"WINNER II Channel Models", ver 1.1, Sept

WINNER II D1.1.2 V1.1
5.2 Space-time concept in simulations
5.2.1 Time sampling and interpolation
Channel sampling frequency has to be finally equal to the simulation system sampling frequency. To
have feasible computational complexity it is not possible to generate channel realisations on the
sampling frequency of the system to be simulated. The channel realisations have to be generated on
some lower sampling frequency and then interpolated to the desired frequency. A practical solution is
e.g. to generate channel samples with sample density (over-sampling factor) two, interpolate them
accurately to sample density 64 and to apply zero order hold interpolation to the system sampling
frequency. Channel impulse responses can be generated during the simulation or stored on a file
before the simulation on low sample density. Interpolation can be done during the system simulation.
To be able to obtain the deep fades in the NLOS scenarios, we suggest using 128 samples per
wavelength (parameter ‘SampleDensity’ = 64). When obtaining channel parameters quasi-stationarity
has been assumed within intervals of 10-50 wavelengths. Therefore we propose to set the drop
duration corresponding to the movement of up to 50 wavelengths.
5.3 Radio-environment settings
5.3.1 Scenario transitions
In the channel model implementation it is not possible to simulate links from different scenarios
within one drop. This assumes that all propagation scenarios are the same for all simulated links. The
change of the scenario in time can be simulated by changing the scenario in the consecutive drop.
Similarly, to obtain different scenarios within radio-network in the same drop, multiple drops could be
simulated – one for each scenario. Afterwards, merging should be performed.
5.3.2 LOS\NLOS transitions
Mix of LOS and NLOS channel realizations can be obtained by first calculating a set of LOS drops
and after it a set of NLOS drops. This can be done by setting the parameter ‘PropagCondition’ to ‘LOS’
and later to ‘NLOS’.
5.4 Bandwidth/Frequency dependence
5.4.1 Frequency sampling
The WINNER system is based on the OFDM access scheme. For simulations of the system, channel
realizations in time-frequency domain are needed. The output of WIM is the channel in time-delay
domain. The time-frequency channel at any frequency can be obtained by applying next two steps:
• define a vector of frequencies where the channel should be calculated
• by use of the Fourier transform calculate the channel at defined frequencies
5.4.2 Bandwidth down scaling
The channel models are delivered for 100 MHz RF band-width. Some simulations may need smaller
bandwidths. Therefore we describe below shortly, how the down-scaling should be performed. In
doing so we assume that the channel parameters remain constant in down-scaling as indicated in our
analyses.
5.4.2.1 Down-scaling in delay domain
There is a need for down-scaling, if the minimum delay sample spacing in the Channel Impulse
Response (CIR) is longer than 5 ns in the simulation. Five nanoseconds is the default minimum
spacing for the channel model samples (taps) and defines thus the delay grid for the CIR taps. For all
smaller spacings the model shall be down-scaled. The most precise way would be filtering by e.g. a
FIR filter. This would, however, create new taps in the CIR and this is not desirable. The preferred
method in the delay domain is the following:
- Move the original samples to the nearest location in the down-sampled delay grid.
- In some cases there are two such locations. Then the tap should be placed in the one that has
the smaller delay.
- Sometimes two taps will be located in the same delay position. Then they should be summed
as complex numbers.
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