Technical Justification

Site Details

Nominal Number / PE0114 / Candidate Number / N/A
Site Name / Boston
Site Address / Trainer Sales
Riverside Industrial Estate, Marsh Lane, Boston PE21 7ST
Easting / 533562 / Northing / 342479

Introduction

This document will describe the basic operational characteristics of a 3G network and the constraints that affect the placement of sites within. It is hoped that the information provided will enable the reader to have a general understanding of the nature of technology and an appreciation of its operational limitations. As a frame of reference, comparisons will be made with the existing GSM networks.

Background

Hutchison 3G is obliged to install, maintain and provide third generation service to an area where at least 80% of the population of the UK live, by no later than 31 December 2007.

Hutchison 3G has to comply with the relevant performance criteria outlined in the ETSI standards[1]. Toward this end, service will be provided through a contiguous network of base transceiver stations (base stations). To operate efficiently the base stations will need to cover a given area well without overlapping significantly onto neighbouring sites.

A Cellular Network

A mobile phone sends and receives information by radio communication. Radiofrequency signals are transmitted from the phone to the nearest base station and conversely from the base station to the phone. The radio signals are usually confined to distances from several hundred metres to several tens of kilometres depending on the technology and frequency. An ideal cellular network can be envisaged as a mesh of hexagonal cells each with a base station at its centre. But, in practice, the coverage and placement of the base stations will depart appreciably from this because of the topography on the ground and the availability of suitable locations for base stations.

Figure 1 - Network of hexagonal cells with base station at the centre

2G - Frequency and Time Division

In a GSM or 2G system, each user, within each cell, is assigned a specific frequency. Frequencies are reused several cells away and the capacity of the network (the number of simultaneous phone calls which can be made) depends on the total number of frequency channels available to each operator, the frequency reuse scheme and coverage area of each base station. Each GSM operator in the UK has more than 70 frequency channels available for use. GSM works under the principal that the signal level of the intended carrier will be much stronger than the interference coming from the neighbouring base station which reuses the same frequencies.

Figure 2 – Example of frequency in a GSM network

To increase the capacity, the GSM system employed a technique called Time Division Multiple Access (TDMA) that allows up to 8 users on each carrier frequency by dividing it into 8 separate time-slots. This is achieved by compressing 4.6 ms of information into a 0.58 ms digitised format, which pulses every 4.6 ms.

3G - Code Division

The third generation of mobile telecommunications technology is called Universal Mobile Telecommunication System (UMTS). It deploys a modulation technique called Wideband Code Division Multiple Access (WCDMA). Just as its name implies, each user is separated by a unique code.

Figure 3 - No frequency division or time division between base station and users

A WCDMA network allows users to transmit on the same frequency and at the same time as all other users. Each user is distinguished by a unique orthogonal code known only to the base station. In a conventional sense, everyone in the network interferes with each other since they are sharing the same carrier frequency. However, due to the unique properties of the codes and the coding process, the intended receiver can be “heard” or the data extracted from below the noise level. In other words the interference in the environment can be much stronger or “louder” than the intended receiver. In WCDMA, the control information is multiplexed with the user’s information resulting in a continuous transmission where no pulse occurs

There are limits as to how much noise a WCDMA network can tolerate before the communication link fails. The limits are dependent upon the type of service in use and the mobility of the mobile device.

Coverage Limitations 2G vs. 3G

For a variety of reasons, a given area will require more 3G base stations than 2G. These are described below.

Signal Propagation

The frequencies assigned to GSM operators (900 MHz & 1800 MHz) are lower than those assigned to 3G (2100 MHz.) The lower frequencies have longer wavelengths, which have the quality of being more resilient to diffractions and refractions caused by the physical obstructions prevalent in suburban and urban environments. Hence, GSM, by and large, will cover a larger area than 3G with the same number of base stations.

Capacity - Voice vs. Data

GSM systems separate users by frequency and time division, whereas in a 3G system, the users are separated by unique orthogonal codes. Although GSM operators have more than 20 times more frequency channels available for network planning than 3G, 3G remains a much more efficient technology for voice communication. In fact, 3G can carry voice traffic 2 to 3 times more efficiently than GSM.

With regard to data transmissions, GSM can provide a minimal service. However, it is hampered by quite low data rates in comparison with 3G. By comparison 3G systems were designed from the outset for data communications and can facilitate ubiquitous 256kbps coverage as well as 2Mbps localise data coverage. The only drawback is with respect to interference. The higher the data rate, the more susceptible the user will be to interference. Having few frequencies, 3G operators do not have the luxury of allocating a different channel to resolve interference. 3G networks are, therefore, less flexible with regard to interference than a 2G system.

To counteract this inherent susceptibility, 3G base stations are of a lower height in comparison with GSM. In medium to high data traffic areas the lower height will result in a much small coverage areas and likewise more sites per given area than GSM.

Cell Breathing

Cell breathing is a particular phenomenon associated with WCDMA systems. It refers to the contraction of the coverage area of the base station during periods of high usage. As the number of users and the data load increases so does the noise in the environment. Beyond a certain threshold, the noise will no longer able the user to decode the information coming from the base station. Most susceptible are the points near to the coverage boundary limit. Although 3G deploys several technological “tricks” (i.e. soft handovers and multiplexed rake receiving) to minimise this phenomenon it does continue to exist.

Hence, from a design perspective the placement of the 3G base stations are such so that there is a well-managed coverage overlap between them. Well managed because too much overlap will increase interference and reduce dramatically capacity, too little will cause coverage holes to appear in the systems during times of high traffic demand.

Conclusion

In areas with high customer demand, densely located base stations with low height and low transmission power offer the greatest capacity. In summation, more 3G sites are require to cover a given area than GSM because:

  • Higher frequencies travel less far.
  • High data rates are more sensitive to interference.
  • Cell breathing reduces coverage boundaries.

These are just a few examples of the important aspects that must be considered in the rollout of a third generation mobile data network.

The Hutchison 3G proposed location takes into account all of the above constraints by choosing the optimum location for providing a contiguous mobile data service to the area, whilst minimizing the impact on the environment.

ACQ/PLE/FOR/086 / Page 1 of 1 / Prepared by: Arnob Gon
Date Prepared:07/01/2010

[1] ETSI TS 125.104: UTRA (BS) FDD & ETSI TS 125.105: UTRA (BS) TDD; Radio Transmission and Reception