A testing time for 3G: accurate UMTS radio planning, along with compatible interpretations of standards, are pre-requisites for keeping 3G costs down and improving revenue prospects - Monitoring Network Performance

The high price of auction-based 3G licences is putting enormous pressure on manufacturers and operators to configure the next-generation mobile network in the most cost-effective way possible. That means taking great care over UMTS radio planning and ensuring terminal and base station compatibility between different vendors.

Network coverage and management

3G network operators need to clearly define the network coverage objectives for each of the data rate services or radio access bearer (RAB) combinations they plan to offer within the different environments.

UMTS network planning should not be underestimated, as it is a very complex area involving optimal site location, sectorisation and hand-over issues. It requires the use of both advanced planning and analysis tools, coupled with tremendous skill and effort from planning engineers in order to achieve a high capacity pre-optimised network.

One key factor that influences the available UMTS network capacity is the choice of soft handover margin. This factor determines the number of subscribers that would enter soft handover. Experience is showing that, while a large margin can provide a certain reduction in the uplink and downlink inter-cell interference -- and therefore an improvement in capacity -- additional downlink connections from other base stations must be made available, thus reducing overall downlink capacity.

On the other hand, a lower soft handover margin reduces the load on additional downlink resources but increases the uplink and downlink inter-cell interference. The optimum soft handover margin therefore needs to be set depending on where the traffic is located with respect to the base stations.

3G RF planning tools, such as CDMA area testers and drive test systems, can be used to measure and verify these 3G path losses and coverage, based on the calculated maximum path loss for each of these services. Soft-handover margins can also be evaluated by displaying the received signal level fluctuation measured relative to the available serving UMTS base station's location.

By effectively deploying a pre-optimised UMTS network, an operator may be able to avoid or delay the need to install additional base stations due to capacity limitation issues and manage his capital expenditure more efficiently.

3G systems using W-CDMA achieve high capacity (many users and high data rate) by careful management of the RF resource. One of the key factors defined in the 3GPP specifications is power control. This is important because in a W-CDMA system, all users will appear as noise to other users in the network. To achieve maximum capacity requires minimum noise, and so the effect of other users must be limited (this is explained by Shannon's law which links data rate, bandwidth, signal/noise ratio and power).

To cope with all users co-existing on the same frequency, each Node B base station (as used in UMTS systems) has a separate scrambling code on the downlink to isolate it from other node Bs and all user channels within the scrambling code are separated by channel codes, On the uplink, each user has a separate scrambling code to separate it from other users. Either way, other users are seen as noise.

Back to Shannon: if higher data rate is required at the same signal to noise ratio, with the same available bandwidth, then more power is required. Therefore the high data rate user must transmit at a higher power output. This creates more 'noise' for other users. These other users must then increase output power to overcome the higher noise floor, and then the high data rate user must increase power to overcome the higher power of other users. This is a positive feedback system that can very quickly run out of control with all users trying to run at full power to overcome the noise of other users.

Hence the power control algorithm in Node B is critical to ensure that the correct balance of transmit powers between all users is enabled, so that they can all get through above the noise while still keeping the noise floor at a minimum level.

Cell breathing issues

An important issue here is a user near the outer edge of a cell. Such a user is running at full power to overcome the noise floor and atmospheric propagation loss. This is acceptable and a call can be placed and held. If a high data rate user comes into the cell, he will raise the noise floor of the entire cell, possibly to such on extent that the distant user's call cannot be maintained. If the high data rate user has a guaranteed service level agreement for certain data rates, he will require to be connected, and so the other user will be dropped.

But how will it be accepted if one user has to be dropped to accept another 'more important' user? What happens if both have guaranteed SLAs? The first (distant user) is on a call, and so would not expect to be thrown off the network. The second (close, high data rate) is close to the Node B base station and knows that he gets goad coverage, so he will expect the call to connect.

The effect of the noise floor rising (and hence cell radius shrinking) is called 'cell breathing' (Figure 11. In effect, the cell coverage is a function of the number of users and type of service being used within the cell. So if a cell is 'mapped' by the operator tracing the coverage of a 'typical' call, this will not represent the true coverage with many other users on the cell. Also, a customer may find that on one day he has coverage and can connect, and then a short time later from the same location he cannot connect. This problem can be managed by the fact that the level of 'interference', or noise floor, is a function of the data rate being offered to the user. So a user can be accepted at a certain data rate, but that rate can be lowered dynamically to support other users coming on.

Radio resource management (RRM) is a process defined within 3GPP that is able to manage this issue, controlling how users are admitted onto the network, what types of services are being used at any time by the network, and the impact on other users. UMTS network simulators are being used to help evaluate some of the basic radio resource management (RRM) algorithms that need to be implemented for the initial network rollouts. These include considering scenarios such as real-time and non real-time 3G services, variable bit rates up to 384Kbps, dedicated or common or shared channels, switching and multiplexing between voice and data transport channels.

Third generation networks will rely predominantly on packet-switched traffic. The management of this traffic is more complex and requires the RRM to be able to control the setting up and control of many different services including many simultaneously. As network traffic changes, the allocation of resources needs to change, so as well as static testing of radio bearers, scenarios including loading and interference, simulating real world environments, need to be tested.

Cell site rollout

To meet coverage requirements, network operators need to consider and implement a variety of rollout solutions that combine high performance Node B base stations, offering low noise floor and high power output, with innovative antenna and antenna feeder designs.

Many existing GSM operators will be building up UMTS networks. As site acquisition becomes more difficult -- often with several operators competing for the same site -- co-location of GSM and UMTS Node B base stations will become commonplace.

Co-location of GSM900/1800 and UMTS base stations can be realised with a number of different cell site solutions. These include deploying single band, broadband, dual/triple band antennas or some combination that then requires higher performance and more complex duplex or triplex transmit and receive filtering to reduce co-site interference. The key technical performance implications that have to be considered in these co-location situations include the effect of spurious emissions, inter-modulation and receiver blocking, as these will all have a direct impact on the radio access performance and user experience.

Interference, especially within the UMTS receiver band and GSM1800 band, by third order and fifth order inter-modulation products must be minimised. This involves a careful and tidy installation performed by trained operatives, and requires thorough testing using equipment such as field-portable transmission line/antenna analysers and spectrum analysers for the off-air measurements.

In view of the massive costs and the increasing pressure by environmentalists, coupled with difficulty in acquiring new cell-sites, it is easy to see why network sharing is viewed as an attractive means of reducing expenditure. Some estimates claim that an operator can save 40 per cent of its rollout costs by having a shared network. Network sharing also offers operators the opportunity to roll out services more quickly.

User equipment interoperability

One of the main problems the telecoms industry has had to face is that, to provide widespread services, it must work to standards to which any manufacturer can build equipment. In reality, written standards will always be open to multiple interpretations and misunderstanding. So, the industry has had to face the problems of how to ensure that one manufacturer's interpretation is compatible with another's.

In the case of cellular radio, each terminal must work with each base-station for network operators to be able to provide a usable service. Over time, many different approaches have been taken to ensure interoperability. In some areas, where there are few equipment manufacturers, each one will take his terminal to each base-station manufacturer and run extensive tests. This is sometimes called 'lockdown'. This system works well when there are few manufacturers, but as the number increases, the problem of resolving which interpretation is correct, together with the logistic complexities, make it increasingly unusable.

Anritsu has been heavily involved in the 3GPP standards process and fully supports the need to develop a conformance test suite for user equipment (UE] that defines the minimum capabilities a UE must perform to work on a 3GPP network. The 600+ 3GPP conformance test cases (TS34.123 part 3) are still a goal that should be met by all 3GPP UEs -- eventually.

Being pragmatic, the GSM Certification Forum (GCF), which is driven by network operators, has realised that this is not likely to provide UE in time for networks that are being deployed now. It has identified deliverable packages of the 3GPP test cases that will allow interim conformance testing. Once a UE has met the conformance tests, it will need to be tested against the real world conditions that include interference and marginal conditions. Also, different applications will behave differently, so testing is required to meet the specific demands.

The aim of 3GPP is to provide a set of approved test cases that are manufacturer and tool-set independent, and can be used on a variety of test equipment to check the conformance of a terminal. The provision and correct operation of these test cases is critical for the successful deployment of W-CDMA globally. Although some non-approved test cases may prove useful in specific applications, the use of these does not guarantee true conformance to the 3GPP spec, and therefore is only likely to result in non-interoperability and other issues in the longer term.

In the rush to provide 3G services, network operators may be tempted to take shortcuts and orthogonal paths that do nothing to provide global interoperability. Selecting a quality test regime that meets 3GPP requirements is essential. 4

SteveTurner, European strategic marketing manager, mobile communications, Anritsu

Jonathan Borrill, product manager. digital mobile radio, Anritsu

Chris Foreman, product marketing manager, Anritsu

COPYRIGHT 2002 Horizon House Publications, Inc.
COPYRIGHT 2002 Gale Group