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Monday, June 4, 2012

3G Simulation based Approach to 3G Radio Network Planning

Radio Network Planning is used to identify the geographic locations of the Node B. It is also used to identify the antenna configurations in terms of antenna type, height, azimuth and tilt. The high-level interaction of radio network planning with dimensioning, site acquisition, site design and site build is illustrated below;

The radio network planning process is preceded by system dimensioning. Dimensioning results are used to generate an estimate of the expected site density. This provides a guide to the number of sites which should be necessary to achieve the coverage requirements across a specific geographic area.

If an operator has already deployed a 2G network then it is likely to be beneficial to re-use as many of the existing site locations as possible. Re-use of existing sites introduces the option of sharing antenna subsystems between 2G and 3G, i.e. feeder cables and antennas can be shared. In general, this requires swapping existing 2G single band antennas for new 2G/3G dual band antennas (or swapping existing 2G dual band antennas for new 2G/3G triple band antennas). The benefit of sharing the antenna subsystem is a reduced requirement for antennas and feeder cables. This can be important at sites which have limited physical space or planning restrictions. Sharing feeder cables requires a diplexor to combine the downlink signals and to separate the uplink signals. A potential drawback associated with sharing the same antenna subsystem is that it may limit the radio network planner’s ability to apply independent 2G and 3G RF optimisation, e.g. if the 3G system requires mechanical antenna down tilt then the impact upon the 2G system must also be evaluated. However, some antennas allow independent adjustment of the 2G and 3G electrical down tilts.

There are two fundamental approaches to 3G radio network planning: the path loss based approach and the 3G simulation based approach.

The 3G simulation based approach to radio network planning requires a 3G radio network planning tool. The majority of 3G radio network planning tools make use of Monte Carlo simulations. Monte Carlo simulations are static rather than dynamic. This means that system performance is evaluated by considering many independent instants (snap shots) in time. A dynamic simulation evaluates performance by considering a series of consecutive instants in time. In general, dynamic simulations are more time consuming than static simulations. The general principle of a static simulation is illustrated below.

Each simulation snap shot is started by distributing a population of UE across the simulation area. This distribution could be based upon a uniform random distribution or a weighted random distribution. Weightings can be based upon Erlang maps generated from the traffic belonging to an existing 2G network. Alternatively, weightings could be environment type dependent so for example, urban areas could be specified to have a higher traffic density than suburban areas. Once the population of UE have been distributed, the set of uplink and downlink transmit powers are converged. Transmit power convergence requires an iterative process because the transmit power of one connection has an impact upon the transmit power of other connections. For the example of two UE, the uplink transmit power of the first UE is calculated based upon the uplink C/I requirement and the uplink interference floor. The transmit power of the second UE is then calculated based upon the uplink C/I requirement and the uplink interference floor which has now been increased by the first UE. The transmit power of the first UE then has to be recalculated because the uplink interference floor has been increased by the second UE. Repeating these calculations results in convergence of both the UE transmit powers and the uplink interference floor. A similar process can be completed in the downlink direction. The uplink and downlink transmit powers can be converged independently although there is some dependence in the case of a UE failing to maintain its connection. For example, if a UE has insufficient uplink transmit power to maintain its connection then the downlink transmit power for that UE can be cleared and made available to the remaining UE. UE which are not able to achieve their C/I requirements are categorised as being in outage. Outage may also be caused by factors such as inadequate Node B baseband processing resources or reaching the maximum allowed increase in uplink interference. The results are recorded at the end of a simulation snap shot and the process is repeated. This allows the simulation to generate probability distributions and to quantify the probability of certain events occurring, e.g. the probability that a UE will be able to establish a connection at a specific location. The number of snap shots necessary to generate statistically stable simulation results tends to depend upon the quantity of traffic distributed during each snap shot. Distributing relatively little traffic tends to increase the number of snap shots required.

The inputs to the planning tool for the 3G simulation based approach are illustrated beside.

The first three inputs are similar to those required by the path loss based approach. The 3G site data may be more complex in terms of requiring greater information to describe the Node capability, e.g. baseband processing capability. Propagation modelling is also more complex for a 3G simulation if slow fading is modelled. The path loss based approach makes use of a link budget threshold which includes a slow fade margin. 3G simulations typically model slow fading explicitly making it necessary to specify a standard deviation and correlation factor. The correlation factor is used to specify the coherence of the fading experienced by the signals between a UE and the set of surrounding Node B. A high correlation factor means that when one signal is experiencing a fade there is a high probability that the other signals will also be experiencing a fade. The 3G simulation tool may allow correlation factors to be configured separately for signals originating from the same Node B and signals originating from different Node B, i.e. the correlation is likely to be greater for signals originating from the same Node B because the propagation paths will be relatively similar. The slow fading correlation factor has an impact upon the soft handover gain which is maximised when soft handover radio links are uncorrelated. Uncorrelated radio links increase the probability of the receiver always having at least one signal which is not experiencing a fade.

Typical 3G parameter assumptions include maximum uplink and downlink transmit powers, common channel transmit powers, uplink and downlink noise figures, uplink and downlink Eb/No requirements, maximum allowed increase in uplink interference, orthogonality, soft handover window and soft handover gains. The soft handover gain used for 3G simulations should be interpreted differently from the soft handover gain used for link budgets. Link budgets use a soft handover gain which includes the diversity gain resulting from both fast and slow fading. 3G simulations usually model slow fading explicitly and so it is not necessary to include the slow fading diversity gain within the soft handover gain parameter. The soft handover window used for static 3G simulations tends to represent an average of the addition and deletion windows. The addition and deletion windows provide hysteresis in the live network to help avoid ping-pong in terms of cells being added and deleted from the active set. It is not possible to model this hysteresis using a static simulation tool because only snap shots in time are considered. When a simulated neighbouring cell is between the addition and deletion windows there is no historical information to indicate whether the cell is already in the active set and the signal quality is decreasing or the cell is outside the active set and the signal quality is increasing.

3G traffic profiles are specified in terms of the services used by the population of UE. The full range of services can be categorized as speech, circuit switched data and packet switched data. One or more bit rates can be associated with each of these categories. Defining accurate traffic profiles can be difficult and it is reasonable to start a simulation exercise by modeling one service at a time, e.g. generate coverage and capacity results for the 12.2 kbps speech service and then generate similar results for the 384 kbps packet switched data service. This approach provides an indication of the variance which can be expected when changing the traffic profile. Identifying the network capacity can be an iterative process, requiring the network planner to increase the quantity of traffic loading the network until the probability of blocking reaches a realistic maximum. The absolute maximum network capacity can be quantified by distributing very large quantities of traffic, but the blocking probability will become unrealistically high. The 3G traffic profile also requires the geographic distribution of the UE to be defined. This includes specifying the percentage of UE which are indoors and experience a building penetration loss.

The 3G simulation approach to radio network planning is more time consuming than the path loss based approach. Results take longer to generate and longer to interpret. Simulation time depends upon the size and resolution of the geographic area, the site density and the quantity of traffic loading the network. It is important to ensure that sufficient simulation snap shots have been completed to generate statistically stable results. 3G simulation tools may include functionality for a passive scan terminal. Passive scan terminals are used to increase the rate at which graphical geographic results are generated. At the end of each simulation snap shot a passive scan terminal is placed within each pixel across the simulated area and connection establishment is attempted without modifying the already converged uplink and downlink transmit powers. This approach allows every simulation snap shot to generate a result for every pixel.

Otherwise, it is only possible to generate results at the pixels where UE have been distributed. The results generated by a passive scan terminal are used to update the graphical geographic results but typically are not used to update the numeric results. This means that passive scan terminals can increase the rate at which graphical results are generated, but decrease the rate at which numerical results are generated.

The main benefit of completing 3G simulations is the relatively large quantity of information which is generated. This information can help guide planning decisions as well as provide more extensive expectations of network performance. The main results generated by a 3G simulation are presented below;
3G simulations can also be used for investigative studies aimed at evaluating the impact of specific network configurations. The impact of increasing sectorisation can be quantified for a range of different antenna gains and beamwidths. Some of the results can be used as an input to dimensioning exercises, e.g. the inter-cell interference ratio and soft handover overheads. Studies can also be completed to help quantify the benefit of deploying an additional RF carrier, or the impact of using mechanical rather than electrical antenna down tilts.

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