Boosting Data Rates in LTE
The next standardization step is joint TDD-FDD carrier aggregation.
Carrier aggregation (CA) is used in LTE-Advanced to increase the throughput for the user. As a technique, carrier aggregation takes advantage of the fact that assigning more spectrum (or bandwidth) to the user, increases the user data rate.
For example, an analogy is that we would like to heat a room. To start with, we have only a single heater in the room, i.e. the case without carrier aggregation. The single heater is turned up as much as possible, but it is still too cold. In order to further increase the temperature in the room, we would need to place an additional heater in the room. We now have two heaters in the room and we can then further increase the temperature in the room. The last case would resemble the case of carrier aggregation. Going back to operating a base station and user equipment (UE): If we take the example that we are operating a base station without CA, this would yield the possibility to deliver a certain data rate to a user over a single carrier. Instead, if we would use carrier aggregation, it would be possible to increase the user data rate as it would be possible to communicate multiple carriers to the same user (more bandwidth/spectrum is used). If we, for example, use two carriers that are equal in bandwidth and compare this to only use a single carrier, the user data rate would double.
3GPP is currently finalizing the work on TDD-FDD carrier aggregation. Carrier aggregation is used in LTE to boost the UE's data rate. This can be achieved by the user equipment receiving or transmitting data on pieces of spectrum that is spread out in different frequency bands. Previously, this has only been possible between FDD and FDD spectrum or between TDD and TDD spectrum. This post will give you some insight into what this means, as well as what benefits this brings.
Figure 1: Example of TDD-FDD carrier aggregation
Carrier aggregation was first introduced in LTE with Rel-10 and implies that the UE aggregates a number of cells operating on different frequencies. The primary cell is used to handle mobility, physical control signaling and broadcast information in addition to user data. All the other cells are viewed as secondary cells with the main functionality of providing higher data throughput. The Rel-10 carrier aggregation supports aggregation of up to 5 carriers in both DL and UL within either FDD or TDD (assuming the same upload/download (UL/DL) configuration for all aggregated carriers) in the core parts. In terms of spectrum it is possible to aggregate up to 100 MHz of spectrum with the Rel-10 design. In practice however the individual requirements are needed for each aggregation band combination and currently the focus is mainly on either 2 or 3 DL carriers.
Within Rel-11 the carrier aggregation was further enhanced in several aspects. One of the main aspects was to allow the aggregation of TDD carriers in different TDD bands with different UL/DL configurations.
For Rel-12 we are now taking another step in carrier aggregation and are specifying the possibility to aggregate FDD and TDD carriers jointly. The main target with introducing the support for TDD-FDD CA is to allow the network to boost the user throughput by aggregating both TDD and FDD towards the same UE. This will allow the network to boost the UE throughput independently from where the UE is in the cell (at least for DL CA). Furthermore TDD and FDD CA would also allow dividing the load more quickly between the TDD and FDD frequencies. The Rel-12 TDD-FDD CA design support that both a TDD or FDD cell can be the primary cell.
There are several different target scenarios in 3GPP for TDD-FDD CA, but there are two main scenarios that 3GPP is targeting to support. The first scenario assume that the TDD-FDD CA is done from the same physical site that is typically a macro Evolved Node B (eNB) (scenario 1-3 in ) and the second scenario is a scenario wherein the Macro eNB provides either a TDD and FDD frequency and the other frequency is provided from a remote radio head (RRH, scenario 4 in ) deployed at another physical location. The typical use case for the second scenario is that the macro eNB provides the FDD frequency and the TDD frequency from the RRH.
Figure 2: Collocated (left) and non-collocated (right) CA deployments 
The work in 3GPP on the physical layer and the protocol layer are in principle completed with the remaining aspects of finalizing specification text. The main remaining work is to define the first band combination examples which are planned for completion before the end of this year.
Figure 3: Explanation of FDD and TDD
Senior Researcher, Ericsson Research
 TS 36.300 V12.1.0 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (Release 12)
 4G LTE/LTE-Advanced for Mobile Broadband, Erik Dahlman, Stefan Parkvall, Johan Sköld
UE: User Equipment
TDD: Time Division Duplex
FDD: Frequency Division Duplex
RRH: Remote Radio Head
CA: Carrier Aggregation
eNB: Evolved Node B