The building penetration loss and the delay spread were measured as a function of elevation angle and building entry angle. All measurements were performed with the helicopter facing the wall behind which the receiver was placed. The distance of the helicopter from each building was 1 km or 2 km. The range of elevations and azimuths covered by the helicopter was from 15° to 60° and from −80° to 80°, respectively. Building entry angle incorporates both elevation and azimuth; it is zero for grazing incidence and is equal to 90° when the transmitter is in the direction of the normal to the building surface.
To clarify the data analysis process, the different data sets and preprocessing steps taken before the actual analysis are summarized. The data set consists of a number of "products." The Propsound channel sounder [12] from Elektrobit, (Oulu, Finland) was used, providing an "instantaneous" data set consisting of one "complex" (
and
) channel response versus delay per patch antenna and polarization. These instantaneous measurements are called cycles. The channel sounder transmits a pseudorandom sequence where different code lengths are possible. Depending on the code length chosen, the update rate of such set (full scan over all antennas and both polarizations) varies giving rates ranging from several tens of cycles per second to a few hundreds.
The transmit antenna was circularly polarized (RHCP) while the receive antenna consisted of a set of patch antennas (SIMO) with two orthogonal linear polarizations covering a surface that approximates a semisphere. From each of the two linearly cross-polarized measurements, both the received copolar (RHCP) and cross-polar (LHCP) components can be calculated. Thus, one instantaneous measurement or cycle consists of an ensemble of instantaneous individual antenna complex channel responses obtained through cross-correlation between a transmitted pseudorandom sequence and an identical, internally generated sequence in the receiver.
The channel sounding process tries to measure the channel impulse response,
. In channel modeling, it is usual to assume a sufficiently large bandwidth so that the impulse response is made up of complex deltas (ideal channel response). The band limiting effects are considered later when the impulse response is converted into a tapped delay line (TDL); fitting the bandwidth requirements of whatever system has to be simulated. In general, the results obtainable from the analyses reported should be valid for systems with RF bandwidths smaller than that of the channel sounder, that is, 200 MHz (chip rate 100 Mcps). Moreover, the impulse response is not constant in time but time-variant, that is,
. This imposes some constraints on the sounding process (the duration of one full instantaneous measurement or cycle) whereby sounding has to be carried out at a rate consistent with the rate of change of the channel (its coherence time). Assuming that the sounding process is fast enough, it is possible to consider the channel impulse response to be constant over the duration of one cycle.
To sound the channel, pseudorandom, noise-like sequences were used. Ideally, noise has a delta autocorrelation function. Pseudorandom sequences show very narrow autocorrelation peaks: a base of the order of
, the chip duration, and an amplitude equal to the sequence length, m. The measurement of the channel impulse response is carried out by computing the cross-correlation between the received signal and an identical sequence synchronized with the one at the transmitter. The measurement process does not exactly provide the ideal channel impulse response but is approximately the result of the convolution between the channel impulse response and the code autocorrelation pulse. Given the narrowness of the cross-correlation pulse, this processing permits a good approximation of
. The in-phase and quadrature parts are obtained in this process. The so-called power delay profile (PDP) is in fact the most frequently used characterization parameter for the wideband channel and is given by
where
is the channel sunder correlation pulse.
Two kinds of parameters were extracted from the measurements; the first group has to do with the entry loss and the other with the time dispersion effects. For extracting the entry loss, the measured averaged power delay profiles (APDPs) were compared with a reference measurement carried out outside each building. Averaging took place over several cycles to remove possible enhancements and cancellations at particular delays.
In the beginning, to measure the entry loss, the sum of all samples above the noise floor was calculated both for the reference outdoor measurement,
, and the indoor measurements
, that is,
This criterion was later changed to take into account only the direct, LOS contribution as illustrated in Figure 1.
The entry loss is given by
where
is a distance correction factor needed when the reference measurement has been carried out at a different distance than that used in the indoor measurement.
and
are the measured APDPs.
Similarly, the averaged PDPs were normalized with respect to the outdoor reference parameter
, that is,
This implies that the direct, line-of-sight (LOS) contribution power outdoors is assumed to be 0 dB, thus the extracted model parameters (see Section 3) will be referred to the conditions just outside the building. Their transformation into absolute values is straightforward.