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Keysight Technologies
Addressing the Challenges of
Deploying Single Frequency
Networks DVB-T & DVB-T2
Application Note
Contents
Introduction................................................................................................ 3
SFN Echo Scenarios ................................................................................. 4
Post-echo scenario ............................................................................. 4
Pre-echo scenario ............................................................................... 4
Pre-echoes and post-echoes ............................................................ 5
Echo mask requirements ................................................................... 5
SFN Measurements.................................................................................. 6
Echo measurement with delay within the GI ................................. 7
Extending the echo delay measurement range ............................. 7
Keysight's SFN Measurement Solution ................................................ 8
X-Series signal analyzers and the N/W6153A DVB-T/H/T2
measurement application .................................................................. 8
Test configuration and system parameters.................................. 10
Post-echo scenario ...................................................................... 10
Pre-echo scenario ........................................................................ 11
Pre-echo and post-echo scenario ............................................. 12
Scenario with long delay echoes .............................................. 13
SFN measurements for DVB-T2 MISO systems .......................... 15
Post-echo scenario ...................................................................... 16
Pre-echo scenario ........................................................................ 17
Conclusion................................................................................................ 18
References ............................................................................................... 18
2
Introduction
Single Frequency Network (SFN) television broadcasting systems have been
widely deployed, especially for OFDM based systems like DVB-T and DVB-T2
[1][2], DTMB, and ISDB-T, due to their high frequency efficiency and simple
frequency planning. However, building a successful SFN system is challenging
because it requires precise transmitter deployment in order to ensure good
coverage.
In an SFN, two or more transmitters radiate the same signal on the same
frequency simultaneously and repeaters are used to ensure good signal cover-
age. The receiver is thus likely to receive multiple echoes from many of these
transmitters. Echo scenarios in SFN systems in the real world can generally be
categorized as one of the following: post-echoes, pre-echoes, pre-echoes and
post-echoes.
For an OFDM system, a guard interval (GI) is inserted between symbols to
prevent inter-symbol-interference (ISI). As long as the maximum delay of the
echoes is within the GI, there should theoretically be no ISI. The principle
behind SFN deployment is that the echoes from different transmitters must lie
within the GI and echoes outside the GI must be low enough to avoid detection
so that no interference is received.
For SFN system optimization, it is necessary to distinguish problems that come
from the receiver from ones that come from the network itself. Some receiver
test specifications, such as MBRAI/NorDig [3-5], define the scenarios, including
echo power, echo delay, and named echo mask, under which all receivers must
achieve Quasi Error Free (QEF) reception with a sufficient Signal to Noise Ratio
(SNR). Otherwise, reception cannot be guaranteed.
BER measurements are always important for transmitter test and operators also
like to use this metric to evaluate the signal quality of the network. However, the
BER metric is not very valuable for network optimization because the achievable
BER depends strongly on the receiver's performance. It is more important for
operators to be aware of the network characteristics that can significantly affect
the ability of the receiver to recover the desired data. These characteristics
include the echo power ratio and the delay spread and it is important to perform
measurements at different geographic locations in order to guarantee good
reception. With the help of these network metrics, operators can adjust the
transmission parameters of the transmitters in the system in order to achieve
optimal signal coverage.
In addition to ISI, there can be both constructive and destructive interference
among the received echoes, resulting in fading and leading to self-interference.
In an SFN, signals with close power strength from two transmitters may
exhibit a significant power loss at the receiver because of potential destructive
interference. To mitigate such a problem, the DVB-T2 standard has adopted the
well-known Alamouti Multiple Input Single Output (MISO) [6] technology. In a
DVB-T2 MISO SFN system, network optimization is required, just as it is for a
SISO system.
3
SFN Echo Scenarios
Post-echo scenario
The transmission scenario and the channel impulse response for post-echoes
are shown in Figure 1. In the post-echo scenario, the strongest path (main path)
is from the nearest transmitter tower and the other paths are either from trans-
mitters located further away or from reflections which arrive after the main path
and have lower power levels. In Figure 1, just one echo is shown.
Figure 1. Post-echo scenario and the associated channel impulse response.
Pre-echo scenario
The transmission scenario and channel impulse response for pre-echoes are
given in Figure 2. Pre-echoes often occur when there are repeaters nearby. For
the pre-echo scenario, the strongest path is re-transmitted from the repeater
and doesn't arrive first. The LOS (line of sight) path arrives first but has a lower
power level.
Figure 2. Pre-echo scenario and the associated channel impulse response.
4
Pre-echoes and post-echoes
A pre-echo and post-echo scenario occurs when both a pre-echo and a post-
echo exist in the transmission channel. The transmission scenario and channel
impulse response are shown in Figure 3. The strongest path is re-transmitted
from the repeater. The LOS signal arrives first but it has a lower power level and
the paths that arrive later may be from reflections.
Figure 3. Pre-echo and post-echo scenario and the associated channel impulse response.
Echo mask requirements
According to the test specifications like MBRAI[3] for DVB-T, as long as the
requirements of the echo mask can be met, all the receivers should achieve QEF
reception. Figure 4 is an example of the echo mask defined in MBRAI[3]. The
receiver must use equalization to overcome the echoes. The performance of the
equalizer is mainly affected by two factors: echo delay and echo power. So it is
critical for measurement instruments to provide the echo power and echo delay
accurately during network deployment in order to guarantee good coverage.
Figure 4. Example of echo mask as defined in MBRAI.
5
SFN Measurements
In SFN systems, the channel impulse response h(n) can be expressed as
L -1
h ( n)
= ( n - ) ,
i i
0 n N -1
i =0
(1)
L = the number of echoes
i = the complex gain
i = the delay of the ith echo
The echoes in the channel impulse response will result in overlapping OFDM
symbols and subsequent ISI, which may cause degradation in reception.
Therefore, measurement instruments need to be able to detect the echoes and
provide gain and delay for each detected echo.
As mentioned previously, in OFDM systems, a GI is inserted in order to prevent
ISI. As long as the maximum delay is within the GI, theoretically there is no ISI.
In order to get accurate echo patterns, an SFN analyzer should not only find the
best FFT window position to do demodulation and analysis, but it should also
provide the flexibility to adjust the FFT window to see the effect using different
FFT window positions.
Figure 5 shows a scenario with one pre-echo and one post-echo and demon-
strates how to adjust FFT window position. The FFT start position, as shown in
Figure 5, is set as the starting point of the FFT window. In order to measure the
echo pattern accurately, the FFT window should be set to ensure that minimal
ISI is introduced and to include as many echoes as possible. "No ISI range"
means that if the FFT start position is set in this range, there is no overlap with
the preceding and subsequent symbols of the various echoes.
Figure 5. FFT window selection.
6
Echo measurement with delay within the GI
In OFDM systems, pre-defined comb-type pilot carriers are usually used to
estimate and track the channel status. By using these pilot carriers for channel
estimation, and with a carefully designed filter, we can detect all echoes accu-
rately for various scenarios when the maximum echo delay is within the GI.
Extending the echo delay measurement range
Echoes in an SFN system may not conform to the echo masks defined by
MBRAI/NorDig without optimization. The capability to detect echoes that vio-
late the masks is therefore essential. Keysight Technologies Inc. measurement
instruments and software offer the high degree of capability required to easily
meet this advanced measurement challenge by decoding the data to provide
accurate reference signals for channel estimation.
7
Keysight's SFN Measurement Solution
X-Series signal analyzers and the N/W6153A DVB-T/H/T2
measurement application
X-Series signal analyzers range from the high-performance (PXA), to the
mid-range (MXA), the economy-class (EXA), and the low-cost (CXA). Power
measurements like channel power, ACP (adjacent channel power), spectrum
emission mask and modulation accuracy tests can be supported with all of
these instruments. With X-Series signal analyzers, you can make measurements
that are compliant with digital video broadcasting standards such as CMMB,
DTMB, DVB-T/H/T2, ISDB-T/Tsb/Tmm, DVB-C, J.83 Annex A/B/C and audio
broadcasting standards like DAB, FM Stereo/RDS and more.
Figure 6. N6153A DVB-T/H/T2 measurement results example showing shoulder
attenuation and an I/Q measured polar graph.
8
The N6153A DVB-T/H/T2 measurement application is one of more than 25
measurement applications in the Keysight X-Series. It runs inside the PXA, MXA,
EXA and CXA signal analyzers and provides one-button, standard-based power
and modulation analysis for DVB-T/H/T2 signals. It can be used to make the
SFN measurement for DVB-T/H, SISO, and MISO DVB-T2 networks.
Key parameters for the DVB-T/H/T2 measurement application:
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