B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

622 SPREAD SPECTRUM COMMUNICATIONS
Figure 11.6
FHSS modulation
in 802.11 and
Bluetooth.
NRZ binary data
LIU
GFSK signal
Gaussian 1-- - Frequency 1-- - •
low-pass filter m(t)
modulator s(t)
TABLE 11.1
Major Specifications of 802.1 1 FHSS and Bluetooth.
802. 11 FHSS Bluetooth (basic rate)
Frequency band
Duplex format
Single-channel bandwidth
Number of nonoverlapping channels
BTs product
Minimum hopping distance
Modulation
Data rate
Hopping rate
ISM (2.4 - 2.4835 GHz)
TDD
1 MHz
79
0.5
6
GFSK-2 and GFSK-4 GFSK-2
1 Mbit/s and 2 Mbit/s 723.1 kbit/s
2.5-160 Hz 1600 Hz
in Fig. 11.6, a simple binary FSK replaces the Gaussian low-pass filter with a direct path. The
inclusion of the Gaussian low-pass filter generates what is known as the Gaussian FSK ( or
GFSK) signal. GFSK is a continuous phase FSK. It achieves better bandwidth efficiency by
enforcing phase continuity. Better spectral efficiency is also achieved through partial response
signaling (PRS) in GFSK. The Gaussian filter response stretches each bit over multiple symbol
periods.
More specifically, the Gaussian LPF impulse response is ideally given by
1 2;2 2
h(t) = -- e - t a
--.fiira
vln2
a=-- 211:B
where B is the 3 dB bandwidth of the Gaussian low-pass filter. Because this response is
noncausal, the practical implementation truncates the filter response to 4Ts seconds. This way,
each bit of information is extended over a window 3 times broader than the bit duration Ts.
Note that the selection of Bis determined by the symbol rate 1 /Ts . In 802.11 and Blue tooth,
B = 0.5/T., is selected. The FM modulation index must be between 0.28 and 0.35. The GFSK
symbol rate is always 1 MHz; binary FSK and four-level FSK can be implemented as GFSK-2
and GFSK-4, achieving data throughput of 1 and 2 Mbit/s, respectively. Table 11.1 summarizes
the key parameters and differences in IEEE 802.11 and the Bluetooth (basic rate).
We note that our discussions on Bluetooth have focused on the (basic rate) versions 1.1
and 1.2. More recently, version 2 of Bluetooth has been released. 4 Version 2.0 implementations
feature Bluetooth Enhanced Data Rate (EDR) and reach 2.1 Mbit/s. Technically, version 2.0
devices retain the FHSS feature but resort to the more efficient (differential) PSK modulations.
SINCGARS
SINCGARS stands for single channel ground and airborne radio system. It represents a family
of VHF-FM combat radios used by the U.S. military. First produced by ITT in 1983,
SINCGARS transmits voice with FM and data with binary CPFSK at 16 kbit/s, occupying a
bandwidth of 25 kHz. There can be as many as 2320 channels within the operational band of
30 to 87.975 MHz.