Abbreviations used herein shall have the following meanings:
AF Amplify-and-Forward
CRC Cyclic Redundancy Check
DCI Downlink Control Information
DF Decode-and-Forward
DFTS Decode and Forward TS
DL Downlink
FSR Frequency Selective Relay (Repeater)
FTR Frequency Translating Relay (Repeater)
OFDM Orthogonal Frequency Division Multiplex
OFR On-Frequency Relay (Repeater)
PDCCH Physical Downlink Control CHannel
CRNTI Cell Radio Network Temporary Identifier
TTI Transmission Time Interval
UL Uplink
One recent development within modern telecommunication standards is the so-called Long Term Evolution (LTE) radio interface, and a further development of the LTE, namely LTE Advanced. These are both OFDM based systems.
One of the most important improvement areas in the so-called LTE-Advanced technology is the increase of data rates available for users at the cell edge and indoor. A very promising technique to achieve high data rates in such difficult locations is the deployment of relays. Relays are usually classified into Layer 1 (L1), Layer 2 (L2), and Layer 3 (L3) relays depending on which Open System Interconnection (OSI) layer they operate on. The OSI model is a conceptual model for telecommunication consisting of 7 layers (physical, data-link, network, transport, session, presentation, and application) Note however that the different layers refer to the user plane of the relay node and a L1 relay may use e.g. L3 control plane signaling.
L1 relays are commonly denoted Amplify-and-Forward (AF) relays or sometimes equivalently repeaters. An AF repeater operates in the physical layer and its basic functionality is, as the name suggests, amplifying and then forwarding the received signal, including any received noise and interference.
L2 relays operate in the data link layer and have the ability to detect and possibly correct errors that have occurred in the physical layer. L2 relays are therefore commonly called Decode-and-Forward (DF) relays as they decode the received data prior to retransmission. DF relays will, at the expense of an increased delay, not forward noise and interference.
L3 relays operate in the network layer and are by the Third Generation Partnership Program (3GPP) regarded as being equivalent to eNodeBs (eNBs) that are wirelessly connected to a donor cell via self-backhauling. L3 relays have the same characteristics as L2 relays in the sense that they do not forward noise and interference as they perform decoding and error correction of the received signal prior to retransmission.
There are several known methods of utilizing various repeaters or relays to further improve the quality of performance in telecommunication systems. Some of the most common include:
Cooperative relaying which enables multiple relays to cooperate during transmissions to users. For example, the cooperation may be used for increased diversity or multiplexing of data.
Multi-hop relaying which enables signals to be conveyed from a source to a destination over two or more wireless hops. The multiple hops are achieved by relaying the signal via one or more relay(s)/repeater(s). It may be used to reduce the end-to-end path loss and thus extending the coverage.
On frequency relays (or repeaters) (OFR) which are relays (repeaters) that forward on the same frequency band occupied by the received signal.
Frequency translating relays (or repeaters) (FTR) which are relays (repeaters) that translate the retransmitted signal to another frequency band that is different from that occupied by the received signal.
Frequency selective relays (or repeaters) (FSR) which are relays (repeaters) that may dynamically retransmit coordinated parts of the received signal bandwidth.
The increased path gain that comes from splitting of the signal path in two hops by repeating or relaying in an intermediate node brings several benefits: Data rates can be significantly increased; transmit power can be reduced and inter-cell interference falls rapidly. A multi-hop solution based on AF repeaters has some interesting characteristics compared to other DF relaying solutions. Since a repeater can receive and transmit on the same radio resource, which is not possible for DF types of relays, it is possible to operate without any duplex coordination loss between the two hops. A decode-and-forward (DF) relay can forward the data on the same frequency resource. However, since the decoding operation will result in an unavoidable delay the forwarding must take place at a later time instance, i.e. on another radio resource. In contrast, an amplify-and-forward (AF) repeater has a delay that typically is negligible compared to the transmission time interval, hence it can forward on the same radio resource. Repeaters also introduce less delay than DF relays which is beneficial for the performance of higher layer protocols such as TCP. Furthermore, a repeater is a simple device that typically is fairly cost efficient.
In particular, the use of OFR in OFDM based systems is interesting if the delay of the repeater is limited to the length of cyclic prefix of the OFDM modulation. In the air, the repeated signal path and the direct signal path add in the same way as normal multi-path does. In case of LTE, the additional time dispersion induced by the repeater does not result in any additional receiver complexity and/or reduced performance due to increased self interference as long as the total time dispersion is limited to the length of the cyclic prefix. Note that is not the case for single-carrier systems without cyclic prefix e.g. HSPA, where additional time dispersion typically increases the receiver complexity (i.e. more rake fingers are required) as well as the self-interference (i.e. signal components with a relative delay difference are non-orthogonal).
Despite the benefits of utilizing AF repeaters, there are a few disadvantages that prevent the use from providing the full benefit of them.
One drawback with repeaters compared to DF relays is that they forward not only signals but also noise and interference. Furthermore, a major challenge for on-frequency repeaters is to sufficiently suppress the self-interference they induce.
Repeaters (and relays) are efficient for both providing coverage in areas without coverage (see upper part of FIG. 1) and also to provide increased data rates to areas with weak signal strength (lower part of FIG. 1). This distinction is important since in the data-rate extension case the users will receive both a direct signal part as well as a repeated signal path, witch in the case of DF relaying will interfere with each other and in case of AF repetition and OFDM will add like multi-path. In addition, since AF repeaters amplify noise and interference they are only beneficial in case it is possible to replace one weak radio link with two significantly better radio links. This is more likely to be possible when the original radio link is weak due to some obstacle (e.g. a wall) that hinders the radio waves rather than pure propagation distance.
In addition, in the data rate extension scenario it is possible to dynamically turn the repeater on and off without losing coverage. It is also possible to do frequency selective repetition in the data-rate extension case without destroying the communication on the uplink and downlink control channels. Furthermore, the data-rate extension case is also particularly relevant for LTE and LTE-Advanced since, in order to compete with HSPA, their main business case is to provide high data-rates, which are only achievable in case the signal strength also is high.
In the coverage extension scenario, the options when it comes to advanced repeater behavior are more limited. It is not possible to e.g. turn the repeater off even when the repeater does not serve any UEs since that would leave the area with no coverage. In that case a UE wanting to perform an initial access would not be able to read the broadcast channel (BCH) and the system information blocks (SIBs) required for random access. Furthermore, an idle UE would not detect any paging messages sent from the network. In the coverage area extension case it is also not possible to perform frequency selective repetition on the downlink since that would hinder the UE from receiving the physical downlink control channel (PDCCH) that covers the whole downlink bandwidth. Also any frequency selective operation by the repeater on the uplink band must assure that the resources used for the physical uplink control channel (PUCCH) as well as the physical random access channel (PRACH) are always repeated.
Consequently, there is a need for a more efficient use of repeaters.