Mts long term evolution (lte)

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Rohde & Schwarz Products: FSQ, FSG, FSV, FSQ-K100, FSV-K100, FSQ-K101, FSV-K101, FSQ-K102,
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SMJ-K55, SMJ-K255, WinIQSIM2, AFQ100A/B, AFQ-K255, AMU200A, AMU-
K55, AMU-K255, CMW500, TS8980
UMTS Long Term Evolution (LTE)
Technology Introduction
Application Note 1MA111
Even with the introduction of HSPA, 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) has been
introduced 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 FDD
and TDD technology and testing aspects.
Subject to change – C.Gessner 09.2008 – 1MA111_2E
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..................................................................... 7
Downlink data transmission ............................................................. 10
Downlink control channels ............................................................... 11
Downlink reference signal structure and cell search........................ 15
Downlink Hybrid ARQ (Automatic Repeat Request) ........................ 17
4 LTE Uplink Transmission Scheme ........................................................ 17
SC-FDMA......................................................................................... 17
SC-FDMA parametrization ............................................................... 18
Uplink data transmission.................................................................. 20
Uplink control channel PUCCH ........................................................ 23
Uplink reference signal structure ..................................................... 24
Random access ............................................................................... 26
Uplink Hybrid ARQ (Automatic Repeat Request)............................. 28
5 LTE MIMO Concepts ............................................................................. 28
Downlink MIMO modes in LTE......................................................... 30
Reporting of UE feedback ................................................................ 32
Uplink MIMO .................................................................................... 33
6 LTE Protocol Architecture...................................................................... 33
System Architecture Evolution (SAE)............................................... 33
E-UTRAN ......................................................................................... 33
Layer 3 procedures .......................................................................... 35
Layer 2 structure .............................................................................. 37
Transport channels .......................................................................... 38
Logical channels .............................................................................. 39
Transport block structure (MAC Protocol Data Unit (PDU))............. 40
7 UE capabilities....................................................................................... 41
8 LTE Testing............................................................................................ 41
LTE RF testing ................................................................................. 41
LTE layer 1 and protocol test ........................................................... 47
9 Abbreviations......................................................................................... 49
10 Additional Information ........................................................................... 52
11 References............................................................................................ 52
12 Ordering Information ............................................................................. 53
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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 Generators R&S® AFQ100A/B are referred to as the
AFQ100A/B.
- 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.
- The Signal Analyzer R&S® FSG is referred to as FSG.
- The Signal Analyzer R&S® FSV is referred to as FSV.
- The Wideband Radio Communication Tester R&S® CMW500 is
referred to as the CMW500.
- The RF test system R&S® TS8980 is referred to as the TS8980.
1 Introduction
Currently, UMTS networks worldwide are being upgraded to High Speed
Packet Access (HSPA) in order to increase data rate and capacity for
packet data. HSPA refers to the combination of High Speed Downlink
Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA).
While HSDPA was introduced as a 3GPP release 5 feature, HSUPA is an
important feature of 3GPP release 6.
However, even with the introduction of HSPA, evolution of UMTS has not
reached its end. HSPA+ will bring significant enhancements in 3GPP
release 7 and 8. 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
introduced in 3GPP release 8. Objective is a high-data-rate, low-latency and
packet-optimized radio access technology. LTE is also referred to as E-
UTRA (Evolved UMTS Terrestrial Radio Access) or E-UTRAN (Evolved
UMTS Terrestrial Radio Access Network).
This application note focuses on LTE/E-UTRA technology. In the following,
the terms LTE or E-UTRA are used interchangeably.
LTE has ambitious requirements for data rate, capacity, spectrum
efficiency, and latency. In order to fulfill these requirements, LTE is based
on new technical principles. LTE uses new multiple access schemes on the
air interface: OFDMA (Orthogonal Frequency Division Multiple Access) in
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downlink and SC-FDMA (Single Carrier Frequency Division Multiple
Access) in uplink. Furthermore, MIMO antenna schemes form an essential
part of LTE. In order 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 referred to as
3GPP System Architecture Evolution (SAE).
LTE includes an FDD (Frequency Division Duplex) mode of operation and a
TDD (Time Division Duplex) mode of operation. LTE TDD which is also
referred to as TD-LTE provides the long term evolution path for TD-SCDMA
based networks. This application note gives an introduction to LTE
technology, including both FDD and TDD modes of operation.
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 UE capabilities.
Chapter 8 explains test requirements for LTE.
Chapters 9-12 provide additional information including literature
references.
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 were identified in the
beginning of the standardization work on LTE and have been captured in
3GPP TR 25.913 [Ref. 1]. They 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. Note: These requirement values
are exceeded by the LTE specification, see chapter 7.
- 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, i.e. 1.4 MHz and 3 MHz.
- 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
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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. Note: E-MBMS
specification has been largely moved to 3GPP release 9.
- 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
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 subcarriers. Each of these
subcarriers 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 [Ref. 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 subcarriers. 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-
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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 [Ref. 2]
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 subcarrier modulated data symbol, during the time
period mTu < t (m+1)Tu.
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 [[Ref. 2]
The vector sm is defined as the useful OFDM symbol. It is the time
superposition of the N narrowband modulated subcarriers. Therefore, from
a parallel stream of N sources of data, each one independently modulated,
a waveform composed of N orthogonal subcarriers is obtained, with each
subcarrier 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.
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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 [Ref. 2]
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
Two frame structure types are defined for E-UTRA: frame structure type 1
for FDD mode, and frame structure type 2 for TDD mode. The E-UTRA
frame structures are defined in [Ref. 3].
For the frame structure type 1, the 10 ms radio frame is divided into 20
equally sized slots of 0.5 ms. A subframe consists of two consecutive slots,
so one radio frame contains ten subframes. 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 Frame structure type 1 [Ref. 3]
For the frame structure type 2, the 10 ms radio frame consists of two half-
frames of length 5 ms each. Each half-frame is divided into five subframes
of each 1 ms, as shown in Figure 5 below. All subframes which are not
special subframes are defined as two slots of length 0.5 ms in each
subframe. The special subframes consist of the three fields DwPTS
(Downlink Pilot Timeslot), GP (Guard Period), and UpPTS (Uplink Pilot
Timeslot). These fields are already known from TD-SCDMA and are
maintained in LTE TDD. DwPTS, GP and UpPTS have configurable
individual lengths and a total length of 1ms.
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One radio frame Tf =10 ms
One half- frame Thf = 5 ms
T = 1 ms
Subframe #0 Subframe #2 Subframe #3 Subframe #4 Subframe #5 Subframe #7 Subframe #8 Subframe #9
One slot, One subframe,
Tslot = 0.5 ms Tsf = 1 ms
DwPTS GP UpPTS DwPTS GP UpPTS
Figure 5 Frame structure type 2 (for 5 ms switch-point periodicity) [Ref. 3]
Seven uplink-downlink configurations with either 5 ms or 10 ms downlink-
to-uplink switch-point periodicity are supported. In case of 5 ms switch-point
periodicity, the special subframe exists in both half-frames. In case of 10
ms switch-point periodicity the special subframe exists in the first half frame
only. Subframes 0 and 5 and DwPTS are always reserved for downlink
transmission. UpPTS and the subframe immediately following the special
subframe are always reserved for uplink transmission. Table 1 shows the
supported uplink-downlink configurations, where “D” denotes a subframe
reserved for downlink transmission, “U” denotes a subframe reserved for
uplink transmission, and “S” denotes the special subframe.
Table 2 Uplink-Downlink configurations for LTE TDD [Ref. 3]
Figure 6 shows the structure of the downlink resource grid for both FDD
and TDD.
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Figure 6 Downlink resource grid [Ref. 3]
The subcarriers in LTE have a constant spacing of f = 15 kHz. In the
frequency domain, 12 subcarriers form one resource block. The resource
block size is the same for all bandwidths. The number of resource blocks
for the different LTE bandwidths is listed in Table 3.
Table 3 Number of resource blocks for different LTE bandwidths
(FDD and TDD) [Ref. 4]
Channel
1.4 3 5 10 15 20
bandwidth [MHz]
Number of
6 15 25 50 75 100
resource blocks
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To each OFDM symbol, a cyclic prefix (CP) is appended as guard time,
compare Figure 1. One downlink slot consists of 6 or 7 OFDM symbols,
depending on whether extended or normal cyclic prefix is configured,
respectively. The extended cyclic prefix is able to cover larger cell sizes with
higher delay spread of the radio channel. The cyclic prefix lengths in
samples and µs are summarized in Table 4.
Table 4 Downlink frame structure parametrization (FDD and TDD) [Ref. 3]
Resource block Number of
RB symbols Cyclic Prefix length Cyclic Prefix length in
Configuration size N sc
DL in samples µs
N symb
Normal cyclic prefix 160 for first symbol 5.2 µs for first symbol
12 7
Pf=15 kHz 144 for other symbols 4.7 µs for other symbols
Ext. cyclic prefix
12 6 512 16.7 µs
Pf=15 kHz
Downlink data transmission
Data is allocated to the UEs in terms of resource blocks, i.e. one UE can be
allocated integer multiples of one resource block in the frequency domain.
These resource blocks do not have to be adjacent to each other. In the time
domain, the scheduling decision can be modified every transmission time
interval of 1 ms. The scheduling decision is done in the base station
(eNodeB). The scheduling algorithm has to take into account the radio link
quality situation of different users, the overall interference situation, Quality
of Service requirements, service priorities, etc. Figure 1 shows an example
for allocating downlink user data to different users (UE 1 – 6).
The user data is carried on the Physical Downlink Shared Channel
(PDSCH).
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Figure 7 OFDMA time-frequency multiplexing (example for normal cyclic
prefix)
Downlink control channels
The Physical Downlink Control Channel (PDCCH) serves a variety of
purposes. Primarily, it is used to convey the scheduling decisions to
individual UEs, i.e. scheduling assignments for uplink and downlink.
The PDCCH is located in the first OFDM symbols of a subframe. For frame
structure type 2, PDCCH can also be mapped onto the first two OFDM
symbols of DwPTS field.
An additional Physical Control Format Indicator Channel (PCFICH) carried
on specific resource elements in the first OFDM symbol of the subframe is
used to indicate the number of OFDM symbols for the PDCCH (1, 2, 3, or 4
symbols are possible). PCFICH is needed because the load on PDCCH can
vary, depending on the number of users in a cell and the signaling formats
conveyed on PDCCH.
The information carried on PDCCH is referred to as downlink control
information (DCI). Depending on the purpose of the control message,
different formats of DCI are defined. As an example, the contents of DCI
format 1 are shown in Table 5. DCI format 1 is used for the assignment of a
downlink shared channel resource when no spatial multiplexing is used (i.e.
the scheduling information is provided for one code word only). The
information provided contains everything what is necessary for the UE to be
able to identify the resources where to receive the PDSCH in that subframe
and how to decode it. Besides the resource block assignment, this also
includes information on the modulation and coding scheme and on the
hybrid ARQ protocol.
The Cyclic Redundancy Check (CRC) of the DCI is scrambled with the UE
identity that is used to address the scheduled message to the UE.
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Table 5 Contents of DCI format 1 carried on PDCCH [Ref. 5]
Number of bits on
Information type Purpose
PDCCH
Resource allocation
1 Indicates whether resource allocation type 0 or 1 is used
header
Depending on
Resource block
resource allocation Indicates resource blocks to be assigned to the UE
assignment
type
Indicates modulation scheme and, together with the
Modulation and coding
5 number of allocated physical resource blocks, the
scheme
transport block size
HARQ process Identifies the HARQ process the packet is associated
3 (TDD), 4 (FDD)
number with
Indicates whether the packet is a new transmission or a
New data indicator 1
retransmission
Identifies the redundancy version used for coding the
Redundancy version 2
packet
Transmit power control (TPC) command for adapting the
TPC command for
2 transmit power on the Physical Uplink Control Channel
PUCCH
(PUCCH)
Downlink assignment number of downlink subframes for uplink ACK/NACK
2
index (TDD only) bundling
In order to save signaling resources on PDCCH, more DCI formats to
schedule one code word are defined which are optimized for specific use
cases and transmission modes, for example scheduling of paging channel,
random access response, and system information blocks. DCI formats 2
and 2A provide downlink shared channel assignments in case of closed
loop or open loop spatial multiplexing, respectively. In these cases,
scheduling information is provided for two code words within one control
message. Additionally there is DCI format 0 to convey uplink scheduling
grants, and DCI formats 3 and 3a to convey transmit power control (TPC)
commands for the uplink.
There is different ways to signal the resource allocation within DCI, in order
to trade off between signaling overhead and flexibility. For example, DCI
format 1 may use resource allocation types 0 or 1 as described in the
following. An additional resource allocation type 2 method is specified for
other DCI formats.
In resource allocation type 0, a bit map indicates the resource block
groups that are allocated to a UE. A resource block group (RGB) consists
of a set of consecutive physical resource blocks (1…4 depending on
system bandwidth). The allocated resource block groups do not have to be
adjacent to each other. Figure 8 illustrates the definition of resource block
groups for the 20MHz bandwidth case.
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In resource allocation type 1, a bitmap indicates physical resource blocks
inside a selected resource block group subset. The information field for
the resource block assignment on PDCCH is therefore split up into 3 parts:
one part indicates the selected resource block group subset. 1 bit indicates
whether an offset shall be applied when interpreting the bitmap towards the
resource blocks. The third part contains the bitmap that indicates to the UE
specific physical resource blocks inside the resource block group subset.
These resource blocks do not have to be adjacent to each other. Figure 8
for the 20 MHz case shows the definition of p=4 resource block group
subsets and which resource block groups are part of each subset.
Figure 8 Resource block groups for resource allocation type 0/1 (example:
20 MHz bandwidth, 1 resource block group contains P=4 resource blocks)
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In resource allocation type 2, physical resource blocks are not directly
allocated. Instead, virtual resource blocks are allocated which are then
mapped onto physical resource blocks. The information field for the
resource block assignment carried on PDCCH contains a resource
indication value (RIV) from which a starting virtual resource block and a
length in terms of contiguously allocated virtual resource blocks can be
derived. Both localized and distributed virtual resource block
assignment is possible which are differentiated by a one-bit-flag within the
DCI.
In the localized case, there is a one-to-one mapping between virtual and
physical resource blocks.
Example: Let’s assume a 10 MHz signal, i.e. 50 resource blocks are
available. A UE shall be assigned an allocation of 10 resource blocks
(LCRBs=10), starting from resource block 15 (RBstart=15) in the frequency
domain. According to the formula in [Ref. 6], a value of RIV=465 would
then be signaled to the UE within DCI on PDCCH, and the UE could
unambiguously derive the starting resource block and the number of
allocated resource blocks from RIV again. For the given bandwidth of 10
MHz, 11 bits are available for signaling the RIV within the DCI. Signaling
LCRBs and RBstart explicitly would require 12 bits for the 10 MHz case. By
focusing on the realistic combinations of LCRBs and RBstart using RIV, 1 bit
can therefore be saved and signaling is more efficient.
In the distributed case of resource allocation type 2, the virtual resource
block numbers are mapped to physical resource block numbers according
to the rule specified in [Ref. 3], and inter-slot hopping is applied: The first
part of a virtual resource block pair is mapped to one physical resource
block, the other part of the virtual resource block pair is mapped to a
physical resource block which is a pre-defined gap distance away (which
causes the inter-slot hopping). By doing so, frequency diversity is achieved.
This mechanism is especially interesting for small resource blocks
allocations, because these inherently provide less frequency diversity.
Besides PCFICH and PDCCH, additional downlink control channels are the
Physical Hybrid ARQ Indicator channel (PHICH) and the Physical Broadcast
Channel (PBCH). PHICH is used to convey ACK/NACKs for the packets
received in uplink, see the section on uplink HARQ below. PBCH carries
the Master Information Block, see the section on cell search below. Table 6
shows a summary of downlink control channels.
Table 6 Downlink control channels
Modulation
Downlink control channel Purpose
scheme
Physical Downlink Control Carries downlink control information (DCI), e.g.
QPSK
Channel (PDCCH) downlink or uplink scheduling assignments
Physical Control Format Indicates format of PDCCH (whether it occupies 1, 2,
QPSK
Indicator Channel (PCFICH) 3, or 4 symbols)
Physical Hybrid ARQ Indicator
Carries ACK/NACKs for uplink data packets BPSK
Channel (PHICH)
Physical Broadcast Channel
Carries Master Information Block QPSK
(PBCH)
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Downlink reference signal structure and cell search
The downlink reference signal structure is important for channel estimation.
Figure 9 shows the principle of the downlink reference signal structure for 1
antenna, 2 antenna, and 4 antenna transmission. Specific pre-defined
resource elements (indicated by R0-3 in Figure 9) in the time-frequency
domain are carrying the cell-specific reference signal sequence.
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 9 Downlink reference signal structure (normal cyclic prefix) [Ref. 3]
The reference signal sequence is derived from a pseudo-random sequence
and results in a QPSK type constellation. Cell-specific frequency shifts are
applied when mapping the reference signal sequence to the subcarriers.
During cell search, different types of information need to be identified by the
UE: symbol and radio frame timing, frequency, cell identification, overall
transmission bandwidth, antenna configuration, cyclic prefix length.
The first step of cell search in LTE is based on specific synchronization
signals. LTE uses a hierarchical cell search scheme similar to WCDMA.
Thus, a primary synchronization signal and a secondary
synchronization signal are defined. The synchronization signals are
transmitted twice per 10 ms on predefined slots, see Figure 10 for FDD and
Figure 11 for TDD. In the frequency domain, they are transmitted on 62
subcarriers within 72 reserved subcarriers around DC subcarrier.
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The 504 available physical layer cell identities are grouped into 168 physical
layer cell identity groups, each group containing 3 unique identities (0, 1, or
2). The secondary synchronization signal carries the physical layer cell
identity group, and the primary synchronization signal carries the physical
layer identity 0, 1, or 2.
10 ms radio frame
Primary synchronization signal
Secondary synchronization signal
Physical Broadcast Channel (PBCH)
0.5 ms slot
1 ms subframe
Figure 10 Primary/secondary synchronization signal and PBCH structure
(frame structure type 1 / FDD, normal cyclic prefix)
10 ms radio frame
Primary synchronization signal
Secondary synchronization signal
Physical Broadcast Channel
0.5 ms slot
1 ms subframe
Figure 11 Primary/secondary synchronization signal and PBCH structure
(frame structure type 2 / TDD, normal cyclic prefix)
As additional help during cell search, a Primary Broadcast Channel (PBCH)
is available which carries the Master Information Block with basic physical
layer information, e.g. system bandwidth, number of transmit antennas, and
system frame number. It is transmitted within specific symbols of the first
subframe on the 72 subcarriers centered around DC subcarrier. PBCH has
40 ms transmission time interval.
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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 Hybrid ARQ (Automatic Repeat Request)
Downlink Hybrid ARQ is also known from HSDPA. It is a retransmission
protocol. The UE can request retransmissions of data packets that were
incorrectly received on PDSCH. ACK/NACK information is transmitted in
uplink, either on Physical Uplink Control Channel (PUCCH) or multiplexed
within uplink data transmission on Physical Uplink Shared Channel
(PUSCH). 8 HARQ processes can be used.
The ACK/NACK transmission in FDD mode refers to the downlink packet
that was received four subframes before. In TDD mode, the uplink
ACK/NACK timing depends on the uplink/downlink configuration. For TDD,
the use of a single ACK/NACK response for multiple PDSCH transmissions
is possible (so-called ACK/NACK bundling).
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 12.
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 subcarriers. In E-UTRA uplink, only localized
transmission on consecutive subcarriers 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.
1MA111_2E 17 Rohde & Schwarz
LTE/E-UTRA
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 N-point Add cyclic Parallel to
M-point f M / 2 1
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 12 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 subcarrier used for
transmission contains information of all transmitted modulation symbols,
since the input data stream has been spread by the DFT transform over the
available subcarriers. In contrast to this, each subcarrier of an OFDMA
signal only carries information related to specific modulation symbols.
SC-FDMA parametrization
The LTE uplink structure is similar to the downlink. In frame structure type
1, an uplink radio frame consists of 20 slots of 0.5 ms each, and one
subframe consists of two slots. The slot structure is shown in Figure 13.
Frame structure type 2 consists also of ten subframes, but one or two of
them are special subframes. They include DwPTS, GP and UpPTS fields,
see Figure 5.
Each slot carries 7 SC-FDMA symbols in case of normal cyclic prefix
configuration, and 6 SC-FDMA symbols in case of extended cyclic prefix
th
configuration. SC-FDMA symbol number 3 (i.e. the 4 symbol in a slot)
carries the reference signal for channel demodulation.
1MA111_2E 18 Rohde & Schwarz
LTE/E-UTRA
Figure 13 Uplink resource grid [Ref. 3]
Table 7 shows the configuration parameters in an overview table.
Table 7 Uplink frame structure parametrization (FDD and TDD) [Ref. 3]
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
Pf=15 kHz 144 for other symbols 4.7 µs for other symbols
Extended cyclic prefix
6 512 16.7 µs
Pf=15 kHz
1MA111_2E 19 Rohde & Schwarz
LTE/E-UTRA
Uplink data transmission
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. The scheduling decisions may be based on
QoS parameters, UE buffer status, uplink channel quality measurements,
UE capabilities, UE measurement gaps, etc.
In uplink, data is allocated in multiples of one resource block. Uplink
resource block size in the frequency domain is 12 subcarriers, 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. Unlike in the downlink, UEs are always assigned contiguous
resources in the LTE uplink.
The uplink transmission time interval is 1 ms (same as downlink).
User data is carried on the Physical Uplink Shared Channel (PUSCH).
By use of uplink frequency hopping on PUSCH, frequency diversity
effects can be exploited and interference can be averaged.
The UE derives the uplink resource allocation as well as frequency hopping
information from the uplink scheduling grant that was received four
subframes before. DCI (Downlink Control Information) format 0 is used on
PDCCH to convey the uplink scheduling grant, see Table 8.
Table 8 Contents of DCI format 0 carried on PDCCH [Ref. 5]
Number of bits
Information type Purpose
on PDCCH
Flag for format 0 / format
1 Indicates DCI format to UE
1A differentiation
Hopping flag 1 Indicates whether uplink frequency hopping is used or not
Indicates whether to use type 1 or type 2 frequency
Resource block Depending on
hopping and index of starting resource block of uplink
assignment and hopping resource
resource allocation as well as number of contiguously
resource allocation allocation type
allocated resource blocks
Indicates modulation scheme and, together with the
Modulation and coding number of allocated physical resource blocks, the
scheme and redundancy 5 transport block size
version
Indicates redundancy version to use
New data indicator 1 Indicates whether a new transmission shall be sent
1MA111_2E 20 Rohde & Schwarz