Small-Scale Multipath Measurements

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5.3 Small-Scale Multipath Measurements

Because of the importance of the multipath structure in determining the small-scale fading

effects, a number of wideband channel sounding techniques have been developed.

These techniques may be classified as direct pulse measurements, spread spectrum sliding

correlator measurements, and swept frequency measurements.

5.3.1 Direct RF Pulse System

A simple channel sounding approach is the direct RF pulse system (see Figure 5.6). This

technique allows engineers to determine rapidly the power delay profile of any channel,

as demonstrated by Rappaport and Seidel [Rap89], [Rap90]. Essentially a wideband pulsed

bistatic radar, this system transmits a repetitive pulse of width T_bb s, and uses a receiver

with a wide bandpass filter (BW=2/T_bb Hz).  The signal is then amplified, detected with an

envelope detector, and displayed and stored on a high speed oscilloscope. This gives an

immediate measurement of the square of the channel impulse response convolved with the

probing pulse (see Equation (5.17)).

If the oscilloscope is set on averaging mode, then this system can provide a local average power

delay profile. Another attractive aspect of this system is the lack of complexity, since off-theshelf

equipment may be used.

The minimum resolvable delay between multipath components is equal to the probing

pulse width T_bb. The main problem with this system is that it is subject to interference and

noise, due to the wide passband filter required for multipath time resolution. Also, the pulse

system relies on the ability to trigger the oscilloscope on the first arriving signal. If the first

arriving signal is blocked or fades, severe fading occurs, and it is possible the system may not

 

Figure 5.6 Direct RF channel impulse response measurement system.

trigger properly. Another disadvantage is that the phases of the individual

multipath components are not received, due to the use of an envelope detector.

However, use of a coherent detector permits measurement of the multipath phase

using this technique.

5.3.2 Spread Spectrum Sliding Correlator Channel Sounding

The basic block diagram of a spread spectrum channel sounding system is shown in Figure 5.7.

The advantage of a spread spectrum system is that, while the probing signal may be wideband,

it is possible to detect the transmitted signal using a narrowband receiver preceded by a

wideband mixer, thus improving the dynamic range of the system as compared to the direct

RF pulse system.

In a spread spectrum channel sounder, a carrier signal is “spread” over a large bandwidth

by mixing it with a binary pseudo-noise (PN) sequence having a chip duration T_c and a

chip rate R_cequal to 1/T_c Hz. The power spectrum envelope of the transmitted spread

spectrum signal is given by [Dix84] as

and the null-to-null RF bandwidth is

 

[+] Enlarge Image

Figure 5.7 Spread spectrum channel impulse response measurement system.

The spread spectrum signal is then received, filtered, and despread using a PN sequence

generator identical to that used at the transmitter. Although the two PN sequences are

identical, the transmitter chip clock is run at a slightly faster rate than the receiver chip clock.

Mixing the chip sequences in this fashion implements a sliding correlator [Dix84]. When the

PN code of the faster chip clock catches up with the PN code of the slower chip clock, the two

chip sequences will be virtually identically aligned, giving maximal correlation. When the two

sequences are not maximally correlated, mixing the incoming spread spectrum signal with the

unsynchronized receiver chip sequence will spread this signal into a bandwidth at least as large

as the receiver’s reference PN sequence. In this way, the narrowband filter that follows the

correlator can reject almost all of the incoming signal power. This is how processing gain is

realized in a spread spectrum receiver and how it can reject passband interference, unlike the

direct RF pulse sounding system.

Processing gain (PG) is given as

where T_bb 1 R_bb, is the period of the baseband information. For the case of a sliding correlator

channel sounder, the baseband information rate is equal to the frequency offset of the PN

sequence clocks at the transmitter and receiver.

When the incoming signal is correlated with the receiver sequence, the signal is collapsed

back to the original bandwidth (i.e., “despread”), envelope detected, and displayed on

an oscilloscope.

Since different incoming multipaths will have different time delays, they will maximally

correlate with the receiver PN sequence at different times. The energy of these individual paths

will pass through the correlator depending on the time delay. Therefore, after envelope detection,

the channel impulse response convolved with the pulse shape of a single chip is displayed

on the oscilloscope. Cox [Cox72] first used this method to measure channel impulse responses

in outdoor suburban environments at 910 MHz. Devasirvatham [Dev86], [Dev90a] successfully

used a direct sequence spread spectrum channel sounder to measure time delay spread of multipath

components and signal level measurements in office and residential buildings at 850 MHz.

Bultitude [Bul89] used this technique for indoor and microcellular channel sounding work, as

did Landron [Lan92], while Newhall and Saldanha measured campuses and train yards

[New96a]. A detailed description of a practical sliding correlator is given in [New96b].

The time resolution (Δτ) of multipath components using a spread spectrum system with

sliding correlation is

In other words, the system can resolve two multipath components as long as they are equal

to or greater than two chip durations, or 2T_c seconds apart. In actuality, multipath components

with interarrival times smaller than 2T_c can be resolved since the rms pulse width of a chip is

smaller than the absolute width of the triangular correlation pulse, and is on the order of T_c .

The sliding correlation process gives equivalent time measurements that are updated every

time the two sequences are maximally correlated. The time between maximal correlations (ΔT )

can be calculated from Equation (5.30)

The slide factor is defined as the ratio between the transmitter chip clock rate and the difference

between the transmitter and receiver chip clock rates [Dev86]. Mathematically, this is

expressed as

where n is the number of shift registers in the sequence generator [Dix84].

Since the incoming spread spectrum signal is mixed with a receiver PN sequence that is

slower than the transmitter sequence, the signal is essentially down-converted (“collapsed”) to a

low-frequency narrowband signal. In other words, the relative rate of the two codes slipping past

each other is the rate of information transferred to the oscilloscope. This narrowband signal

allows narrowband processing, eliminating much of the passband noise and interference. The

processing gain of Equation (5.28) is then realized using a narrowband filter (BW = 2(α – β)).

The equivalent time measurements refer to the relative times of multipath components as

they are displayed on the oscilloscope. The observed time scale on the oscilloscope using a sliding

correlator is related to the actual propagation time scale by

This effect is due to the relative rate of information transfer in the sliding correlator. For

example, ΔT of Equation (5.30) is an observed time measured on an oscilloscope and not actual

propagation time. This effect, known as time dilation, occurs in the sliding correlator system

because the propagation delays are actually expanded in time by the sliding correlator.

Caution must be taken to ensure that the sequence length has a period which is greater

than the longest multipath propagation delay. The PN sequence period is

The sequence period gives an estimate of the maximum unambiguous range of incoming

multipath signal components. This range is found by multiplying the speed of light with τ_PNseq

in Equation (5.34).

There are several advantages to the spread spectrum channel sounding system. One of the

key spread spectrum modulation characteristics is the ability to reject passband noise, thus

improving the coverage range for a given transmitter power. Transmitter and receiver PN

sequence synchronization is eliminated by the sliding correlator. Sensitivity is adjustable by

changing the sliding factor and the post-correlator filter bandwidth. Also, required transmitter

powers can be considerably lower than comparable direct pulse systems due to the inherent

“processing gain” of spread spectrum systems.

A disadvantage of the spread spectrum system, as compared to the direct pulse system, is

that measurements are not made in real time, but they are compiled as the PN codes slide past

one another. Depending on system parameters and measurement objectives, the time required to

make power delay profile measurements may be excessive. Another disadvantage of the system

described here is that a noncoherent detector is used, so that phases of individual multipath components

can not be measured. Even if coherent detection is used, the sweep time of a spread

spectrum signal induces delay such that the phases of individual multipath components with different

time delays would be measured at substantially different times, during which the channel

might change.

5.3.3 Frequency Domain Channel Sounding

Because of the dual relationship between time domain and frequency domain techniques, it is

possible to measure the channel impulse response in the frequency domain. Figure 5.8 shows a

frequency domain channel sounder used for measuring channel impulse responses. A vector network

analyzer controls a synthesized frequency sweeper, and an S-parameter test set is used to

monitor the frequency response of the channel. The sweeper scans a particular frequency band

(centered on the carrier) by stepping through discrete frequencies. The number and spacings of

these frequency steps impact the time resolution of the impulse response measurement. For each

frequency step, the S-parameter test set transmits a known signal level at port 1 and monitors the

received signal level at port 2. These signal levels allow the analyzer to determine the complex

response (i.e., transmissivity S₂₁(ω)) of the channel over the measured frequency range. The

transmissivity response is a frequency domain representation of the channel impulse response.

This response is then converted to the time domain using inverse discrete Fourier transform

(IDFT) processing, giving a band-limited version of the impulse response. In theory, this technique

works well and indirectly provides amplitude and phase information in the time domain.

However, the system requires careful calibration and hardwired synchronization between the

transmitter and receiver, making it useful only for very close measurements (e.g., indoor channel

sounding). Another limitation with this system is the non-real-time nature of the measurement.

For time varying channels, the channel frequency response can change rapidly, giving an erroneous

impulse response measurement. To mitigate this effect, fast sweep times are necessary to

Figure 5.8 Frequency domain channel impulse response measurement system.

keep the total swept frequency response measurement interval as short as possible. A faster

sweep time can be accomplished by reducing the number of frequency steps, but this sacrifices

time resolution and excess delay range in the time domain. The swept frequency system has

been used successfully for indoor propagation studies by Pahlavan [Pah95] and Zaghloul et al.

[Zag91a], [Zag91b].

Relevant NI products

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