Relays in LTE-Advanced

Increasing capacity and reducing capital and operational costs are major goals of every mobile operator throughout the world. With the introduction of Long Term Evolution – Advanced, a 3GPP standard, a number of techniques to improve capacity and coverage while reducing the cost to the operator have been introduced. Amongst these techniques is the introduction of advanced relays.

Figure 1: Relay Nodes

A relay is similar to a repeater which is currently in widespread use for 2G/3G technologies. Relay nodes are basically low power base stations which enhance coverage in areas where coverage is poor or to provide coverage in rural areas without the need for a wired backhaul connection.  The cost to purchase and implement a relay station is significantly less than that of an eNodeB. The backhaul between the relay and the eNodeB is wireless which reduces costs in the implementation of infrastructure.

A repeater (also known as a Layer 1 relay) extends the coverage of an existing base station to areas where the base station cannot reach or there is a high SINR such as at the edge of the cell, areas where shadowing occurs or to provide coverage indoors. A Layer 1 relay amplifies and forwards downlink and uplink signals between the UE and the base station. A major disadvantage of a Layer 1 relay is that not only is the desired signal amplified and forwarded but also unwanted interference such as inter-cell interference and noise.

A Layer 2 Relay acts by demodulating and decoding the incoming signal and re-modulating and re-encoding the signal before the amplified version is transmitted. This process overcomes the problems associated with the Layer 1 relay as no inter-cell interference or noise is being amplified and re-transmitted.

A Layer 3 relay operates in a similar manner to a Layer 2 relay with additions such as having a unique Physical Cell ID to ensure the UE knows it is connected to a relay node and treats the relay node as a base station. Layer 3 relays have been standardised by 3GPP for release with LTE Advanced (Rel. 10).

Figure 2: Implementation Scenarios

The figure above shows a scenario where relaying is utilised. In scenario (a) the UE communicates directly with the eNodeB. In scenario (b) a relay is utilised to extend the coverage area of the eNodeB and the UE communicates with the eNodeB via a relay node. In scenario (c) a relay node is utilised to overcome excessive shadowing, the signal from the eNodeB would not be able to reach the desired location behind tall buildings if it was not for the use of this relay node. In scenario (d) the area is considered to be a “hotspot” where there are a high number of users and the relay node is utilised to increase the available throughput to these users.

The installation of new eNodeB’s in a network can be very expensive both in the cost of installing and the cost of operation. The installation of relays in a network are a cheaper alternative to improving network performance while reducing costs and energy consumption. Relays do not need to be as high as a standard eNodeB and they also connect back to the eNodeB wirelessly which reduces the need for a complicated wired interface. They are also not as large as eNodeB’s and do not require air conditioned rooms or cooling units.

Will Relays play a big part in future high speed mobile networks? Let us know what you think.

– Laurence Fyfe