A Condensed Review of Spread Spectrum
TM Techniques for ISM Band Systems
The approval by the IEEE of the
802.11 Standard for wireless LANs has
given the WLAN industry a needed
boost. Manufacturers of WLAN
systems are now cooperating in performing interoperability
testing. Such testing is providing assurance that 802.11
compliant equipment purchased from one manufacturer will
in data transmission rate when compared with that for DQPSK,
and radios utilizing the MBOK modulation format have now
achieved FCC certification. This application note provides an
overview of several frequency hopping and direct sequence
techniques used for wireless data transmission in the ISM
band. Included is a discussion of how the MBOK modulation
technique is able to achieve Ethernet speeds over three
separate band-limited channels within the ISM band.
interoperate with 802.11 radios manufactured by other OEMs.
It will be shown that for a given increase in waveform
This is an important consideration for MIS managers who
desire to use WLAN technology to provide mobility with
connectivity to their workforce.
complexity the MBOK modulation adds very little system
complexity in terms of the number of components and radio
bill of materials (BOM) cost, when compared to a 2Mbit/s
The November 1997 approved 802.11 Standard defines the
protocol and compatible interconnection of data communication
equipment via radio or infrared air interface in a local area
radio with QPSK modulation.
A Variable Data Rate Radio
for network. The radio implementation of the PHY, a subject of this
Figure 2 depicts a system implementation of a 2.4GHz DSSS
paper, specifies the use of either Frequency-Hopping Spread
radio called PRISM™ designed for operation in the unlicensed
Band Spectrum (FHSS) or Direct Sequence Spread Spectrum
industrial, scientific and medical (ISM) band. By unlicensed
(DSSS) modulation. For frequency-hopping radios the IEEE
specifies a minimum requirement of 1Mbit/s data rate using
two-level Gaussian frequency shift keying (2GFSK) modulation.
An optional rate of 2Mbit/s is supported using four-level
band, we mean that frequency band of 2400MHz to
2483.5MHz allocated by the FCC wherein an intentional,
unintentional or incidental radiator may be operated without an
individual license . This international band is also defined by
Gaussian FSK (4GFSK) modulation. Figure
of the signaling schemes for 802.11 2GFSK
a4GcoFmSKpa. DIrEisaEotnEaSheet4Ura.ecfgeouwmlactooruynatrgieesnmcieinsoirndCifafenraednace, sEuinroaplleo,caanteddJfarepqauneanltchieosugahrein
words Std 802.11 speciﬁes h2, the deviation factor, as 0.32 nominal
found. Table 1 shows the operating frequency ranges in effect
(Inter- for 2GFSK and h4 = 0.45 x h2 nominal for 4GFSK.
internationally for the 2.4GHz ISM band .
sil For direct sequence systems two modulation formats and
TABLE 1. ISM BAND OPERATING FREQUENCIES
data rates are supported, a basic access rate of 1Mbit/s
and an enhanced access rate of 2Mbit/s. Both data rates
utilize phase shift keying modulation with differential binary
phase shift keying (DBPSK) used for the 1Mbit/s basic
access rate and differential quadrature phase shift keying
(DQPSK) used for the enhanced access rate. These two
techniques, FHSS and DSSS, constitute the currently
approved standard for IEEE 802.11.
Another modulation technique known as M-ary Biorthogonal
Keying (MBOK) has been used to achieve a 5.5X improvement
LIMIT (GHz) LIMIT (GHz)
2.400 to 2.4835 North America
2.471 to 2.497
2.445 to 2.475
2.4465 to 2.4835
DataSheet4 U .com
SYMBOL TIME INTERNAL
FIGURE 1. SIGNALING SCHEMES FOR 2GFSK AND 4GFSK MODULATION
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