|
<
Digital Modulation
A brief overview of digital modulation techniques.
A brief overview of the Coded Orthogonal
Frequency Division Multiplex (COFDM) modulation, currently offered by Gigawave:
Thanks to the unique modular design of all Gigawave digital microwave
links the 'plug-in' encoder and modulator modules can easily be changed
on-site, or upgraded as new modulation techniques evolve. DVB-T COFDM
COFDM has been chosen for both Digital Video Broadcasting – Terrestrial
(DVB-T) and for Digital Audio Broadcasting (DAB) because of its tolerance
to multipath, in fact multipath can even enhance the overall performance
of the received signal. It is because of these properties that COFDM
is being exploited for other applications like wireless cameras and mobile
vision links.
COFDM spreads the digital information over a large number of orthogonal
carriers, using Frequency Division Multiplex. Complex Forward Error Correction
Coding (the ‘C’ in COFDM) is then applied.
DVB-T and DAB are accepted ETSI standards for use in Europe and the
rest of the world.
The terrestrial digital standard is referred
to as DVB-T. There are similar standards
for satellite distribution (DVB-S),
and for cable distribution (DVB-C).
DVB-T COFDM
is available in 2000(2k) and 8000(8k) carrier versions
| Delay with Frequency Interleaving (as DVB-T) |
5ms approx |
| Delay if time interleaving was added (not available
on DVB-T) |
0.5 secs approx |
DVB-T Multiplexer
(64QAM, 2/3 FEC, 1/32 symbol) |
24 Mb/s
(6 - 8 Chs)
|
Scaleable options:-
Variable Payload
(See - Table of DVB-T non-hierarchical bit rates)
| Maximum payload (64 QAM, 7/8 FEC, 1/32 symbol) |
31.67 Mb/s |
| Selectable modulation options |
|
| Selectable FEC |
|
| Selectable guard bands |
|
| Output Bandwidth |
7.61 MHz |
| Channel spacing |
8 MHz |
8-VSB Vestigial Sideband
This is the standard terrestrial digital television
distribution system adopted in USA. 8-VSB is not considered to be as good
as COFDM for use in radio camera and mobile links.
Types of modulation:
| Modulation |
Bandwidth ratio
(bits/Hz) |
C/N for threshold |
| Analogue FM |
0.5 (Hz/Hz) |
-70 |
| PSK |
0.833 |
-90 |
| QPSK |
1.66 |
-90 |
| 16 QAM |
3.33 |
-84 |
| 64 QAM |
5.00 |
-78 |
| 256 QAM |
6.66 |
-72 |
Forward Error Correction (FEC)
Normally takes the form of: 1/2, 2/3, 3/4, 5/6,
7/8.
The net payload of:
6Mbits with 3/4 FEC, requires a gross payload
capacity of 8 Mb/s
6Mbits with 1/2 FEC, requires a gross payload
capacity of 12Mb/s
Typical 'Link Budget' Calculation
Received signal = Tx power + Antenna ‘Gains' – Path Loss (all in dBs)
P = 20 log |
|
or D = 10 P/20 x |
|
Where:
P = Path Loss in dBs
F = the Frequency in MHz
D = the Path Length in meters
| |
Doubling
the power = +3dB |
| |
Doubling
the size/elements of TX or RX antenna = +3dB |
| |
Doubling
the diameter of antenna dish = +6dB |
| |
Doubling
the range = -6dB |
Example
1*:
D-CAM Radio Camera
2.5GHz, 6 Mbits MPEG, 1/2
FEC Omni to omni over 1.6Kms (one mile).
| 100 mW TX output power |
+20dBM |
| Tx omni 3dB |
+3 |
| Rx omni 3dB |
+3 |
| Clear path attenuation 1 mile @ 2.5GHz |
-104 |
| Received signal |
-78 |
| Threshold |
-92 |
| Threshold margin |
+14dB |
Example 2*:
D-CAM Radio Camera
2.5GHz, 6 Mbits MPEG, 1/2 FEC Omni to 20dB helix
over 40Kms (25 miles).
| 100 mw TX output power |
+20dBM |
| Tx antenna - 3dB omni |
+3 |
| Rx antenna - 20dB helix |
+20 |
| Clear path attenuation 40Kms
@ 2.5GHz |
-132 |
| Received signal |
-89 |
| Threshold |
-92 |
| Threshold margin |
+3dB
|
*These distances
has been verified under actual operational conditions.
Technical information on COFDM
A Guide to Digital Microwave Technology:
Digital Compression Techniques
Digital Modulation Techniques
Digital RF Transmission
A Glossary of Terms |
