802.11g

83180 Wireless LANs Page 1 of 24802.11g PHY

802.11gPhysical Layer

Hristo Brachkov

16-Feb-2005


Outline• Introduction• Extended Rate PHY• 802.11g Optional mechanisms

– PBCC– DSSS-OFDM

• Single carrier to multicarrier transition• Baseband implementation• 802.11(a, b, g) comparison• Standard co-existence• References


Introduction

• After IEEE 802.11a and IEEE 802.11b standards were approved, a work on higher data rate physical layer started.

• Two major competitive proposals:– Extension of PBCC– DSSS-OFDM, which uses preamble of IEEE 802.11b for

backwards compatibility

• IEEE 802.11g is a compromise - mandatory and optional mechanisms


Introduction (cont.)

• 802.11g is very similar to IEEE 802.11a, especially from PHY point of view but– IEEE 802.11a 5 GHz– IEEE 802.11g 2.4 GHz

• Very small differences between the mandatory PHY of 802.11g and 802.11a– SIFS for 802.11a is 16 us, whereas for 802.11g it is 10us– An 802.11g packet is followed by 6 us of silence

• Backwards-compatible with 802.11b – compatibility achieved at MAC layer


ERP(Extended Rate PHY)

• ERP builds on the payload data rates of 1 and 2 Mbit/s, as described in original IEEE 802.11 using DSSS modulation

• DSSS, CCK and optional PBCC modulation is used for building on the payload data rates of 1, 2, 5.5, and 11 Mbit/s, as described in 802.11b

• The ERP draws from IEEE 802.11a to provide additional payload data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s.

• Of these rates, transmission and reception capability for 1, 2, 5.5, 11, 6, 12, and 24 Mbit/s data rates is mandatory.

• Two optional ERP-PBCC modulation modes with payload data rates of 22 and 33 Mbit/s are defined.

• An optional modulation mode DSSS-OFDM is also incorporated with payload data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s.


ERP (cont.)• The 2.4 GHz ISM band is a shared medium, and coexistence with

other devices such as IEEE 802.11 and IEEE 802.11b standards is an important issue for maintaining high performance in ERP.

• The ERP modulations (ERP-OFDM, ERP-PBCC, and DSSS-OFDM) have been designed to coexist with existing IEEE 802.11 and IEEE802.11b.


ERPMandatory operational modes

• DSSS/CCK – uses the capabilities of 802.11b PHY with the following exceptions:– Support of short PLCP PPDU header format is mandatory

– Maximum signal input level is –20 dBm

– Locking the Tx central frequency and the symbol clock frequency to the same reference oscillator is mandatory

• OFDM – uses the capabilities of 802.11a PHY with the following exceptions– Frequency plan is according to 802.11b instead of 802.11a

– The frequency accuracy is 25 ppm

– Maximum input signal level is –20 dBm

– Time slot is 20 us as in 802.11b, except that an optional 9 us slot time might be used when BSS consists only of EPR standards

– SIFS (Short interframe space) time is 10 us in accordance with 802.11b


PBCC• In the PBCC encoder, incoming data are first encoded with a packet binary

convolutional code. A cover code (as defined in PBCC modes in 802.11b ) is applied to the encoded data prior to transmission through the channel.

• Achieves data rates of 22 and 33 Mb/s• PBCC-22

– uses 256 state binary code with rate of 2/3 and a cover sequence– The input bits are divided into adjacent bits

• PBCC-33 achieves the higher data rate by increasing the clock frequency by 50% from 11 MHz to 16.5 MHz only for the data portion of the packet.

22/33 Mbit/s ERP-PBCC convolutional encoder PBCC-22 and PBCC-33 cover code mapping


PBCC (cont.)• When the clock is switched from 11 MHz to 16.5 MHz, the clock

switching structure in the figure below is used.

PBCC 33 Mbit/s clock switching


DSSS-OFDM

• DSSS-OFDM - Hybrid modulation combining a DSSS preambule and header with an OFDM payload transmission

• As a result, for DSSS-OFDM, the PPDU format described in 802.11b is relatively unchanged. The major change is to the format of the PSDU.

• The 802.11b single carrier PSDU is replaced by a PSDU that is very similar to the PSDUs described in 802.11a.

• In addition, 802.11g specifies the radio and physical layer behavior of the transition from the Barker symbol-modulated preamble and the OFDM-modulated data for PSDU.


DSSS-OFDMPPDU Format

Long preamble PPDU format for DSSS-OFDM


DSSS-OFDMPPDU Format

Short preamble PPDU format for DSSS-OFDM


DSSS-OFDM PLCP PSDU Encoding process

DSSS-OFDM PSDU


Single carrier to multicarrier transition

• The single carrier signal segment of the packet shall have a coherent relationship with the multicarrier (OFDM) segment of the packet.

• All characteristics of the signal shall be transferable from one symbol to the next, even when transitioning to the OFDM segment.

• This enables high-performance, coherent receiver operation across the whole packet. This requirement is no different in nature than that stated in 802.11, 802.11a, and 802.11b. The distinction being that those clauses use a signalling scheme that is either just single carrier or just multicarrier. In contrast, for this mode, both single carrier and multicarrier signalling are used within the context of a single packet.


Single carrier to multicarrier transition

• The ideal transition would provide – a constant carrier frequency and phase, – constant power– constant spectrum– constant timing


Baseband Practical Implementation

Example of IEEE 802.11g implementation


802.11(a, b, g) comparison

Standards 802.11g 802.11b 802.11a

Data Rate Support 54, 48, 36, 24, 18, 12, 9, 6,11, 5.5, 2, 1 Mbps

11, 5.5, 2, 1 Mbps 54, 48, 36, 24, 18, 12, 9, 6 Mbps

Max. Data Rate 54 Mbps 11 Mbps 54 Mbps

Frequency Band 2.4 GHz (2.4 GHz to 2.4835 GHz) 2.4 GHz (2.4 GHz to 2.4835 GHz)

5 GHz (5.725 GHz to 5.850 GHz)

Channels 3 non-overlapping channels, up to 13 overlapping

3 non-overlapping channels, up to 13 overlapping

12 non-overlapping channels

Technique OFDM/CCK (6,9,12,18,24,36,48,54)OFDM (6,9,12,18,24,36,48,54)DQPSK/CCK (22, 33, 11, 5.5 Mbps)DQPSK (2 Mbps)DBPSK (1 Mbps)

DQPSK/CCK (11, 5.5 Mbps)DQPSK (2 Mbps)DBPSK (1 Mbps)

BPSK (6, 9 Mbps)QPSK (12, 18 Mbps)16-QAM (24, 36 Mbps)64-QAM (48, 54 Mbps)

Max. Range* Up to 1,000 ft Up to 1,000 ft Up to 500 ft

Backward Compatibility

802.11b N/A N/A

Features Replacement for 802.11b with higher data rate and better security

Most widely deployed today Ideal for high-density environments


802.11(a, b, g) comparison (cont)


802.11(a, b, g) comparison (cont)


802.11g/b coexistence

Maximum throughput for IEEE 802.11 environments


802.11g/b coexistence (cont.)• 802.11g-only• 802.11g AP, mixed clients• 802.11b AP, 802.11g client


802.11g/b coexistence (cont.)• Multiple 802.11g APs, mixed clients


References• [1] “Wireless LAN Medium Access Control (MAC) and Physical

Layer (PHY) specifications Amendment 4: Further Higher Data Rate Extension in the 2.4 GHz Band”, IEEE std 802.11g, 2003

• [2] T. Cooklev, ”Wireless Communications Standards, A Study of 802.11, 802.15, and 802.16”, IEEE Press, 2004

• [3] W. Carney, “IEEE 802.11g New Draft Standard Clarifies Future of Wireless LAN”, Texas Instruments White Paper, May 2002

• [4] http://www.54g.org

• [5] http://www.vocal.com


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