Frequency Hopping Spread Spectrum subpart of the 802.11 Wireless LAN Standard
FCC - 802.11 meeting, Jan. 5, 1996

Naftali Chayat
Chief Scientist
802.11-FCC Meeting Objectives
l	To inform FCC about the 802.11 WLAN Standard and its status
l	To avoid situation in which 802.11 standard is not acceptable by FCC
l	To get better understanding of what FCC does permit, in order to produce a better standard
Collision Avoidance
l	The Physical layer (PHY) is able to provide Carrier Detect indication, when not transmitting.
l	Randomized Inter-Frame Space (IFS) for reducing collision probability
l	Exponential Backoff is used to stabilize the network when collisions occur
Virtual Carrier Sense mechanism
l	In radio network, some stations may not hear a transmission from a certain station and therefore collide with it
l	The RTS-CTS exchange contains timing information that signals other stations for how long the medium will be busy
l	Virtual Carier Sense mechanism is implemented in the MAC layer
Frequency Hopping Sp.Sp.
l	Operates according to FCC 15.247 ruling:
–	max 1 MHz bandwidth at -20 dB PSD
–	hopping over 79 frequencies in a pseudo-random manner
–	max 1 Watt power and 36 dBm EIRP 
802.11 FHSS Modulation Objectives
l	Achieving at least 1 Mbit/sec rate
l	Low cost/familiar technology - FSK
–	Constant Envelope- Saturated Amplifiers
–	Limiter-Discriminator detection
l	Multichannel operation -shaping to reduce adjacent channel interference
l	Multiple rates - taking advantage of short range scenarios to increase rate
802.11 FHSS Modulation
l	Gaussian shaped FSK (GFSK)
–	1 Msymbol/sec
–	NRZ data is filtered with BT=0.5 Gaussian filter
l	1 or 2 Mbit/sec with multilevel GFSK
–	1 Mbit/sec:	2 level GFSK      h2=0.34
–	2 Mbit/sec:	4 level GFSK      h4=0.45h2=0.15
–	Preamble and Header always at 1 Mbit/sec; Data at 1 or 2 Mbit/sec
GFSK vs GMSK comparison
l	GMSK is popular with cellular telephony
l	GMSK is a binary GFSK with h2=0.5
l	802.11 GFSK is h2=0.34 due to both
–	1 Mb/s objective
–	1 MHz 20 dB bandwidth 15.247 ruling
l	Results:
–	Sensitivity reduced by 4-5 dB
–	Increased first adjacent channel interference
802.11 FHSS Frame Format
l	PHY header indicates payload rate and length; CRC16 protected
l	Data is formatted to limit DC offset variations, for PLL based implementations
FHSS Modulation Specifications
l	Deviation:
–	Upper bounded by FCC requirements
–	Lower bounded to maintain sensitivity
l	Shape accuracy:
–	Zero crossing instants accuracy
–	Level accuracy
l	Center Frequency accuracy - 60 KHz
Receiver Performance
CCA- Clear Channel Assessment
l	CCA is used to:
–	Initiate frame reception
–	Avoid transmitting when the channel is busy
l	CCA Sensitivity:
–	-80 dBm for PT<20 dBm, reduced by 0.5 dB for each dB of power increase
–	Detection during 0101 pattern within 20 usec with 95% probability
Frequency Hopping Sequences
l	Design Criteria:
–	Assured minimum hop distance for multipath diversity performance
–	Minimizing hits and adjacent channel hits between different hopping patterns
–	Minimizing consecutive hits between different hopping patterns
l	FCC 15.247 requirement: Pseudorandomly ordered frequency list
Previous Hopping Sequences
l	Previous 802.11 hopping sequences were based on constant frequency increment (mod 79).
l	Had better worst case hit, adjacent hit and consecutive hit performance
l	Were rejected by FCC as insufficiently pseudorandom (due to constant frequency increment)
New Frequency Hopping Sequences
l	Passed FCC approval for BreezeNet
l	Predesigned computer generated pseudorandom list of 79 frequencies
l	Minimum hop distance of 6 channels
l	Additional hopping sequences derived from the base predesigned sequence.
l	78 hopping patterns organized in 3 sets of 26 patterns each.
New Frequency Hopping Sequences- cont.
l	Denote frequency as 2402+b[i], b[i] is the base sequence in range 0.. 78.
l	k-th sequence is formed from the base sequence as 2402+(b[i]+k) mod 79
l	Example:
–	Base seq: 	2402, 2456, 2472, 2447, ...
–	30-th seq:	2432, 2407, 2423, 2477, ...
Hopping - Multiple AP’s
l	Unspecified in the standard
l	System level optimization prefers network-wide hop synchronization
–	more efficient system - less collisions, less “air usage”
l	Folklore saya that FCC forbids this. If so, what is the rationale?
l	What is the truth?
Wish List
l	Synchronized hopping
l	Wider channels (Petition by Symbol Technology