Литература / UMTS-Report

Source: SMG2

Concept Group Alpha -

Wideband Direct-Sequence CDMA (WCDMA)

EVALUATION DOCUMENT (3.0)

Part 4:

WCDMA/ODMA description

This document was prepared during the evaluation work of SMG2 as a possible basis for the UTRA standard. It is provided to SMG on the understanding that the full details of the contents have not necessarily been reviewed by, or agreed by, SMG2.

1. INTRODUCTION

WCDMA 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 WCDMA 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. Findings from the Epsilon evaluation work are recorded in this text.

A feasibility study was jointly conducted by the Alpha and Epsilon concept groups to determine the practicality of supporting relaying using the basic WCDMA design. The outcome of the study is included in this text and the basic conclusion was that the WCDMA design was sufficiently flexible to support relaying with negligible increase to the MS complexity or cost. WCDMA can therefore offer 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 WCDMA given the following pre-conditions;

∙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

2.1 WCDMA + ODMA Scenario

Figure 1 shows a WCDMA cell using an ODMA enhancement. Relaying is used to route packet data services to and from mobiles close to the WCDMA BTS - using a separate unpaired spectrum band. The relay nodes in range of the BTS will toggle between ODMA and WCDMA modes. Relaying extends the range of the high rate data services and can be used to provide coverage in dead spots.

Figure 2 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.

3.3 Relaying to Avoid Shadowing

Radio channels are more complicated than the previous example suggest and show considerable temporal variation of shadow(slow) fading. This fading makes it hard to predefine an optimum route as the shortest paths may be heavily attenuated. However an intelligent relay can make the best local choice of route.

Consider the situation shown in Figure 3 - it is possible to use a number of paths to reach the BTS but the least cost route is to relay around the buildings. To put this into perspective @ 2GHz it is possible to experience 30dB fluctuations around “Manhattan“ street corners. Combine this with fast fading variations and burst noise/interference then the ability to identify least cost routing becomes even more desirable.

Figure 3 Relaying to Avoid Shadowing

Even if the complexities of real scenarios are ignored and the simple log normal variation model of UMTS 04.02 is used then a gain can be extracted by being allowed to chose the best of several routes.

Figure 4 Benefits of Path Selection - Space Diversity

Figure 4 shows the probability of signal loss for a single path but also for the cases where you can chose the best from a number of paths e.g. @ a probability of 0.5 there is a 10dB benefit in being able to chose 1 from 10 paths.

3.4 Radio Resource Re-use

To relay information requires the use of radio resources such as codes. In a conventional structure resources are used once per cell however in ODMA the resources can be used many times within the basic cell area. This is because transmissions are lower power and so interference has only localised effect.

Figure 5 shows just 2 resources being re-used several times along a relay route and elsewhere in the cell - if the transmission circles of a single resource do not overlap interference is avoided.

Relay Route

Maximum Cell Range

Figure 5 Re-using Resources Within a Cell Area

3.5 Potential For Capacity Gain

The potential for reducing TX power and extending the range of high data rate coverage is an obvious consideration for a relaying solution however it may be less clear that there is potential for increasing capacity. Figure 6 shows a scenario with 3 cells - normally they would be planned to avoid gaps between coverage areas. The resulting overlap will create intercell interference which ultimately reduces capacity.

Figure 6 Normal Overlapping

Cell Coverage

If the cell areas are restricted so that they do not overlap they become more spectrally efficient (spot coverage) but coverage is poor. Relaying would then be used to fill in the coverage gaps. (This would also be a useful technique at country borders).

Relay Path

Relay Path

Figure 7 Discontinuous Cell Coverage

The above example does not imply that only a single relay stream can be brought to the vicinity of a BTS. For example there could be multiple ODMA nodes or “data buckets” distributed around the BTS each collecting and distributing significant amounts of traffic (as shown in

Figure 8). In this case it is the final hop to the BTS that becomes critical and so the high efficiency of WCDMA is of great benefit especially if relaying has reduced the level of intercell interference.

ODMA ODMA

Figure 8 Cluster of

ODMA Nodes around

WCDMA BTS