LTE-Advanced Tutorial LTE Breakthrough to Real 4G

In the spring of 2009, 3GPP LTE Release 8 (LTE Rel.8) specification was completed, and it has triggered LTE service deployment by leading mobile network operators. LTE Rel.8 has set various target requirements for LTE, designed to achieve higher system performance than HSPA in 3GPP Release 6. It has improved system capacity, cell edge user throughput and lower C/U-plane latency, supported by introduction of new radio interface technologies, such as OFDM, frequency domain scheduling and MIMO. In the following year, spring of 2010, 3GPP LTE Release 9 (LTE Rel.9) has also been completed to extend various functionalities in LTE Rel.8. The area of enhancement includes closed subscriber group (CSG), self-organizing network (SON), and new functionalities such as location information service and MBMS (Multimedia Broadcast and Multicast Service).

The next important milestone is the standardization of LTE-Advanced (LTE Rel.10 and beyond). To keep up with the today's rapidly growing traffic, especially by the wide spread of smart phone devices, it became necessary to further enhance LTE Rel.9 and achieve much higher level of system performance, while keeping the backward compatibility. Accordingly, the radio access interface specifications for LTE-Advanced has been developed in the beginning of 2011. ITU-T has announced new requirements including spectral efficiency, higher bandwidth, and lower latency. To meet these competitive requirements, a series of new technologies have been discussed to introduce into LTE-Advanced, such as Carrier Aggregation, Enhanced Uplink Multi-antenna Transmission, and CoMP Transmission/Reception. LTE-Advanced will enable 1Gbps downlink bandwidth in addition to the existing LTE service and open a new era of true wireless broadband services in the near future.

Figure 1: Standardization Schedule

LTE-Advanced services will become available from leading mobile network operators around 2014. Additional features include higher downlink bandwidth and non-contiguous spectra usage to further enhance the current LTE services.

Evaluation Results against ITU-R Technical Criteria

ITU-R has specified minimum requirements and evaluation criteria for IMT-Advanced in the eight technical areas as listed below.

Peak spectral efficiency Cell spectral efficiency Cell edge user spectral efficiency Bandwidth Latency Mobility Handover interruption time VoIP capacity

3GPP TR 36.912 V9.0.0 (2009-09) describes the detailed evaluation results for the ITU-R technical criteria. The 3GPP self-evaluation concluded that LTE Rel.10 & beyond (LTE-Advanced), SRIT (Set of Radio Interface Technology), individual FDD RIT (FDD Radio Interface Technology) and TDD RIT (TDD Radio Interface Technology) components completely satisfied the criteria of the decision step and should move forward to Step 7 of the process. It concluded that, "consequently, the 3GPP LTE Release 10 & beyond (LTE-Advanced)

technology should be included in the ITU-R IMT-Advanced terrestrial component radio interface Recommendation(s)."

DL/UL Acceleration Technologies

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.

Figure 2: New Technology Adaptation into LTE-Advanced

Figure 3: DL Acceleration with CA and MIMO

Carrier Aggregation

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.

Figure 4: Carrier Aggregation

Figure 5: Three Types of Carrier Aggregation

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.

(a) Multiple CCs over contiguous bandwidth (Figure 6).

Figure 6: Overlapped Coverage

(b) CCs over different bands with different coverage in cells (Figure 7).

Figure 7: Different Coverage

(c) CCs cover cell edges of different CC cells (Figure 8).

Figure 8: Cell Edge Beamforming

(d) Macro-coverage with lower CC and Hotspot with RRH (Remote Radio Head) (Figure 9).

Figure 9: RRH Integration

New MIMO Techniques

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.

Figure 10: Closed-Loop MIMO (4x4 MIMO rank-2)

Figure 11: Rank Adaptation

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.

CoMP Techniques

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.

Figure 12: Centralized/Autonomous Decentralized Control

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.

Figure 13: Centralized/Autonomous Decentralized Control

Figure 14: Centralized/Autonomous Decentralized Control

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.

Deployment Possibilities

Deployment of the first LTE-Advanced service is expected to be around 2013 to 2014, with limited set of features. Not all the advanced technologies will be implemented in the beginning, but the rest will be "added-on" as the various conditions are cleared. Such conditions may include radio regulations, frequency allocations, and some technical barriers. Many of the new features are expected to be software upgrades on LTE nodes.

Unlike the general expectations of 4G, LTE-Advanced will not initially reach 1 Gbps or even 600 Mbps. One of the biggest reasons is the spectrum, which is strictly controlled by the governments. Currently, only the 3.5 GHz spectrum is going to be assigned for wireless broadband (in Japan), but it will not become available until 2014 to 2015. NTT docomo has announced its plan to start LTE-Advanced service with 40 MHz bandwidth on the 3.5GHz

spectrum, which is enough to achieve 1.2 Gbps speed theoretically. However, 8x8 MIMO required for such link speed will not be technically possible by 2015.

On the other hand, LTE services on 2 GHz spectrum will be able to provide around 100 Mbps bandwidth in 2015. LTE-Advanced will only achieve the same speed as the existing LTE services at this point, or in the best case, it may be able to reach up to 300 Mbps with 2x2 MIMO and Carrier Aggregation on 40 MHz bandwidth. Thus, LTE-Advanced will not initially achieve full spec services, but it only indicates the roadmap for the future enhancements.

LTE-Advanced is the real 4G specification that greatly enhances the performance of existing LTE services. Although the maximum link speed may not greatly exceed the current LTE in the initial stage, deployment of LTE-Advanced will be an important milestone to set clear path for the beginning of true wireless broadband.

LTE-Advanced TutorialCarrier AggregationNew MIMO TechniquesCoMP Techniques