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LTE-Advanced: An introduction

Posted: 04 Jul 2011 ?? ?Print Version ?Bookmark and Share

Keywords:LTE-Advanced? 3GPP? 4G?

LTE-Advanced is designed to operate in spectrum allocations of different sizes, including allocations wider than the 20MHz in Release 8, in order to achieve higher performance and target data rates. Although it is desirable to have bandwidths greater than 20MHz 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.

LTE Advanced key technologies
Carrier aggregation: Achieving the 4G target downlink peak data rate of 1Gbit/s will require wider channel bandwidths than are currently specified in LTE Release 8. At the moment, LTE supports channel bandwidths up to 20MHz, and it is unlikely that spectral efficiency can be improved much beyond current LTE performance targets. Therefore the only way to achieve significantly higher data rates is to increase the channel bandwidth. IMT-Advanced sets the upper limit at 100MHz, with 40MHz the expectation for minimum performance.

Because most spectrum is occupied and 100MHz of contiguous spectrum is not available to most operators, the ITU has allowed the creation of wider band- widths through the aggregation of contiguous and non-contiguous component carriers. Thus spectrum from one band can be added to spectrum from another band in a UE that supports multiple transceivers. The figure 1 an example of contiguous aggregation in which two 20MHz channels are located side by side. In this case the aggregated bandwidth covers the 40MHz minimum requirement and could be supported with a single transceiver. However, if the channels in this example were non-contiguousthat is, not adjacent, or located in different frequency bandsthen multiple transceivers in the UE would be required.

The term component carrier used in this context refers to any of the bandwidths defined in Release 8/9 LTE. To meet ITU 4G requirements, LTE-Advanced will support three component carrier aggregation scenarios: intra-band contiguous, intra-band non-contiguous, and inter-band non-contiguous aggregation. The spacing between center frequencies of contiguously aggregated component carriers will be a multiple of 300kHz to be compatible with the 100kHz frequency raster of Release 8/9 and at the same time preserve orthogonality of the subcarriers, which have 15kHz spacing. Depending on the aggregation scenario, the n x 300kHz spacing can be facilitated by inserting a low number of unused subcarriers between contiguous component carriers. In the case of contiguous aggregation, more use of the gap between component carriers could be made, but this would require defining new, slightly wider component carriers.

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Figure 1: Contiguous aggregation of two uplink component carriers.

An LTE-Advanced UE with capabilities for receive and/or transmit carrier aggregation will be able to simultaneously receive and/or transmit on multiple component carriers. A Release 8 or 9 UE, however, can receive and transmit on a single component carrier only. Component carriers must be compatible with LTE Release 8 and 9.

In Release 10, the maximum size of a single component carrier is limited to 110 resource blocks, although for reasons of simplicity and backwards compatibility it is unlikely that anything beyond the current 100 RB will be specified. Up to 5 component carriers may be aggregated. An LTE-Advanced UE cannot be config- ured with more uplink component carriers than downlink component carriers, and in typical TDD deployments the number of uplink and downlink component carriers, as well as the bandwidth of each, must be the same.

For mapping at the physical layer (PHY) to medium access control (MAC) layer interface, there will be one transport block (in the absence of spatial multiplex- ing) and one hybrid-ARQ entity for each scheduled component carrier. (Hybrid ARQ is the control mechanism for retransmission.) Each transport block will be mapped to a single component carrier only. A UE may be scheduled over mul- tiple component carriers simultaneously. The details of how the control signaling will be handled across the multiple carriers are still being developed.

Aggregation techniques are not new to 4G; aggregation is also used in HSPA and 1xEV-DO Release B. However, the 4G proposal to extend aggregation to 100MHz in multiple bands raises considerable technical challenges owing to the cost and complexity that will be added to the UE. Moreover, operators will have to deal with the challenge of deciding what bands to pick for aggregation and it may be some time before consensus is reached allowing sufficient scale to drive the vendor community. 3GPP initially identified 12 likely deployment scenarios for study with the intention of identifying requirements for spurious emissions, maximum power, and other factors associated with combining different radio frequencies in a single device. However, because of the number of the scenarios and limited time, the study for Release 10 LTE-Advanced was initially limited to two scenarios, one intra-band TDD example and one inter-band FDD example. In June 2010 a third scenario was added for bands 3 and 7. This scenario is an important combination for Europe, where re-farming of the underused 1800MHz band currently allocated to GSM is a significant possibility.

The physical layer definition for CA is considered 80% complete and although the CA concept is simple, the details of the physical layer changes to support the signaling are complex and involve changes to the PCFICH, PHICH, PDCCH, PUCCH, UL power control, PUSCH resource allocation, and the UCI on the PUSCH. The radio performance aspects are only at 30% completion. This is significant, as Table above just begins to describe the possible scope of CA. To get some idea of the number of combinations requested by operators, refer to Annex A of TR 36.807. Every combination introduced into the specifications has to be assessed for aspects such as required guard bands, spurious emissions, power back off, and so forth.

One of the new challenges that CA introduces to the radio specifications is the concept of variable TX/RX frequency separation. This attribute impacts specifications for reference sensitivity and receiver blocking, among others. In Release 8 and Release 9, the TX and RX separation for each of the 19 defined FDD bands is fixed. The introduction of CA changes that, since asymmetric uplink and downlink allocations will be commonplace. The asymmetry is driven by three scenarios; different numbers of CCs in the uplink and downlink, different bandwidths of CC in the uplink and downlink, and finally a combination of different bandwidths and numbers of CCs. How to limit the allowed allocations in order to minimize the number of test scenarios is still under study.

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