Transcript DTV: an opportunity for Research?

Mobile Digital TV Technology
for the Terminal
Dave Evans, Sri Andari Husen, Hans Brekelmans, Peter Massey
Philips Research Laboratories
Philips first with a mobile
phone demonstration.
3GSM, February 2005
Technology for the Terminal
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Technical challenge
RF Tuner
Antenna
TV coexistence in the phone
Channel decoding – dealing with Doppler
What next?
Conclusion
Philips Research Laboratories
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Technical Challenge
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Reception in all situations
Good picture quality
Limited impact on phone battery life
Global usage
Mobile TV is now addressing the issues
that are familiar in the design of mobile
terminals
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Size
Performance
Reception on the move
Low power
Multi-standard
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TV in the phone – Generalised architecture
Cellular
transceiver
Display
TV
tuner
Additional elements for
broadcast TV reception
Channel
decoder
Baseband
controller
+ interface
+ Software
Media
processor
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Philips supplies complete
system solutions for the
mobile terminal
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RF Tuner
• Major challenge was power consumption
– Starting point ~500mW
• Now >100mW (>5% with DVB-H time slicing)
• Low/zero IF design
• Minimal off-chip components
• 470 to 860 MHz operation
– Separate on chip LNA for 1452 to 1675 MHz operation
• On-going work to improve performance
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Mobile DTV Antenna
• Two issues:
– Close proximity between mobile DTV and GSM antennas
– Common ground plane
• Coupling between them disturbs the GSM antenna and affects
its performance
• Co-design of the GSM and Mobile DTV antennas is essential
• Signal coupling from GSM to mobile DTV antenna is high
– Need to incorporate GSM signal blocking
• Ideally continuous operation from 470 to above 700 MHz
– Limited to ~700 MHz to assist GSM coexistence
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Interaction between GSM & DVB-H
Strong coupling, poor isolation
DVB-H
port
GSM
DVB-H
TV filter must be
reflective at GSM
GSM
feed
Reflection phase matters!
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Interaction between GSM & DVB-H
open circuit at
DVB-H monopole
DVB-H
port
GSM
feed
short circuit at
DVB-H monopole
GSM S11
• GSM seriously effected by impedance of DVB-H circuit. Codesign is necessary.
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Antenna + RF Tuner
Feed tab
Integrated GSM filter
• Compact PIFA
• 470 to 700 MHz continuous
operation
• Antenna includes a GSM trap
Antenna & RF tuner
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TV Coexistence in the Phone
• Interference from GSM900 transmissions due to very close co-location
– DTV receiver blocking
– 58 dB isolation between GSM TX and mobile DTV receiver is required
• Potential solutions
– Isolation between antennas – limited to between 6 to 10 dB
– Can be improved by use of GSM trap within mobile DTV antenna, ~20 dB
– Managing GSM transmission at the terminal – limited scope
– Managing DVB-H transmissions – not possible
– Power cancellation – not very promising
– Receiver filter, good solution but requires frequency separation – restricts
channel usage. TV channel 50 (~700 MHz) OK, extending this to 54/55 desired
• Coexistence best achieved by filter before TV RX + antenna with GSM trap
• Out-of-band noise – high pass filter at GSM TX output
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Broadband matching + filtering, PIFA to LNA
NF <4dB, 480 to 720 MHz
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G >17dB, 470 to 710 MHz
50dB attenuation above 877MHz
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Channel Decoding
• Key issue for mobile TV
– Reception at high vehicle speed
• Problem
– Impact of Doppler effects on OFDM
– Channel changes during symbol period
– Inter carrier interference (ICI)
• 150 kph equates to typically 100 Hz Doppler
• 8k DVB-T mode has 1.1 kHz subcarrier spacing
• Solution
– Channel estimation and Doppler compensation
– ICI cancellation
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Mobile multipath channel
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The faster the vehicle, the more severe the ICI, the poorer the reception.
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Challenge: DVB-T/H 8K mode (fs = 1.12 kHz) reliable high throughput reception
under high Doppler frequency (10% fs) with low complexity.
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Channel Estimation
The received signal in frequency domain is approximated as follows:
y  diag{H} a    diag{H '} a  n
Wanted received signal
Inter-Carrier
Interference
Noise
where:
• H is the complex channel transfer function vector for all the subcarriers
• H’ is the the temporal derivative of H (proportional to vehicle speed)
• Ξ is the fixed Inter-Carrier Interference spreading matrix
• a is the transmitted data vector
• n is a complex circular white Gaussian noise vector
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Channel Estimation
pilots
data carriers
empty carriers
OFDM symbol
time
frequency
• Estimation of H: rather than time interpolation, frequency interpolation
• Estimation of H’: calculated from H estimation of past and future symbols
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Inter-carrier Interference
• ICI level is not constant but
varies over frequency
• ICI level per sub-carrier can
be estimated from H’
• Soft demapper takes into
account ICI level per subcarrier, rather than average ICI
power
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Basic Channel Decoding Scheme
channel
estimation
H'
H
y
Data Estimation
aˆ
Soft demapper
Log likelihood ratio per bit
To de-interleaver
& Viterbi decoder
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Overall Scheme
channel
estimation
H'
H
y
Data Estimation
aˆ
ICI cancellation
y'
(Using regenerated ICI)
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Soft QAM demapper
Log likelihood
ratio per bit
To de-interleaver
& Viterbi decoder
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Performance after Viterbi decoder when H
is known
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Final points on channel decoding
• Channel model
– MBRAI specification defines the use of COST 207 TU6 profile
– Modeling of the Doppler spectrum is not defined
• System performance is very sensitive to model parameters
– No conformance tests are defined in for the complete channel model
– Caution needs when comparing performance
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What next?
• On-going work to improve performance
– Further reductions in power consumption
– Move to a CMOS architecture
• Single chip solution that includes channel decoder
– Emerging RF filter technologies including MEMS
– Antenna diversity, extra dBs are very useful – gain of a few dBs?
• Technology will evolve to meet that in the terminal, convergence!
• Multi-standard solutions
– Needed now to support multi-standard multi-band cellular requirements
– Also required for WLAN/BT, mobile DTV and GPS
– Reconfigurable, highly digitised radios
• Coexistence in the phone
– Exploitation of multiple radios to assist mobile DTV reception - diversity
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Conclusion
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Keys issues and challenges are understood
Solutions are available now
On-going process of performance improvement
Continuing to maintain the leading position of Philips
Complete systems
solution shown at IFA,
Berlin, September 2005
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