1. GCM Communications Technology
September 2001
Building the Global Link
Over 90Mbps IEEE applications using GCM
Vicente Díaz and Daniel Hernanz
GCM Communications Technology
Divino Vallés 2, 2-F
28805 Alcalá de Henares
Madrid (Spain) ,
V. Diaz, GCMcom Technology

2. Outline Introduction to GCM. Main characteristics. Why using GCM?
September 2001
Outline
Introduction to GCM.
Golay sequences properties.
How it works.
Main characteristics.
Aspects to certification.
Processing gain and jamming properties.
Preambles and throughput.
Hardware complexity.
Scalability and complexity.
Why using GCM?
Advantages.
Summary.
V. Diaz, GCMcom Technology

3. September 2001
Golay Code Modulation
Uses binary Golay complementary sequences to encode symbols.
For 11 Mbps:
One pair of sequences.
Length 16 (higher PG than 11 bits barker).
Same bandwidth (QASK modulation spectrum).
For 22 Mbps:
Two pairs of fully orthogonal 16 bits sequences.
V. Diaz, GCMcom Technology

4. September 2001
Golay sequences
A pair of, length N, Golay sequences (A,B) has an ideal aperiodic autocorrelation function:
Then, a pair of Golay sequences (A,B) are orthogonal themselves if they are, at least, a chip apart.
So if symbols overlap, the maximum rate will raise a symbol per chip.
Overlapping does not mean a high PMEPR or crest factor.
Using 2n length sequences, maximum PMEPR equals 2 [Popovic 1991].
V. Diaz, GCMcom Technology

5. GCM (Golay Code Modulation)
September 2001
GCM (Golay Code Modulation)
Example: GCM 1/16 (11Mbps)
16 chips (11 Mchip/s)
Data symbol
Data symbol
687..5Kbps
Transmission (spreading data)
1.454usec
687.5Kbps
Recepcion (correlation)
V. Diaz, GCMcom Technology

6. GCM (Golay Code Modulation)- Cont. Recepcion (correlation)
September 2001
GCM (Golay Code Modulation)- Cont.
Example: GCM 1/16 (11Mbps)
16 chips (11 Mchip/s)
Data symbol flux 1
Data symbol Flux 2
Data symbol flux 3
Data symbol Flux 16
687.5Kbps
Total: 11Mbps
Data symbol Flux 16
Transmission
687.5Kbps
Total: 11Mbps
Recepcion (correlation)
V. Diaz, GCMcom Technology

7. GCM (Golay Code Modulation)- Cont.
September 2001
GCM (Golay Code Modulation)- Cont.
OFDM is composed by N simultaneous fluxes with a rate R on each carrier so total Rate is NxR. So it is called...
Orthogonal Frequency Div. Multiplex.
GCM is composed by N simultaneous fluxes with rate R each, then total Rate is NxR. Example: (22Mbps GCM 2/16)
- 32 fluxes of 11/16(687.5Kbps)=22Mbps.
So GCM should be also called...
Orthogonal Time Div. Multiplexing
Correlation receiver
Transmission t + -
V. Diaz, GCMcom Technology

8. GCM as a correlation based communication technique
September 2001
GCM as a correlation based communication technique
LTI
System
Golay
sequences
generator
[n ]
N-QAM
MODULATOR
aN[n ]
bN[n ]
e[n ]
DEMOD.
r[n ]
Efficient
correlator
I[n ]
Q[n ] +
Rra[n ]
Rrb[n ]
[n-no]
LTI system with process delay n0
M-PAM
x[n ]
x[n- no]
Emitter
Receiver
Transmission path
noise r 
V. Diaz, GCMcom Technology

9. GCM as a correlation based communication technique (cont.)
September 2001
GCM as a correlation based communication technique (cont.)
Each symbol is encoded by means of sequences which aperiodic autocorrelation is an ideal Krönecker delta of value N.
Such a system is equivalent to a Pulse Amplitude Modulation system in which receiver noise standard deviation r is reduced by a factor of
As a result, Bit Error Rate reduces as follows:
Being M the number of levels per symbol, A the maximum symbol level, and  the standard deviation of input noise.
V. Diaz, GCMcom Technology

10. GCM processor components
September 2001
GCM processor components
GCM encoder
data
signal
QASK Tx
data
signal
GCM decoder
QASK Rx
V. Diaz, GCMcom Technology

11. Efficient Golay Generator [Popovic]
September 2001
GCM encoder
Same encoder for all rates (Mbps): 11, 22, 33, etc...
For N=16, 2log2N = 8 additions per symbol.
N-1 memory positions.
Efficient Golay Generator [Popovic]
The sequence a is sent on phase, and b is sent on quadrature.
V. Diaz, GCMcom Technology

12. Efficient Golay Correlator [Popovic]
September 2001
GCM decoder
Same decoder for all rates (Mbps): 11, 22, 33, etc...
For N=16, 4log2N = 16 additions per symbol.
2(N-1) memory positions.
Efficient Golay Correlator [Popovic]
V. Diaz, GCMcom Technology

13. Aspects to Certification
September 2001
Aspects to Certification
GCM should be certified under old and new rules
Transmission: The Power Spectrum
Reception: Robustness at the receiver
Noise
Processing Gain
Measured with respect to a reference
Interference Rejection
CW Jamming Margin
V. Diaz, GCMcom Technology

14. The Power Spectrum Similar Spectrum of actual DSSS options.
September 2001
The Power Spectrum
Similar Spectrum of actual DSSS options.
A General QASK spectrum with chip rate 11MHz.
V. Diaz, GCMcom Technology

15. In GCM, Information is Spread
September 2001
In GCM, Information is Spread
For sequences of length N, each symbol is influenced by 2N symbols.
Noise is averaged. S/N ratio is reduced.
Spreading should be 2N?
Data received
Transmitted symbols t
2N data
V. Diaz, GCMcom Technology

16. Robustness Reference for GCM: QPSK. Processing gain.
September 2001
Robustness
Reference for GCM: QPSK.
Processing gain.
Is defined as the difference between the SNR (Es/ No) required to achieve a threshold BER or PER with the reference scheme and the SNR (Es/ No) required to achieve the same threshold BER or PER when the signal is processed.
For 22Mbps, N=16: SNR (Es/ No)=-1.5dB for PER=10-2.
V. Diaz, GCMcom Technology

17. September 2001
P.G. Comparison
Scheme
Rate
Mod
Code
C. G.
R. G.
W.G.
P.G
(Mbps)
(dB)
(dB)
(dB)
(dB)
Barker-1 1
BPSK
Barker
0.00
13.40
2.60
16.00
Barker-2 2
QPSK
Barker
0.00
10.40
2.60
13.00
CCK-11 11
QPSK
CCK
2.00
3.01
2.60
7.61
PBCC-11 11
QPSK
64 state BCC
5.90
3.01
2.60
11.51
PBCC-22 22
8PSK
256 state BCC
8.10
1.76
2.60
12.46
GCM-22 22
QPSK
Golay 2/16
0.00
15.05
2.60
17.65
Processing Gain = Coding Gain + Rate/Spreading Gain Waveform/Spreading Gain
Scheme
Eb/No
Coding
PER = 10e-2
Gain
Uncoded QPSK
10.5
Uncoded 8PSK
13.8
Barker
0.0
CCK-11
8.5
2.0
PBCC-11
4.6
5.9
PBCC-22
5.7
8.1
GCM-22
-4.5
-4.5= (2 bits/symb)
V. Diaz, GCMcom Technology

18. Comparison 10.5-10log10(16)= -1.5dB x September 2001 Shannon
Shannon (r = 11)
Barker 1
Barker 2
CCK 11
PBCC 11
PBCC 22
GCM 22 (2/16) x
log10(16)= -1.5dB
log10(16)=-1.5dB
V. Diaz, GCMcom Technology

19. Continuous Wave margin
September 2001
Continuous Wave margin
22Mbps GCM 2/16 has better CW margin than Barker 1Mbps.
It has a higher spreading ratio: 16/11=1.6dB.
Theoretically, higher immunity to Bluetooth.
Further:
If g needs more immunity, uses longer sequences without reducing rate:
Example: N=32, (22Mbps GCM 2/32).
64 fluxes of 11/32 (343.75Kbps)=22Mbps.
CW=Continuous Wave
V. Diaz, GCMcom Technology

20. Preamble Structures: Long and Short Preambles identical
September 2001
Preamble Structures: Long and Short Preambles identical
HRb LONG PREAMBLE
PSDU SELECTABLE
GCM Symbols
11, 22, 33, 44, 55, 66, 77, 88, 99 Mbps
PREAMBLE/HEADER
192 usecs
Data Payload
HRb SHORT PREAMBLE
PREAM/HDR
72 BITS
@ 1 Mbps
PSDU SELECTABLE
GCM Symbols
11, 22, 33, 44, 55, 66, 77, 88, 99 Mbps
96 usecs
V. Diaz, GCMcom Technology

21. Preamble Structures: Ultra-short proposed preambles
September 2001
Preamble Structures: Ultra-short proposed preambles
HRb LONG PREAMBLE (identical 192bits)
PREAMBLE/HEADER
USING GCM 11MBPS
192 BITS
PSDU SELECTABLE
GCM Symbols
11, 22, 33, 44, 55, 66, 77, 88, 99 Mbps
17.4 usecs
Data Payload
HRb SHORT PREAMBLE (identical 72 bits)
PREAM/HDR
72 BITS
GCM 11 Mbps
PSDU SELECTABLE
GCM Symbols
11, 22, 33, 44, 55, 66, 77, 88, 99 Mbps
6.5 usecs
Higher immunity to noise. Easier synchronization
V. Diaz, GCMcom Technology

22. Throughput performance
September 2001
Throughput performance
Shorter preamble obviously means better throughput.
Without ultra-short preambles, the throughput:
100 bytes / 22-> 192usec + PSDU=36.3 usec -> 3.5Mbps.
1000 bytes /22-> 192usec + PSDU=363 usec -> 14.4Mbps.
2346 bytes/22 -> 192usec + PSDU=853 usec -> 18Mbps.
With ultra-short preambles, the throughput:
100 bytes / 22-> 6.5usec + PSDU=36.3 usec -> 18.7Mbps.
1000 bytes /22-> 6.5usec + PSDU=363 usec -> 21.6Mbps.
2346 bytes/22 -> 6.5usec + PSDU=853 usec -> 21.8Mbps.
Other cases:
100/99->54.9Mbps.
2346/99 -> 95.7 Mbps.
V. Diaz, GCMcom Technology

23. Hardware complexity of 22Mbps baseband processor
September 2001
Hardware complexity of 22Mbps baseband processor
RF stages:
Identical elements of IEEE b for all rates.
Baseband:
Encoder only needs 2log2N additions (No products).
Decoder only needs 4log2N additions (No products).
Total resources for a 22Mbps GCM 2/16 processor:
(8+16)*2*11=528M additions/sec.
45*2 memory positions.
V. Diaz, GCMcom Technology

24. Scalability/Complexity
September 2001
Scalability/Complexity
How to increase rate:
M levels instead of binary. Higher PEMPR but always lower than OFDM without coding.
Example: M=4, K=2 pairs, results 44Mbps. 528Madd/s.
K sequences instead of two.
Example: K=3, binary M=2, results 33Mbps. 792Madd/s.
Combination of both.
Example: K=3, M=4, results 66Mbps. 792Madd/s.
Then:
M=8, K=3, results in 99Mbps. 792Madd/s.
Encoder/decoder complexity increases only with K.
V. Diaz, GCMcom Technology

25. Why Using GCM? Spectral efficiency over 4.5bps/Hz.
September 2001
Why Using GCM?
Spectral efficiency over 4.5bps/Hz.
Easier scalability: Ready for future needs.
Lower complexity:
Lower implementation costs.
Lower consumption.
Higher immunity to noise:
Higher range...Or lower emission power: Healthy.
Better performance in Bluetooth environments.
Three bands in US 2.4GHz. band.
V. Diaz, GCMcom Technology

26. GCM offers many advantages
September 2001
GCM offers many advantages
Additive Gaussian noise.
Does not need from FEC to improve performance.
Better performance than Barker 1.
It has unlimited spreading gain without reducing rate.
Multipath distortion.
It has, at least, the same advantages and limitations 1Mbps technique has. It uses the same chip rate.
Bandwidth.
Same bandwidth of actual applications.
Chip rate can be reduced without reducing rate, improving multipath distortion and reducing bandwidth.
V. Diaz, GCMcom Technology

27. September 2001
Summary
Golay Code Modulation is possibly the next future option to OFDM because it has fundamental characteristics of:
Simplicity.
Spectral efficiency.
Scalability.
Lower implementation costs.
Future possibilities:
Increasing number of bands by means of reducing chip rate, but maintaining transmission rates.
Increasing range in high rate modes.
GCM Communications Technology is working to develop a high rate baseband processor prototype aiming to demonstrate a future 100Mbps wireless Ethernet equivalent.
V. Diaz, GCMcom Technology