NTT DoCoMo has announced that it will launch the world’s first combined HSDPA/LTE femtocells in December 2012, to enable improved indoor coverage for both 3G and 4G mobile handsets. Shops and offices will be able to offer high quality 3G and 4G coverage and users will be able to drop back to slower HSDPA services when they reach the monthly limit of their LTE subscriptions.
From the outset, LTE and LTE-Advanced have been designed to take advantage of mixed network architectures (often referred to as Heterogeneous Networks, or HetNets) comprising macrocells, picocells and femtocells. New features of the LTE radio interface, such as Orthogonal Frequency Domain Multiplexing (OFDM), Multiple Input Multiple Output (MIMO) antennas and advanced error correction help to improve the performance of its macrocells. However, they can never entirely solve the problems of delivering coverage and capacity everywhere, particularly indoors and towards the edge of cells. Therefore new network architectures will be crucial to maximising the potential of LTE.
LTE and LTE-Advanced already include features to facilitate the introduction of HetNets and further developments are underway. A significant step in 3GPP Release 10 (frozen in 2011) was the the introduction of relay nodes as a new component of the LTE network architecture. Relay nodes provide a highly flexible and effective way of introducing new cells into a network quickly and easily, without the need for fixed backhaul.
As the name suggests, a relay node acts as a relay for a “donor” base station (eNB), to which it is connected by nothing more than an LTE radio link. However, rather than simply relaying the radio signal (along with any noise and interference it may have accumulated), like a traditional repeater, a relay node can demodulate, decode and error correct a received signal before retransmitting it.
A number of operating modes are possible for relay nodes. For example, a Type 1 relay has its own base station identity and synchronisation channel, and appears to a mobile terminal to be a normal LTE base station. A Type 2 relay node has no identity of its own and as far as the mobile terminal is concerned it appears to be the donor base station. An inband relay node uses the same carrier frequencies for the link between the mobile terminal and the relay as between the relay and the donor base station, whereas an outband relay uses different frequencies. A half-duplex relay node can carry data in only one direction at a time, whereas a full-duplex node can carry data in both directions simultaneously.
Relay nodes have a number of potential roles, including:
- extending the coverage of existing cells (which is the main focus of the Release 10 implementation)
- enabling rapid roll-out of services, because there is no need to deploy fixed backhaul
- improving data throughput for users located near the edges of cells
- increasing the overall capacity delivered by a cell by achieving higher average throughput across the cell
As a wide variety of new HetNet products come onto the market in the coming years, we are likely to see significant changes to the shape of cellular networks.