C010341216

IOSR Journal of Electronics and Communication Engineering (IOSR-JECE)
e-ISSN: 2278-2834,p- ISSN: 2278-8735.Volume 10, Issue 3, Ver. IV (May - Jun.2015), PP 12-16
www.iosrjournals.org
DOI: 10.9790/2834-10341216 www.iosrjournals.org 12 | Page
Physical Layer Frame Structure in 4G LTE/LTE-A Downlink
based on LTE System Toolbox
Sherry Varghese George1
, Luxy Mathews2
, Sakuntala S. Pillai3
1,2,3
(Department of Electronics and Communication Engineering,
Mar Baselios College of Engineering and Technology, Trivandrum, India)
Abstract: LTE/LTE-A is the standard for next generation wireless communications which provides high data
rates of 1 Gbps. LTE-A is an extension of LTE providing five times higher bandwidth compared to LTE. Release
10 LTE-A is backward compatible with Release 8 LTE making it a highly dissolvable system. Traditional system
models in LTE used the 3GPP spatial channel model where the parameters have to be changed at the backend
which is a slightly complex process. In this paper, a new approach for the modeling of LTE/LTE-A systems in
the downlink is done where the physical signals and physical channels are generated and plotted onto the
specified indices. This is much easier compared to the models used traditionally.
Keywords: 3GPP, LTE, LTE-A, LTE system toolbox, Physical frame structure
I. Introduction
The communication scenario in the world is growing at a large pace and the industry is gearing up for a
thousand times more faster and reliable data rates. So, the standards of wireless communication are taking a
transition to meet the demands of the rapidly growing customer base. The 3GPP (Third Generation Partnership
Project) is an organisation of telecommunication associations for the development and improvement of GSM
standard and was later extended for the development of newer standards of wireless communication.
Multiple Input Multiple Output (MIMO) is a default feature in LTE systems to boost the overall data
rates. It offers superior data rates without requiring additional bandwidth and transmit power. The number of
antennas at the transmitter or receiver will decide the number of data streams supported. Traditional cellular
systems provide best performance under line-of-sight conditions but MIMO tries to provide best performance
under rich scattering conditions. LTE systems make the promise of delivering large data rate to users which is
substantially larger than what the current 3G systems are offering. This is a challenging proposition as wireless
networks are subject to interference and multipath. So, MIMO techniques have emerged as a solution to provide
high data rates by exploiting multipath characteristics of wireless channel. This is accomplished by using several
antennas at the transmitter and receiver leveraging spatial dimensions. When the signals are properly combined at
the receiver, data rate of each MIMO user will be improved [1]. The different configurations possible for MIMO
are 2X2,4X4, 8X8. Higher configuration of MIMO antennas will lead to higher data rates.
OFDM (Orthogonal Frequency Division Multiplexing) is a multi-carrier modulation technique where the
total bandwidth is split into a large number of smaller and narrow bandwidth units known as subcarriers which
are orthogonal to each other. Orthogonality means that the maximum of one subcarrier is at the minimum of the
next subcarrier thereby eliminating inter symbol interference (ISI). Frequency representation of one OFDM
subcarrier is a sinc function. OFDM converts high speed data channel into a number of low speed channels so
that processing becomes easier. OFDMA (Orthogonal Frequency Division Multiple Access) is an access scheme
that uses OFDM principle to organize the distribution of scarce radio resources among several users enabling
multi-user communication [2]. A set of carriers known as pilot carriers are used to track the residual phase error if
present after correction of the frequency. The carriers of OFDM are having different frequencies and Discrete
Fourier Transform (DFT) is applied to generate orthogonal subcarriers. The reason for choosing OFDMA in the
downlink is the bandwidth flexibility it offers, since changing the number of subcarriers can increase or decrease
the used frequency bandwidth.
SCFDMA (Single Carrier Frequency Domain Multiple Access) is used in the uplink transmission
scheme of LTE. It is a very desirable technique for the uplink to have an efficient usage of power amplifier in the
transceiver antenna. This provides a high battery life to the devices used by the customers. SCFDMA provides
low peak to average power ratio (PAPR) which is an advantage over the OFDMA technique used in the
downlink.
MIMO-OFDM is the dominant air interface for 4G and 5G broadband systems. It combines MIMO
technology which multiplies capacity by transmitting different signals over multiple antennas and OFDM which
divides a radio channel into a large number of closely spaced subchannels to provide more reliable
communication at higher speeds. MIMO provides a high data rate along with OFDM eliminating the interference
makes the MIMO-OFDM medium a highly reliable one [3].

Physical Layer Frame Structure in 4G LTE/LTE-A Downlink based on LTE System Toolbox
II. Long Term Evolution (LTE)
The LTE (Long Term Evolution) became a standard for wireless communications under the 3GPP after
the GSM and CDMA. These standards are structured as release versions by 3GPP board and the first LTE was
released in 2008. First generation (1G), second generation (2G) and third generation (3G) of mobile
communication works on GSM standard and 4G and higher generations work on LTE standard.
LTE follows a flat architecture which is completely different from the architecture of GSM which is
having a two dimensional architecture. It is an all IP architecture where all the devices are having an IP address
and it is called as Evolved Packet Core. LTE provides a bandwidth of 20 MHz which is capable of a downlink
peak rate of 300 Mbps and an uplink peak rate of 75 Mbps. LTE follows an all IP based network architecture
called as Evolved Packet Core (EPC).
In LTE systems, the base station is called as eNB (evolved node B) which directly communicates with
the mobile station or UE (user equipment). Evolved node B is the hardware in the mobile phone network that
communicates directly with the mobile. It is similar to Base Transceiver Station (BTS) in GSM which is
controlled by the Radio Network Controller (RNC). There are three processes which occurs at the eNB: first is
the transmission mode selection, second is precoding, and the third is resource allocation and scheduling. In this
paper, resource allocation and scheduling is considered based on the mobility of users [4].
III. Long Term Evolution – Advanced (LTE-A)
LTE-A (Long Term Evolution-Advanced) is an extension of LTE which provides a bandwidth of 100
MHz which is capable of a downlink peak rate of 3.3 Gbps and an uplink peak rate of 500 Mbps. LTE-A uses a
technique known as carrier aggregation for obtaining the 100 MHz bandwidth which in turn reflects in high data
rates. It means that a number of different component carriers are combined at the device so as to increase the data
rate and bandwidth. A maximum of five component carriers can be combined to get a bandwidth of 100 MHz.
The bandwidth can be split and used like 1.4 MHz, 3.5 MHz, 10 MHz, 15 MHz and 20 MHz. It includes
smoother handoff procedures, new transmission protocols and stuffing more bits per second into a single hertz of
spectrum.
In LTE/LTE-A systems, the multiple access schemes are based on FDM (Frequency Domain
Multiplexing). OFDMA is widely employed in downlink and it has very robust characteristics against frequency
selective channels. The reason for using OFDMA in the downlink is the bandwidth flexibility it offers, since
changing the number of subcarriers used can be increased or decreased. SCFDMA (Single Carrier Frequency
Division Multiple Access) is a very desirable technique for the uplink transmission.
IV. Simulation Scenario
The simulation scenario under consideration is MATLAB R2014a. It is the penultimate version of
MATLAB released in 2014. The main highlight of this version of the software is the addition of LTE system
toolbox which consists of more than 200 functions. These functions are inbuilt in the software and it can be used
for the design, simulation and verification of LTE wireless communication systems. There are several
propagation models which can be configured, simulated, measured and analysed for end-to-end communication
links.
TABLE 1 shows the configuration and parameters used for simulation. The simulation parameters in
the table correspond to those parameters defined at the base station or eNB. These parameters are assigned with
the values specified in the table. The number of downlink resource blocks of 6 corresponds to 1.4 MHz of
system bandwidth in LTE system. The parameter name should be same as that in the table and each parameter
should be prefixed with „enb.‟.
Table 1: Simulation configuration and parameters
Simulation Parameters Value
CyclicPrefix Normal
PHICHDuration Normal
Ng Sixth
NDLRB 6
System bandwidth 1.4 MHz
DuplexMode FDD
NCellID 1
NSubframe 0
CFI 1
V. Physical Frame Structure of LTE
Scheduling is the process of allocating resource blocks to users. One resource block has duration of 7
OFDM symbols (0.5 ms) in the time domain and 12 subcarriers in the frequency domain. Two such resource
blocks constitute a PRBP (Physical Resource Block Pair) which has duration of 14 OFDM symbols (1 ms) in

the time domain. A number of such PRBP‟s constitute a PRBG (Physical Resource Block Group) and usually
resources are allocated as PRBG‟s so as to reduce the system overheads. The first and foremost step in an LTE
system is to create physical signals and physical channels for the transmission of control data and user data.
Specified slots in the time and frequency domain are allocated by default by this toolbox [5].
5.1 Physical signals
There are two physical signals in LTE which are Primary Synchronisation Signal (PSS) and Secondary
Synchronisation Signal (SSS). These are used by the user equipment (UE) to obtain the identity of the cell and
frame timing. Fig. 2 shows the constellation of PSS which is identical to that of Zadoff Chu sequence. These
physical signals and their indices are generated by the inbuilt functions and the physical signals are mapped to
the indices [2].
Fig. 1 Constellation diagram of Primary Synchronisation Signal
5.2 Physical channels
There are five physical channels in LTE which carry the different control data and user data. These
channels and their indices are generated by the functions and the channel symbols are mapped to the indices [6].
The different physical channels are:
5.2.1 Physical Broadcast Channel (PBCH)
MIB is the first information to be broadcasted by eNB irrespective of the users present in the cell. This
is a 24 bit value which describes the system bandwidth and is transmitted through PBCH channel. If MIB cannot
be configured, the UE considers the cell as barred and no communication will be possible.
5.2.2 Physical HARQ Indicator Channel (PHICH)
LTE uses HARQ scheme for correction of errors in the received control and user data. The eNB will
send a HARQ indicator to UE to indicate a positive or negative acknowledgement for the data. This HARQ
indicator value is sent through PHICH channel. The data sent through this channel is either a 1 or 0 depending
on whether the data is correctly received or not.
5.2.3 Physical Control Format Indicator Channel (PCFICH)
PCFICH is the control channel in LTE which is to be decoded by the UE to decode the other control
channels and data channels. This channel transmits the information regarding the number of OFDM symbols
used by the control data. It is represented in terms of Control Format Indicator (CFI) which can take values 1, 2
and 3. If CFI value is 1, there is only one OFDM symbol that carries the control data in each sub-frame.
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Inphase component
Quadraturephasecomponent

5.2.4 Physical Downlink Control Channel (PDCCH)
The downlink scheduling and control information (DCI) is carried by the PDCCH to each of the UE in
the cell. It gives the information about resource block carrying the data and the demodulation scheme that
should be used to decode the data. The user data can be decoded only if the DCI is decoded.
5.2.5 Physical Downlink Shared Channel (PDSCH)
PDSCH is the channel which transmits all the user data. The majority of the OFDM symbols in the
time domain carry the user data in each sub-frame. In Fig. 2, the OFDM symbol indices 2, 3, 4, 11, 12. 13 are
allotted for user data and all the 72 subcarriers in those OFDM symbols carry the data to the UE.
Transmit resource grid when plotted shows which all physical signals and channels are transmitted in
which all frames in the time and frequency domain. Fig. 2 shows the transmit resource grid in the downlink for a
single sub-frame (1 ms). The X-axis shows the OFDM symbol index in the time domain and Y-axis shows the
number of subcarriers in the frequency domain. PSS and SSS are transmitted in the 6th
OFDM symbol of every
0th
sub-frame and 5th
sub-frame. PBCH is transmitted in 0th
sub-frame of every radio frame. PCFICH is
transmitted in the 0th
OFDM symbol of every radio frame and the number of OFDM symbols used by it depends
on the CFI value which ranges from 1 to 3. Here, CFI value is taken as 1, so only one OFDM symbol carries
PCFICH. PHICH and PDCCH are transmitted in the 0th
OFDM symbol of every radio frame. PDSCH is
transmitted in OFDM symbol indices 2, 3, 4, 11, 12, 13 in every sub-frame.
Fig. 2 clearly shows which OFDM symbols are carrying what type of data. The number of resource
blocks can take values of 6, 15, 25, 50 and 100. As the number of resource blocks increases, the number of
subcarriers increases. For 100 resource blocks, there will be 1200 subcarriers in the frequency domain which
corresponds to a system bandwidth of 100 MHz. So, for a duration of 14 OFDM symbols (1 ms), there will be
16800 resource elements.
I. CONCLUSIO
Fig. 2 Downlink transmit resource grid for a sub-frame (1 ms)
Fig. 3 shows the transmit resource grid for a single radio frame (10 ms). Here, 10 consecutive resource
grids, each of 1 ms constitute a resource gird for a radio frame (10 ms). This can be obtained by changing the
simulation parameter NSubframe to 9. The figure shows the information that is transmitted by the eNB in one
single radio frame or 10 ms. Like this, there will be several resource grids transmitted in say 10 seconds or 1
minute or 1 hour as long as the communication is progressing between the UE and eNB in each cell.
0 2 4 6 8 10 12 14
0
10
20
30
40
50
60
70
OFDM symbol index
Subcarrierindex
unused
Cell RS
PSS
SSS
PBCH
PCFICH
PHICH
PDCCH
PDSCH


Fig. 3 Downlink transmit resource grid for a radio frame (10 ms)
VI. Conclusion
The physical frame structure of LTE is highly complex but at the same time it is highly flexible which
makes it very much useful for modeling an LTE downlink system according to our requirements. As the number
of resource blocks increases, the downlink transmit resource grid varies correspondingly. The physical frame
structure of LTE is a combination of time domain OFDM symbols and frequency domain subcarrier indices.
Traditionally, the system models were made using the 3GPP spatial channel model for LTE and a number of
parameters have to be varied at the backend. But now, with the introduction of LTE system toolbox [7], the
parameters can be varied at the front end which makes the system modeling of LTE wireless communication
systems much easier than before.
References
[1]. J. P. Niu, D. W. Lee, T. Su, Geoffrey Y. Li, and Yusun Fu, “Joint Transmission Mode Selection and Scheduling in LTE Downlink
MIMO Systems”, IEEE Wireless Communications Letters, Vol. 3, pp. 173-176, April 2014.
[2]. J. P. Niu, D. W. Lee, T. Su, Geoffrey Y. Li, and X. F. Ren, “User classification and scheduling in LTE downlink systems with
heterogeneous user mobilities”, IEEE Transactions on Wireless Communications, Vol. 12, pp. 6205-6213, December 2013.
[3]. J. P. Niu, D. W. Lee, T. Su, Geoffrey Y. Li, and X. F. Ren, “Scheduling exploiting frequency and multi-user diversity in LTE
Downlink Systems”, IEEE Transactions on Wireless Communications, Vol. 4, pp. 1843–1849, April 2013.
[4]. J. C. Ikuno, M. Wrulich, and M. Rupp, “System level simulation of LTE networks”, IEEE 71st
Vehicular Technology Conference
(Spring), Vol. 71, pp. 1-5. May 2010.
[5]. LTE System Toolbox, The Mathworks, Inc., January 2015.
[6]. LTE System Toolbox Introduction, The MathWorks, Inc., April 2014.
[7]. Houmann Zarrinkoub, “Modelling a 4G-LTE System in MATLAB”, The MathWorks, Inc., November 2012.
0 20 40 60 80 100 120 140
0
10
20
30
40
50
60
70
OFDM symbol index
Subcarrierindex
unused
Cell RS
PSS
SSS
PBCH
PCFICH
PHICH
PDCCH
PDSCH

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