802.11g

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802.11gPhysical Layer

Hristo Brachkov

16-Feb-2005

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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

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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

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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

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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.

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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.

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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

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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

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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

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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.

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DSSS-OFDMPPDU Format

Long preamble PPDU format for DSSS-OFDM

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DSSS-OFDMPPDU Format

Short preamble PPDU format for DSSS-OFDM

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DSSS-OFDM PLCP PSDU Encoding process

DSSS-OFDM PSDU

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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.

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Single carrier to multicarrier transition

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

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Baseband Practical Implementation

Example of IEEE 802.11g implementation

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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

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

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

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802.11g/b coexistence

Maximum throughput for IEEE 802.11 environments

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

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

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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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Questions & Comments