Some Radio Implementation Challenges in 3G-LTE … sources/WT1-3_RF_DSP...Interference • 3G-LTE TX...
Transcript of Some Radio Implementation Challenges in 3G-LTE … sources/WT1-3_RF_DSP...Interference • 3G-LTE TX...
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Dirty-RF ThemeSome Radio Implementation Challenges
in 3G-LTE Context
Dr. Mikko Valkama
Tampere University of TechnologyInstitute of Communications Engineering
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General ”Dirty-RF” Paradigm
• General challenge: Low-cost, low-power, small-size yet flexible radio implementations (TX and RX) for future wireless systems.
• Cost and size of individual radios (are or should be) going down− especially important in multiantenna (MIMO) systems with multiple parallel
radios− simplified radio architectures and low-cost analog electronics, especially for
the RF parts
• With simplified analog RF front-ends, several RF impairments or non-idealities <=> ”dirty-RF” theme− getting more important also when the waveform structure gets more complex
(higher-order modulations, etc.), e.g. 3G-LTE downlink with 64QAM-OFDMA or 3G-LTE uplink with 16QAM-SC-FDMA
• Some important example problems: − I/Q imbalance, PA nonlinearities, oscillator phase noise, timing jitter in IF/RF
sampling, mixer and LNA nonlinearities in receivers, etc.
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General ”Dirty-RF” Paradigm (cont’d)
• Instead of increasing the design efforts and cost of the used analog electronics, use sophisticated DSP for restoring and enhancing the quality of the RF parts; DSP-based mitigation of RF impairments
• In general, the dirty-RF theme calls for expertise on both sides of the A/D and D/A interfaces; high synergy by merging the radio engineering and baseband DSP communities and knowledge!
BPF
LPF
LPF
A/D
A/D
I
Q
I/Q LO
DIGITALFRONT-END
ANALOG FRONT-END
RF COMP.
SELECTIVITY
DEMOD.
SYNCH.
I’
Q’
CHAN. EQ.
DETECTION
DECODING
SYNCH.
DIGITALBASEBAND
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General ”Dirty-RF” Paradigm (cont’d)
• In the earlier systems, with dedicated hardware and narrowband low-order modulated waveforms, these RF impairments did not actually play that big a role.
• But the story is really totally different in the emerging futuresystems− increasing bandwidths, more and more complex multicarrier type waveforms
(with increased sensitivity to all distortion and interference)− relative levels of RF impairments increasing due to simplified RF parts and
electronics, and also due to flexibility and reconfigurability requirements
In general, seen to play a major role in the future wireless evolution !
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One Important Practical Example Problem: I/Q Imbalance and Mirror-Frequency Interference
• I/Q imbalance: unintentional amplitude and phase errors between transceiver I and Q signal branches
• Results in mirror-frequency interference− practical mirror-frequency attenuation in the order of 20-40dB, depending on
the quality (amplitude and phase error levels) of the analog front-end− the role depends heavily on the applied transceiver architecture (zero-IF, low-
IF, etc.) and on the used communications waveforms− most critical in wideband multicarrier IF transmitters and receivers, in which
the mirror-frequency band is another signal band (high dynamic range)− high modulation order also increases sensitivity
• Illustrated in the following figures, for both TX and RX
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I/Q Imbalance and Mirror-Frequency Interference
• Mirror-frequency interference due to I/Q imbalances on TX side
Interference to other signal bands (low-IF) or within the spectrum itself (zero-IF)
I
Q
D/ALPF
D/ALPF
I/QLO
RF
0f
fIF
0 f
0f
fRFf fRF IF�2
fLO
0f
f fRF LO=
RF
RF
IQ
IQ
Zero-IF
Low-IF
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I/Q Imbalance and Mirror-Frequency Interference
• 3G-LTE TX scenarios:− for fast frequency domain scheduling and frequency hopping capabilities, the
focus in 3G-LTE uplink TX (mobile) is most likely on the latter IF transmitter case => tuning within the total system/operator band (5-20MHz) on the digital side=> big challenges to obtain sufficient attenuation for the mirror-
frequencies, at least using purely analog techniques !− the individual mobile signals in the corresponding downlink TX (basestation)
also (anyway) follow the IF transmitter model=> again big challenges
− additional mirror-frequency attenuation using DSP-based calibration techniques
− notice also that due to limited power-control, the mobile transmitter case is more challening (from the TX side point of view)
− additional mirror-frequency attenuation using DSP-based calibration techniques
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I/Q Imbalance and Mirror-Frequency Interference
• Mirror-frequency interference due to I/Q imbalances on RX side
Interference between different carriers or parts of the used radio spectrum
ILPFA/D
Q
I/QLO
RF
LPFA/D
0f
fLO
RF
fRF,ifIF,i0
f
IQ
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I/Q Imbalance and Mirror-Frequency Interference
• 3G-LTE RX scenarios:− (again) for fast frequency domain scheduling and frequency hopping
capabilities, the overall system/operator band (5-20MHz) is mostly likely I/Q downconverted as a whole (wideband direct-conversion / low-IF radio architecture)=> this applies most likely also to mobile receivers as well
(=> base-station is, of course, demodulating all the carriers anyway)− so in both mobile as well as base-station receivers, the individual mobile
signals follow essentially the IF model=> very big challenges to obtain sufficient mirror-frequency attenuation ! => can and should be enhanced using proper DSP, I/Q imbalance
compensation theme=> can and should be enhanced using proper DSP, I/Q imbalance
compensation theme
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Example case study #1: TX I/Q Calibration Using Pre-Distortion and Feedback
Mobile TX
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Example case study #1: TX I/Q Calibration Using Pre-Distortion and Feedback
• Proper pre-distortion of the digital signal such that generated RF signal is essentially free from mirror-frequency interference
• Pre-distortion coefficients estimated based on the statisticsbetween the feedback signal and the known TX data− real down-conversion based feedback to IF to avoid excess I/Q errors− can handle, by design, also frequency-dependent I/Q imbalances
• No technical details or math here, simply iIllustrated using a simple example− this example focuses on IF transmitter, but similar principles apply also in
case of zero-IF transmitter and/or wideband multicarrier TX
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Example case study #1: TX I/Q Calibration Using Pre-Distortion and Feedback
• Example: 3G-LTE SC-FDMA low-IF transmitter with 16-QAM data, roughly 2 MHz mobile bandwidth, IF frequency 3 MHz.
• Frequency-selective I/Q mismatches (analog front-end) varying within the overall signal band− results in frequency-selective analog front-end mirror-frequency
attenuation varying between ~25dB ... ~40dB
• Estimator block-size 100,000 and pre-distortion filter length = 3.
• Figures below show the average obtained IRR (in many independent realizations) together with 10 individual realizations− exceptionally good performance, stemming partially from the analytic
nature of the overall waveform (signal energy only at the other side of the spectrum)
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Example case study #1: TX I/Q Calibration Using Pre-Distortion and Feedback
− Analog front-end in dark-dashed, overall (calibrated) in colors-solid
-1 -0.5 0 0.5 120
30
40
50
60
70
80
90Average IRR vs. Frequency
Normalized frequency ω/π
IRR
[dB
]
-1 -0.5 0 0.5 120
30
40
50
60
70
80
90
10010 IRR Realizations vs. Frequency
Normalized frequency ω/πIR
R [d
B]
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Example case study #2: RX I/Q Compensation Using the Rich Statistics of the Received Complex Signal
• Contrary to TX, receiver front-end signals pretty challenging for estimation purposes, due to− channel noise (low SNR’s), multipath, (mis)synchronization, other
interferences, etc.
• Here we focus on purely blind signal processing utilizing the rich statistical nature of the received complex signal under RX I/Q imbalance (no explicit waveform structure assumed)
• To put it in short: The ordinary and complementary (pseudo) correlation functions of the received signal offer sufficient information to extract I/Q compensation parameters− can be estimated using sample statistics directly from the received
complex signal (overall down-converted signal), rather independently of the exact waveform structure (type of modulation, etc.)
− unaffected by noise, multipath, synhronization, etc.− can again handle also the challenging case of frequency-selective I/Q
imbalances
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Example case study #2: RX I/Q Compensation Using the Rich Statistics of the Received Complex Signal
BPF
RF LNA
AGC
A/D
A/D
I
Q
LPF
LPF
AGC
I/Q LO
w( )t
I jQ+
(.)*
Post-Processing Compensator
fIF,i0f
IQ
fIF,i0f
IQ
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• Example: Base-station receiver with 4 rather wideband mobiles.
• 10MHz 3G-LTE uplink mode assumed and here used by 4 mobiles− 16-QAM SC-FDMA waveforms for each mobile− individual mobile bandwidths of {4.5, 2.16, 1.26, 1.08}MHz− corresponding RF power levels at BS input are {0, 10, 15, 20}dB
• Individual multipath fading channels plus noise for each mobile− drawn from the Extended Vehicular A power-delay profile− the velocities of the four mobiles are {30, 3, 120, 240}km/h and the RF
carrier range is 2GHz− RX frequency offset (CFO) also included
• Frequency-selective I/Q mismatches are assumed for the base-station RX front-end, varying smoothly between 30-40dB.
• A three-tap post-processing compensation filter is used and a block of 100,000 received samples is used for sample statistics evaluations.
Example case study #2: RX I/Q Compensation Using the Rich Statistics of the Received Complex Signal
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−6 −4 −2 0 2 4 610
20
30
40
50
60
70
80Mirror−Frequency Attenuation vs. Frequency
Relative Frequency [MHz]
Atte
nuat
ion
[dB
]
Compensated, AverageCompensated, RealizationsAnalog Front−End
MS#1
0 dB
MS#2
10 dB
MS#3
15 dB
MS#4
20 dB
Example case study #2: RX I/Q Compensation Using the Rich Statistics of the Received Complex Signal
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• Clearly, good compensation performance is obtained, especially at the most weak signal band (here the most wideband mobile)− automatically most mirror-frequency attenuation to those bands really
needing it
• Next, laboratory radio signal measurements are carried out for further demonstration and verifications− similar waveforms (4 SC-FDMA mobiles, etc.), state-of-the-art receiver
RF-IC chip, high-quality laboratory signal generators, etc. − example obtained results illustrated in the following, in terms of the
detection error rate (BER/SER) when detecting the most wideband mobile signal in the base-station
Example case study #2: RX I/Q Compensation Using the Rich Statistics of the Received Complex Signal
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−8 −6 −4 −2 0 2 4 6 8−50
−40
−30
−20
−10
0
10
20
30
40
50Spectrum of the measured composite SC−FDMA signal
Frequency [MHz]
Am
plitu
de [d
B]
MS 1
MS 2
MS 3
MS 4
Example case study #2: RX I/Q Compensation Using the Rich Statistics of the Received Complex Signal
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Example case study #2: RX I/Q Compensation Using the Rich Statistics of the Received Complex Signal
5 8 11 14 17 20 2310
−4
10−3
10−2
10−1
100
Average SNR [dB]
SE
RMeasured localized SC−FDMA waveforms, 16−QAM
Uncompensated
Iterative, Nw=1
Iterative, Nw=3
Block, Nw=1
Block, Nw=3
No imbalance (simul.)
© Mikko Valkama, [email protected]
• Pioneering work in the dirty-RF field since 1999, focus on understanding and mitigation of the various analog RF impairmenteffects using DSP.
• Altogether 7 PhD/MSc researchers at the moment, led and coordinated by Dr. Mikko Valkama.
• Several research projects focusing on the dirty-RF topics, funded by, e.g., the Academy of Finland and Nokia-Siemens Networks.
• Happy to make new openings and establish new research cooperation ...
• Contact:Dr. Mikko Valkama− Tampere Univ. Technology, P.O.Box 553, Tampere, Finland− Email: [email protected], Tel: +358-40-8490-756
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RF-DSP Research Group at TUT, Finland