There are series of technologies in LTE-Advanced as shown in Figure 2. But new MIMO and Carrier Aggregation (CA) are the two key technologies for DL/UL acceleration. These technologies will improve communication performance and expand the effective bandwidth, enabling the maximum downlink speed of even up to 3 Gbps as shown Figure 3.
Carrier Aggregation (CA) is an innovative approach to create wider bandwidth by using multiple aggregated Carrier Components (CCs). LTE Rel.10 adopts the CA technique to increase spectral bandwidth up to 100 MHz using multiple CCs. The aggregated CCs must be on compatible spectral bandwidth supported by LTE Rel.8 (i.e., 1.4 MHz/3 MHz/5 MHz/10 MHz/15 MHz/20 MHz). It allows seamless migration into LTE Rel.10 by re-utilizing LTE Rel.8 eNB along with radio frequency (Figure 4), adjacent channel leakage ratio (ACLR), spectrum emission mask (SEM), adjacent channel selectivity (ACS) and blocking.
As the LTE Rel.10 UEs are backward-compatible to LTE Rel.8 standards, it has great advantage on reducing redundant implementation with this approach. Thus, CA-enabled LTE Rel.10 UE would achieve higher user throughput than LTE Rel.8.
There are three types of CA, depending on CC combination as shown in Figure 5.
1. Intra-band Contiguous CA
Contiguous bandwidth wider than 20 MHz is used in this scenario. For example, wideband such as 3.5 GHz band would fit this model.2. Inter-band Non-contiguous CA
Non-contiguous band over multiple bands is used in this scenario. Network with two spectrum bands (i.e., 2 GHz and 800 MHz) would fit this model. This scenario would have advantage on having higher throughput simply by two carriers as well as the improvement on stable transmission by two different spatial paths on different spectrum bands.3. Intra-band Non-contiguous CA
Non-contiguous band in same band is used in this scenario. This model would fit operators in North America or Europe, who have fragmental spectrum in one band or share same cellular network.CA Scenarios
There are four possible CA Scenarios in real LTE-Advanced deployment.
LTE Rel.8 supported up to four layers of MIMO multiplexing for downlink and no MIMO for uplink. LTE-Advanced supports single user MIMO (SU-MIMO) scheme up to eight layers (8x8 MIMO) for downlink and four layers (4x4 MIMO) for uplink. With this technology, it achieves peak spectral efficiency of 30 bit/s/Hz for downlink and 15 bit/sec/Hz for uplink. In other words, single 20MHz bandwidth to achieve up to 600Mbps downlink speed.
Multi-user MIMO (MU-MIMO) is also the important technology to increase peak data rate as well as the system capacity and cell edge user throughput. MU-MIMO and CoMP transmission, which will be described later, are applying various advanced signal processing techniques, e.g. dedicated downlink beamforming, adaptive transmission power control, and multi cell simultaneous transmission.
Coordinated multi-point transmission/reception (CoMP) is a DL/UL orthogonalization technique to improve system capacity and cell edge user throughput. Currently, there are two different approaches for CoMP techniques (Figure 12). One approach is a decentralized autonomous control based on independent eNB architecture, and the other is a centralized control based on remote radio equipment (RRE) architecture.
In the approach with independent eNB architecture, CoMP is performed by signaling between eNBs. This technique can utilize legacy cells, but the disadvantage is signaling delay and other overheads. In the second approach with RRE technique, the eNB can centralize and control all the radio resource by transmitting baseband data directly between eNB and RREs on optical fiber connections. There is little signaling delay or other overheads in this technique, and Intra-cell radio resource control is relatively easy. However, CAPEX on optical fibers is not negligible, and centralized eNB must be able to accept higher load according to the number of RREs. Therefore, both approaches are under consideration for LTE-Advanced.
1. Downlink CoMP
Downlink CoMP also has two approaches under consideration for LTE-Advanced, i.e., Coordinated Scheduling/Beamforming (CS/CB) (Figure 13) and Joint Processing (Figure 14).
In CS/CB, the transmission to a single UE is performed from the serving cell, exactly as in the case of non-CoMP transmission. However, the scheduling is dynamically coordinated between the cells, including any beamforming functionality. In that way, the interference between different transmissions can be controlled and reduced. In principle, schedule optimization will be performed based on the serving set of users, so that the transmitter beams are constructed to reduce interference to other neighboring user, while increasing the served users’ signal strength.
In Joint Transmission feature of Joint Processing, the transmission to a single UE is simultaneously performed from multiple transmission points in practice cell sites. The multi-point transmissions will be coordinated as a single transmitter with multiple antennas that are geographically separated. This scheme has the potential for higher performance, compared to CS/CB, but comes at the expense of more stringent requirement on backhaul communication.
2. Uplink CoMP
Uplink CoMP utilizes geographically separated antennas for signal reception from UE, and scheduling decisions are coordinated by multiple cells to control interference from each other. UE is not aware of multi-cell reception of its signal, so that impact on radio interface specification is at minimal. Implementation of Uplink CoMP largely depends on scheduler and receiver in the cells.