LTE-Advanced Tutorials Part-1

The LTE (Long Term Evolution) technology was standardized within the 3GPP (3rd Generation Partnership Project) as part of the 3GPP Release 8 feature set. Since end 2009, LTE mobile communication systems are deployed as an evolution of GSM (Global system for mobile communications), UMTS (Universal Mobile Telecommunications System) and CDMA2000, whereas the latter was specified in 3GPP2 (3rd Generation Partnership Project 2). An easy-to-read LTE technology introduction can be found in [1]. The ITU (International Telecommunication Union) coined the term IMT-Advanced to identify mobile systems whose capabilities go beyond those of IMT 2000 (International Mobile Telecommunications). 3GPP responded on IMT-Advanced requirements with a set of additional technology components specified in 3GPP Release 10, also known as LTE-Advanced (see [3] for details). In October 2010 LTE-Advanced (LTE-A) successfully completed the evaluation process in ITU-R complying with or exceeding the IMT-Advanced requirements and thus became an acknowledged 4G technology. Existing mobile technologies have always been enhanced over a significant time period. As an example, GSM after more than 20 years of operation is still improved. LTE / LTE-A is in its infancy from a commercial operation perspective and one can expect further enhancements for many years to come. This white paper summarizes additional technology components based on LTE, which are included in 3GPP Release 11 specifications. Each technology component is described in detail in section 2. The technology component dependencies from LTE Release 8 to 11 are illustrated in Fig. 1-1 below. 

Technology Components of LTE-Advanced Release 11

Naturally the LTE/LTE-Advanced technology is continuously enhanced by adding either new technology components or by improving existing ones. LTE-Advanced as specified in the 3GPP Release 11 timeframe comprises a number of improvements based on existing features, like LTE carrier aggregation enhancements or further enhanced ICIC. Among the new technology components added, CoMP is clearly the feature with most significant impact for both end user device and radio access network. CoMP was already discussed in the 3GPP Release 10 time frame. However it was finally delayed to 3GPP Release 11. Note that many of the enhancements in 3GPP Release 11 result from the need to more efficiently support heterogeneous network topologies. 2.1 LTE carrier aggregation enhancements Within the LTE-Advanced feature set of 3GPP Release 10 carrier aggregation was clearly the most demanded feature due to its capability to sum up the likely fragmented spectrum a network operator owns. Naturally further enhancements of this carrier aggregation technology component were introduced in 3GPP Release 11. These are illustrated in the following sections. 2.1.1 Multiple Timing Advances (TAs) for uplink carrier aggregation As of 3GPP Release 10 multiple carriers in uplink direction were synchronized due to the fact that there was only a single Timing Advance (TA) for all component carriers based on the PCell. The initial uplink transmission timing on the random access channel is determined based on the DL reference timing. The UE autonomously adjusts the timing based on DL timing. This limits the use of UL carrier aggregation to scenarios when the propagation delay for each carrier is equal. However this might be different in cases when repeaters are used on one frequency band only, i.e. in case of inter-band carrier aggregation. Also repeaters/relays may introduce different delays on different frequency bands, if they are band specific. ). Another typical scenario might be a macro cell covering a wide area aggregated with a smaller cell at another frequency for high data throughput. The geographical location of the antennas for the two cells may be different and thus a difference in time delay may occur. Additionally and potentially even more important, if UL carrier aggregation is used in combination with UL CoMP (see section 2.2), the eNodeB receiving entities may be located at different places, which also requires individual timing advance for each component carrier (see Fig. 2-1). 

Fig. 2-1: CoMP scenario 3 requires different timing advance if multiple carriers are used in UL

To enable multiple timing advances in 3GPP Release 11, the term Timing Advance Group (TAG) was introduced [4]. A TAG includes one or more serving cells with the same UL timing advance and the same DL timing reference cell. If a TAG contains the PCell, it is named as Primary Timing Advance Group (pTAG). If a TAG contains only SCell(s), it is named as Secondary Timing Advance Group (sTAG). From RF (3GPP RAN4) perspective in 3GPP Release 11 carrier aggregation is limited to a maximum of two downlink carriers. In consequence only two TAGs are allowed. The initial UL time alignment of sTAG is obtained by an eNB initiated random access procedure the same way as establishing the initial timing advance for a single carrier in 3GPP Release 8. The SCell in a sTAG can be configured with RACH resources and the eNB requests RACH access on the SCell to determine timing advance. I.e. the eNodeB initiates the RACH transmission on the secondary cells by a PDCCH order send on the primary cell. The message in response to a SCell preamble is transmitted on the PCell using RA-RNTI that conforms to 3GPP Release 8. The UE will then track the downlink frame timing change of the SCell and adjust UL transmission timing following the timing advance commands from the eNB. In order to allow multiple timing advance commands, the relevant MAC timing advance command control element has been modified. The control element consists of a new 2 bit Timing Advance Group Identity (TAG Id) and a 6 bit timing advance command field (unchanged compared to 3GPP Release 8) as shown in Fig. 2-2. The Timing Advance Group containing the PCell has the Timing Advance Group Identity 0. TAG Id Timing Advance Command Oct 1 Fig. 2-2: Timing Advance Command MAC control element [10] As of 3GPP Release 8 the timing changes are applied relative to the current uplink timing as multiples of 16 TS. The same performance requirements of the timing advance maintenance of the pTAG are also applicable to the timing advance maintenance of the sTAG.