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PDF AN9820.1 Data sheet ( Hoja de datos )
| Número de pieza | AN9820.1 | |
| Descripción | A Condensed Review of Spread Spectrum Techniques for ISM Band Systems | |
| Fabricantes | Intersil Corporation | |
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Application Note 9820
Using the integral operator
T
∫0 [""]φj(t)dt on both sides of EQ. 1 yields:
TT
∫ ∫0 Si(t)φj(t)dt = 0 [Σaijφj(t)]φj(t)dt
and so:
∫T
aij = 0 Si(t)φj(t)dt =
i = 1, 2, ...M
j = 1, 2, ...N
N≤M
Orthogonality will be discussed later in the context of both
FH and DS systems.
Frequency Hopping Spread Spectrum
(FHSS)
In a frequency hopping system, the carrier is caused to jump
around in a pseudorandom fashion under the control of a
synthesizer that is driven by a pseudonoise (pn) code
generator. Figure 3 illustrates the concept.
generate a sequence by the inclusion of a feedback loop which
computes a new term for the first stage based on the previous
N terms. Because the sequence of ones and zeros generated
by the shift register is deterministic and repetitive, the resulting
random-like sequence is designated as pseudorandom [6].
We will now investigate in some detail the characteristics and
limitations of FH signals in the ISM band of 2400MHz to
2483.5MHz. Since it is difficult for the hopping synthesizer to
maintain phase coherence over the wide hopping bandwidth,
the FSK waveform is widely used in FH systems because it is
relatively easy to demodulate non-coherently. Therefore FSK
modulation is assumed throughout the following discussion
because of its widespread use in FH systems. An FSK signal
can be thought of as the sum of two amplitude shift key (ASK)
signals [7]. To see this analogy refer to Figure 4.
In Figures 4A and 4B we have two ASK signals which can be
represented mathematically by:
ASK1 (t) = A cos 2πf1t + θ1 0 < t ≤ T
0 elsewhere
INFORMATION
SOURCE
DIGITAL
MODULATOR
FHSS
SIGNAL
ASK2 (t) = A cos 2πf2t + θ2 0 < t ≤ T
0 elsewhere
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CARRIER
FREQUENCY
SYNTHESIZER
The FSK waveform of Figure 4C is the linear sum of the two
ASK waveforms of Figures 4A and 4B. Thus the FSK
waveform is represented mathematically as:
PN - CODE
FSK (t) = A cos 2πf1t + θ1
GENERATOR
A cos 2πf2t + θ2
FIGURE 3A. BLOCK DIAGRAM OF A FREQUENCY HOPPDINaGtaSheet4U.com
SYSTEM
Where: f1 = fC + fD
f2 = fC - fD
For Binary 1
For Binary 0
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As with any FM signal, the bandwidth of the FSK signal
depends on the modulation index. Figure 5 shows the typical
FSK magnitude spectrum for a carrier frequency fC and
deviation fD. Having described the FSK waveform
graphically and mathematically we now refer to Figure 6 to
see the typical spectrum of an ISM band FH Signal.
FIGURE 3B. FREQUENCY HOPPING SIGNAL
In Figure 3 the information signal is used by a digital modulator
to modulate a carrier signal typically using FSK modulation.
The FSK modulated carrier signal is then hopped over a very
wide bandwidth compared to the bandwidth of the information
signal. The hopping sequence is generated by the pn code
generator which sets the synthesizer output. The pn code
generator generates what appears to be a random sequence of
ones and zeros, but it is not truly random. To understand why,
consider the random process of flipping a coin. If we assign a
logical one to the occurrence of a head and a logical zero to the
occurrence of a tail, then the sequence of ones and zeros
produced by flipping a coin is a random process due to the
unpredictable fashion in which the sequence is generated. In
the case of a pn code generator, a shift register is used to
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Figure 6A shows ideal line spectra for the 79 hop channels
defined by IEEE Std 802.11 for North America and most of
Europe. The center frequencies for the 79 channels are
defined by IEEE Std 802.11 in 1MHz steps beginning at
2.402GHz and ending at 2.480GHz (see IEEE Std 802.11
for channels and center frequencies in France, Spain, and
Japan). This is in compliance with the FCC Part 15
regulation specifying the use of at least 75 hopping
frequencies for FHSS systems operating in the 2400MHz to
2483.5MHz band. The minimum hop rate is governed by
regulatory authorities and is specified by the FCC in terms of
a maximum dwell time of 400ms on any one channel. This
equates to a minimum hop rate of 2.5 hops/s. The minimum
802.11 hop is 6MHz for North America and Europe
(including France and Spain) and 5MHz minimum for Japan.
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Application Note 9820
SIGNAL BEFORE SPREADING
SIGNAL AFTER SPREADING
JAMMER GETS SPREAD
WHEN SIGNAL GETS DESPREAD
fC
fC - 11MHz
MAIN LOBE NULL-TO-NULL
BANDWIDTH
fC + 11MHz
FIGURE 12. POWER SPECTRUM OF DSSS BEFORE AND AFTER SPREADING
LSYS are cumulative systems losses due to filtering,
synchronization, tracking, etc.
We can solve Equation 3 for the jamming margin MJ as
follows:
MJ = GP – (S/N)0 – LSYS
(EQ. 4)
To maximize system utilization it was desired to keep the
spread rate the same as that for 802.11 in order to maintain
at least three non-overlapping channels in the band. This is
the minimum necessary for co-located networks because it
allows for frequency reuse. Figures 13A and 13B illustrate
the concept of frequency reuse in co-located networks.
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Now, given a BPSK signal with (S/N)0 = 9.6dB and
LSYS = 2dB and assuming the minimum allowed GP of
10dB, Equation 3 yields a system jamming margin of -1.6dB.
Consequently, we would not expect the system to operaDteataSheet4U.com
reliably with an interfering signal more than -1.6dB above the
desired data signal. This is the meaning of the system
jamming margin.
2400MHz
22MHz
F1 30MHz
22MHz
22MHz
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F2 30MHz F3 2483.5MHz
Choice of MBOK Modulation
The selection of a particular modulation technique involves
making trade-offs among various constraints and conflicting
goals. The communications engineer generally will attempt to:
• Maximize spectral efficiency (bits/Hz)
FIGURE 13A. ILLUSTRATION OF NON-OVERLAPPING
CHANNELS IN THE ISM BAND
F2 F1
• Minimize power required
• Maximize system utilization
• Minimize system cost
F1 F3 F2
Trading off among these four criteria was indeed the case in
choosing MBOK modulation for the high data rate radio of
Figure 2. First of all, it was desired that the high rate radio be
backward compatible with the IEEE Std 802.11 basic access
and enhanced access rates of 1Mbit/s and 2Mbit/s. Thus the
radio had to be capable of using the 802.11 preamble and
header for signal acquisition and then do on the fly rate
switching to the high data rate. Conveniently, the 802.11
protocol already accommodates rate changing. On the fly
rate switching offers the added benefit of allowing the radio
to downshift to lower, more robust data rates in high
multipath environments such as might be found in large
open areas like a supermarket, or factory floor.
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F3 F2 F1 F3
FIGURE 13B. ILLUSTRATION OF FREQUENCY REUSE FOR
CELL PLANNING
Figure 13A shows three DSSS channels occupying the ISM
band. In Figure 13B we see how having a minimum of three
channels in the band allows the network planner to
implement a cellular network via frequency reuse. Each
adjacent cell is assigned a different channel and therefore
interference between adjacent cells is minimized for the
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| Número de pieza | Descripción | Fabricantes |
| AN9820.1 | A Condensed Review of Spread Spectrum Techniques for ISM Band Systems | Intersil Corporation |
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