Some Radio Implementation Challenges in 3G-LTE … sources/WT1-3_RF_DSP...Interference • 3G-LTE TX scenarios: −for fast frequency domain scheduling and frequency hopping capabilities,

© Mikko Valkama, [email protected]

1 (12)

23.5.2007

Dirty-RF ThemeSome Radio Implementation Challenges

in 3G-LTE Context

Dr. Mikko Valkama

Tampere University of TechnologyInstitute of Communications Engineering

[email protected]


2 (21)

23.5.2007

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.


3 (21)

23.5.2007

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


4 (21)

23.5.2007

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 !


5 (21)

23.5.2007

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


6 (21)

23.5.2007

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


7 (21)

23.5.2007


• 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


8 (21)

23.5.2007


• 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


9 (21)

23.5.2007


• 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


10 (21)

23.5.2007

Example case study #1: TX I/Q Calibration Using Pre-Distortion and Feedback

Mobile TX


11 (21)

23.5.2007


• 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


12 (21)

23.5.2007


• 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)


13 (21)

23.5.2007


− 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]


14 (21)

23.5.2007

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


15 (21)

23.5.2007


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


16 (21)

23.5.2007

• 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.



17 (21)

23.5.2007

−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



18 (21)

23.5.2007

• 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



19 (21)

23.5.2007

−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



20 (21)

23.5.2007


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.)


• 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

21 (21)

23.5.2007

RF-DSP Research Group at TUT, Finland

Some Radio Implementation Challenges in 3G-LTE … sources/WT1-3_RF_DSP...Interference • 3G-LTE TX scenarios: −for fast frequency domain scheduling and frequency hopping capabilities,

Documents