FHSS PMD Operation
Apr 30,2007 00:00 by admin
FHSS PMD Operation
The operation of the PMD translates the binary representation of the PPDUs into a radio signal
suitable for transmission. The FHSS PMD performs these operations via a frequency hopping
function and frequency shift keying modulation technique. The following sections explain the
FHSS PMD.
Frequency Hopping Function
The 802.11 standard defines a set of channels that are evenly spaced across the 2.4GHz ISM
band. The number of channels depends on geography. For example, the number of channels for
North America and most of Europe is 79, and the number of channels for Japan is 23.
The channels are spread across a band of frequencies, depending on geography. For example,
802.11-compliant stations in North America and most of Europe operate from 2.402 and
2.480GHz, and stations in Japan operate from 2.473 and 2.495GHz. Each channel is 1MHz
wide; therefore, the center operating frequency for channel 2 (the first channel) in the U.S. is
2.402GHz, channel 3 is 2.403GHz, and so on.
The FHSS-based PMD transmits PPDUs by hopping from channel to channel according to a
particular pseudo-random hopping sequence that uniformly distributes the signal across the
operating frequency band. Once the hopping sequence is set in an access point, stations automatically
synchronize to the correct hopping sequence. The 802.11 standard defines a particular
set of hopping sequences. For example, it specifies 78 sequences for North America and
most of Europe and 12 sequences for Japan. The sequences avoid prolonged interference with
one another. This enables designers to collocate multiple PMDs to improve performance.  The hop rate is adjustable, but the PMD must hop at a minimum rate that regulatory bodies
within the country of operation specify. For the U.S., FHSS must operate at a minimum hop
rate of 2.5 hops per second. In addition, the minimum hop distance in frequency is 6MHz in
North America and most of Europe and 5MHz in Japan.
FHSS Frequency Modulation Function
The FHSS PMD transmits the binary data at either 1Mbps or 2Mbps using a specific modulation
type for each, depending on which data rate is chosen. The PMD uses two-level Gaussian
frequency shift key (GFSK) modulation, as shown in Figure 5.5, for transmitting data streams
at 1Mbps. The concept of GFSK is to vary the carrier frequency to represent different binary
symbols. Thus, changes in frequency maintain the information content of the signal. Noise
usually affects the amplitude of the signal, not the frequency. As a result, the use of GFSK
modulation reduces potential interference.
The input to the GFSK modulator is either a 0 or 1 coming from the PLCP. The modulator
transmits the binary data by shifting the transmit frequency slightly above or below the center
operating frequency (Fc) for each hop. To perform this operation, the modulator transmits on a
frequency using the following rules:
Transmit frequency = Fc+fd for sending a 1 bit
Transmit frequency = Fc–fd for sending a 0 bit
In the equation, Fc is the operating center frequency for the current hop, and fd is the amount
of frequency deviation. The value of fd will be greater than 110KHz. The 802.11 specification
explains how to calculate exact values for fd, but the nominal value is 160KHz. The FHSS PMD uses four-level GFSK modulation, as shown in Figure 5.6, for transmitting
data streams at 2Mbps. Stations implementing the 2Mbps version must also be able to operate
at 1Mbps for the entire MSDU. For 2Mbps operation, the input to the modulator is combinations
of two bits (00, 01, 10, or 11) coming from the PLCP. Each of these two-bit symbols is
sent at 1Mbps, meaning each bit is sent at 2Mbps. Thus, the four-level modulation technique
doubles the data rate while maintaining the same baud rate as a 1Mbps signal. Similar to two-level GFSK, the modulator transmits the binary data bits by shifting the transmit
frequency slightly above or below the center operating frequency for each hop. In this
case, though, the transmitter can transmit at four possible frequencies, one for each two-bit
combination. To perform this operation, the modulator will transmit on the operating center
frequency with a frequency deviation equal to fd. There are two values of fd that move the
transmit frequency above Fc and two values of fd that move the transmit frequency below Fc.
The 802.11 standard describes how to calculate the exact value of fd. Overall, the transmit power of the FHSS radio must comply with IEEE standard C95.1-1991.
The 802.11 specification also limits the maximum amount of transmitter output power to 100
milliwatts of isotropically radiated power (meaning that the measurements are taken with an
antenna having no gain). Apparently, this limit enables 802.11 radio products to comply with
transmit power limits in Europe. The effective power will be higher, though, using antennas
that offer higher directivity (gain).
The 802.11 specification also says that all PMDs must support at least 10 milliwatts of transmit
power. Most access points and radio cards allow you to select multiple transmit power levels
via initialization parameters
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