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Rohde & Schwarz Products: FSQ, FSQ-K100, FSQ-K101, SMU200A, SMU-K55, SMU-K255,
SMATE200A, SMATE-K55, SMATE-K255, SMJ100A, SMJ-K55, SMJ-K255,
WinIQSIM2, AFQ100A, AFQ-K255, AMU200A, AMU-K55, AMU-K255
UMTS Long Term Evolution (LTE)
Technology Introduction
Application Note 1MA111
Even with the introduction of HSDPA and HSUPA, evolution of UMTS has not reached its end. To ensure
the competitiveness of UMTS for the next 10 years and beyond, UMTS Long Term Evolution (LTE) is being
specified in 3GPP release 8. LTE, which is also known as Evolved UTRA and Evolved UTRAN, provides
new physical layer concepts and protocol architecture for UMTS. This application note introduces LTE
technology and testing aspects.
Subject to change – C.Gessner 03.2007 – 1MA111_0E
LTE/E-UTRA
Contents
1 Introduction.............................................................................................. 3
2 Requirements for UMTS Long Term Evolution ....................................... 4
3 LTE Downlink Transmission Scheme...................................................... 5
OFDMA .............................................................................................. 5
OFDMA parametrization..................................................................... 6
Downlink Data Transmission.............................................................. 8
Downlink Reference Signal Structure and Cell Search ...................... 8
Downlink Physical Layer Procedures ............................................... 10
4 LTE Uplink Transmission Scheme ........................................................ 11
SC-FDMA......................................................................................... 11
SC-FDMA Parametrization............................................................... 12
Uplink Reference Signal Structure ................................................... 13
Uplink Physical Layer Procedures ................................................... 13
5 LTE MIMO Concepts ............................................................................. 15
Downlink MIMO................................................................................ 15
Uplink MIMO .................................................................................... 17
6 LTE Protocol Architecture...................................................................... 17
System Architecture Evolution (SAE)............................................... 17
E-UTRAN ......................................................................................... 18
Layer 2 structure .............................................................................. 20
Transport channels .......................................................................... 21
Logical channels .............................................................................. 21
7 LTE MBMS Concepts ............................................................................ 22
8 LTE Testing............................................................................................ 23
LTE RF testing ................................................................................. 23
LTE Layer 1 and Protocol Test ......................................................... 27
9 Abbreviations......................................................................................... 27
10 Additional Information ........................................................................... 30
11 References............................................................................................ 30
12 Ordering Information ............................................................................. 30
The following abbreviations are used in this application note for R&S test
equipment:
- The Vector Signal Generator R&S® SMU200A is referred to as the
SMU200A.
- The Vector Signal Generator R&S® SMATE200A is referred to as the
SMATE200A.
- The Vector Signal Generator R&S® SMJ100A is referred to as the
SMJ100A.
- SMU200A, SMATE200A, and SMJ100A in general is referred to as the
SMx.
- The IQ Modulation Generation R&S® AFQ100A is referred to as the
AFQ100A.
- The Baseband Signal Generator and Fading Simulator R&S®
AMU200A is referred to as the AMU200A.
- The Signal Analyzer R&S® FSQ is referred to as FSQ.
1MA111_0E 2 Rohde & Schwarz
LTE/E-UTRA
1 Introduction
Currently, UMTS networks worldwide are being upgraded to High Speed
Downlink Packet Access (HSDPA) in order to increase data rate and
capacity for downlink packet data. In the next step, High Speed Uplink
Packet Access (HSUPA) will boost uplink performance in UMTS networks.
While HSDPA was introduced as a 3GPP release 5 feature, HSUPA is an
important feature of 3GPP release 6. The combination of HSDPA and
HSUPA is often referred to as HSPA.
However, even with the introduction of HSPA, evolution of UMTS has not
reached its end. HSPA+ will bring significant enhancements in 3GPP
release 7. Objective is to enhance performance of HSPA based radio
networks in terms of spectrum efficiency, peak data rate and latency, and
exploit the full potential of WCDMA based 5 MHz operation. Important
features of HSPA+ are downlink MIMO (Multiple Input Multiple Output),
higher order modulation for uplink and downlink, improvements of layer 2
protocols, and continuous packet connectivity.
In order to ensure the competitiveness of UMTS for the next 10 years and
beyond, concepts for UMTS Long Term Evolution (LTE) have been
investigated. Objective is a high-data-rate, low-latency and packet-
optimized radio access technology. Therefore, a study item was launched in
3GPP release 7 on E-UTRA (Evolved UMTS Terrestrial Radio Access) and
E-UTRAN (Evolved UMTS Terrestrial Radio Access Network). LTE/E-
UTRA will then form part of 3GPP release 8 core specifications.
This application note focuses on LTE/E-UTRA technology. In the following,
the terms LTE or E-UTRA are used interchangeably.
In the context of the LTE study item, 3GPP work first focused on the
definition of requirements, e.g. targets for data rate, capacity, spectrum
efficiency, and latency. Also commercial aspects like costs for installing and
operating the network were considered. Based on these requirements,
technical concepts for the air interface transmission schemes and protocols
were studied. Notably, LTE uses new multiple access schemes on the air
interface: OFDMA (Orthogonal Frequency Division Multiple Access) in
downlink and SC-FDMA (Single Carrier Frequency Division Multiple
Access) in uplink. Furthermore, MIMO antenna schemes form an essential
part of LTE. In an attempt to simplify protocol architecture, LTE brings some
major changes to the existing UMTS protocol concepts. Impact on the
overall network architecture including the core network is being investigated
in the context of 3GPP System Architecture Evolution (SAE).
This application note gives an introduction to LTE technology.
Chapter 2 outlines requirements for LTE.
Chapter 3 describes the downlink transmission scheme for LTE.
Chapter 4 describes the uplink transmission scheme for LTE.
Chapter 5 outlines LTE MIMO concepts.
Chapter 6 focuses on LTE protocol architecture.
Chapter 7 introduces LTE MBMS (Multimedia Broadcast Multicast Service)
concepts.
Chapter 8 explains test requirements for LTE.
Chapters 9-12 provide additional information including literature
references.
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2 Requirements for UMTS Long Term Evolution
LTE is focusing on optimum support of Packet Switched (PS) Services.
Main requirements for the design of an LTE system have been captured in
3GPP TR 25.913 [1] and can be summarized as follows:
- Data Rate: Peak data rates target 100 Mbps (downlink) and 50 Mbps
(uplink) for 20 MHz spectrum allocation, assuming 2 receive antennas
and 1 transmit antenna at the terminal.
- Throughput: Target for downlink average user throughput per MHz is
3-4 times better than release 6. Target for uplink average user
throughput per MHz is 2-3 times better than release 6.
- Spectrum Efficiency: Downlink target is 3-4 times better than release
6. Uplink target is 2-3 times better than release 6.
- Latency: The one-way transit time between a packet being available at
the IP layer in either the UE or radio access network and the availability
of this packet at IP layer in the radio access network/UE shall be less
than 5 ms. Also C-plane latency shall be reduced, e.g. to allow fast
transition times of less than 100 ms from camped state to active state.
- Bandwidth: Scaleable bandwidths of 5, 10, 15, 20 MHz shall be
supported. Also bandwidths smaller than 5 MHz shall be supported for
more flexibility.
- Interworking: Interworking with existing UTRAN/GERAN systems and
non-3GPP systems shall be ensured. Multimode terminals shall support
handover to and from UTRAN and GERAN as well as inter-RAT
measurements. Interruption time for handover between E-UTRAN and
UTRAN/GERAN shall be less than 300 ms for real time services and
less than 500 ms for non real time services.
- Multimedia Broadcast Multicast Services (MBMS): MBMS shall be
further enhanced and is then referred to as E-MBMS.
- Costs: Reduced CAPEX and OPEX including backhaul shall be
achieved. Cost effective migration from release 6 UTRA radio interface
and architecture shall be possible. Reasonable system and terminal
complexity, cost and power consumption shall be ensured. All the
interfaces specified shall be open for multi-vendor equipment
interoperability.
- Mobility: The system should be optimized for low mobile speed (0-15
km/h), but higher mobile speeds shall be supported as well including
high speed train environment as special case.
- Spectrum allocation: Operation in paired (Frequency Division Duplex /
FDD mode) and unpaired spectrum (Time Division Duplex / TDD
mode) is possible.
- Co-existence: Co-existence in the same geographical area and co-
location with GERAN/UTRAN shall be ensured. Also, co-existence
between operators in adjacent bands as well as cross-border co-
existence is a requirement.
- Quality of Service: End-to-end Quality of Service (QoS) shall be
supported. VoIP should be supported with at least as good radio and
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backhaul efficiency and latency as voice traffic over the UMTS circuit
switched networks
- Network synchronization: Time synchronization of different network
sites shall not be mandated.
3 LTE Downlink Transmission Scheme
OFDMA
The downlink transmission scheme for E-UTRA FDD and TDD modes is
based on conventional OFDM. In an OFDM system, the available spectrum
is divided into multiple carriers, called sub-carriers, which are orthogonal to
each other. Each of these sub-carriers is independently modulated by a low
rate data stream.
OFDM is used as well in WLAN, WiMAX and broadcast technologies like
DVB. OFDM has several benefits including its robustness against multipath
fading and its efficient receiver architecture.
Figure 1 shows a representation of an OFDM signal taken from [2]. In this
figure, a signal with 5 MHz bandwidth is shown, but the principle is of
course the same for the other E-UTRA bandwidths. Data symbols are
independently modulated and transmitted over a high number of closely
spaced orthogonal sub-carriers. In E-UTRA, downlink modulation schemes
QPSK, 16QAM, and 64QAM are available.
In the time domain, a guard interval may be added to each symbol to
combat inter-OFDM-symbol-interference due to channel delay spread. In E-
UTRA, the guard interval is a cyclic prefix which is inserted prior to each
OFDM symbol.
5 MHz Bandwidth
FFT
Sub-carriers
Guard Intervals
Symbols

Frequency

Time
Figure 1 Frequency-Time Representation of an OFDM Signal
In practice, the OFDM signal can be generated using IFFT (Inverse Fast
Fourier Transform) digital signal processing. The IFFT converts a number N
of complex data symbols used as frequency domain bins into the time
domain signal. Such an N-point IFFT is illustrated in Figure 2, where
th
a(mN+n) refers to the n sub-channel modulated data symbol, during the
time period mTu < t (m+1)Tu.
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mTu (m+1)Tu
time
a(mN + 0)
mTu (m+1)Tu
a(mN + 1) time
a(mN + 2)
sm(0), sm(1), sm(2), …, sm(N-1)
. IFFT
.
. sm
a(mN + N-1)
Figure 2 OFDM Useful Symbol Generation using an IFFT
The vector sm is defined as the useful OFDM symbol. It is the time
superposition of the N narrowband modulated sub-carriers. Therefore,
From a parallel stream of N sources of data, each one independently
modulated, a waveform composed of N orthogonal sub-carriers is obtained,
with each sub-carrier having the shape of a frequency sinc function (see
Figure 1).
Figure 3 illustrates the mapping from a serial stream of QAM symbols to N
parallel streams, used as frequency domain bins for the IFFT. The N-point
time domain blocks obtained from the IFFT are then serialized to create a
time domain signal. Not shown in Figure 3 is the process of cyclic prefix
insertion.
QAM symbol rate =
N/T u symbols/sec
N symbol OFDM
QAM streams symbols Useful OFDM
Source(s) 1:N IFFT N:1
Modulator 1/Tu 1/Tu symbols
symbol/sec symbols/s
Figure 3 OFDM Signal Generation Chain
In contrast to an OFDM transmission scheme, OFDMA allows the access
of multiple users on the available bandwidth. Each user is assigned a
specific time-frequency resource. As a fundamental principle of E-UTRA,
the data channels are shared channels, i.e. for each transmission time
interval of 1 ms, a new scheduling decision is taken regarding which users
are assigned to which time/frequency resources during this transmission
time interval.
OFDMA parametrization
A generic frame structure is defined for both E-UTRA FDD and TDD
modes. Additionally, an alternative frame structure is defined for the TDD
mode only. The E-UTRA frame structures are defined in [3]. For the generic
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LTE/E-UTRA
frame structure, the 10 ms radio frame is divided into 20 equally sized slots
of 0.5 ms. A sub-frame consists of two consecutive slots, so one radio
frame contains 10 sub-frames. This is illustrated in Figure 4 (Ts is
expressing the basic time unit corresponding to 30.72 MHz).
One radio frame, Tf = 307200×Ts=10 ms
One slot, Tslot = 15360×Ts = 0.5 ms
#0 #1 #2 #3 #18 #19
One subframe
Figure 4 Generic frame structure in E-UTRA downlink
Figure 5 shows the structure of the downlink resource grid for the duration
DL
of one downlink slot. The available downlink bandwidth consists of N BW
sub-carriers with a spacing of f = 15 kHz. In case of multi cell MBMS
transmission (see chapter 7), a sub-carrier spacing of f = 7.5 kHz is also
DL
possible. N BW can vary in order to allow for scalable bandwidth operation
up to 20 MHz. Initially, the bandwidths for LTE were explicitly defined within
layer 1 specifications. Later on a bandwidth agnostic layer 1 was
DL
introduced, with N BW for the different bandwidths to be specified by 3GPP
RAN4 to meet performance requirements, e.g. for out-of-band emission
requirements and regulatory emission limits.
One downlink slot, Tslot
Resource element
One resource block,
NRB subcarriers
BW
NDL subcarriers
DL
N symb OFDM symbols
Figure 5 Downlink resource grid
DL
One downlink slot consists of N symb OFDM symbols. To each symbol, a
cyclic prefix (CP) is appended as guard time, compare Figure 1.
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LTE/E-UTRA
DL
N symb depends on the cyclic prefix length. The generic frame structure with
DL
normal cyclic prefix length contains N symb = 7 symbols. This translates into
a cyclic prefix length of TCP 5.2µs for the first symbol and TCP 4.7µs for
the remaining 6 symbols. Additionally, an extended cyclic prefix is defined in
order to cover large cell scenarios with higher delay spread and MBMS
transmission. The generic frame structure with extended cyclic prefix of
DL
TCP-E 16.7µs contains N symb = 6 OFDM symbols (sub-carrier spacing 15
kHz). The generic frame structure with extended cyclic prefix of TCP-E
DL
33.3µs contains N symb = 3 symbols (sub-carrier spacing 7.5 kHz).
Table 1 gives an overview of the different parameters again for the generic
frame structure.
Table 1 Parameters for downlink generic frame structure
Number of symbols Cyclic Prefix length in Cyclic Prefix length in
Configuration DL
N symb samples µs
Normal cyclic prefix 160 for first symbol 5.2 µs for first symbol
7
Nf=15 kHz 144 for other symbols 4.7 µs for other symbols
Extended cyclic prefix
6 512 16.7 µs
Nf=15 kHz
Extended cyclic prefix
3 1024 33.3 µs
Nf=7.5 kHz
Downlink Data Transmission
Data is allocated to the UEs in terms of resource blocks. A physical
resource block consists of 12 (24) consecutive sub-carriers in the frequency
domain for the Nf=15 kHz (Nf=7.5 kHz) case. In the time domain, a physical
DL
resource block consists of N symb consecutive OFDM symbols, see Figure 5.
DL
N symb is equal to the number of OFDM symbols in a slot. The resource
block size is the same for all bandwidths, therefore the number of available
physical resource blocks depends on the bandwidth.
Depending on the required data rate, each UE can be assigned one or
more resource blocks in each transmission time interval of 1 ms. The
scheduling decision is done in the base station (eNodeB).
The user data is carried on the Physical Downlink Shared Channel
(PDSCH). Downlink control signaling on the Physical Downlink Control
Channel (PDCCH) is used to convey the scheduling decisions to individual
UEs. The PDCCH is located in the first OFDM symbols of a slot.
Downlink Reference Signal Structure and Cell Search
The downlink reference signal structure is important for cell search, channel
estimation and neighbor cell monitoring. Figure 6 shows the principle of the
downlink reference signal structure for 1 antenna, 2 antenna, and 4 antenna
transmission. Specific pre-defined resource elements in the time-frequency
domain are carrying the reference signal sequence. Besides first reference
symbols, there may be a need for second reference symbols. The different
colors in Figure 6 represent the sequences transmitted from up to 4
transmit antennas.
1MA111_0E 8 Rohde & Schwarz
LTE/E-UTRA
R0 R0
One antenna port
R0 R0
R0 R0
R0 R0
l =0 l =6 l =0 l =6
Resource element (k, l )
R0 R0 R1 R1
Two antenna ports
R0 R0 R1 R1 Not used for transmission on this antenna port
R0 R0 R1 R1 Reference symbols on this antenna port
R0 R0 R1 R1
l =0 l =6 l =0 l =6 l =0 l =6 l =0 l =6
R0 R0 R1 R1 R2 R3
Four antenna ports
R0 R0 R1 R1 R2 R3
R0 R0 R1 R1 R2 R3
R0 R0 R1 R1 R2 R3
l =0 l =6 l =0 l =6 l =0 l =6 l =0 l =6 l =0 l =6 l =0 l =6 l =0 l =6 l =0 l =6
even-numbered slots odd-numbered slots even-numbered slots odd-numbered slots even-numbered slots odd-numbered slots even-numbered slots odd-numbered slots
Antenna port 0 Antenna port 1 Antenna port 2 Antenna port 3
Figure 6 Downlink reference signal structure (normal cyclic prefix)
The reference signal sequence carries the cell identity. Each reference
signal sequence is generated as a symbol-by-symbol product of an
OS
orthogonal sequence r (3 of them existing) and a pseudo-random
PRS
sequence r (170 of them existing). Each cell identity corresponds to a
unique combination of one orthogonal sequence rOS and one pseudo-
PRS
random sequence r , allowing 510 different cell identities.
Frequency hopping can be applied to the downlink reference signals. The
frequency hopping pattern has a period of one frame (10 ms).
During cell search, different types of information need to be identified by the
handset: symbol and radio frame timing, frequency, cell identification,
overall transmission bandwidth, antenna configuration, cyclic prefix length.
Besides the reference symbols, synchronization signals are therefore
needed during cell search. E-UTRA uses a hierarchical cell search scheme
similar to WCDMA. This means that the synchronization acquisition and the
cell group identifier are obtained from different SCH signals. Thus, a
primary synchronization signal (P-SCH) and a secondary synchronization
signal (S-SCH) are defined with a pre-defined structure. They are
transmitted on the 72 centre sub-carriers (around DC sub-carrier) within the
same predefined slots (twice per 10 ms) on different resource elements,
see Figure 7.
1MA111_0E 9 Rohde & Schwarz
LTE/E-UTRA
0.5 ms slot
Figure 7 P-SCH and S-SCH structure
As additional help during cell search, a Common Control Physical Channel
(CCPCH) is available which carries BCH type of information, e.g. system
bandwidth. It is transmitted at pre-defined time instants on the 72 sub-
carriers centered around DC sub-carrier.
In order to enable the UE to support this cell search concept, it was agreed
to have a minimum UE bandwidth reception capability of 20 MHz.
Downlink Physical Layer Procedures
For E-UTRA, the following downlink physical layer procedures are
especially important:
Cell search and synchronization:
See above.
Scheduling:
Scheduling is done in the base station (eNodeB). The downlink
control channel PDCCH informs the users about their allocated
time/frequency resources and the transmission formats to use. The
scheduler evaluates different types of information, e.g. Quality of
Service parameters, measurements from the UE, UE capabilities,
buffer status.
Link Adaptation:
Link adaptation is already known from HSDPA as Adaptive
Modulation and Coding. Also in E-UTRA, modulation and coding for
the shared data channel is not fix, but it is adapted according to
radio link quality. For this purpose, the UE regularly reports Channel
Quality Indications (CQI) to the eNodeB.
Hybrid ARQ (Automatic Repeat Request):
Downlink Hybrid ARQ is also known from HSDPA. It is a
retransmission protocol. The UE can request retransmissions of
incorrectly received data packets.
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LTE/E-UTRA
4 LTE Uplink Transmission Scheme
SC-FDMA
During the study item phase of LTE, alternatives for the optimum uplink
transmission scheme were investigated. While OFDMA is seen optimum to
fulfil the LTE requirements in downlink, OFDMA properties are less
favourable for the uplink. This is mainly due to weaker peak-to-average
power ratio (PAPR) properties of an OFDMA signal, resulting in worse
uplink coverage.
Thus, the LTE uplink transmission scheme for FDD and TDD mode is
based on SC-FDMA (Single Carrier Frequency Division Multiple Access)
with cyclic prefix. SC-FDMA signals have better PAPR properties compared
to an OFDMA signal. This was one of the main reasons for selecting SC-
FDMA as LTE uplink access scheme. The PAPR characteristics are
important for cost-effective design of UE power amplifiers. Still, SC-FDMA
signal processing has some similarities with OFDMA signal processing, so
parametrization of downlink and uplink can be harmonized.
There are different possibilities how to generate an SC-FDMA signal. DFT-
spread-OFDM (DFT-s-OFDM) has been selected for E-UTRA. The principle
is illustrated in Figure 8.
For DFT-s-OFDM, a size-M DFT is first applied to a block of M modulation
symbols. QPSK, 16QAM and 64 QAM are used as uplink E-UTRA
modulation schemes, the latter being optional for the UE. The DFT
transforms the modulation symbols into the frequency domain. The result is
mapped onto the available sub-carriers. In E-UTRA uplink, only localized
transmission on consecutive sub-carriers is allowed. An N point IFFT where
N>M is then performed as in OFDM, followed by addition of the cyclic prefix
and parallel to serial conversion.
m1 bits 0
Bit to x(0,n) fo 0
Constellation 0
Mapping 0
f1
m2 bits 0
Bit to x(1,n)
Incoming Bit Constellation
Stream Serial to Mapping M-point f M / 2 1 N-point Add cyclic Parallel to
Parallel IFFT prefix Serial
Converter FFT
fM / 2 converter
fM 2
mM bits 0
Bit to x(M- 1,n) 0
Constellation
Mapping fM 1 0
0
0
Channel BW
Figure 8 Block Diagram of DFT-s-OFDM (Localized transmission)
The DFT processing is therefore the fundamental difference between SC-
FDMA and OFDMA signal generation. This is indicated by the term DFT-
spread-OFDM. In an SC-FDMA signal, each sub-carrier used for
transmission contains information of all transmitted modulation symbols,
1MA111_0E 11 Rohde & Schwarz
LTE/E-UTRA
since the input data stream has been spread by the DFT transform over the
available sub-carriers. In contrast to this, each sub-carrier of an OFDMA
signal only carries information related to specific modulation symbols.
SC-FDMA Parametrization
The E-UTRA uplink structure is similar to the downlink. An uplink radio
frame consists of 20 slots of 0.5 ms each, and 1 subframe consists of 2
slots. The slot structure is shown in Figure 9.
UL UL
Each slot carries N symb SC-FDMA symbols, where N symb = 7 for the normal
UL
cyclic prefix and N symb = 6 for the extended cyclic prefix. SC-FDMA symbol
th
number 3 (i.e. the 4 symbol in a slot) carries the reference signal for
channel demodulation.
One uplink slot,Tslot
UL UL
0 1 2 N symb 2 Nsymb 1
Modulation symbol a u , Nsymb
UL
2
Figure 9 Uplink slot structure
Also for the uplink, a bandwidth agnostic layer 1 specification has been
selected.
Table 2 shows the configuration parameters in an overview table.
Table 2 Parameters for uplink generic frame structure
Number of symbols Cyclic Prefix length in Cyclic Prefix length in
Configuration UL
N symb samples µs
Normal cyclic prefix 160 for first symbol 5.2 µs for first symbol
7
Nf=15 kHz 144 for other symbols 4.7 µs for other symbols
Extended cyclic prefix
6 512 16.7 µs
Nf=15 kHz
Uplink Data Transmission
In uplink, data is allocated in multiples of one resource block. Uplink
resource block size in the frequency domain is 12 sub-carriers, i.e. the
same as in downlink. However, not all integer multiples are allowed in order
to simplify the DFT design in uplink signal processing. Only factors 2,3, and
5 are allowed.
The uplink transmission time interval is 1 ms (same as downlink).
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User data is carried on the Physical Uplink Shared Channel (PUSCH) that
is determined by the transmission bandwidth NTx and the frequency hopping
pattern k0.
The Physical Uplink Control Channel (PUCCH) carries uplink control
information, e.g. CQI reports and ACK/NACK information related to data
packets received in the downlink. The PUCCH is transmitted on a reserved
frequency region in the uplink.
Uplink Reference Signal Structure
Uplink reference signals are used for two different purposes: on the one
hand, they are used for channel estimation in the eNodeB receiver in order
to demodulate control and data channels. On the other hand, the reference
signals provide channel quality information as a basis for scheduling
decisions in the base station. The latter purpose is also called channel
sounding.
The uplink reference signals are based on CAZAC (Constant Amplitude
Zero Auto-Correlation) sequences.
Uplink Physical Layer Procedures
For E-UTRA, the following uplink physical layer procedures are especially
important:
Non-synchronized random access:
The random access may be used to request initial access, as part
of handover, when transiting from idle to connected, or to re-
establish uplink synchronization. The structure is shown in Figure
10.
Figure 10 Random access structure, principle
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LTE/E-UTRA
Multiple random access channels may be defined in the frequency
domain within one access period TRA in order to provide a sufficient
number of random access opportunities.
For the random access, a preamble is defined as shown in Figure
11. The preamble sequence occupies TPRE = 0.8 ms and the cyclic
prefix occupies TCP = 0.1 ms within one subframe of 1 ms. During
the guard time TGT, nothing is transmitted. The preamble bandwidth
is 1.08 MHz (72 sub-carriers). Higher layer signaling controls in
which sub-frames the preamble transmission is allowed, and the
location in the frequency domain. Per cell, there are 64 random
access preambles. They are generated from Zadoff-Chu
sequences.
TRA
CP Preamble
TCP TPRE TGT
Figure 11 Random access preamble
The random access procedure uses open loop power control with
power ramping similar to WCDMA. After sending the preamble on a
selected random access channel, the UE waits for the random
access response message. If no response is detected then another
random access channel is selected and a preamble is sent again.
Uplink scheduling:
Scheduling of uplink resources is done by eNodeB. The eNodeB
assigns certain time/frequency resources to the UEs and informs
UEs about transmission formats to use. Scheduling decisions
affecting the uplink are communicated to the UEs via the PDCCH in
the downlink. The scheduling decisions may be based on QoS
parameters, UE buffer status, uplink channel quality
measurements, UE capabilities, UE measurement gaps, etc.
Uplink link adaptation:
As uplink link adaptation methods, transmission power control,
adaptive modulation and channel coding rate, as well as adaptive
transmission bandwidth can be used.
Uplink timing control:
Uplink timing control is needed to time align the transmissions from
different UEs with the receiver window of the eNodeB. The eNodeB
sends the appropriate timing-control commands to the UEs in the
downlink, commanding them to adapt their respective transmit
timing.
Hybrid ARQ:
Uplink Hybrid ARQ protocol is already known from HSUPA. The
eNodeB has the capability to request retransmissions of incorrectly
received data packets.
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5 LTE MIMO Concepts
Multiple Input Multiple Output (MIMO) systems form an essential part of
LTE in order to achieve the ambitious requirements for throughput and
spectral efficiency. MIMO refers to the use of multiple antennas at
transmitter and receiver side.
Downlink MIMO
For the LTE downlink, a 2x2 configuration for MIMO is assumed as
baseline configuration, i.e. 2 transmit antennas at the base station and 2
receive antennas at the terminal side. Configurations with 4 antennas are
also being considered.
Downlink MIMO modes
Different MIMO modes are envisaged. It has to be differentiated between
spatial multiplexing and transmit diversity, and it depends on the channel
condition which scheme to select.
Spatial Multiplexing
Spatial multiplexing allows to transmit different streams of data
simultaneously on the same downlink resource block(s). These data
streams can belong to one single user (single user MIMO / SU-MIMO) or to
different users (multi user MIMO / MU-MIMO). While SU-MIMO increases
the data rate of one user, MU-MIMO allows to increase the overall capacity.
Spatial multiplexing is only possible if the mobile radio channel allows it.
Figure 12 shows the principle of spatial multiplexing, exploiting the spatial
dimension of the radio channel which allows to transmit the different data
streams simultaneously.
Figure 12 Spatial multiplexing
In Figure 12, each transmit antenna transmits a different data stream. Each
receive antenna may receive the data streams from all transmit antennas.
The channel (for a specific delay) can thus be described by the following
channel matrix H:
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LTE/E-UTRA
In this general description, Nt is the number of transmit antennas, Nr is the
number of receive antennas, resulting in a 2x2 matrix for the baseline LTE
scenario. The coefficients hij of this matrix are called channel coefficients
from transmit antenna j to receive antenna i, thus describing all possible
paths between transmitter and receiver side.
The number of data streams that can be transmitted in parallel over the
MIMO channel is given by min {Nt, Nr} and is limited by the rank of the
matrix H. The transmission quality degrades significantly in case the
singular values of matrix H are not sufficiently strong. This can happen in
case the 2 antennas are not sufficiently de-correlated, for example in an
environment with little scattering or when antennas are too closely spaced.
In LTE, up to 2 code words can be mapped onto different so-called layers.
The number of layers for transmission is equal to the rank of the matrix H.
There is a fixed mapping between code words to layers.
Precoding on transmitter side is used to support spatial multiplexing, see
Figure 13. This is achieved by applying a precoding matrix W to the signal
before transmission.
Figure 13 Precoding principle
The optimum precoding matrix W is selected from a predefined “codebook”
which is known at eNodeB and UE side. Unitary precoding is used, i.e. the
H
precoding matrices are unitary: W W = I. The UE estimates the radio
channel and selects the optimum precoding matrix. The optimum pre-
coding matrix is the one which offers maximum capacity. The UE provides
feedback on the uplink control channel regarding the preferred precoding
matrix (precoding vector as a special case). Ideally, this information is
made available per resource block or at least group of resource blocks,
since the optimum precoding matrix varies between resource blocks
Figure 14 gives an overview of EUTRA downlink baseband signal
generation including the above-mentioned steps relevant for MIMO
transmission.
code words layers antenna ports
Modulation OFDM OFDM signal
Scrambling
Mapper Mapper generation
Layer
Precoding
Mapper
Modulation OFDM OFDM signal
Scrambling
Mapper Mapper generation
Figure 14 Overview of downlink baseband signal generation
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LTE/E-UTRA
Transmit Diversity
Instead of increasing data rate or capacity, MIMO can be used to exploit
diversity. Transmit diversity schemes are already known from WCDMA
release 99 and will also form part of LTE as one MIMO mode. In case the
channel conditions do not allow spatial multiplexing, a transmit diversity
scheme will be used instead, so switching between these two MIMO modes
is possible depending on channel conditions. Transmit diversity is used
when the selected number of streams (rank) is one.
Uplink MIMO
Uplink MIMO schemes for LTE will differ from downlink MIMO schemes to
take into account terminal complexity issues. For the uplink, MU-MIMO can
be used. Multiple user terminals may transmit simultaneously on the same
resource block. This is also referred to as spatial domain multiple access
(SDMA). The scheme requires only one transmit antenna at UE side which
is a big advantage. The UEs sharing the same resource block have to apply
mutually orthogonal pilot patterns.
To exploit the benefit of two or more transmit antennas but still keep the UE
cost low, antenna subset selection can be used. In the beginning, this
technique will be used, e.g. a UE will have two transmit antennas but only
one transmit chain and amplifier. A switch will then choose the antenna that
provides the best channel to the eNodeB.
6 LTE Protocol Architecture
System Architecture Evolution (SAE)
SAE is a study within 3GPP targeting at the evolution of the overall system
architecture. Objective is “to develop a framework for an evolution or
migration of the 3GPP system to a higher-data-rate, lower-latency, packet-
optimized system that supports multiple radio access technologies. The
focus of this work is on the PS domain with the assumption that voice
services are supported in this domain". This study includes the vision of an
all-IP network [5]. Clear requirement is the support of heterogeneous
access networks in terms of mobility and service continuity. Figure 15 gives
an overview of the evolved system architecture.
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LTE/E-UTRA
Figure 15 System architecture evolution
E-UTRAN
E-UTRAN stage 2 description can be found in [4]. The E-UTRAN consists
of eNodeBs (eNBs), providing the E-UTRA user plane
(PDPC/RLC/MAC/PHY) and control plane (RRC) protocol terminations
towards the UE. The eNBs are interconnected with each other by means of
the X2 interface. The eNBs are also connected by means of the S1
interface to the EPC (Evolved Packet Core), more specifically to the MME
(Mobility Management Entity) and to the SAE Gateway. NAS protocols are
terminated in MME.
The following figure illustrates the functional split between eNodeB (eNB)
and Evolved Packet Core.
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LTE/E-UTRA
eNB
Inter Cell RRM
RB Control
Connection Mobility Cont.
Radio Admission Control MME
eNB Measurement
NAS Security
Configuration & Provision
Dynamic Resource Idle State Mobility
Allocation (Scheduler) Handling
RRC PDCP SAE Bearer Control
RLC
SAE Gateway
MAC
S1
PHY Mobility Anchoring
internet
E-UTRAN EPC
Figure 16 Functional split between E-UTRAN and Evolved Packet Core
The base station functionality has increased significantly in E-UTRAN, e.g.
compared to WCDMA release 99. The base station hosts functions for
radio bearer control, admission control, mobility control, uplink and downlink
scheduling as well as measurement configuration.
User plane protocol stack is shown in Figure 17.
Figure 17 User plane protocol stack
Control plane protocol stack is shown in Figure 18.
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LTE/E-UTRA
Figure 18 Control plane protocol stack
Layer 2 structure
Figure 19 and Figure 20 show the downlink and uplink structure of layer 2.
The service access points between the physical layer and the MAC
sublayer provide the transport channels. The service access points
between the MAC sublayer and the RLC sublayer provide the logical
channels. Radio bearers are defined on top of PDCP layer. Multiplexing of
several logical channels on the same transport channel is possible. In both
uplink and downlink, only one transport block is generated per transmission
time interval in the non-MIMO case. E-UTRAN provides ARQ and HARQ
functionalities. The ARQ functionality provides error correction by
retransmissions in acknowledged mode at layer 2. The HARQ functionality
ensures delivery between peer entities at layer 1. The HARQ is an N-
channel stop-and-wait protocol with asynchronous downlink
retransmissions and synchronous uplink retransmissions. ARQ
retransmissions are based on RLC status reports and HARQ/ARQ
interaction.
Figure 19 Downlink layer 2 structure
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