Thuyết trình Orthogonal Frequency Division Multiple Access in WiMAX and LTE A Comparison

S. Srikanth and P. A. Murugesa Pandian, Anna University
Xavier Fernando, Ryerson University
Orthogonal Frequency Division
Multiple Access
in WiMAX and LTE: A Comparison
Group 4 – D10VT6

INTRODUCTION

USE OF OFDMA IN WIMAX AND LTE

NETWORK ENTRY STEPS IN WIMAX AND LTE

PERFORMANCE BOUNDS FOR SYNCHRONIZATION AND CHANNEL ESTIMATION

EVOLUTION OF WIMAX AND LTE

CONCLUSION
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INTRODUCTION
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- IEEE 802.16e-based WiMAX and Third Generation Partnership Project (3GPP)-based Long Term Evolution (LTE)
are the two standards that are likely to dominate the fourth generation (4G) wireless landscape.
- Both LTE and WiMAX standards use several common technologies with subtle differences.The main common
technology is orthogonal frequency-division multiple access (OFDMA).
- There are many good reasons for choosing OFDMA such as multipath handling capability, scalability of operation
in different bandwidths, the ability to handle different data rates, and the ability to easily combine with multiple
antenna techniques.
- The focus of this article is on the comparative use of OFDMA in LTE and WiMAX systems.

FRAME STRUCTURE
USE OF OFDMA IN WIMAX AND LTE
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RESOURCE MAPPING FROM SUBCARRIERS
USE OF OFDMA IN WIMAX AND LTE
Subcarriers are the smallest granular units in the frequency domain, OFDM symbol duration is the smallest granular unit in the
time domain in OFDMA systems.
In WiMAX, subchannels are formed from a group of
subcarriers in an OFDM symbol.
A slot is formed by combining a subchannel with different
numbers of OFDM symbols.
In LTE, 12 adjacent subcarriers are grouped as a unit in the
frequency axis, and 7 OFDM symbols (or 6 OFDM symbols in
special cases) are considered as a unit in the time axis
In LTE, the uplink uses SCFDMA, which can be viewed
asdiscrete Fourier transform (DFT)-spread OFDMA. The data
is passed through a DFT block before being input to the
OFDMA module.
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USE OF FREQUENCY DIVERSITY
USE OF OFDMA IN WIMAX AND LTE
- In WiMAX, use PUSC (Partially Used Sub-Carrier), subchannels
are formed by grouping 24 subcarriers that are present in different
parts of the spectrum.
- This pseudorandom selection of the positions of the subcarriers.
- Control messages are also sent using this diversity-based
subchannelization method.
- In LTE, an RB (Resource Block) contains the same 12 contiguous

subcarriers for 7 OFDM symbols.
- Another RB can be used in the second slot of the subframes
- Control messages use small bunches of contiguously placed
subcarriers spread out over the entire bandwidth
- Within an OFDM symbol, no FD.
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USE OF MULTIUSER DIVERSITY
USE OF OFDMA IN WIMAX AND LTE
- In WiMAX, the BAMC organize groups of 9 contiguous subcarriers.
- Each bin has 8 data &1 pilot subcarriers, and 4 such contiguous bins are
grouped in a band. user feeds back the best 4 bands
- Based on this, BS chooses 2 bins in one of these bands and allocates the
same bin over three consecutive OFDM symbols.
- Frequency diversity and MUD-based transmission cannot coexist in time.
- In LTE, BTS uses the channel feedback from the mobile to schedule an
RB for the user in a frame.
- The feedback can be periodic.
- Different modes of channel feedback.
- Channel feedback can be sent as one value for the entire operating
bandwidth or as a sequence of values for a sequence of subbands covering
the entire bandwidth.
- In LTE both can be used.
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USE OF INTERFERENCE DIVERSITY
USE OF OFDMA IN WIMAX AND LTE
- The subcarriers used in a reference subchannel in a reference cell are
distributed in different subchannels in the reuse cells. The subcarriers
used in a subchannel in a certain cell are unlikely to be repeated in a

particular
subchannel in another cell provided the downlink perm base values are
different.
- In LTE, there can be a clash in the positions of the data subcarriers that
can be used in neighboring cells.
- There is no interference diversity.
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SYNCHRONISATION
NETWORK ENTRY STEPS IN WIMAX AND LTE
- In WiMAX, a preamble is transmitted at the start of every frame.
- The preamble signal is generated by using every third subcarrier in the
allowed bandwidth.
- The pseudorandom sequence to be sent on the subcarriers is specified in
the IEEE 802.16e standard.
- A mobile uses the time domain properties of the preamble sequence,
along with the structure of the cyclic prefix (CP).
- Since the duration of the CP is not fixed, a search procedure might have
to be performed to obtain the CP duration.
- In LTE, the frame synchronization is obtained by detecting the primary
synchronization sequence (PSS).
- Unlike the preamble in WiMAX, the number of subcarriers used for PSS
is fixed.
- The Zadoff-Chu sequence loaded onto the subcarriers is specified in the
standard.
- ID of the PSS in the received signal gives two potential starting points in
the frame as there are two PSS transmissions in the frame.
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NETWORK ENTRY

NETWORK ENTRY STEPS IN WIMAX AND LTE
In WiMAX, a mobile has to search for a valid preamble to acquire
frame synchronization. Once synchronized, the mobile reads the
frame control header (FCH) message, which points to the length of
the DL-MAP message that contains the various allocations in the
frame.
In LTE, irrespective of bandwidth and the number of subcarriers, the
first step remains the same for all mobiles: locating the primary
synchronization signal (PSS) and secondary synchronization signal
(SSS).
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WiMAX LTE
Maximum delay spread
PUSC mode:


Considering both the pilots carrying symbols:
~22us
Maximum Doppler spread





WiMAX LTE
Maximum delay spread
Maximum Doppler spread
PERFORMANCE BOUNDS FOR SYNCHRONIZATION AND CHANNEL
ESTIMATION

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PHYSICAL LAYER OVERHEAD
In WiMAX
The overhead due to the use ofpreambles and pilots in every OFDM symbol isgiven by the
ratio :

In LTE
The subcarriers used for physical layer processing purposes are the ones that carry reference
symbols, plus primary and secondary synchronization sequences
In LTE
The subcarriers used for physical layer processing purposes are the ones that carry reference
symbols, plus primary and secondary synchronization sequences
PERFORMANCE BOUNDS FOR SYNCHRONIZATION AND CHANNEL
ESTIMATION
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EVOLUTION OF WIMAX AND LTE
The WiMAX and LTE camps have submitted candidate proposals to satisfy the official requirements of the ITU IMT-
Advanced criterion for 4G wireless systems.
WiMAX to WiMAXv2
Base on the IEEE 802.16m standard
Forming fixed size physical resource units (PRUs)
Forming distributed and contiguous localized
resourceunits (DRUs and LRUs)
LTE to LTE-A
The aggregation of multiple carriers to obtain wider
channel bandwidths.
the OFDMA related features are very similar in both
WiMAXv2 and LTE-A

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CONCLUSION
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This article has compared the use of OFDMA in WiMAX and LTE standards in detail
Thanks For Listening