4G LTE Advanced Relay

- 4G LTE Advanced relay technology, how LTE relaying works and details about relay nodes, RNs.

Relaying is one of the features being proposed for the 4G LTE Advanced system. The aim of LTE relaying is to enhance both coverage and capacity.

The idea of relays is not new, but LTE relays and LTE relaying is being considered to ensure that the optimum performance is achieved to enable the expectations of the users to be met while still keeping OPEX within the budgeted bounds.

Need for LTE relay technology

One of the main drivers for the use of LTE is the high data rates that can be achieved. However all technologies suffer from reduced data rates at the cell edge where signal levels are lower and interference levels are typically higher.

The use of technologies such as MIMO, OFDM and advanced error correction techniques improve throughput under many conditions, but do not fully mitigate the problems experienced at the cell edge.

As cell edge performance is becoming more critical, with some of the technologies being pushed towards their limits, it is necessary to look at solutions that will enhance performance at the cell edge for a comparatively low cost. One solution that is being investigated and proposed is that of the use of LTE relays.

LTE relay basics

LTE relaying is different to the use of a repeater which re-broadcasts the signal. A relay will actually receive, demodulates and decodes the data, apply any error correction, etc to it and then re-transmitting a new signal. In this way, the signal quality is enhanced with an LTE relay, rather than suffering degradation from a reduced signal to noise ratio when using a repeater.

For an LTE relay, the UEs communicate with the relay node, which in turn communicates with a donor eNB.

Relay nodes can optionally support higher layer functionality, for example decode user data from the donor eNB and re-encode the data before transmission to the UE.

The LTE relay is a fixed relay - infrastructure without a wired backhaul connection, that relays messages between the base station (BS) and mobile stations (MSs) through multihop communication.

There are a number of scenarios where LTE relay will be advantageous.

  • Increase network density:   LTE relay nodes can be deployed very easily in situations where the aim is to increase network capacity by increasing the number of eNBs to ensure good signal levels are received by all users. LTE relays are easy to install as they require no separate backhaul and they are small enabling them to be installed in many convenient areas, e.g. on street lamps, on walls, etc.

    LTE relay used to increase network density

  • Network coverage extension :   LTE relays can be used as a convenient method of filling small holes in coverage. With no need to install a complete base station, the relay can be quickly installed so that it fills in the coverage blackspot.

    LTE relay coverage extension
    LTE relay coverage extension - filling in coverage hole

    Additionally LTE relay nodes may be sued to increase the coverage outside main area. With suitable high gain antennas and also if antenna for the link to the donor eNB is placed in a suitable location it will be able to maintain good communications and provide the required coverage extension.

    LTE relay coverage extension
    LTE relay coverage extension - extending coverage

    It can be noted that relay nodes may be cascaded to provide considerable extensions of the coverage.
  • Rapid network roll-out:   Without the need to install backhaul, or possibly install large masts, LTE relays can provide a very easy method of extending coverage during the early roll-out of a network. More traditional eNBs may be installed later as the traffic volumes increase.

    LTE relay to provide fast rollout & deployment
    LTE relay to provide fast rollout & deployment

LTE relaying full & half duplex

LTE relay nodes can operate in one of two scenarios:

  • Half-Duplex:   A half-duplex system provides communication in both directions, but not simultaneously - the transmissions must be time multiplexed. For LTE relay, this requires careful scheduling. It requires that the RN coordinates its resource allocation with the UEs in the uplink and the assigned donor eNB in the downlink. This can be achieved using static pre-assigned solutions, or more dynamic ones requiring more intelligence and communication for greater flexibility and optimisation.
  • Full Duplex:   For full duplex, the systems are able to transmit and receive at the same time. For LTE relay nodes this is often on the same frequency. The relay nodes will receive the signal, process it and then transmit it on the same frequency with a small delay, although this will be small when compared to the frame duration. To achieve full duplex, there must be good isolation between the transmit and receive antennas.

When considering full or half duplex systems for LTE relay nodes, there is a trade-off between performance and the relay node cost. The receiver performance is critical, and also the antenna isolation must be reasonably high to allow the simultaneous transmission and reception when only one channel is used.

LTE relay types

There is a number of different types of LTE relay node that can be used. However before defining the relay node types, it is necessary to look at the different modes of operation.

One important feature or characteristic of an LTE relay node is the carrier frequency it operates on. There are two methods of operation:

  • Inband:   An LTE relay node is said to be "Inband" if the link between the base station and the relay node are on the same carrier frequency as the link between the LTE relay node and the user equipment, UE, i.e. the BS-RN link and the BS-UE link are on the same carrier frequency.
  • Outband:   For Outband LTE relay nodes, RNs, the BS-RN link operates of a different carrier frequency to that of the RN-UE link.

For the LTE relay nodes themselves there are two basic types that are being proposed, although there are subdivisions within these basic types:

  • Type 1 LTE relay nodes:  These LTE relays control their cells with their own identity including the transmission of their own synchronisation channels and reference symbols. Type 1 relays appear as if they are a Release 8 eNB to Release 8 UEs. This ensures backwards compatibility. The basic Type 1 LTE relay provides half duplex with Inband transmissions.

    There are two further sub-types within this category:

    • Type 1.a:   These LTE relay nodes are outband RNs which have the same properties as the basic Type 1 relay nodes, but they can transmit and receive at the same time, i.e. full duplex.
    • Type 1.b:   This form of LTE relay node is an inband form. They have a sufficient isolation between the antennas used for the BS-RN and the RN-UE links. This isolation can be achieved by antenna spacing and directivity as well as specialised digital signal processing techniques, although there are cost impacts of doing this. The performance of these RNs is anticipated to be similar to that of femtocells.
  • Type 2 LTE relay nodes:  These LTE relaying nodes do not have their own cell identity and look just like the main cell. Any UE in range is not able to distinguish a relay from the main eNB within the cell. Control information can be transmitted from the eNB and user data from the LTE relay.

LTE Relay Class Cell ID Duplex Format
Type 1
Inband half duplex
Type 1.a
Outband full duplex
Type 1.b
Inband full duplex
Type 2
Inband full duplex

Summary of Relay Classifications & Features in 3GPP Rel.10

There is still much work to be undertaken on LTE relaying. The exact manner of LTE relays is to be included in Release 10 of the 3GPP standards and specifications.

By Ian Poole

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