WN Blog 004 – Modulation & Encoding Techniques

Hey! In this short blog post we’ll share our notes about older modulation techniques used in WiFi (up to OFDM and 256 QAM).

To learn more about OFDMA and 1024 QAM (both used in WiFi 6), see our other blog post about 802.11ax here: https://wifininjas.net/index.php/2019/07/03/wn-blog-003-wifi-6-deep-dive-real-world-testing/

Modulation Summary

• How to use wireless waves’ physical attributes like amplitude and phase to represent binary data and send it over a wireless medium
• 802.11 Tx signals must use physical filters to limit the signals sent away from the main target frequency:

• Radio and attached antenna is called radio chain
• Possible ways of modulating the waveform:
  • Amplitude – uses strength of the signal to modulate it with power (AM)
  • Phase – timing of the signal between peaks
  • Frequency – how often the signal repeats in one second (Hz, MHz, GHz)

DSSS Chip Sequence

• DSSS – Direct-Sequence Spread Spectrum
• Each bit is transformed into sequence, called ‘chip’ or ‘symbol’
• DSSS sends a single data bit as a string of chips
• Barker 11 was one of the first encoding methods
  • 1=01001000111
  • 0=10110111000
  • Sending ‘1’ or ‘0’ is represented by 11 ‘chips’
  • 11 ‘chips’ is called a ‘symbol’
  • By expanding original data in this way; with some signal lost due to noise; the original data can still be understood
  • Each bit takes 2MHz of frequency width, sending 11 in parallel takes 22 MHz
  • Supports 1, 2, 5.5 and 11 Mbps data rates
• Round at the top in spectrum analyzer

DBPSK / DQPSK Modulation

• DBPSK – Differential Binary Phase Phase Shift Keying
  • alter the signal phase by 180 degrees
  • max 1Mbps
  • ‘0’ – no change in direction
  • ‘1’ – change in direction

• DQPSK – Differential Quadrature Phase Phase Shift Keying
  • alter the signal phase by 90 degrees in four quadrants
  • max 2Mbps
  • Each ‘chip’ represents 2 bits instead of one as in PSK (hence twice as quick)

CCK Encoding

• Complimentary Code Keying
• Code word (wave shape) represents a symbol
• 2 bits used to check consistency
• Still uses one 22MHz wide wave
• CCK 4 – coding 4 bits per symbol (in a 6-bit chip) gave 5.5 Mbps
• CCK 8 – coding 8 bits per symbol (in a 6-bit chip) gave 11 Mbps
• CCK loses some of the extra bits used by Barker code to recover information received in a noisy or low SNR environment
  • CCK achieves faster data rates at the expense of requiring a stronger, less noisy signal

OFDM

• Orthogonal Frequency-Division Multiplexing
• Alternative to DSSS
• Square at the top in spectrum analyzer
• Instead of sending one wide combo-channel transmission, frequencies are broken up into sub-channels (called sub-carriers or tones)
• Using a 20 MHz wide spectrum, each sub-carrier is 312.5 KHz apart from one another
  • 64 waves (tones / sub-carriers), 312.5 KHz apart
    • 12 ‘Guard’ subcarriers are – used to help set one channel apart from another and to help receivers lock onto the channel
      • 6 head and 5 trail have no power / not used (isolate neighbouring channels)
      • 1 in the middle have no power / not used (identify centre of the signal)
    • 4 ‘Pilot’ subcarriers  – equally spaced and always transmitted to help receivers evaluate the noise state of the channel
    • 48 ‘Data’ subcarriers – devoted to carrying data
      • Each data subcarrier can ‘carry’ 1125 kbps
• To differentiate between the small sub-carriers, adjacent sub-carriers will have 90 degree changed polarization
• Data is sent simultaneously over subcarriers in parallel
• Some of the subcarriers are used to protect/guard against interference and control the signal as a whole
• Inside OFDM, each tone can use BPSK or QPSK
  • BPSK – allows 250 kbps per tone (250 x 48 = 12 000 kbps = 12 Mbps per OFDM wave)
  • QPSK – allows 500 kbps per tone (500 x 48 = 24 000 kbps = 24 Mbps per OFDM wave)
  • No Barker 11 coding used, repeating allowed
  • Depending on the percentage of repeated signals, total throughput is:
    • BPSK – 9 Mbps (25% repeat) or 6 Mbps (50% repeat)
    • QPSK – 18 Mbps (25% repeat) or 9 Mbps (50% repeat)
• Used in 802.11 a/g/n/ac

QAM – Quadrature Amplitude Modulation (freaking brilliant!)

• To go faster than 18 Mbps, OFDM uses another technique, called QAM
• Uses 90 degrees phase shifts like QPSK, but now combines that with Amplitude Modulation (or changing the amplitude) to accomplish more possible differentiations of simultaneous transmission
• So then with the 4 ‘quads’, depending on how many differing amplitudes are varied will determine how many signals can be sent and therefore how much additional data can be realized
• 16-QAM (4^2) used in 802.11a/g/n
• 64-QAM (4^3) used in 802.11a/g/n
• 256-QAM (4^4) used in 802.11ac
• EVM (Error Vector Magnitude) used for improved reliability
  • if you miss the spot, vector length from the centre of expected spot is measured and used to decide what you wanted to hit
• Denser QAM requires better (quieter) environment, noise can prevent the signal from being understood
• With more noise introduced, signal will have to downgrade
• With more advanced QAM modulation (16 > 64 > 256 > 1024), it is more and more challenging to ‘hit’ the right spot to code the right data, therefore more SNR is required to decode it correctly

See some graphical representations of QAM below!

Now, try to imagine how much more complexity would using 1024-QAM modulation bring!