz for W-CDMA). No comparable focus exists for LTE and LTE-Advanced, with Release 10 having upwards of 30 bands defined from the outset.

System performance requirements

The system performance requirements for LTE-Advanced will in most cases exceed those of IMT-Advanced. The 1 Gbps peak data rate required by the ITU will be achieved in LTE-Advanced using 4x4 MIMO and transmission bandwidths wider than approximately 70 MHz [8]. In terms of spectral efficiency, today’s LTE (Release 8) satisfies the 4G requirement for the downlink, but not for the uplink.

Table 3. Performance targets for LTE, Advanced-LTE,
and IMT-Advanced Table 3 compares the spectral efficiency targets for LTE, LTE-Advanced, and IMT-Advanced. Note that the peak rates for LTE-Advanced are substantially higher than the 4G requirements, which highlights a desire to drive up peak performance in 4G LTE, although targets for average performance are closer to ITU requirements. It’s worth noting that peak targets, because they can be met in ideal circumstances, are often easier to demonstrate than average targets. However, TR 36.913 states that targets for average spectral efficiency and for cell-edge user throughput efficiency should be given higher priority than targets for peak spectral efficiency and other features such as VoIP capacity5. Thus the work of LTE-Advanced should be focused on the very real challenges of raising average and cell-edge performance.

Spectrum flexibility

In addition to the bands currently defined for LTE Release 8, TR 36.913 identifies the following new bands:

• 450–470 MHz band
  
• 698–862 MHz band
  
• 790–862 MHz band
  
• 2.3–2.4 GHz band
  
• 3.4–4.2 GHz band
  
• 4.4–4.99 GHz band

Some of these bands are now formally included in the 3GPP Release 9 and Release 10 specifications. Note that frequency bands are considered release independent features, which means that it is acceptable to deploy an earlier release product in a band not defined until a later release.

LTE-Advanced is designed to operate in spectrum allocations of different sizes, including allocations wider than the 20 MHz in Release 8, in order to achieve higher performance and target data rates. Although it is desirable to have bandwidths greater than 20 MHz deployed in adjacent spectrum, the limited availability of spectrum means that aggregation from different bands is necessary to meet the higher bandwidth requirements. This option has been allowed for in the IMT-Advanced specifications.

References

[5] ITU-R M [IMT-TECH], “Requirements related to technical performance for IMT-Advanced radio interface(s),” August 2008.
[6] 3GPP TR 36.913 V9.0.0 (2009-12): http://www.3gpp.org/ftp/Specs/archive/ 36_series/36.913.
[7] http://www.3gpp.org/LTE-Advanced
[8] http://www.3gpp.org/IMG/pdf/2009_10_3gpp_IMT.pdf