One advanced feature of 3GPP LTE-Advanced is carrier aggregation. It is an essential mechanism for LTE Release 10/11 to meet the peak-data rate, 1Gbps, requirement of IMT-Advanced of ITU-R. Beside this, it also helps LTE maintain backward compatibility, support symmetric/asymmetric operation modes and contiguous/non-contiguous aggregations. All these features help enable a variety of network deployment scenarios, such as hot-spot operations and flexible duplex.
Historically carrier aggregation is not something brand-new in standards. On 3GPP2 side, the IS-2000 standard supports a multi-carrier operation, Spreading Rate 3 or 3x. Spreading 3 is used when higher data rates are desired with more bandwidth available. Spreading 3 of multi-carrier operation can make IS-2000 not only is backward compatible with its IS-95 predecessor but also satisfies the requirements set forth by IMT-2000/3G at that time. With adding additional guard band, IS-2000 can be harmonization-ready with UMTS. Since IS-2000 essentially is circuit-switch based mobile system and is optimized for delay sensitive services, especially voice service. It assumes symmetric traffic between its uplink and downlink. Therefore its multi-carrier extension is also symmetric and very straightforward. It can be implemented either with a single RF carrier of direct spread of 3.6864 Mcps or with three separated RF carriers of 1.2288 Mcps each. Naturally this maintains backward compatibility and can even reuse existing channel cards and many hardwares.
In 2006, 3GPP2 upgraded its CDMA2000 high rate pack data (HRPD) standard, also known as EV-DO, from Rev. A to Rev. B. One key feature of Rev. B is the multi-carrier operation supporting a bandwidth from 1.25MHz up to 20MHz bandwidth and accordingly resulting a much higher peak data rate, 4.9xMbps downlink. This makes EV-DO not only directly competing but also compatible with UMTS. Different to IS-2000 1x RTT, EV-DO is a packet-switch based mobile system optimized for data services. It doesn't assume the balance between uplink and downlink. Though it can be symmetrically extended in the similar fashion as IS-2000 does and maintain nice backward compatibility, it is not optimal.
Similar to HSPA, EV-DO Rev. B supports three multi-carrier operation modes, symmetric mode, asymmetric modes and enhanced asymmetric mode. For the symmetric mode, additional mechanism, Multi-Link RLP, is introduced to help solve the MAC queue hole issue. This feature is software upgradable. In order to achieve higher efficient and flexibility, asymmetric operation mode and its enhancement are introduced with merging control overheads of difference carriers. ( Note: EV-DO Rev. B = Multi-Carrier EV-DO Rev. A + more )
For LTE-Advanced, story can go much further. The symmetric operation mode essentially is similar to EV-DO Rev. B and IS-2000 1XRTT except the extension of OFDMA signaling is much naturally scalable. The interesting part is on asymmetric operation mode. LTE-Advanced proposed a concept of carrier segmentation, which can also be found in the enhanced asymmetric mode of EV-DO Rev. B. LTE carrier segment is a pool of adjacent component carriers with a total bandwidth up to 110 resource block or 20MHz while it is up to 4 forward link carriers in EV-DO Rev. B.
And ... story will never stop here. One of the reasons is that carrier aggregation is naturally related to interference management and heterogeneous networks. I am sure that carrier aggregation with dynamic component carrier activation/de-activation and X2 interface between eNodeBs will enable a lot of innovative applications in the future.