integrated transmitter architectures for 4g networks … · integrated transmitter architectures...

INTEGRATED TRANSMITTER ARCHITECTURES

FOR 4G NETWORKS AND BEYOND

Martin Schleyer, Adel Fatemi | Fachgebiet Mikroelektronik

Literature Recommendations

[Dahlman2011] Dahlman, E.; Parkvall, S. & Skold, J.:

4G: LTE/LTE-Advanced for Mobile Broadband

[Holma2007] Holma, H. & Toskala, A:

WCDMA for UMTS: HSPA Evolution and LTE:

[Holma2009] Holma, H. & Toskala, A:

LTE for UMTS - OFDMA and SC-FDMA Based Radio Access

[Myung2008] Myung, H. G. & Goodman, D.:

Single Carrier FDMA: A New Air Interface for Long Term Evolution

[Sauter2011] Sauter, M.: From GSM to LTE:

An Introduction to Mobile Networks and Mobile Broadband:

Available at http://www.meis.tu-berlin.de/?id=141359

Integrated Transmitter Architectures for 4G Networks and beyond | M. Schleyer, A. Fatemi | Session 2

Seite 2

http://www.meis.tu-berlin.de/?id=141359

EDGE AND EDGE-EVO3GPP DATA SERVICES


Seite 3

EDGE - Enhanced Data Rates for GSM Evolution

Pure GSM only allows 9.6 kBit/s in circuit switched data mode.

How to increase data rate within same ecosystem?

– GPRS added packed data mode, and more efficient (i.e. less

redundant) error coding better sprectral efficiency

– Multislot transmission higher data rate

With EDGE, new modulation scheme 8PSK is added to GSM

– Today, EDGE is used mostly in terms of EGPRS:

• Coding MCS-1..4 GMSK + less redudancy

• Coding MCS-5..8 8PSK + less redudancy

The newest enhancment EGPRS2 uses 16QAM and 32QAM

– GERAN Release 7: 16QAM + 32QAM added

– Typical 2.5G technology works within existing RANs


Seite 4

EDGE 8PSK Modulation Scheme

Classical 8PSK Modulation

– 8PSK = 8 different phase states

– 3 bit/symbol – better spectral efficiency

– Problem: High crest factor,

especially due to zero crossings


Seite 5

Classic 8PSK Modulation [Agilent2005]Classic QPSK Modulation – Time Domain [Wiki2007]

EDGE 8PSK Modulation Scheme

Differential 8PSK Modulation

– Even and odd symbols are shifted by 3π/8

– Hence, zero-crossings are avoided…

– …and dynamic range is reduced

Crest Factor – Peak Voltage to RMS:

Peak-To-Averate-Power Ratio:

PAPR and CF are important quantitites in power

amplifier design (will be discussed next week)

PAPR in D8PSK ≈ 3.2 dB typically


Seite 6

3π/8 8PSK Modulation Idea [Agilent2005]

EDGE and EVM – Error Vector Magnitude

The 8PSK/16QAM/32QAM modulation schemes of EDGE and

EDGE-EVO are no constant-envelope techniques

– Phase error is not sufficient to determine TX performance

– Signal has a significant PAPR (peak-to-average power ratio)

as seen in previous slides

Concept of EVM (Error Vector Magnitude):

– Signals are „noisy“ due to TX characteristics

• AM/AM Amplitude Errors

• AM/PM Phase deviations

– EVM reflects both– measured in complex domain


Seite 7

[Agilent2005]

[Agilent2005a]

SNR in high-order modulation techniques

High-order modulation has better spectral

efficiency than simple BPSK…

– More bits/symbol = less Hz/bit

…but higher SNR for same BER required

– Shannon Theorem predicts this trade-off

– For BER=10-3:

8PSK ≈ SNR +7dB required

16PSK ≈ SNR +10dB required

Higher transmitting power or smaller

cells required for sufficient throughput


Seite 8

Bit-error rate curves for phase-shift keying: BPSK,

QPSK, 8-PSK and 16-PSK [Wiki2007a]

UMTS DATA SERVICES3GPP DATA SERVICES


Seite 9

Why do we need a 3G Network?

GSM/EDGE – 2nd Generation Network:

– Optimized for circuit-switched voice

– High delay (180 ms round-trip)

– Small band, inflexible assignment

of data rates

– Low to medium data rates

– Suboptimal use of radio resources

(spectral efficiency)

– Complicated RF planning

(layout of frequency usage)

– Standard set by Europe, but used

almost globally as 2G standard

UMTS – 3rd Generation Network

– Focus on packet-switched data

– Lower latency

– Flexible assignment of spectrum with

variable data rates

– Higher data rates for multimedia

services

– Higher capacity of radio system

– Simplified RF engineering

(no frequency planning)

– Worldwide agreed standard and

roaming by design


Seite 10

[Mitschele2013]

UMTS System Architecture

UMTS/3G is based on packet switched architecture, as in IP networks


Seite 11

UMTS System Architecture

UE User EquipmentUser Terminal (Cell Phone, Data Card, …)

Node B Base Transceiver StationRadio frontend on operator side

RNC Radio Network ControllerRadio Resource Management, Encryption

RNS Radio Network SubsystemMin. one RNC and several Node B‘s

SGSN Serving GPRS Support Node Location register, authentication, billing

Core Core NetworkSupporting Functions (EIR/HLR/AUC…)

RNSRNS

RNS

UE

Node B

Node B

Node B

RNC

Core

Network

SGSN

GGSN

HLREIR

MSC/VLR

AUC

GGSN

RNS

UMTS Radio Access Network - UTRAN

GSM used a TDMA/FDMA scheme. 3GPP UMTS standard uses

Wideband Code Division Multiplex Access (WCDMA)

– User information is multiplied with a quansi-random bit

sequence, spreading the original spectrum

– Chip Rate 3.84 Mbit/s leads to 5 MHz carrier bandwidth

– Resources are split in 10ms time slots


Seite 12

Allocation of bandwidth in WCDMA in the

time–frequency–code space [Holma2007]Spreading and despreading in DS-CDMA [Holma2007]

CDMA Modulation and Demoodulation - Principle


Seite 13

[Mitschele2013]

Advantages and Challenges of WCDMA Techniques

CDMA spreads the narrow band signal into a broad band signal using the spreading code

Protection against narrow band interference

All users transmit on same channel W no guard bands required, wider filters

Signals are less sensitive to channel fading W working well in urban areas

Spreading techniques has some additional costs

Higher receiver complexity

Restrictive power control to avoid Node B desensitization


Seite 14

f

Power Interferer

Spread

Signal

f

Power

Spreaded

Interferer

Signal

(despreaded)

Detection at

Receiver

[Mitschele2013]

The UMTS Uplink in Standard Mode

UMTS combines two BPSK signals in quadrature modulation

– In contrary to the downlink direction, data is not muliplexed before the IQ modulator

– For equal bandwidth compared to upload, only half the spreading factor is required

– Signaling in DPCCH has SF=256 (≈ 15kBit/s)

– Payload in DPDCH has SF=4…256 (≈ 15…960 kBit/s)

TX is always transmitting the DPCCH!

Power control allows adaption of relative DPCCH power


Seite 15

DPDCH

DPCCH

I

Q

I+j⋅Q

CD

CSCRAMBLE

to QPSK

modulator

CC

PQ

PIDPDCH

DP

CC

H

G=1

G=1/2

I

Q

[Holma2007]

Effect of IQ Imbalances on Quadrature Modulation Signals

Phase Error Gain Error


Seite 16

[Pfeiffer2013]

HSUPA – High-Speed Upload Packet Access

HSUPA enhanced UMTS upload speed up to 35.5 Mbit/s

– Release 6: QPSK, four channel codes in parallel,

Coding 1/1 5.76 Mbit/s max.

– Release 7: QAM16 doubles bit per symbol,


– Release 9: Intraband Carrier Aggregation,

using two channels in parallel


– Release 11: 2x2 MIMO Techniques + QAM64

Two parallel TXs + Carrier Aggregation

Coding 1/1 70 Mbit/s max.

HSUPA uses same methods

as in LTE, main difference:

WCDMA vs. OFDM


Seite 17

[NSN2010]

QAM – Quadrature Amplitude Modulation

QAM uses both phase and amplitude modulation

– 4 bit/symbol – hence, improved spectral efficiency

– QAM requires better SNR, and is much more

sensitive regarding AM/AM errors (EVM!)

– Most challenging in TX design:

16QAM/64QAM has high CF/PAPR!


Seite 18

QPSK

I

Q

QPSK and 16QAM Modulation

LTE AND LTE ADVANCED3GPP DATA SERVICES


Seite 19

LTE and LTE-Advanced – The 4G Networks

UMTS with HSPA+ offers significant higher data rates

compared to GSM or early UMTS Implementations

– However, UTRAN has limited capabalities due to its

channel bandwidth (5 MHz) and WCDMA strategy

– Higher transmission speed / larger bandwidth require

shorter timesteps

– Hence, multipath fading gets more and more critical and

reduces the overall transmission accuracy

With LTE – Long Term Evolution – a new air interface was

specified by the 3GPP Consortium

– Instead of spreading signal over carrier bandwidth, LTE

uses Orthogonal Frequency Division Multiplexing

– Instead of one single and fast data transmissions,

several slow streams are transmitted simultaneously


Seite 20

Orthogonal Carriers [Rayal2012]

LTE Logo [Flynn2008]

OFDM – Orthogonal Frequency Division Multiplexing

Frequency Domain: Overlapping sinc functions

– Overall spectrum is divided in several

subcarriers (up to some hundreds…)

– Bandwidth typically very narrow (e.g. 15 kHz)

– Modulation scheme can be adapted to channel

fading properties

Time Domain: Multiple gated sine functions

– Orthogonality: integer number of cycles per

symbol

– Fundamental frequency f0 = 1/TSymbol

– Other carriers with fk = k∙f0


Seite 21

Subcarriers in frequency domain [Mitschele2013]

Subcarriers in time domain [Mitschele2013]

LTE – Resource Block Allocation

Resource allocation is based on frequency and time

– One frame is 10ms and consists of 10 sub-frames

– One subframe is 1ms and contains 2 slots

– One slot is 0.5ms in time domain and each 0.5ms

assignment can contain N resource blocks

[6 < N < 110] depending on the BW allocation

– One resource block is 0.5ms and contains

12 subcarriers for each OFDM symbol in

frequency domain.

– There are 7 symbols (normal cyclic prefix)

per time slot in the time domain or

6 symbols in long cyclic prefix. 1ms Allocation

Period

180 kHz

Total

System

Bandwidth

Resource Blocks for UE1

Single

Resource

Block

Subcarriers in

180kHz resource

block

Time

Fre

qu

en

cy


Seite 22

Resource Block Allocation [according to Homa2009]

[Teletopix2013]

LTE Uplink – Single-carrier FDMA

OFDM signals have high crest factors: subcarriers interfere

constructively and destructively with varying amplitude

– The more subcarriers, the higher the crest factor

– High CF/PAPR requires larger backoff in LTE Pas

SC-FDMA adds additional DFT step into the modulator chain

– Actually, a single symbol is transmitted in parallel,

although all OFDMA properties are kept

– Allocation strategy is same as in DL

– Modulation scheme is chosen depending on UL quality

Peak

Average

Back-Off

PA Characteristics


Seite 23

Peak

Average

Back-Off

PA Characteristics

Power amplifier back-off requirements

for different input waveforms

[according to Holma2009]

a) Low PAPR

b) High PAPR

OFDMA and SC-FDMA in Frequency-Time-Plot


Seite 24

OFDMA and SCDMA symbols in Frequency-Time Domain [Rumney2008]

OFDMA and SC-FDMA Signal Generation

Typical OFDM Transmitter chain, using an RF-DAC for D/A conversion in RF domain

– channel coder adds error correcting code and cyclic redundancy check to data

– digital baseband modulator performs QPSK or QAM and builds symbols of m bits

– symbols are mapped on subcarriers according to resource block allocation

– signal processor performs an IDFT on N symbols to one OFDM symbol (N∙m bits)

– cyclic prefix added to get guard interval τGuard, reducing Inter-Symbol Interference (ISI)

– the pulse shaping attenuates signal energy outside of the nominal OFDM bandwidth


Seite 25

Typical OFDM TX Structure [based on Hyung2008]

[Hyung2008]

Par.

to

Serial

Add

cyclic

prefix

N-

point

IDFT

M-QAM

Modulator

RF-DAC PA

LO

Data In Pulse

Shaper

Channel

Coding

OFDM-Core

Su

bc

arr

ier

Ma

pp

ing

N

OFDMA and SC-FDMA Signal Generation

The SC-FDMA Transmitter adds an additional DFT before subcarrier mapping

resulting signal is having N symbols serially instead of parallel

Additional DFT required

lower PAPR due to single carrier modulation at TX – no constructive/destructive interference

better robustness against nulls in channel spectrum

Lower sensitivity to carrier frequency offset


Seite 26

Typical SC-FDM TX Structure [based on Hyung2008]

[Hyung2008]

Par.

to

Serial

Add

cyclic

prefix

N-

point

IDFT

M-QAM

Modulator

RF-DAC PA

LO

Data In Pulse

Shaper

Channel

Coding

SC-FDM-Core

Su

bc

arr

ier

Ma

pp

ing

N

N-

point

DFT

Timing Advance in OFDMA / SC-FDMA Schemes

The LTE standard uses uplink orthogonality as

multiple access technique

– Requirement for orthogonality: signals arrive

approximately time aligned at Node B

– LTE uses transmit-timing advance to ensure

proper uplink transmittion timing

Example:

– first terminal UE1 is located close to BS,

small propagation delay TP,1 small offset TA,1

– second terminal UE2 is located close to Node B,

larger propagation delay TP,2 large offset TA,2

– TA,x is determined by Node B and referred to DL

time slots


Seite 27

[Dahlman2011]

TP,2

TA,2

UL UE2

DL

TA,1

TP,1

UL UE1

DL

1 ms

UL UE1

UL UE2

DL

LTE and LTE-Advanced – Speed Acceleration Techniques

LTE as defined by 3GPP Rel. 8 allows data rates

up to ≈300 Mbit/s DL and ≈75 Mbit/s UL

– Downlink uses up to 4x4 MIMO

Spatial Multiplexing - MIMO

– LTE terminals are equipped with multiple

antennas RX and TX Diversity

– Additionally, antennas can be used for spatial

multiplexing, e.g. by beamforming technologies


Seite 28

[Yackulic2013]

[Dahlman2011]

LTE and LTE-Advanced – Speed Acceleration Techniques

Carrier Aggregation allows bandwidth enhancements

to achieve tranmissions > 20MHz

– Intra-band aggregation with frequency-

contiguous component carriers

– Intra-band aggregation with non-contiguous

component carriers

– Inter-band aggregation with non-contiguous

component carriers.

CA allows efficient usage of a fragmented spectrum;

operators with a fragmented spectrum can

provide high-data-rate services

allows addition of free “spots” in the frequency

allocation, even if band is very small

Requires much flexibility on RF side


Seite 29

[Dahlman2011]

Band A Band B

Band A Band B

Band A Band B

Sources and Literature

[Agilent2005] Agilent Technologies, I.: EGPRS Test: Meeting

the Challenge of 8PSK Modulation, Online,

2005

[Dahlman2011] Dahlman, E.; Parkvall, S. & Skold, J.: 4G:

LTE/LTE-Advanced for Mobile Broadband:

Academic Press., 2011

[Flynn2008] Flynn, K.: 3GPP Marketing and

Communications Plan, Presentation at 3GPP

PCG #21, 2008

[Mitschele2013]Mitschele-Thiel, A. & Mückenheim, J.: Cellular

Communication Systems, Lecture Notes, 2013

[Myung2008a] Myung, H. G.: Single Carrier FDMA Tutorial,

Presentation, 2008

[Myung2008] Myung, H. G. & Goodman, D.: Single Carrier

FDMA: A New Air Interface for Long Term

Evolution (Wireless Communications and

Mobile Computing): Wiley., 2008

[Rayal2012] Rayal, F.: The Number of Sub-Carriers in

OFDM impacts NLOS Backhaul Performance

(More than You Think), Online, 2012

[Rumney2008] Rumney, M.: 3GPP LTE: Introducing Single-

Carrier FDMA. In: Agilent Measurement Journal

(2008), Nr. 4, S. 18-27

[Teletopix2013] Teletopix: LTE Frame Structure and Resource

Block Architecture, 2013

[Walke2003] Walke, B. H.; Seidenberg, P. & Althoff, M. P.:

UMTS: The Fundamentals: Wiley., 2003

[Wiki2007] Wikipedia: QPSK Timing Diagramm,

[Yackulic2013] Yackulic, C.: 4G – is it working?, Online, 2013

[Holma2009] LTE for UMTS - OFDMA and SC-FDMA Based

Radio Access. In: Holma, H. & Toskala, A.

(Hrsg.): Wiley., 2009

[Holma2007] WCDMA for UMTS: HSPA Evolution and LTE.

In: Holma, H. & Toskala, A. (Hrsg.): Wiley.,

2007

[Wiki2007a] Bit-error rate curves for phase-shift keying:

BPSK, QPSK, 8-PSK and 16-PSK, 2007


Seite 30

Summary

Cellular communications has its roots at the early 1900

– Starting with Marconi‘s telegraphy aparatus, radio communication further evolved

– In the 1960‘s to 1990‘s, analog and hybrid 1G networks were publically available

– The interoperable GSM paved the to road what we have in our pockets today

With digital power and analog integration, capabilites of cellular networks grew

– First „handsets“ actually filled a whole car booth

– With microprocessors, digitally assists networks grew and allowed cellular concepts (AMPS)

– Starting with GSM and EDGE, data services enhanced the user experience

Each evolutional step brought new requirements to RF design

– From analog modulation to burst oriented GMSK…

– …ending with high PAPR schemes as 8PSK, 16QAM, 32QAM

Next Week: Basics of RF Power Amplifier

A Class E Power Amplifier Design in RFCMOS technology


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