System-level Challenges in the Design of a
Wideband RF Transceiver
for LTE and LTE-A
Dr. Jin Wang
Senior Algorithm Engineer
Aeroflex Test Solutions
Stevenage, UK
Aeroflex Company Confidential
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Agenda
▼
1. Design Objectives
▼
2. Design Challenges
▼
3. Summary
▼
4. Q&A
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1.1 3GPP LTE Air Interface Overview
Modulation
DL: OFDM
UL: DFTS-OFDM
Signal Bandwidth
1.4,3,5,10,15,20 MHz
Signal PAPR (Crest Factor)
DL: ~11 dB, UL: ~8 dB
FFT Size
2048 (Normal CP)
Sub-Carrier Spacing
15 kHz (Normal CP)
DL MIMO
2x2 (Rel-8)
4x4, 4x2 (Rel-9)
UL MIMO
2x2 (Rel-9)
Max Data Rate
DL: 150~300 Mbps
UL: 50~100 Mbps
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1.2 Product Overview: TM500
▼
Industry Standard Base Station Tester for
LTE and HSPA
▼
LTE Rel-8,9,10 and beyond
▼
From RF to Protocol Layers
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1.3 Wideband Radio Card
▼
▼
▼
▼
Operating Frequencies: 400MHz~4GHz
Signal Bandwidth: up to 20 MHz
Transceiver Units: 2 RX, 1 TX
Form Factor: double height and double width of a
uTCA slot
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1.4 Translate System Req. to RF Req.
System Engineers
Receiver Sensitivity
Noise Figure
Maximum Throughput
EVM Floor
System Bandwidth
Filter Spec.
MIMO
LO Phase Noise
Hand-over
LO Settling Time
…
Product Managers/End Users
…
RF Engineers
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2. RF Design Challenges
▼
Homodyne or Heterodyne?
▼
What’s the minimum requirement on Noise
Figure?
▼
What’s the minimum requirement on EVM
floor?
▼
The biggest blocker might be your own TX!
▼
Do we need to worry about IQ imbalance?
▼
What about phase noise?
▼
Further challenges in LTE-A
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2.1 Homodyne or Heterodyne?
▼
Homodyne (direct conversion/zero-IF)
Pros:
- fewer processing stages
- No image frequency problem
- Mainstream design in recent years
Cons:
- IQ imbalance
- DC offset or carrier leakage
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2.2 Noise Figure < ?
▼
Max noise figure allowed depends on the
RX sensitivity requirement, e.g.
3GPP requires that no less than 95% of maximum throughput is
achieved on a reference measurement channel (BW=10MHz)
when minimum input power of PREFSENS=-97dBm is applied (from
3GPP 36.101).
100
90
80
70
T-put (%)
(I)
SNRmin = -1 dB
60
50
40
30
20
10
0
-2
-1.8
-1.6
-1.4
-1.2
SNR (dB)
-1
-0.8
-0.6
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2.2 Noise Figure (Cont’d)
PREFSENS
SNR
PN = Pthermal + NF
NF
NF<7 dB
Pthermal = kTB
(II) NF = PREFSENS – Pthermal – SNRmin
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2.3 EVM Floor <?
 
 
E e2
rm s ErrorLevel
EVM 

2
Es
rm s Signal Level
SNR  20 Log(EVM) (dB)
▼
EVM results from various RF non-idealities: carrier
leakage, IQ imbalance, gain compression, phase
noise, frequency error, etc;
▼
Overall EVM floor limits the max achievable T-put!
▼
Given a EVM value, the error power increases linearly
with the signal power;
▼
The effect on BLER/T-put may be treated as noise,
hence EVM can be converted to SNR;
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2.3.1 EVM Floor, Noise Figure and SNR
SNR clamped
by the EVM
floor.
Noise Figure=7dB, EVM Floor=1.8%
50
40
1%
30
3%
1.8%
20
10%
10
32%
0
-10
-130
Noise limited
EVM limited
Combined
-120
-110
-80
-90
-100
RSRP (dBm/sc)
-70
-60
-50
EVM
SNR (dB)
SNR increases
with the input
signal power.
LTE requires
near 30 dB SNR
to achieve the
max T-put
(150Mbps with
2 layers).
Note: SNR is defined at the output of the RF front-end, i.e.
baseband
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2.3.2 EVM Floor, Noise Figure and T-put
Consider three RF front-end with different
NF and EVM characteristics.
No effect on
T-put.
EVM is the
50
differential
40
1%
factor.
NF is the
differential
factor.
30
3%
20
10%
10
32%
0
-10
-130
EVM
SNR (dB)
▼
NF=4dB,EVM=1%
NF=7dB,EVM=1.8%
NF=7dB,EVM=3%
-120
-110
-100
-90
-80
RSRP (dBm/sc)
-70
-60
-50
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2.4 TX Blocking
TX Max Power: ~ 23 dBm
Transmitter
duplex
Receiver
RX REFSENS: ~ -97 dBm
▼
▼
The TX power can be 120 dB higher than the RX
power;
The TX and RX frequency separation can be as
small as 30 MHz;
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2.4 TX Blocking (Cont’d)
Consequences:
▼ Particularly serious for wideband transceivers
▼ Cause compression in the RX amplifiers and
demodulator
▼ Desensitize the receiver
▼ Limit the max TX power allowed
Solutions:
▼ Application-specific:
- Block-tolerant front-end;
- TX Power back-off for lab operations;
- Half-duplex mode for budget handsets;
▼ Advanced techniques:
- Adaptive interference cancellation duplexer
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2.5 IQ Imbalance
ε:
Amplitude
error
θ:
Phase
error
Wanted Signal
xRF (t )  I (t ) cos(c t )  Q(t ) sin(c t ),
x LPF (t )  cos( )  j sin( ) I (t )  jQ(t ) 
  cos( )  j sin( ) I (t )  jQ(t ) 
 x L (t )  x L* (t )
Image Signal
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2.5.1 Self-Interference induced by IQ Imbalance
140
X: 4.8
Y: 134.9
120
X: -4.8
Y: 93.87
Mag (dB)
100
SIR=41 dB
80
60
40
20
-80
-60
-40
-20
0
Freq (MHz)
20
40
60
80
Single tone measurement
- Input: cos(2π(fc+fm)t)
- Output Expected : cos(2πfmt)+j sin(2πfmt)
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2.5.2 SIR of IQ Imbalance
6
18
20
20
5
18
20
22
4
24
3
26
26
28
2
30
1
32

SIR  10log10
 

3
2
2
4
5
6
Amplitude Error (%)
7
8
20
2
22
1
26
30
0
24
28
36 34
40
38
0
20
24
22
Desired Region
Phase Error (degree)
22
9
10
2
2


  
1


t
an
  10log10
  2  t an2   




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2.6 LO Phase Noise
f2
P   L(f)df
f1
 rms 
180

EVM 

2 P (deg)
 rms 100%
180
Source: Analog Devices® ADF4350 datasheet
Integrated Phase Noise Power (15KHz~10MHz): P = -44.1 (dBc)
RMS Phase Error: θRMS = 0.50 (deg)
EVM = 0.88%
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2.6.1 Phase Noise on OFDM Constellation
CPE dominated
ICI dominated
Loop Bandwidth=10kHz
Loop Bandwidth=40kHz
1.5
1.5
1
1
0.5
0.5
0
0
-0.5
-0.5
-1
-1
-1.5
-1.5
-1
▼
▼
▼
-0.5
0
0.5
1
1.5
-1.5
-1.5
-1
-0.5
0
0.5
1
1.5
Two types of effects:
- Common Phase Error (CPE)
- Inter sub-Carrier Interference (ICI)
CPE can be easily corrected, ICI not
Loop BW ↓ lock time ↑
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2.7 LTE-A: High Order MIMO
Downlink: 8x8
- 8 RX processing chains – high density
- L1 data rate 600 Mbps
- ADC Sample data rate:
30.72MSamp/s x (2x16 bits/sample)x8 = 7.86 Gbps
Uplink: 4x4
- 4 TX processing chains – high density
- L1 data rate 300 Mbps
- DAC sample data rate:
30.72MSamp/s x (2x16 bits/sample)x4 = 3.93 Gbps
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2.8 LTE-A: Carrier Aggregation
Contiguous CA
N*300kHz
20 MHz
20 MHz
20 MHz
20 MHz
Band 4
Band 3
20 MHz
Band 7
~ 40 MHz
Non-contiguous CA
CC for operator-B
N*100
kHz
20 MHz
5 MHz
10 MHz
15 MHz
10 MHz
20 MHz
Band 3
Band 4
N*300
kHz
Non-contiguous
aggregated CC for
operator-A
Band 7
LTE-A allows up to 5 component carriers. Each component carrier can
be 1.4, 3, 5, 10, 15 and 20 MHz. The maximum aggregated system
bandwidth is 100 MHz.
The three possible carrier aggregation types are:
- Intra-band contiguous carrier aggregation
- Intra-band non-contiguous carrier aggregation
- Inter-band carrier aggregation
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LTE-A in the News
▼
World-record 1.4 Gbps in LTE-Advanced
demo (03/2012 source: http://4g-portal.com )
- 5 component carriers
- 20MHz 4x4 MIMO each
- and TM500!
▼
TM500 supports the
development of
multiband lightRadio®
technology.
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Summary
▼
Introduction to LTE and the Test Mobile:
TM500;
▼
How to determine various RF system
parameters such as: noise figure, EVM
floor, TX leakage, IQ imbalance and phase
noise;
▼
Further challenges from LTE-A: high order
MIMO and CA;
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