WO2001037446A1

WO2001037446A1 - Cdma mobile communication system

Info

Publication number: WO2001037446A1
Application number: PCT/GB2000/004306
Authority: WO
Inventors: Fatemeh Shirin Dehghan; David Lister
Original assignee: Vodafone Limited
Priority date: 1999-11-17
Filing date: 2000-11-10
Publication date: 2001-05-25
Also published as: AU1165901A; GB0027578D0; GB2356527A; GB2356531A; GB9927200D0; GB2356531B

Abstract

A technique to balance the load across a CDMA network and hence increase its capacity for a given network grade of service (GoS). The technique applies varying uplink and downlink attenuation factors to the cells in the network, in order to change their effective coverage area. This has the effect of changing their load (or carried traffic) and hence the level of interference experienced by the cells. In this way, the carried traffic is forced to be redistributed uniformly across the cells and is of particular importance in a non-homogeneous network. The level of attenuation applied can be determined using an adaptive/optimisation algorithm either dynamically in real-time at network level or as part of the radio planning process of a CDMA/UMTS network. The source of data used to obtain the attenuation factors may be derived using system simulation and/or real network data.

Description

CDMA MOBILE COMMUNICATION SYSTEM

This invention relates to mobile communication systems deployed in a manner based on Code Division Multiple Access (CDMA) or 'spread spectrum' technology.

CDMA is a means of transmission in which, over time, the transmitted signal occupies a bandwidth in excess of the minimum necessary to send the information. The 'spread' is accomplished by transmitting data encoded on a relatively narrowband signal at frequencies which are varied in a predetermined way and according to a predetermined pseudorandom code. The code is independent of the data and the frequency variations occur at a much higher rate than the data rate. Synchronised reception, using the same code at the receiver, is used for despreading and recovering the original data.

In accordance with the invention, there is provided a method of increasing capacity in a CDMA mobile communications network having a plurality of base stations associated with respective cells, comprising the step of applying gain adjustments to adjust the uplink and/or downlink gain of one or more of the base stations to achieve a generally equal load in each base station and to maximise the capacity of the network for a predetermined grade of service.

In accordance with a second aspect of the invention, there is provided a CDMA mobile communications network, comprising a plurality of base stations associated with respective cells, and gain adjustment means operable to apply gain adjustments to adjust the uplink and/or downlink gain of one or more of the base stations to achieve a generally equal load in each base station and to maximise the capacity of the network for a predetermined grade of service. This system offers a cost-effective solution to increase capacity of a network without the need to add extra base stations. It allows network operators to adopt a progressive rollout. For example, in the case of third generation system UMTS, base stations may initially be placed in relatively high geographical locations to maximise coverage. These sites may then be intermixed with macrocells at lower geographical heights. In the case of operators with second generation networks, UMTS base stations may be co-located with second generation sites which can range between 200 metres above ground to 15 metres above ground. This technique allows a mix of such sites and increases the capacity of the network in such deployments.

Embodiments of CDMA networks in accordance with the invention will now be described by way of example with reference to the drawings in which:

Figure 1 is a chart showing the typical noise rise with load for a CDMA base station (BS);

Figure 2 is a schematic diagram showing the operation of four base stations of a prior art network;

Figure 3 is a chart showing the distribution of noise rise, carried traffic, cell height and failed traffic in a simulated UMTS network, the numbers on the x-axis being used as identification for the cells in the simulated network;

Figure 4 is a chart showing the distribution of the F factor (intracell interference / (intra+inter cell interference) for the network of Figure 3, the numbers on the x- axis being used as identification for the cells in the simulated network; Figure 5 is a flow chart of steps for network planning in accordance with the invention;

Figure 6 is a flow chart of steps showing dynamic application of the invention to a network; and

Figure 7 is a schematic diagram showing the operation of four base stations of a CDMA network in accordance with the invention.

An inherent feature of CDMA is that all users have access to the whole bandwidth all of the time. Thus a frequency reuse of one is a well-known feature of CDMA based systems. This means also that CDMA is an interference-limited system. Hence any reduction in interference has a direct impact on increasing the capacity of the network.

In CDMA networks, the noise rise (defined as the total interference [intracell plus intercell interference] excluding the background noise) at a base station (BS) is a critical factor in determining how well the power control loop can perform. In practice, due to power control instability issues, the maximum noise rise at the BS is limited to what is known as an operating region. In most networks, noise rises above 6dB (which corresponds to 75% load) are not allowed at a BS. This is illustrated in Figure 1. To maintain loadings below 75%, existing BSs use a so- called 'admission control' mechanism to block new calls.

Figure 2 illustrates the problem in a prior art network. Due to the non- homogenous nature of mobile networks, which occurs, for example, due to variations in terrain, vegetation, man-made structures such as buildings and variability of cell height,, the noise rise will be non-uniform across the network. For example, a geographically high cell BSA tends to suffer more from a rise in its noise than its surrounding lower cells BSB, BSC and BSD. This is due to a more favourable propagation environment, such as less obstruction by surrounding buildings. Due also to its consequently enhanced coverage area, the high cell, BSA, tends to carry a larger proportion of the local traffic than the surrounding lower cells. Furthermore the contribution of intercell interference to noise rise is also higher for these cells. Figure 3 shows the distribution of noise rise, carried traffic, cell height and failed traffic in a simulated UMTS network based on real positions of Nodafone (Registered Trade Mark)BSs in a UK city. The cells considered are a mixture of directional and omnidirectional cells. Figure 4 shows the distribution of the F factor (intracell interference / (intra+inter cell interference) for the same network. In both Figures 3 and 4, the numbers on the x-axis are used to identify the cells in the simulated network. It will be seen that the highest BS (18) has the highest noise rise and carries the most users. It also has one of the lowest F Factors.

The invention preferably uses estimation of the so-called 'F factor' and noise rise at the cell. The F Factor is defined as uplink intracell interference divided by total uplink interference (intracell interference plus intercell interference). It has been found that capacity can be improved by applying uplink attenuation to cells with high noise rise and low F factor. The amount of up/downlink attenuation applied to the sites can be determined in two ways.

With reference to Figure 5, a first iterative method is to use a CDMA system simulator (step 5) from which an estimate (step 6) of noise rise, F factor and carried traffic for a given network grade of service (GoS) (i.e. the percentage of successful call attempts averaged over a statistically representative number of cells) may be derived. These estimates are used with an optimisation algorithm (step 7), such as a so-called 'simulated annealing' algorithm, to choose suitable attenuation factors which will maximise carried traffic (step 8) within the network. This process may be carried out as part of the network planning process and the attenuation factors may then be applied to BSs (step 9). The nature of traffic used within the simulations would typically be aligned with the expected traffic mix when the network is in operation. Several simulation runs may be required depending on the variability of traffic and nature of the service mix throughout the day. This method would provide the option of adapting the network according to the time of day and hence the level and mix of traffic.

Alternatively, the method shown in Figure 6 may be used. In this case, a CDMA system simulator (step 5) produces estimates (step 6), for various network arrangements, of noise rise, F factor and carried traffic. These estimates are used as described above with reference to Figure 5 with an optimisation algorithm (step 7) to choose suitable attenuation factors which will maximise carried traffic (step 8) with the network. From this is produced a look-up table (step 1 1) that provides gain factors for uplinks and downlinks for given input parameters of noise rise, traffic carried (number of users, services), F factor for the cell and surrounding cells. The look-up table is used by the network planning process (step 10) together with live network data, noise rise traffic (number of users, services) carried and F factor for the cell and surrounding cells, to set dynamically the attenuation factors on the uplinks and downlinks to the cells.

By introducing attenuation on up and/or downlinks and thereby reducing the coverage area of a cell which is carrying a large proportion of the traffic (see Figure 7), the amount of intercell interference is reduced and the users are redistributed so that they are registered with other surrounding cells. Thus the load is re-distributed across the cells with the aim of achieving a generally uniform load across all cells. This results in a substantial increase in the number of users supported for a given network GoS. The technique also has the added benefit of reducing the number of mobile telephones in soft handoff (i.e. those mobile telephones which are simultaneously in communication with more than one cell), which in turn reduces interference on the downlink as well as reducing the traffic carried from the BS to the Radio Network Controller (RNC); the so-called "back- haul traffic".

The application of the up/downlink attenuation factors described in connection with Figure 5 may be performed in a similar way to the setting of frequencies of cells in a GSM network. It may be performed by causing the network to communicate the relevant attenuation factors to the appropriate base stations thereby to cause the relevant base station to adjust the sensitivity of its receiver and in the case of the downlink, its transmission power. On the downlink, the control and pilot channels are preferably adjusted by the same factor such that the up and downlinks of the cells are balanced.

In summary, there has been described a technique for balancing the load across a CDMA network ard hence increase its capacity for a given network grade of service (GoS). The technique applies varying uplink and downlink attenuation factors to the cells in the network, in order to change their effective coverage area. This has the effect of changing their load (or carried traffic) and hence the level of intra and intercell interference experienced by the cells. In this way, the carried traffic is forced to be redistributed uniformly across the cells and is of particular importance in a non-homogeneous network. The level of attenuation applied can be determined using an adaptive/optimisation algorithm either dynamically in real- time at network level or as part of the radio planning process of a CDMA/UMTS network. The data used to obtain the attenuation factors may be derived using system simulation and/or real network data. It will be appreciated by the skilled person that various other modifications and variations could be employed in relation to the embodiments described above, without departing from the scope of the present invention.

Claims

1. A method of increasing capacity in a CDMA mobile communications network having a plurality of base stations (BS) associated with respective cells, characterized by the step of applying gain adjustments to adjust the uplink and/or downlink gain of one or more of the base stations (BS) to achieve a generally equal load in each base station (BS) and to maximise the capacity of the network for a predetermined grade of service.

2. A method according to claim 1, characterized in that the gain adjustments are applied in real-time during network operation.

3. A method according to claim 1 , characterized in that the gain adjustments are predetermined and substantially unchanging during network operation.

A method according to any preceding claim, characterized in that the gain adjustments are calculated in dependence on the noise rise and F factor and/or the number of active users and their corresponding services, of one or more of the cells.

5. A CDMA mobile communications network, comprising a plurality of base stations (BS) associated with respective cells, characterized by gain adjustment means operable to apply gain adjustments to adjust the uplink and/or downlink gain of one or more of the base stations (BS) to achieve a generally equal load in each base station and to maximise the capacity of the network for a predetermined grade of service.

6. A network according to claim 5, characterized in that the adjustment means is operable to perform adjustments in real-time during network operation. A network according to claim 5, characterized in that the adjustment means is operable to perform adjustments periodically such that the adjustments remain substantially unchanged during network operation.