Transcript presentation source

DTTB Transmitter
Ratings
This presentation seeks to explain the relationship to PAL analogue
Transmitters and provides the base for the ratings of
DTTB Transmitters.
Compiled by Wayne Dickson
SMIREE MIEAust. CPEng. Member SMPTE
25 July 1998
WTD
1
DTTB Transmitter
Ratings
LINEAR CONSIDERATIONS
25 July 1998
WTD
2
COFDM
Amplitude Distribution
( CDF = 99.95%)
Peak voltage =2.8 (9 dB) - COFDM
( for CDF = 95% peak voltage = 1.7 (4.7dB) )
( “CDF” - Cumultive Distribution Function )
RMS voltage = 1 (0dB)
9 dB peak to average
DIGITAL TV
25 July 1998
WTD
3
DTTB Peak Amplitude
Distribution
Peak Amplitude Distribution
for COFDM and 8VSB DTTB Systems
10
0.01% 9.63 dB
9
8
0.01% 7.14 dB
7
COFDM
6
Peak to Average
5
ratio (dB)
4
Peak
3
sinewave
2
Noise
8VSB
Measurements by HP VSA
Extrapolation
1
Average
(RMS voltage)
0
0
2
4
6
8
10
12
14
16
18
20
Probability of occurrence (100-CDF) %
25 July 1998
WTD
4
DTTB Peak Amplitude
Distribution
Peak Amplitude Distribution
for COFDM and 8VSB DTTB Systems
(Expanded)
10
0.01% 9.63 dB
9
COFDM
8
7
Peak to Average
ratio (dB)
Peak
sinewave
6
8VSB
5
4
3
2
99.95 % CDF
1
Average
(RMS voltage)
Noise
0.01% 7.14 dB
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Probability of occurrence (100-CDF) %
25 July 1998
WTD
5
PAL-DTTB
Relative Levels
Relative Power Levels for
PAL/DTTB Ratio = - 6 dB
(in dB)
Peak
Peak 3dB
Average Power 0dB(Peak Sync.)
(if continuous) -2.15dB (Black + Sync.)
6 dB
9 dB
0 dB
Difference
Analogue PAL
25 July 1998
COFDM
Average Power
DTTB
WTD
6
PAL-DTTB
Relative Levels
Relative Power Levels for
PAL/DTTB Ratio = - 6 dB
(in KW)
Peak 20KW
(Instantaneous)
Average Power10KW
(if continuous) 6KW
20KW Peak
(Instantaneous)
(Peak Sync.)
(Black + Sync.)
6 dB
9 dB
0 dB
Difference
Analogue PAL
25 July 1998
COFDM
Average
2.5KW Power
DTTB
WTD
7
DTTB Rating
of a PAL Transmitter
• To handle a COFDM signal with a 9 dB peak
to average character, a PAL transmitter needs
to be derated by :
• 6 dB
• That is, a 10 KW peak sync transmitter is
capable of 2.5 KW COFDM power.
– Provided linearity is adequate.
– and other parameters are adequate such as :
25 July 1998
» amplitude and group delay response
» LO phase noise
» noise level
WTD
8
DTTB Transmitter
Ratings
NON - LINEAR CONSIDERATIONS
25 July 1998
WTD
9
DTTB Transmitter
Non - linear considerations
• The non - linear performance is shown by the
intermodulation character displayed in the
transmitted spectrum.
• The required intermodulation performance is
influenced by the multipath performance of
the consumer’s receivers.
• The performance of a receiver is influenced
by the modulation type eg :
– whether 64QAM, 16QAM or QPSK
– and whether FEC is 7/8, 5/6, 3/4, 2/3, or 1/2
25 July 1998
WTD
10
DTTB Receiver
Multipath Performance
• An example of a receiver’s performance
operating with modulation of 64QAM and a
FEC of 2/3 follows. Differences will occur
from :
•
•
•
•
25 July 1998
other receiver implementations
complex static multipath
dynamic multipath eg flutter
impulse noise
WTD
11
DTTB System Multipath
Performance
(Conditions: Static simple multipath, No Co-channel or impulse interference)
C/N Threshold (dB)
Indoor Antennas
35
COFDM
(64QAM,2/3,1/8)
(Nov. 1997)
25
COFDM
Current implementations
(April 1998)
Picture
(above
curve)
19
0
25 July 1998
Outdoor Antennas
3
15
Multipath Level ( - dB)
WTD
30
No Picture
(Below
curve)
12
W.T.Dickson 16 April 98
DTTB System Multipath
Performance under typical
reception conditions
C/N Threshold (dB)
Indoor Antennas
35
COFDM
(64QAM,2/3,1/8)
Complex multipath
(approximate median)
25
19
3
15
Multipath Level ( - dB)
WTD
Picture
(above
curve)
No Picture
(Below
curve)
Simple multipath
0
25 July 1998
Outdoor Antennas
30
13
W.T.Dickson 16 April 98
DTTB COFDM
Decoder Threshold C/N
COFDM THRESHOLD C/N
( d B )
FACTS DTTB Trial Sites
30
29
C / N
28
COFDM (64QAM,2/3,1/8)
27
T h r e s h o ld
26
25
24
23
22
21
20
19
18
17
HP VSA not available
16
15
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
TEST #
System Noise Method
HP VSA Measurement
25 July 1998
WTD
W.TY.D. 15 Jan 1998
14
W.T.Dickson 16 April 98
DTTB Transmitter
Performance requirements
• By knowing the worse threshold C/N required
by the receiver, the required transmission C/N
may be derived.
• The transmission C/N will be determined by the
combination of :
– the noise floor at the transmitted power level
– the intermodulation at the transmitted power level
25 July 1998
WTD
15
DTTB Transmission
C/N requirements
• By observing the previous examples, of a receiver
operating in a typical to a worse reception
condition, the required C/N by the receiver is
varies from approximately 20 dB to 35 dB.
• Hence the transmitted C/N has to be such that the
reception conditions determine the decoding
performance, not the transmission conditions.
• The following plot will be used to derive the
required transmission C/N requirements.
25 July 1998
WTD
16
Transmitter C/N interaction
with decoder threshold C/N
The influence of Transmission C/N
on Decoder C/N
3
Influence on decoder C/N (dB)
2.5
2
1.5
Hence Transmission C/N should be more than 6 dB below
Hence
transmission C/N should be more than 6 dB below
Decoder C/N for less than 1 dB degrading of Decoder C/N
decoder C/N for less than 1 dB degrading of decoder C/N
1
0.2dB influence results
from a 13dB difference.
0.5
0
0
-5
-10
-15
-20
Level of Transmission C/N relative to Decoder C/N (dB)
25 July 1998
WTD
17
The required DTTB
Transmitter C/N
• Considering :
– (A) @ 35dB required decode C/N, an influence of 1dB by
the Transmitted C/N is acceptable. (as such high C/N will not be
common the high 1dB influence maybe acceptable)
– (B) @ 25dB required decode C/N, an influence of 0.2dB by
the Transmitted C/N is acceptable. (as a 25dB decoder C/N
requirement will be potentially common a small influence is demanded)
• The Transmitted C/N needs to be :
– for (A) 41dB
for (B) 38dB
– Hence 41dB Transmitted C/N appears to be indicated.
25 July 1998
WTD
18
Transmitter C/N
versus
Spectrum regrowth
• Although the spectrum regrowth is a reflection of what
is occurring within the band of data, there is a 1 to 2dB
higher level of “N” within the data area. Hence the
regrowth level should be allowed to be 2dB higher than
the required Transmitted C/N.
• Aim for a Transmitted C/N of 43 dB.
• A Transmission C/N of 22 dB is likely to ensure failure
of all decoders !
• These requirements are for 64QAM @ FEC of 2/3.
25 July 1998
WTD
19
DTTB Transmission
Spectrum
Spectrum Transmission
regrowth
C/N
level
Commonly called :
“spectrum regrowth”
or “spectrum spread”.
COFDM
Intermod.
Noise
Notes :
1.
2.
3.
4.
5.
Intermod. level increases at double the rate the input level is increasing.
Noise level increases at the same rate the input level is increasing.
As COFDM is the same as noise, the display of C/N is independent of the resolution B/W.
The shape of the sidebands is influenced by the transmitter’s response.
Either noise or intermod. may dominate.
25 July 1998
WTD
20
DTTB Transmission
Implementations
• Spectrum “regrowth” or “spectrum spread” is a mirror
into what is happening within the modulated spectrum.
• Although filtering of the spectrum spread is required for
the control of adjacent channel interference, such filtering
does not change the the level of intermodulation or noise.
• Implementations must ensure that under conditions of
maintenance or partial failure that the maximum
allowable Transmission C/N of 43 dB is not exceeded.
25 July 1998
WTD
21
COFDM - PAL
Spectrum Analyser Display
Spectrum Analyser display :
Vision Carrier
Dependent upon
Resolution B/W
setting.
eg. If Res. B/W = 300KHz
“A”= D + 10Log (6.6 / 0.3)
= 23.5 dB
Chroma
DTTB
PAL
(COFDM)
Note : It is usually less than this value as
D
resolution B/W shape collects more power
than the ideal rectangular filter. When Res.
B/W approaches or is less than the
separation between the carriers of COFDM :
A = D + 10Log (No. of Carriers)
25 July 1998
Sound Carriers
A
= DTTB to PAL ratio ( eg D = 10 dB )
DTTB power = average heating power
PAL power = the equivalent CW power of
peak sync Vision Carrier power
WTD
22
DTTB Transmission
Spectrum Mask Requirements
PAL ADJACENT CHANNEL CONSIDERATIONS
25 July 1998
WTD
23
DTTB to PAL
Adjacent Channel requirements
• DTTB power and out of band levels will impact
upon the interference into the PAL lower and
upper adjacent channels.
• The required Transmission C/N of 43 dB for
proper operation of DTTB will provide some
inherent protection of the PAL service if it is
guaranteed.
• Deduction from the plot following provides
some worse case figures.
25 July 1998
WTD
24
DTTB to PAL
Interference
COFDM into PAL Protection Ratio
50
Protection ratio (dB)
Ratio D/U (dB)
Protection
45
40
35
Vision
(3 dB)
30
25
Sound
(8 dB)
Co-channel
(45 dB)
20
SCM40 Av
SCM40
Avg
SCM40 max
SCM40
Max
LOP-10dB
LOP-10
dB
15
10
5
0
-5
-10
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
DTTB Frequency Offset (MHz)
3
4
5
6
7
DTTB Frequency Offset (MHz)
25 July 1998
WTD
25
8
DTTB Power Levels
• Without considering spectrum spread from the
DTTB, the power which may exist between DTTB
and PAL without causing interference is :
– for adjacent channel operation
• limited by sound in lower adjacent, DTTB 8 dB below PAL
• limited by vision in upper adjacent, DTTB 3 dB below PAL
– for co - channel operation
• limited by vision, DTTB 45 dB below PAL
25 July 1998
WTD
26
DTTB Power Levels
• Allowing for a variation of +/- 2 dB in the DTTB to
PAL ratio (>4 Km from the two tower transmissions
in Sydney), and combining with the previous -8 dB
restriction, a -10 dB DTTB to PAL ratio can be
allowed without interference into PAL.
• This is cautious worse case analysis with the
information currently available.
• Note that the sound of the lower channel is the
restricting criteria. If the vision was the limiting
factor a -5 dB DTTB to PAL ratio could be allowed.
25 July 1998
WTD
27
DTTB Side Bands
• The side bands generated from the spectrum spread or
regrowth (from noise or intermod.) may be limited by the “Co
- channel” restrictions imposed upon the DTTB to PAL ratio.
• As shown the co - channel DTTB to PAL ratio without causing
interference is - 45 dB.(flat spectrum across PAL)
• As the side bands only need to be down this far, the DTTB to
PAL ratio can be deducted from this figure to arrive at the
Transmission C/N which will produce PAL interference.
• Hence with a DTTB to PAL ratio of 10 dB, the side band level
may be - 35 dB. ( ie 35 dB Transmission C/N)
• Hence the side band levels are controlled by the DTTB
decoding requirements of 43 dB Transmission C/N.
25 July 1998
WTD
28
Braodcast Spectrum
Vision Carrier
(peak sync)
DTTB to PAL
eg -10 dB
0 dB
Carrier only
no modulation
Sound Carriers
-13 dB
Chroma
PAL
(Lower)
25 July 1998
DTTB
COFDM
WTD
-20 dB
PAL
(Upper)
29
DTTB Spectrum
Mask Base
• Based upon DCA Comms. Laboratory subjective testing using
the SCM40 method for vision interference levels and a
deduction from the LOP method for sound interference levels.
• Adjacent channel operation with matching coverage patterns of
DTTB and PAL, transmitted from the same area.
• DTTB to PAL ratio of -10 dB required for adjacent channel
protection of PAL from DTTB.
• The DTTB side bands are required to be 45 dB below PAL
vision to protect PAL.
• The mask is conservative allowing for the introduction of
DTTB into a 100 % PAL market.
25 July 1998
WTD
30
DTTB Spectrum
Mask
P- 42 dBm/4KHz
(P-10-10LogB/W)
-3.5 MHz
@ D = - 10 dB & P = 10 KW
+3.5 MHz
28 dBm/4KHz
43 dB *
P- 77 dBm/4KHz
Intermod.
- 7 dBm/4KHz
(P-10LogB/W- 45 dB)
10.5 MHz
-10.5 MHz
(- 15 dBm/4KHz)
* dependent upon modulation parameters
D = DTTB to PAL ratio (DTTB average power to equivalent peak sync average power) = - 10 dB
B/W = -1dB Bandwidth of COFDM = 6.6 MHz
25 July 1998
WTD
P = 10 KW (70 dBm)
31
DTTB Spectrum
Mask
DTTB Spectrum Mask
PAL “peak sync” vision carrier power
PAL "peak sync" vision carrier power
0
Spectral Density (dB/4 KHz)
Spectral Density (dB/4 kHz)
-10
DTTB “single” carrier power
DTTB "single" carrier power
-20
-30
-40
-50
-60
-70
-80
-90
-100
-12
-10
-8
-6
-4
-2
0
2
4
6
8
10
Frequency
relative
to to
centre
of DTTB
channel (MHz)
Frequency
relative
centre DTTB
channel (MHz)
25 July 1998
WTD
32
12