1. Senior Algorithm Engineer Aeroflex Test Solutions Stevenage, UK
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

2. Agenda
1. Design Objectives
2. Design Challenges
3. Summary
4. Q&A

3. 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

4. 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

5. 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

6. 1.4 Translate System Req. to RF Req.
System Engineers
Receiver Sensitivity
Maximum Throughput
System Bandwidth
MIMO
Hand-over …
Noise Figure
EVM Floor
Filter Spec.
LO Phase Noise
LO Settling Time …
Product Managers/End Users
RF Engineers

7. 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

8. 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

9. 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 ).
(I)
SNRmin = -1 dB

10. (II) NF = PREFSENS – Pthermal – SNRmin
2.2 Noise Figure (Cont’d)
PREFSENS
SNR
PN = Pthermal + NF
NF<7 dB NF
Pthermal = kTB
(II) NF = PREFSENS – Pthermal – SNRmin

11. 2.3 EVM Floor <?
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;

12. 2.3.1 EVM Floor, Noise Figure and SNR
SNR clamped by the EVM floor.
SNR increases with the input signal power.
1.8%
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

13. 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 differential factor.
NF is the differential factor.

14. 2.4 TX Blocking Transmitter Receiver duplex TX Max Power: ~ 23 dBm
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;

15. 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

16. 2.5 IQ Imbalance Wanted Signal ε: Amplitude error θ: Phase error
Image Signal

17. 2.5.1 Self-Interference induced by IQ Imbalance
SIR=41 dB
Single tone measurement
- Input: cos(2π(fc+fm)t)
- Output Expected : cos(2πfmt)+j sin(2πfmt)

18. 2.5.2 SIR of IQ Imbalance
Desired Region

19. 2.6 LO Phase Noise
Source: Analog Devices® ADF4350 datasheet
Integrated Phase Noise Power (15KHz~10MHz): P = (dBc)
RMS Phase Error: θRMS = 0.50 (deg)
EVM = 0.88%

20. 2.6.1 Phase Noise on OFDM Constellation
CPE dominated
ICI dominated
Two types of effects:
- Common Phase Error (CPE)
- Inter sub-Carrier Interference (ICI)
CPE can be easily corrected, ICI not
Loop BW ↓ lock time ↑

21. 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

22. 2.8 LTE-A: Carrier Aggregation
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

23. LTE-A in the News
World-record 1.4 Gbps in LTE-Advanced demo (03/2012 source: )
- 5 component carriers
- 20MHz 4x4 MIMO each
- and TM500!
TM500 supports the development of multiband lightRadio® technology.

24. 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;