Ofdm based wireless lan transmitter

International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME
469
OFDM BASED WIRELESS LAN TRANSMITTER
Ms. S.D. Giri and Prof. Ms A. R. Salunke (IEEE Member)
Dept. of Electronics & Telecommunication, Dr. B. A. M. University
Jawaharlal Nehru Engineering College, Aurangabad
ABSTRACT
OFDM (orthogonal frequency-division multiplexing) signals used in 802.11a and
802.11g wireless LAN systems include provisions for equalization in the receiver to correct
for some types of system and channel impairments. High data rate wireless communication
has improved by a factor of minimum four while migrating from one generation to next
generation. And now we have entered into an era, where fast, accurate and secured data
transmission is of maximum value. The Orthogonal Frequency Division Multiplexing
(OFDM) put its need in the communication for the effective bandwidth utilization with large
data transmission. In this paper, we present a proposed OFDM based wireless LAN
architecture and its implementation using Verilog for an efficient physical layer
implementation of an OFDM technique. Our architecture improves resources utilization,
compared with the other architecture.
Keywords: W-LAN, OFDM, IEEE 802.11a.
I. INTRODUCTION
The main goals in developing next-generation wireless communication systems are
increasing the link throughput (bit rate) and the network capacity. Important improvements in
throughput can be achieved when multiple antennas are applied at both the transmitter and
receiver side, especially in a rich scattering environment. This has been shown for wireless
communication. With the rapid growth of digital communication in recent years, the need for
high- speed data transmission has been increased [1].
The major challenges in future wireless communication system design are increased
spectral efficiency and improved link reliability. Wireless networking shares several
important advantages, no matter how the protocols are designed, or even what type of data
they carry. The most obvious advantage of wireless networking is mobility. Wireless
networks typically have a great deal of flexibility, which can translate into rapid
INTERNATIONAL JOURNAL OF ELECTRONICS AND
COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)
ISSN 0976 – 6464(Print)
ISSN 0976 – 6472(Online)
Volume 4, Issue 2, March – April, 2013, pp. 469-476
© IAEME: www.iaeme.com/ijecet.asp
Journal Impact Factor (2013): 5.8896 (Calculated by GISI)
www.jifactor.com
IJECET
© I A E M E

470
development. Wireless networks use a number of base stations to connect users to existing
networks. The upcoming standard 802.11n WLAN, however, can achieve 250 Mbits/s by
virtue of Multiple Input Multiple Output OFDM (MIMO-OFDM) technology. Wireless
communication is obviously less reliable than wired communication. For example, the IEEE
standard 802.11a. has various communication modes with possible data rates of 6, 9, 12, 18,
24, 36, 48, and 54 Mbits/s [2].
II. BACKGROUND
A. Introduction of OFDM
OFDM was first developed in the 1960s, only in recent years, it has been recognized
as an outstanding method for high-speed cellular data communication where its
implementation relies on very high-speed digital signal processing. This method has only
recently become available with reasonable prices versus performance of hardware
implementation.
Frequency Division Multiplexing (FDM) transmits multiple signals simultaneously
over a single path. Each signal has a unique frequency range (carrier). Orthogonal FDM
(OFDM) is a special case of FDM where a single data stream is distributed over several lower
rate sub-carriers. In other words, one signal is transmitted by multiple carriers. Sub-carriers
are separated by given frequency ranges, to avoid cross-carrier interference. The benefit of
orthogonality is that it gives a high spectral density. A guard band interval is employed to
avoid the Inter-Symbol Interference (ISI) problem
B. Orthogonal Frequency Division Multiplexing
Frequency Division Multiplexing (FDM) transmits multiple signals simultaneously
over a single path. Each signal has a unique frequency range (carrier). Orthogonal FDM
(OFDM) is a special case of FDM where a single data stream is distributed over several lower
rate sub-carriers. In other words, one signal is transmitted by multiple carriers. Sub-carriers
are separated by the given frequency ranges, to avoid cross-carrier interference. The benefits
of orthogonally is that it gives a high spectral density (maximizing channel usage). A guard
band intervals is employed to OFDM (Orthogonal Frequency Division Multiplexing) is a
method of using many carriers waves instead of only one, and using each carrier wave for
only part of the message. OFDM is also called multicarrier modulation (MCM) or Discrete
Multi-Tone (DMT).
OFDM is not really a modulation scheme since it does not conflict with other
modulation schemes. It is more a coding scheme or a transport scheme. Orthogonal
Frequency Division is where the spacing between carriers is equal to the speed(bit rate) of the
message in earlier multiplexing literature, a multiplexer was primarily used to allow many
users to share a communication medium like a phone trunk between two telephone central
offices[3].
A more detailed understanding of Orthogonal arises when we observe that the
bandwidth of a modulated carrier has a so-called sinc shape (sinx/x) with nulls spaced by the
bit rate. In OFDM, the carriers are spaced at the bit rate, so that the carriers fit in the nulls of
the other carrier. Another view of Orthogonal is that each carrier has an integer number of
sinc wave cycles in one-bit period.

471
III. WIRELESS LAN
In just the past few years, wireless LANs have come to occupy a significant niche in
the local area network market. Increasingly, organizations are finding that wireless LANs are
an indispensable adjust to traditional wired LANs, to satisfy requirements for mobility,
relocation, ad hoc networking, and coverage of locations difficult to wire.
IEEE 802.11a
One of today’s biggest high-tech controversies is how best to encourage and deliver
technology innovations. Everyone agrees that standards play an important role in ensuring
interoperability and cost reduction (through volume production of key components). Not
everyone agrees, however, about what makes some standards more successful than others.
The IEEE 802.11a standard defines an OFDM-based wireless LAN that supports raw data
rates between 6 and 54 Mbps in the 5 GHz band. The block structure of the transmitter is
shown in Fig. Multiple data rates are achieved by employing two puncturing patterns (of rate
2/3 and 3/4) in addition to the basic convolutional code of rate 1/2, as well as by using four
different modulation schemes (BPSK, QPSK, 16-QAM, 64-QAM). Of the twelve possible
combinations, only eight are defined by the standard. In particular there is difference in
opinion regarding whether standards drive innovation or innovation drives standards. The
Signal Processing Designer IEEE 802.11a models are based on the High-speed Physical
Layer standard defined over the 5 GHz band [4].
IV. AN OFDM BASED WIRELESS LAN ARCHITECTURE:
In wireless communication when the data is transmitted in air the different types of
precaution you need to take care of in order to protect it from following types error and
noises, which may corrupt or lose your data in a deep fad.
Fig.1 OFDM based Transmitter
Time and Frequency Recovery: This kernel is composed of packet detection, time
synchronization and frequency correction. Packet detection roughly identifies transmitted
signals & time synchronization exactly distinguishes transmitted signals from noise signal.
Frequency correction estimates frequency error in the original transmitted signals, and
recovers the original frequency.
Channel Estimation: An OFDM carrier signal is the sum of a number of orthogonal sub-
carriers. Each sub-carrier is located at a particular frequency, Wireless channels affect on
these sub-carriers to varying degrees. In the ideal channel case, this gain is 1 for all sub-

472
carriers. However, for the real channel environment, the channel estimation block estimates
the amount of gain change, and recovers the original gain for each sub-carrier [5]. General
from of global transmitter block is shown in fig.2.
Fig.2 Global Transmitter
Channel Coding System: Channel coding is often used in digital communication systems
to protect the digital information from noise and interference and reduce the number of bit
errors. Channel Coding is mostly accomplished by selectively introducing redundant bits into
the transmitted information stream. These additional bits will allow detection and correction
of bit errors in the received data stream and provide more reliable information transmission.
The cost of using channel coding to protect the information is a reduction in data rate or an
expansion in bandwidth [6].
With the large amount of applications today, that use digital communication to store
and retrieve data, this information needs to sent and received with minimal errors. Despite the
common perception that digital transmission yields no errors, this is not often the case as
many digital communications contain errors due to noise. This can be combated using error
correction codes.
Coding techniques have long been used for error correction to decrease the bit error
rate (BER) in data transmission systems. This decrease in BER is accomplished by adding
redundant data bits to the transmitted data bits and, in some cases, scrambling the order of the
original data bits. There are many types of coding techniques used to correct different error.
Convolutional encoding method, a good technique used for correcting errors that occur
during data transmission.
The overall purpose of the wireless communication is to transfer information from
point in space and time, called the source, to another point the user called destination. The
information source and the destination point are usually separated in space. The channel
provide the electrical connection between the information source and the user.
Interleaver: Interleaving combats signal fading. The block size corresponds to the number of
bits in a single OFDM symbol. Interleaving enhance the quality of digital transmission over
the radio fading channel. This is usually accomplished by scrambling successive symbols of
the transmitted sequence into different time slots. A channel is considered fully interleaved
when consecutive symbols of the received sequence appear to be independent i.e. not affected
by the same error burst. Interleaving improves the performance of digital radio system at the
cost of increasing memory space requirement, system complexity and time delay [7].
QAM Mapping: The encoded output of Convolutional encoder is fed to modulator or
mapper for modulation. According to IEEE 802.11a the OFDM subcarriers shall be
modulated by using BPSK, QPSK, 16-QAM, or 64-QAM modulation, depending on the
RATE requested. The conversion shall be performed according to Gray-coded constellation
mappings, illustrated in Fig.3. The output values, d, are formed by multiplying the resulting
(I+ j Q) value by a normalization factor KMOD.
d = (I + j Q) x KMOD

0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March
Fig.3 constellation diagram for 16
The normalization factor, KMOD, depends on the base modulation mode
normalization factor is to achieve the same average power for all mappings. The
shows the modulation-dependent
Table 1 Modulation
Modulation
BPSK
QPSK
16-QAM
64-QAM
For 16-QAM, d = (I + j Q) x 1/√10
The correct detection of a signal depends upon a separation between the signal point
signal space. In case of PSK al points lie on the circumference of circle. This is because PSK
signal has constant amplitude throughout
IFFT: IFFT performs the reverse process, transforming a spectrum (amplitude and phase of
each component) into a time domain signal. An IFFT converts a number of complex data
points, of length which is a power of 2, into the time domain signal of the same number
points. Each data point in frequency spectrum used for an FFT or IFFT is called
orthogonal carriers required for the OFDM signal can be easily generated by setting the
amplitude and phase of each bin, then performing the IFFT. IFFT reduces t
complex multiplications from N
computation time. How IFFT works in trans
At the transmitter side, an OFDM system treats the source
in the frequency domain. These symbols are feed to an IFFT block which brings the signal
into the time domain. If the N numbers of subcarriers
6472(Online) Volume 4, Issue 2, March – April (2013), © IA
473
constellation diagram for 16-QAM.
The normalization factor, KMOD, depends on the base modulation mode. The purpose of the
normalization factor is to achieve the same average power for all mappings. The
dependent normalization factor KMOD.
Modulation-dependent normalization factor KMOD.
Modulation KMOD
1
1/√2
QAM 1/√10
QAM 1/√42
√10
The correct detection of a signal depends upon a separation between the signal point
throughout [8].
IFFT performs the reverse process, transforming a spectrum (amplitude and phase of
points, of length which is a power of 2, into the time domain signal of the same number
points. Each data point in frequency spectrum used for an FFT or IFFT is called
amplitude and phase of each bin, then performing the IFFT. IFFT reduces the number of
complex multiplications from N2
to (N/2) log2
N as required in IDFT. It reduces the
computation time. How IFFT works in transmitter side is shown in fig.4.
Fig.4 workings of IFFT
At the transmitter side, an OFDM system treats the source symbols as though they are
These symbols are feed to an IFFT block which brings the signal
If the N numbers of subcarriers are chosen for the system, t
April (2013), © IAEME
. The purpose of the
normalization factor is to achieve the same average power for all mappings. The table 1
The correct detection of a signal depends upon a separation between the signal point in the
IFFT performs the reverse process, transforming a spectrum (amplitude and phase of
points, of length which is a power of 2, into the time domain signal of the same number of
points. Each data point in frequency spectrum used for an FFT or IFFT is called a bin. The
he number of
N as required in IDFT. It reduces the
symbols as though they are
These symbols are feed to an IFFT block which brings the signal
are chosen for the system, the basic

474
functions for the IFFT are N orthogonal sinusoids of distinct frequency and IFFT receive N
symbols at a time. Each of N complex valued input symbols determines both the amplitude
and phase of the sinusoid for that subcarrier. The output of the IFFT is the summation of all
N sinusoids and makes up a single OFDM symbol.
V. IMPLEMENTATION OF OFDM BASED LAN TRANSMITTER USING
VERILOG
Following are the different tools used for implementation of LAN transmitter.
1) Model simulation - For model testing and function simulation
a) Verilog HDL
b) VHDL
2) Active HDL
Active HDL Overview
Active-HDL is an integrated environment designed for development of VHDL,
Verilog, EDIF and mixed VHDL – Verilog – EDIF designs. It comprises three different
design entry tools, VHDL’93 compiler, Verilog compiler, single simulation kernel, several
debugging tools, graphical and textual simulation output viewers, and auxiliary utilities
designed for easy management of resource files, designs, and libraries [9].
Standards Supported
1 .VHDL: The VHDL simulator implemented in Active-HDL supports the IEEE Std.
1076-1993 standard.
2 Verilog: The Verilog simulator implemented in Active-HDL supports the IEEE Std.
1364-1995 standard. Both PLI (Programming Language Interface) and VCD (Value
Change Dump) are also supported to Active-HDL.
Wireless Standards for Wireless LANs
The new uses, as well as the growing number of convectional WLAN users,
increasingly combine to strain existing Wi-Fi networks. Fortunately, a solution is close at
hand. The industry has come to an agreement on the components that will make up 802.11n,
a new WLAN standard that promises both higher data rates and increased reliability, and the
IEEE standards-setting body is ironing out the final details. Tough the specification is not
expected to be finalized yet, the draft is proving to be reasonably stable as it progresses
through the formal IEEE review process [10].
The table 2 shows a brief list of all available LAN standards and their specifications
Table 2 LAN Standards and their specifications.
802.11a 802.11b 802.11g 802.11n
Standard Approved July 1999 July 2003 June 2003 Not yet ratified
Maximum Data Rate 54 Mbps 11 Mbps 54 Mbps 600 Mbps
Modulation OFDM DSSS or CCK DSSS or CCK or
OFDM
DSSS or CCK or
OFDM
RF Band 5 GHz 2.4 GHz 2.4 GHz 2.4 or 5 GHz
Number of Streams 1 1 1 1,2,3 or 4
Channel Width 20 MHz 20 MHz 20 MHz 20 or 40 MHz

475
VI. CONCLUSIONS
The different techniques available for above discussed system of transmitter will
decide the overall performance and specification of the system. Channel encoding is
indispensable in wireless communication system. It is used for error correction, by adding
redundant bits to the original. It is observed through some practical experiments that this
transmitter system increases the row data rate and hence, it increases the bandwidth
requirement. We will also like to make one comment that this increased bandwidth
requirement can be solved by using the concept of Orthogonal Frequency Division
Multiplexing (OFDM) and by using the concept of parallel processing the data rate can be
increased. The dynamic reconfigurable system discussed above can achieve flexibility with
regard to changing data rates, increasing range, and increasing diversity, while offering
efficient resource utilization
REFERENCES
[1] Multi Band OFDM Physical Layer Proposal for IEEE 802.15.3a. September 2004, MBOA
SiG.
[2] “Digital communication: Fundamentals and applications” by Bernard Sklar, Pearson
Education Publication, fourth edition, year 2003, pp. 461-466.
[3] C.Snow, L.Lampe, andR.Schober, “Performance Ana-lysis of Multi band OFDM for UWB
Communication”, IEEE Intl. Conf. on Communication (ICC), VOL.4, Seoul, May 2005.
[4] “802.11 –Wireless networks: The definitive guide”, by Matthew S. Gast, SHROFF
Publication, second edition, July 2004, pp.198-213.
[5] Lee, and L.H.C., “New rate-compatible punctured convolutional codes for Viterbit
Decoding”, IEEE Transactions on communications volume 42 Issue: 12, Dec.
[6] Zou Decai Nat. Time Service Center, Chinese Acad. Of Sci., Lintong, Lu Xiaochum; Wu
Haitao; Xu Jinsong, “Implementation of convolutional code based on FPGA
In OFDM-UWB system” Industrial Electronics and Application, 2008. ICIEA 2008, Pp 1119-
1122.
[7] Davinder Pal Sharms, Jasvir Singh “DSP Based Implementation of Scrambler”, Signal
Processing-An International Journal, Vol.4: issue 2.
[8] Szczecinski,Leszek L. Inst. Nat. de la recherché scientifique, “rectangle QAM arbitrary
constellation mapping.” IEEE Transactions on communications.
[9] “VHDL Programming by example” By Douglas L. Perry.
[10] IEEE 802.11 wireless LAN Medium access control (MAC) and Physical layer (PHY)
Specifications. IEEE-5A 5 April2012.
[11] Sandeep Bidwai, Saylee S. Bidwai, Dr. S.P. Patil and Sunita S. Shinde, “Implementation &
Performance Analysis of Cordic in OFDM Based WLAN System using VHDL”, International
journal of Electronics and Communication Engineering & Technology (IJECET), Volume 3,
Issue 3, 2012, pp. 103 - 111, ISSN Print: 0976- 6464, ISSN Online: 0976 –6472.
[12] T.Priyadarsini, B.Arunkumar, K.Sathish and V.Karthika, “Traffic Information Dissemination
in Vanet using IEEE-802.11”, International Journal of Electronics and Communication
Engineering & Technology (IJECET), Volume 4, Issue 1, 2013, pp. 294 - 303, ISSN Print:
0976- 6464, ISSN Online: 0976 –6472.
[13] Rambabu.A.Vatti and Dr.A.N.Gaikwad, “Throughput Improvement of IEEE 802.15.4 Based
Medical Ad-Hoc Sensor Networks Using Clear Channel Assessment”, International journal of
Electronics and Communication Engineering & Technology (IJECET), Volume 3, Issue 3, 2012,
pp. 43 - 48, ISSN Print: 0976- 6464, ISSN Online: 0976 –6472.

476
AUTHORS
Ms. Sunita D. Giri, a student of ME (EC) at JNEC, Aurangabad,
Maharashtra.
Prof. Aarti R. Salunke, Department of ECT, MGM’s Jawaharlal Nehru
Engineering College, N-6, CIDCo Aurangabad, Maharashtra.

Ofdm based wireless lan transmitter

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (9)

Similar to Ofdm based wireless lan transmitter

Similar to Ofdm based wireless lan transmitter (20)

More from IAEME Publication

More from IAEME Publication (20)

Recently uploaded

Recently uploaded (20)

Ofdm based wireless lan transmitter