LTE-A stands for LTE-Advanced; it is a major enhancement of the Long Term Evolution (LTE) standard and features backward compatibility. LTE-A has the ability to meet the higher requirements and diversified scenarios of wireless communications in future years while maintaining excellent compatibility with the LTE network. LTE-A can deliver data rates that far exceed LTE rates. The max rate of transmission for a standard LTE network is 150Mbps, but for an LTE-A network the transmission can be up to 2Gbps. For example, caching a 2GB movie on a mobile phone via the LTE network takes approximately 20 minutes, while the same task can be completed in a few minutes via the LTE-A network. What makes LTE-A so advanced? The answer is in the five key technologies of LTE-A which include carrier aggregation, multiple-input multiple-output (MIMO), coordinated multi-point (CoMP) transmission and reception, relay, and enhanced inter-cell interference coordination for heterogeneous networks. The eNodeB, communications media and terminal are essential components of a communications system. This article introduces the technologies in these three categories.

Category 1: LTE technologies in eNodeBs

1. Enhanced inter-cell interference coordination for heterogeneous networks (eICIC).

With the deployment of LTE networks different types of eNodeBs are required for various application scenarios, such as the macro eNodeB, micro eNodeB and home eNodeB. The macro eNodeB meets general coverage requirements, the micro eNodeB provides support for a large number of data services and the home eNodeB, which can be regarded as a pico eNodeB, serves a smaller coverage. A heterogeneous eNodeB will be formed by eNodeBs in different modes and with various function levels. Therefore, the future network will be a heterogeneous network. In a heterogeneous network, inter-cell interference will be generated. The conventional inter-cell interference coordination (ICIC) tackles interference in the LTE system, while Enhanced ICIC (eICIC) for the LTE-A system deals with complex interference in a heterogeneous network.

The above diagram shows the interference problem in a heterogeneous network. In scenario A, the macro eNodeB works in the coverage of a cell. As it has no right to access the cell, it is subject to strong downlink interference from the micro eNodeB.  In scenario B, the use of offset makes the cells closer to the macro eNodeB stay in the cell, causing subscribers to be subject to strong downlink interference from the macro eNodeB.

2. Coordinated multi-point (CoMP) transmission and reception.

In the LTE system common-frequency networking is the main networking mode under which inter-cell interference becomes a major factor affecting the experience of the cell edge subscribers. To eliminate inter-cell interference at the cell edge and improve the transmission rate for the edge subscribers, the LTE-A network introduces CoMP.

The CoMP technology converts interference signals into useful transmission signals for improved user experience.

See diagram below:

Category 2: Technology between the network and the terminal

3. Relay

A relay node is added to the communications link between the eNodeB and the terminal to implement data forwarding. This ensures effective control of the interference while improving the network coverage. It is similar to the principle of Wi-Fi relays.

Category 3: Terminal technologies (key points)

4. Carrier aggregation

As we all know, the network rate is directly related to the bandwidth. Similarly, in the communications system the simplest way to achieve a higher rate and enhanced system capacity is to increase the transmission bandwidth of the system. That’s why the LTE-A system adopts the technology for enhancing transmission bandwidth known as carrier aggregation (CA).

The CA technology aggregates two to seven LTE component carriers (CCs) to achieve maximum transmission bandwidth and effectively improve the uplink and downlink transmission rates. The terminal can decide the number of carriers for uplink and downlink transmission based on its own capacity. As shown in the following figure, the bandwidth of a single carrier is 20MHz, but if five CCs are adopted, the bandwidth is 20MHz*5=100MHz. A larger bandwidth means a higher rate.

Below is an analogy to explain this:

Imagine there is a road that has two lanes with two-way traffic. The road allows the passing of only one vehicle at a time in each direction. The following figure shows the situation where no CA is provided.

Then, the road is expanded to have four lanes with two-way traffic. This time, it allows the passing of two vehicles at a time in each direction. If you think about this analogy in relation to bandwidth, an increase in bandwidth would occur. The same rule applies to the LTE-A network, demonstrating as the saying goes that “technology is rooted in life”. Broadening only the downlink road means downlink CA, broadening only the uplink road means uplink CA, and broadening both the uplink and downlink roads means uplink and downlink CA.

5. Multiple-input multiple-output (MIMO)

MIMO refers to the use of multiple antennas both at the transmitting and receiving end. The signals are transmitted and received by multiple antennas at both ends to improve the quality of service (QoS). MIMO can make full use of space resources and implement multiple inputs and multiple outputs through multiple antennas. It has a multiplier effect on system channel capacity and data rates without requiring additional frequency spectrum resources and antenna transmission power.

Let’s start with SISO (Single Input Single Output).

Both the eNodeB and the mobile phone have an antenna. Let’s say that the eNodeB is a train station and the terminal is your home and there is only one way to and from the train station. There would surely be traffic jams during the morning and evening rush hours.

See SISO diagram below:

When it is reported to the local authority that the road from the train station to home is always jammed, suggestions may be offered to broaden the road, which is the aforesaid CA technology, or to make a double-layer road by adding a road on another layer. The 2-output road relieves congestion in the direction from the station to home.

This is SIMO (Single Input Multi Output).

Later it is discovered that congestion still occurs in the direction from home to the train station. The same method is used to modify the road to a double-layer one. Now, two inbound roads are available which relieves congestion in the direction from home to the station.

This is MISO (Multi Input Single Output).

In order to solve both problems, roads in both directions are modified into double-layer roads. This is MIMO (Multi Input Multi Output).

By Oscar Liu,  Section Manager from Hardware BB Department  (II)