- Carrier Frequency:
This is the RF frequency that is modulated by
the PN code.
- Dynamic Range:
At the receiver, the signal must be strong enough to overcome the system noise. At the
same time, it cannot be so strong that it over-drives the system. This range in signal
power levels is referred to as the dynamic range of the receiver.
- Maximum Interval of Disrimination (IOD):
This is the maximum achievable interval between the peak of the impulse and the
processing noise floor.
(See Figure). It is dependant
upon the PN code length, as well as, the processing capabilities of the receiver.
The IOD determines how far, in magnitude, below the peak that a delay can be detected.
Some times the IOD will be a value other than what is maximally achievable and is dependant
on the received signal strength and the noise figure of the system.
- Maximum Delay:
This is the maximum time period for which an impulse delay can be measured. Each
impulse delay represents one or more copies of the transmitted signal as they are
reflected off of objects. And since each reflected signal has a different path, the different
path lengths translates into different times of travel between the transmitter and the
receiver. The first received copy represents the shortest path and the longest delay
represents a copy with the longest path. The maximum measurable difference between
time of the first received copy and the last received copy is dependant on the maximum
delay specification. This specification is, in turn, dependant upon the PN code bit rate
and the length of the PN code in terms of bits. (See Figure)
- Minimum Holdoff:
This is the minimum time between sequential acquisitions for continuous and burst
modes of acquisition.
(See Figure)
- Mode of Acquisition:
There are two modes of data acquisition for the digital probes: continuous and burst.
For both modes, the impulse data is acquired repeatedly at a specified interval (holdoff
period), with only one full impulse attained at each interval. In other words, data may
be collected for every 5th impulse. The difference between continuous and burst is that
for continuous mode, these repeated acquisitions are performed continuously over an
extended period of time. For burst mode the repeated acquisitions are done for a
specified period of time (burst period) after which there is quiescent time when no datais acquired,
followed by another burst of data acquisition. (See Figure)
- Null-to-Null Bandwidth:
The Fourier transform of a PN code produces a line spectrum for which the magnitude
is confined by a sinc function. The first null of the sinc function occurs at frequency
equivalent to the bit rate of the PN code. For example, a bit rate of 10 MBits/s translates
to a frequency at the first null of 10 Mhz. Since the magnitude of the spectrum is
symmetrical around DC, the null-to-null bandwidth is twice the bit rate - 20 MHz in the
example given. (See Figure).
- PN Code Bit Rate:
This is the bit rate of the PN code generator which, in turn, determines the RF bandwidth of the transmitted signal.
The null-to-null bandwidth in Hz translates to twice the PN code rate. For example a PN code rate of 10 Mbits/s
produces a 20 Mhz null-to-null bandwidth.
- PN Code Word Length:
This refers to the length of the PN code in terms of bits. These systems use a maximal
length code and, therefore, the length (in bits) is approximately 2 to the power of n,
where n is an integer. (It is actually one bit less than this). The longer the PN code length
in relation to a specific bit rate, the greater the capability for seeing long delays in the
impulse response. The length of the impulse response is determined by the PN code
length divided by the bit rate. For example, a 511 bit code with a bit rate of 10 Mbits/s
can be used to measure delays up to 51.1 microseconds. The code length also determines
the maximum interval of discrimination (IOD). (See Figure).
The tradeoff for longer code lengths is greater data size and longer time periods of processing.
- Resolution:
This is the minimum time between two discernable impulse delays and is equivalent to
the reciprocal of the PN code bit rate. For example, there is a 100 ns resolution for a PN
code bit rate of 10 MHz. Any two impulse delays that have a time of separation less the
resolution is not discernable.(See Figure).
- Shutter Time:
A channel impulse response is simply an image which characterizes a propagation
channel. And just as a camera cannot take an instantaneous snapshot of an image
(because it requires a certain period of exposer), the picture provided by each impulse
response requires a certain interval of time to characterize the channel for the particular
instance. This interval is the shutter time (also called the "trace repetition time" for
sliding correlators). One of the primary differences between an analog sliding
correlator probe and a digital sampling probe is that the shutter time is generally much
longer for an analog system. The key to a clear picture is the requirement that the
environment not change significantly during the snapshot. Therefore, because the
digital probe has a much shorter shutter time (generally the period of one PN code word
length), it is better suited for rapidly changing environments such as when either the
transmitter or receiver is mobile.
- Time of Travel Capability:
By synchronizing the PN code generator at the transmitter with an absolute timing trigger
device at the receiver it is possible to have the signal digitized at the receiver at the
precise moment that the PN code generator starts the beginning of a code cycle. By doing
this, the system can then measure the time of signal travel between the transmitter and the
receiver. This requires highly stable reference oscillators at both the transmitter and the receiver.
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