frequency hopping spread spectrum

1. The Frequency-Hopping (FH) PHY

2. Contents
• Frequency-Hopping Transmission
• Gaussian Frequency Shift Keying (GFSK)
• FH PHY Convergence Procedure (PLCP)
• Frequency-Hopping PMD Sublayer

3. Frequency-Hopping Transmission
• Depends on rapidly changing the transmission frequency in a
predetermined, pseudorandom pattern
• works on three things:
1. Frequency slots
2. Time slots
3. Hopping pattern
• Timing the hops accurately is the key to success

4. Frequency-Hopping Transmission
• Example
• A hopping pattern controls how the slots
are used
• In the figure, the hopping pattern is
{2,8,4,6}.

5. Frequency-Hopping Transmission
• Similar to FDMA , but FDMA is fixed and FH varies with time.
• Each frequency used for a small amount of time called dwell time.
• Main objective of FH is to avoid interference
• Figure below is an example that explains the interference
avoidance with primary device.
• Hopping sequence = {2,8,4,7}

6. Frequency-Hopping Transmission
Interference avoidance among the FH devices
• when two frequency-hopping systems need to share the same band,
interference can be avoided by configuring system with different hopping
sequences that do not overlap.
• Hopping sequences that do not overlap are called orthogonal
• 802.11 uses orthogonal hopping sequences for the multiple users.
First Hopping sequence = {2,8,4,7},
Second Hopping sequence = {6,3,7,2}

7. Frequency-Hopping Transmission
• ISM band range 2.4-2.5 GHz
• 802.11 divides ISM band into a series of 1-MHz channels.
• Channels are defined by their center frequencies, which
begin at 2.400 GHz for channel 0, 2.401 GHz for channel 2
and so on up to channel 95 at 2.495 GHz
• Successive channels are derived by adding 1-MHz steps
• The dwell time used by 802.11 FH systems is 390 time units,
which is almost 0.4 seconds.
802.11 FH Details

8. Frequency-Hopping Transmission
• 802.11 hop sets are derived from some standard
mathematical function.
• As an example, hopping sequence 1 for North America and
most of Europe begins with the sequence {3, 26, 65, 11, 46,
19, 74, 50, 22, ...}.
• 802.11 further divides hopping sequences into non
overlapping sets, and any two members of a set are
orthogonal hopping sequences.
• In Europe and North America, each set contains 26 members
802.11 Hop Sequences

9. Frequency-Hopping Transmission
802.11 Hop Sequences
Table 11-2. Size of hop sets in each
regulatory domain
Regulatory domain Hop set size
U.S. (FCC) 26
Canada (IC) 26
Europe (excluding France and Spain)
(ETSI)
26
France 27
Spain 35
Japan (MIC) 23
Table 11-2. Size of hop sets in each regulatory domain

10. Frequency-Hopping Transmission
• Joining 802.11 FH network is made possible by standardization of
hop sequences
• Information of the FH network is included in the beacon frame
Joining a 802.11 frequency-hopping network
• By receiving a Beacon frame, a station knows everything it needs to
synchronize its hopping pattern.
• Based on the hop sequence number, the station knows the channel-hopping
order

11. Frequency-Hopping Transmission
Joining a frequency-hopping 802.11 network
Dwell Time
The amount of time spent on each channel in the hopping sequence is
called the dwell time. It is expressed in time units (TUs).
Hop Set
This field, a single byte, identifies the set of hop patterns in use.
Hop Pattern
Stations select one of the hopping patterns from the set. This field, also
a single byte, identifies the hopping pattern in use.
Hop Index
Each pattern consists of a long sequence of channel hops. This field, a
single byte, identifies the current point in the hop sequence.

12. Frequency-Hopping Transmission
Effect of Interference
• 802.11 is a secondary use of the 2.4-GHz ISM band and must
accept any interference from a higher-priority transmission.
• As more channels are affected by interference, the throughput
continues to drop.

13. Gaussian Frequency Shift Keying (GFSK)
What is a frequency shift keying?
It is a modulator for transmitting digital data as analog signals. Logic
1 and 0 are represented with a different frequencies.
Frequency-shift keying

14. Gaussian Frequency Shift Keying (GFSK)
• A GFSK modulator, is similar to a FSK modulator, except that
before pulses go into the FSK modulator, it is passed through a
gaussian filter to make the pulse smoother so to limit its spectral
width to overcome noise.
GFSK

15. Gaussian Frequency Shift Keying (GFSK)
• GFSK confines emissions to a relatively narrow spectral band and
is thus appropriate for secondary uses
• By reducing the potential for interference, GFSK makes it more
likely that 802.11 wireless LANs can be built in an area where
another user has priority
• The Gaussian in GFSK refers to the shape of radio pulses;

16. (GFSK)
• Two different frequencies are used, depending on whether the
data that will be transmitted is a 1 or a 0.
• To transmit a 1, the carrier frequency is increased by a certain
deviation. Zero is encoded by decreasing the frequency by the
same deviation
2-Level GFSK

17. (GFSK)
• Frequency changes with GFSK are not sharp changes.
• Gradual frequency changes allow lower-cost equipment with
lower RF leakage
2-Level GFSK
Figure 1 shows how frequency varies as a result of encoding the
letter M (1001101 binary) using 2GFSK.

18. (GFSK)
• The four symbols (00, 01, 10, and 11) each correspond to a
discrete frequency,
• Therefore 4GFSK transmits twice as much data at the same
symbol rate.
• 4GFSK requires more complex transmitters and receivers.
4-Level GFSK
4GFSK packs multiple bits into a single
symbol

19. (GFSK)
• Figure shows how the letter M might be encoded.
4-Level GFSK
4GFSK encoding of the letter M

20. FH PHY Convergence Procedure (PLCP)
• Before any frames can be modulated onto the RF carrier, the frames
from the MAC must be prepared by the Physical Layer Convergence
Procedure (PLCP).
• PLCP is a relay between the MAC and the physical medium
dependent (PMD) radio interface
PLCP framing in the FH PHY

21. FH PHY Convergence Procedure (PLCP)
Preamble :
In the 802.11 FH PHY, the Preamble is composed of the Sync field
and the Start Frame Delimiter field
Sync
• 80 bits in length
• composed of an alternating zero-one sequence
(010101...01).
• Three purposes:
1. A sync signal indicates that a frame is imminent.
2. Stations that have multiple antennas to combat
multipath fading or other environmental reception
problems can select the antenna with the strongest
signal.
3. The receiver can measure the frequency of the
incoming signal relative to its nominal values and
perform any corrections needed to the received signal.

22. FH PHY Convergence Procedure (PLCP)
Start Frame Delimiter (SFD)
• The FH PHY uses a 16-bit SFD: 0000 1100 1011 1101.
Header :
PSDU Length Word (PLW)
• The 12-bit length field informs the receiver of the length of the
MAC frame
• A MAC frame in the PLCP may be up to 4,095 bytes long.
PLCP Signaling (PSF)
• Bit 0, the first bit transmitted, is reserved and set to 0.
• Bits 1-3 encode the speed at which the payload MAC frame is
transmitted.

23. FH PHY Convergence Procedure (PLCP)
PSF meaning
Bits (1-2-3) Data rate
000 1.0 Mbps
001 1.5 Mbps
010 2.0 Mbps
011 2.5 Mbps
100 3.0 Mbps
101 3.5 Mbps
110 4.0 Mbps
111 4.5 Mbps

24. FH PHY Convergence Procedure (PLCP)
Header Error Check (HEC)
• To protect against errors in the PLCP header, a 16-bit CRC is calculated
over the contents of the header and placed in this field.
• The header does not protect against errors in other parts of the frame

25. Frequency-Hopping PMD Sublayer
• Two standardized PMD layers
1. PMD for 1.0-Mbps FH PHY
2. PMD for 2.0-Mbps FH PHY
• Several features are shared between both PMDs
• Antenna diversity support,
• Allowances for the ramp up and ramp down of the power
amplifiers in the antennas,
• And the use of a Gaussian pulse shaper

26. Frequency-Hopping PMD Sublayer
• Figure shows the general design of the transceiver used in 802.11
frequency-hopping networks

27. PMD for 1.0-Mbps FH PHY
• MAC with PLCP header transmitted at 1.0 Mbps using 2GFSK
• 1 million symbols are transmitted per second.
• 2GFSK is used as the modulation scheme
• 802.11 specifies a minimum power of 10 milliwatts (mW)
• Uses a power control function to cap the radiated power at 100 mW,
if necessary

28. PMD for 2.0-Mbps FH PHY
• MAC with PLCP header transmitted at 1.0 Mbps using
2GFSK
• 4GFSK must be used to support 2.0-Mbps at 1 million
symbols per second.
• Firmware that supports the 2.0-Mbps PMD can fall back
to the 1.0-Mbps PMD if signal quality is too poor to
sustain the higher rate

29. PMD for 2.0-Mbps FH PHY
Carrier sense/clear channel assessment (CS/CCA)
• the physical layer implements the physical carrier
sense
• PCLP includes a function to determine whether the
wireless medium is bus or idle to implement the
CSMA/CA
• 802.11 does not specify how to determine whether a
signal is present; vendors are free to innovate

30. Characteristics of the FH PHY
Parameter Value Notes
Slot time 50ms
SIFS time 28ms
The SIFS is used to derive the value of the
other interframe spaces (DIFS, PIFS, and EIFS).
Contention window size 15-1,023 slots
Preamble duration 96ms Preamble symbols are transmitted at 1 MHz,
so a symbol takes 1 ms to transmit; 96 bits
require 96 symbol times.
PLCP header duration 32ms The PLCP header is 32 bits, so it requires 32
symbol times.
Maximum MAC frame 4,095 bytes 802.11 recommends a maximum of 400
symbols (400 bytes at 1 Mbps, 800 bytes at 2
Mbps) to retain performance across different
types of environments.
Minimum sensitivity -80 dBm