Models in RNPT

Abstract

This document gives detailed technical description of models and procedures used by RNPT.

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Contents

1 Introduction............................................................................................3

2 Coverage models...................................................................................32.1 R99 DCH and A-DCH coverage................................................32.2 HS-DSCH coverage...................................................................32.3 E-DCH coverage........................................................................4

3 Capacity models....................................................................................53.1 R99 DCH and A-DCH capacity..................................................53.2 HS-DSCH capacity....................................................................53.3 E-DCH capacity.........................................................................5

4 Link budget sheet..................................................................................54.1 Uplink interference.....................................................................54.2 CPICH power.............................................................................54.3 Downlink total power..................................................................54.4 Downlink DCH power.................................................................54.5 HS-DSCH power........................................................................54.6 E-DCH noise rise.......................................................................5

5 Axcel.......................................................................................................55.1 Average gain data (A2_gmean vectors)....................................55.2 Multipath and non-orthogonality data (A2_ff vectors)................55.3 Downlink calculations................................................................55.4 Uplink calculations.....................................................................5

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1 Introduction

Basically, all services (dedicated channels) available within RNPT are divided into two categories: R99-like channels HS channels

Because of the big difference in the backgrounds of these two groups of channels, two different methodologies are used:

For R99-like channels, coverage is modelled with the link budgets (taking into account the noise rise margin coming from the active users) while the capacity is modelled through the number of simultaneous users and the Mpole capacities for each service.

For HS channels, both, coverage and capacity are modelled with Axcel, by means of 1000 test points evenly distributed across the cell.

2 Coverage models

In this chapter, coverage models are described. There are three special cases: R99 DCH and A-DCH coverage HS-DSCH coverage E-DCH coverage

2.1 R99 DCH and A-DCH coverage

Requirements for R99 DCH and A-DCH coverage are given as yes or no values. You either require the coverage or you don’t.

Coverage for R99 DCH and A-DCH are assured using standard link budget. For each service type certain Eb/No values are set as target and are used for calculating sensitivities (RBS and UE).

More details on link budget calculations are given in Chapter 4.

2.2 HS-DSCH coverage

Requirements for HS-DSCH coverage are given in term of percentage of user positions achieving bitrate greater or equal to the required one. The percentage is set in HSxPA sheet cells L8-12 and the required bit rates are set in Phases sheet for each clutter type.

As it is mentioned in Chapter 1, Axcel is used to model the coverage and capacity of HS channels.

Using Axcel, achievable bitrate is calculated for number of UE positions within the cell. It is said that coverage requirement is met if given percentage of achieved bitrates is greater or equal to the required bitrate (Figure 1).

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HSDPA throughput CDF

0%

20%

40%

60%

80%

100%

0 100 200 300 400 500

Figure 1 Cumulative distribution function

It is important to state that power is the shared resource when using HSDPA and if power allocated for HS-DSCH is increased, both capacity bitrate and coverage bitrate are increased.

More on Axcel is given in Chapter 5.

2.3 E-DCH coverage

Requirements for E-DCH coverage are given in term of percentage of user positions achieving bitrate greater or equal to the required one. The percentage is set in HSxPA sheet cells L44-49 and the required bit rates are set in Phases sheet for each clutter type.

As for HS-DSCH, coverage requirement is met if given percentage of achieved bitrates is greater or equal to the required bitrate.

The achieved bitrate for each of the users depends on, among other things, the UE transmission power and the interference allowed on the receiving RBS. The higher the interference is, the lower the user bitrate is! On the other hand, the uplink interference is the shared resource that guarantees the capacity for the UL subscribers. The higher the UL interference is, the more users can simultaneously transmit and the cell capacity is increased. As a result, the change in the allowed interference levels has different impacts on the coverage bitrate and the capacity bitrate, i.e. when interference used for capacity is increased, coverage bitrate is decreased and vice versa.

So, hybrid between R99 DCH and HS-DSCH coverage models is used in this case. Design decision was to meet the coverage requirements and maximize capacity.

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In order to guarantee the coverage bitrate, the maximum allowed interference is limited by dynamically changing the RBS sensitivity for E-DCH (named HSDPA 64UL throughout RNPT). This is actually performed by changing the EbNo value, while keeping data rate fixed at required coverage bitrate (plus SRB data rate).

If, for example the obtained coverage bitrate is not enough, the tool will try to increase the EbNo for E-DCH service. In turn, the RBS sensitivity will deteriorate and from the standard link budget equation, lower interference limit will be available for the capacity.

With this lower interference limit, more users will have enough power to achieve higher bitrates and the coverage (single user) bitrate will be increased. The bitrate is read from Axcel.

More on Axcel is given in Chapter 5.

3 Capacity models

In this chapter, capacity models are described. There are two special cases: R99 DCH and A-DCH capacity HS (HS-DSCH and E-DCH) capacity

3.1 R99 DCH and A-DCH capacity

Requirements for R99 DCH and A-DCH channels are given in term of the number of subscribers and the traffic profile per subscriber.

Furthermore, for each of the services, the pole capacity, Mpole, representing the number of simultaneous users that would result in indefinite interference, is given.

Since a few users using different services are supposed to be simultaneously present the required capacity for each of them is normalized through the speech equivalents as shown in Equation 1.

Equation 1 Normalization of services

The overall capacity of the system is also given through the number of speech users (Equation 2).

Equation 2 Speech system capacity

With the defined traffic profile and assuming certain number of subscribers per cell, the required R99 loading can be calculated.

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For CS services, multi-erlang calculation is used, as a function of the number of users, traffic profile, speech equivalents and the required blocking probabilities. Multi-erlang function output is the number of speech equivalents (Npeak) required, so that the most demanding service reaches its blocking probability. As additional output, the blocking probability of other services Pb,service (which is by definition lower or equal than their blocking probability targets) is also given.

For CS and PS services, an average required number of speech equivalents is calculated. For CS services, it is calculated based on the number of subscribers, erlang traffic, effective bandwidth and the blocking probability (result of the multi erlang calculation) – shown in Equation 3.

Equation 3 Speech equivalents for CS services

For PS services the average is calculated based on Equation 4.

Equation 4 Speech equivalents for PS services

As the final result, the required capacity is the maximum between the peak and the average result (Equation 5).

Equation 5 Required R99 capacity

In the capacity sheet, the number of supported subscribers is calculated as the maximum number of subscribers that does not exceed the designed R99 loading. The number is given per site; therefore, the number of subscribers per cell is multiplied by the number of cells.

During the calculation, the tool varies the number of sites. From the number of sites and the total number of subscribers, the number of subscribers per site is calculated and from the capacity sheet, the requested R99 loading is found. With that loading, coverage calculations are run (the R99 loading has impact on the noise rise in the system).

3.2 HS-DSCH capacity

Requirements for HS-DSCH capacity can be given as: traffic profile per subscriber cell capacity

Traffic profile can be expressed as the cell capacity as shown in Equation 6.

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Equation 6 HS capacity

If requirements are given in both ways (traffic profile and cell capacity), the most demanding one is considered when dimensioning.

When using HSDPA, cell capacity and average user throughput are the same because the time multiplexing is used.

3.3 E-DCH capacity

As for HS-DSCH, requirements for E-DCH capacity can be given as: traffic profile per subscriber (instead of A-DCH) cell capacity

Cell capacity is calculated from the traffic profile in same way as in case of HSDPA (Equation 6).

Also, if given both, the most demanding one is considered and cell capacity is the basic requirement so traffic profile is actually used to calculate corresponding cell capacity.

Main difference from HS-DSCH is that time multiplexing is not used so all users are transmitting at the same time. Shared resource is the uplink interference so cell throughput can actually be larger than average user throughput.

More on models and formulae used is given in Chapter 5.

4 Link budget sheet

In this worksheet, most of the coverage calculations are done. Only HS-DSCH and one part of the E-DCH coverage calculations are not shown here.

Generally, link budgets for every environment, for every service and both, for uplink and downlink, are given here.

Each part of the worksheet will be further described in following chapters.

4.1 Uplink interference

This part tries to answer the following question: For given cell range and coverage requirement for certain services, what is the maximum allowed interference margin that can be used for uplink traffic (R99 DCH and E-DCH)?

For each service, required RBS sensitivity is calculated using Equation 7 (logarithmic scale).

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Equation 7 RBS sensitivity

Afterwards, pathloss is calculated from the cell range using specified propagation model (Okumura-Hata, Walfish-Ikegami, etc.).

Interference margin for each service is calculated by starting from UE TX power, subtracting all losses, adding all gains, and, at the end, subtracting pathloss and RBS sensitivity.

Maximum interference margin is calculated as minimum of services’ interference margins.

Equivalent R99 loading is calculated from maximum interference margin (noise rise) using Equation 8 (linear scale).

Equation 8 Equivalent loading

Equivalent R99 loading represents maximum allowed R99 loading if there is no E-DCH capacity requirements.

4.2 CPICH power

This part tries to answer the following question: For given cell range, how much power is used for CPICH and CCHs?

CPICH UE sensitivity is calculated based on the thermal noise, UE noise figure, chip rate and the required Ec/No value for the pilot (Equation 9).

Equation 9 UE CPICH sensitivity

The total signal attenuation (Lsa) on the cell edge (R) is calculated as the sum of the propagation loss and all the DL margins (Equation 10 and Equation 11).

Equation 10 Lsa on the cell edge

and

Equation 11 Downlink CPICH margin

Feeder loss, jumper and connector losses, as well as the ASC insertion loss (if any) are located BEFORE the reference point; therefore, they are taken into account in the calculation of the maximum power at the system reference point (Equation 12).

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Equation 12 Power at system reference point

IF HSDPA is not switched on, the worst case is to have 75% of the output power from each RBS. If HSDPA is activated, the output power can go up to 100%.

Noise rise for pilot is calculated using Equation 13.

Equation 13 CPICH noise rise

The required received signal code power for the pilot is calculated using Equation 14.

Equation 14 Received Signal Code Power for CPICH

Where RSCPdesign is a user specified parameter. If the designed RSCP is not specified, the value of this parameter should be set to very low value.

The actual output power is obtained by scaling up the RSCPCPICH by the Lsa.

Average common channel power is strictly related to the CPICH since all the common channels are parametrically bound to the CPICH power. Different coefficients are used whether the system has R99 only traffic, or the HSDPA common channels are used, or the E-DCH traffic is used.

4.3 Downlink total power

The downlink total power is calculated for each of the selected R99 services, and the worst case is taken as the final result.

In general, total power is expressed with Equation 15.

Equation 15 Total power

Lsa is calculated as in the pilot case (Equation 10), but the DL_margin is somewhat different (Equation 16).

Equation 16 Downlink margin

H term is taking into account the effect of the noise in the overall result.

It is calculated using Equation 17.

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Equation 17 H term

Where φ is the tabulated numerical result of the noise function and SHO is the resource consumption increase due to handover.

During the calculation, after a number of sites is assumed, it is possible to calculate the required site area and the number of subscribers per cell (from the number of sites, total area to be covered and total subscribers figure). From the site area, cell range is found and from the number of subs, the required R99 loading is found.

Using the cell range and R99 loading and assuming certain HSDPA power, it is possible to calculate the required RBS total power. In practice, the tool target is to maximize the allocated HSDPA power while keeping the total RBS power below the limit.

The limit itself is either 100% (if HSDPA is switched on) or 75% (if the system is running with R99 only). These parameters are configurable.

4.4 Downlink DCH power

This part tries to answer the following question: For given cell range, how much power is used by one user on the cell-edge using the most demanding service?

The DL DCH power is calculated similar to the calculation of the CPICH power. The main difference is however that in this calculation, the actual total power (instead of the maximal total power) is taken into account.

The maximal interference received by the UE on the cell edge is given in Equation 18.

Equation 18 Cell edge interference

In a special case, when a very demanding R99 service is considered (PS384) it may happen that the R99 loading would give the result which is lower than the sum of the power needed for the common channels and one single user, using that service. In those cases, an alternative expression is used, which would ensure that on the cell edge, the DCH power is enough to maintain the C/I for a single user:

Equation 19 Single R99 DCH user consideration

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The noise rise for the UE at the cell edge is calculated as given in Equation 20.

Equation 20 Total noise rise for UE on cell edge

The power allocated for a single user, PDCH should be enough to compensate for the NRDCH, thermal noise and the signal attenuation (Equation 21).

Equation 21 Power allocated for single user

Missing terms are given in Equation 22 and Equation 23.

Equation 22 UE sensitivity

Equation 23 Downlink margin

Using the cell range and R99 loading and assuming certain HSDPA power, it is possible to calculate the required DL DCH power. In practice, the tool target is to maximize the allocated HSDPA power while keeping the DL DCH power below the limit.

The limit itself is 40%, but it is configurable.

4.5 HS-DSCH power

As it is explained in Chapter 5.1, Axcel imports Lsa samples for 1000 points evenly distributed across the cell. In order to use that nominal data for different scenarios (different cell ranges, different margins) it is necessary to scale the actual Lsa up or down.

The reference Lsa is calculated from the vector data (Equation 24).

Equation 24 Reference Lsa

And the calculated Lsa is calculated using Equation 10 using downlink margin in Equation 25.

Equation 25 Downlink margin

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The delta between the Lsa,calc and Lsa,ref is calculated and the result is used for scaling the vector data up or down. In this way, it is ensured that on the cell edge, the calculated signal attenuation is equal in both cases.

The total power calculation has already been shown in chapter 4.3. For the assumed cell range, R99 loading and the HSDPA power allocation, it is possible to calculate the total DL power.

For each of the samples in Axcel part, the C/I value is calculated according to Equation 26.

Equation 26 C/I for HSDPA

PHSDPA, Pown, Pothers are obtained as it is explained above. All other parameters are found in the axcel input vectors.

Based on the C/I to bitrate mapping, it is possible to read the achievable bitrate for each of the sampled points in the cell. The cell capacity is calculated as the average value of all the samples.

In case the proportional fair scheduling is used, the scheduling gain is added on top of the average.

4.6 E-DCH noise rise

This part tries to answer the following question: For given cell range, maximum interference margin, how big the E-DCH noise rise is and what capacity bitrate can be achieved in that case?

Main issue here is the distribution of total noise rise on R99 loading and E-DCH noise rise. Distribution is given in Equation 27.

Equation 27 Noise rise distribution (linear scale)

5 Axcel

Axcel [1] is an Excel-based tool for WCDMA radio network dimensioning that estimates HSDPA and EUL capacity and coverage for mixed traffic scenarios.

The approach employed by the tool differs from the standard dimensioning approach in the fundamental aspect that instead of working with average levels, it derives estimates of distributions that are important for dimensioning purposes. In short, the calculations are based on C/I equations and imported pre-generated propagation data. The main output of the tool is HSDPA and EUL bitrate distributions as well as coverage and capacity estimates.

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5.1 Average gain data (A2_gmean vectors)

The gain distribution depends on a number of factors like antenna height and sectorization, and therefore different scenarios might require different data sets. Thus, Axcel is delivered together with a number of data sets holding gain values that match different scenarios. These data sets were prepared in the following manner: First, a large number of user positions were sampled randomly in an area with a specified radio environment. The area was then covered by cells, and for all randomly sampled positions, average path gain values (not including multipath fading) associated with all cells in the area were determined.

All data sets holding average gain values were generated assuming: Homogenous radio environment; Perfect uniform hexagonal network layout, i.e. all site-to-site distances are

equal; Uniform spatial distribution of user positions; 1000 user positions per data set; Distance attenuation according to the Okumura-Hata formula with

reference area type constant equal to 155.1 dB; Shadow fading described by a lognormal distribution; Antenna gain modeled as a multiplicative factor.

The gain data set contains, for each user position, the largest and second and third largest average gain values and the sum of all average gain values. For each user, the data set also includes some site information about the potential handover links (that is, the second and third strongest links). This information is needed when considering handover in the uplink.

5.2 Multipath and non-orthogonality data (A2_ff vectors)

For each channel model, multipath and non-orthogonality factors for five time instances have been pre-generated for each user position. Multipath and non-orthogonality maps produced by Matlab code were used for this purpose. In Axcel, the average gain values are combined with the multipath factors to produce instantaneous gain values.

All data sets holding multipath and non-orthogonality data were generated assuming: Homogenous radio environment; 1000 user positions per data set.

For each sampled user position, different but correlated gain and non-orthogonality factor samples were generated (derived by taking five multipath/non-orthogonality factors from the multipath/non-orthogonality map 17 meters apart, which corresponds to the distance traveled during 20 seconds when moving at a speed of 3 km/h). By averaging over a few time instances a more realistic picture of e.g. the HSDPA bitrate distribution can be obtained since the most extreme values are averaged out.

5.3 Downlink calculations

Main inputs for downlink calculations are: Own cell HSDPA power

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Own cell total power Other cells total power

Given these, cumulative HSDPA bitrate distribution function and capacity bitrate estimate are obtained as results.

5.4 Uplink calculations

Main inputs for uplink calculations are: Maximum UE TX power Own cell total interference Other cell total interference Own cell R99 loading

Given these, cumulative E-DCH bitrate distribution function and capacity bitrate estimate are obtained as results.

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References

[1] Birgitta Olin and Magnus Lundevall, “Axcel 2.0”, Technical report EAB-06:006835

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