Литература / UMTS-Report

SUPPORT FOR RELAYING AND ODMA

1. Introduction

WB-TDMA/CDMA satisfies the high level requirements for the UTRA and particular consideration has been given to the future evolution of the system in terms of performance and flexibility. Many advanced features have already been incorporated into the core design but WB-TDMA/CDMA is also a suitable platform for the support of relaying.

Relaying is a widely used technique for radio packet data transmission both in commercial and military systems but it has so far not been widely used in cellular systems. Relaying has the potential to improve coverage and flexibility but may also increase capacity by lowering transmission powers and associated intercell interference.

Within the ETSI SMG2 process for UTRA evaluation, investigation of relay systems was carried out within the Epsilon Concept Group by considering a technology called Opportunity Driven Multiple Access - ODMA. The protocols used in ODMA are very similar to those of a packet radio system currently being trialed in South Africa. System level simulations were carried out in accordance with UMTS 04.02 which showed that very wide area coverage was possible in all environments using a subscriber relay system and that there was potential for increased capacity when used in a cellular hybrid.

A feasibility study was jointly conducted by the Delta and Epsilon concept groups to determine the practicality of supporting relaying using the basic WB-TDMA/CDMA design. The outcome of the study and the general findings from the epsilon evaluation work are recorded in this text and the basic conclusion was that the WBTDMA/CDMA design was sufficiently flexible to support relaying with negligible increase to the MS complexity or cost. WB-TDMA/CDMA therefore offers the flexibility of simple relaying but also provide a suitable platform for advanced relay protocols such as ODMA.

1.1 Terminology

The definition of terms listed below may help when reading the following text.

2. Relays

Relays add flexibility to a communication system. The figure below illustrates a range of options from simple one hop relays to ODMA which is an intelligent adaptive multi-hop system supporting full mobility of callers and relays.

Figure 1 Relaying Options

All the options shown above are possible providing that from the outset the UTRA can support the direct transfer of information from one MS to another. Potential benefits of relaying include;

♦Extension of high data rate coverage (144-384kbps)

♦Reduction of TX power

♦Overcoming dead-spots

♦Relaying to satellite system

♦Relaying to a vehicle

♦Directly linking terminals (private and uncoordinated systems, home BTS etc.)

The above list is by no means exhaustive and the real motivation for relaying will only become apparent as the UTRA standard develops, however there appears sufficient justification to support relaying within WB-TDMA/CDMA given the following preconditions;

♦Relaying must not degrade the QoS of a UMTS cell

♦Relaying must not add significant cost or complexity to a UMTS terminal

♦Relaying can be enabled or disabled by an operator

3. An Overview of ODMA

The Principle:

Opportunity Driven Multiple Access is a mechanism for maximising the potential for effective communication. This is achieved by distributing intelligence within communicating nodes and providing multiple communication paths between them. The intelligent nodes measure and evaluate their communications options and adapt to exploit the optimum opportunity.

The Practice:

ODMA is an intelligent protocol that sits upon a radio sub-system that supports relaying. The protocol breaks difficult radio paths into a sequence of shorter hops which enables lower transmit powers or higher data rates to be used. It is the goal of the protocol to choose the least cost route through the relaying system when the relays are moving and the radio paths are dynamically changing.

3.1 Multi-hop Transmissions

The transmission of information over a radio channel is subject to some fundamental rules e.g. it becomes more difficult the higher the data rate and the longer the distance. Difficulty maps to a requirement for higher transmitted powers which cause practical problems for mobile equipment and have an associated interference effect which limits the number of possible simultaneous transmissions (capacity). As high data rates are encouraged in UMTS that only leaves the distance aspect to consider.

Relaying is one of the enhancements WB-TDMA/CDMA may use to offset the difficulties of high data rate transmission over significant distances.

3.2 Relaying vs. Direct Transmission

It is not difficult to identify a potential benefit from relaying rather than using direct transmission. Consider the vehicular simulation scenario of UMTS 04.02. The path loss is given by

Loss (dB) = 127 + 37.6LOG(D)

[where D is in km]

Figure 3 below shows the benefits of breaking a long path into a number of shorter hops . It plots effective pathloss for a 1km transmission against the number of relay hops (0 = direct). (Effective pathloss is the loss between hops)

Number of relays

Figure 3 Effective Pathloss v Number of Relays

The graph shows benefits in the range of 10-30dB and that just 2 intermediate relays give a +12dB benefit.