OFDM+CDMA+Stego = Secure Communication: A Review
John Bosco Balaguru Rayappan
Orthogonal Frequency Multiplexing (OFDM) is a multicarrier modulation technique which allows sub channel overlapping for efficient utilization of available frequency spectrum. Demand for high data rate wireless communication is rapidly increasing and there is a need for a technique which overcomes the problems like Intersymbol Interference (ISI) and frequency selective fading. OFDM overcomes the problems of high data rate wireless communication by converting high rate data stream in to number of parallel low rate data streams and this will be a suitable modulation/multiplexing technique for advanced wireless communication systems. In this paper, a review has been made, to detail the progress on OFDM since 1870, it also explains the basic building blocks of OFDM, Code Division Multiple Access (CDMA) and Multi Carrier Code Division Multiple Access (MC-CDMA) Schemes and how it could be modified suitably?, to carry additional confidential Information using steganography.
to cite this article:
K. Thenmozhi, Padmapriya Praveenkumar, Rengarajan Amirtharajan, V. Prithiviraj, R. Varadarajan and John Bosco Balaguru Rayappan, 2012. OFDM+CDMA+Stego = Secure Communication: A Review. Research Journal of Information Technology, 4: 31-46.
Received: September 19, 2011;
Accepted: November 28, 2011;
Published: January 31, 2012
The very basic idea and the long history of OFDM came into existence in the
year 1870 when Thomas Edison invented Multiplex telegraph (Weinstein,
2009). Then the concept of FDM transmission was introduced by Bell
(1876). FDM transmission for analog telephony was first demonstrated in
the year 1910 by Schwartz (2008). It was then implemented
in 1918 by AT&T. The principle behind OFDM was proposed by Chang
(1970) and analysis on the performance of efficient parallel data transmission
system over wireless channels was reported by Saltzberg
(1967). Larger sub channels increases the cost and complexity of FDM systems
which utilizes (Weinstein and Ebert, 1971) Discrete
Fourier Transform (DFT) as it eliminates the need of equalization methods and
a reduction in multipath fading is achieved.
In OFDM, processing of signals can be performed in digital domain and the latest
developments in VLSI technology has enabled high speed large size FFT (Bidet
et al., 1995) chips to be economically affordable. In 1980's OFDM
technique was introduced and used in high speed modems. In this system a pilot
tone was used for the stabilization of carrier frequency and clock frequency
and error correcting codes were implemented to reduce the required carrier to
noise ratio. Cimini (1985) introduced the concept of
the cyclic prefix which forms a pivotal point in the OFDM formulation chain
to combat the effect of frequency selective fading and Doppler shift in mobile
channel. COFDM technique was harnessed by incorporating the addition of a guard
interval (Vahlin and Holte, 1994).
The multimedia data transmission for various data types is provided by Van
Nee and Prasad (2000). Speth et al. (1999)
analyses the problem of multipath channels and multicarrier system in his paper
on "OFDM Receivers for Broadband Transmission". In 1960s, several military
systems were using OFDM (Mosier and Clabaugh, 1958;
Zimmerman and Kirsch, 1967). Hirosaki
(1981) realized the OFDM technique utilizing multiplexed QAM with DFT. Telephone
networks using various high speed modems were developed by Keasler
et al. (1980). In 1990s, COFDM was studied for broadband data
communication, High bit rate HDTV terrestrial broadcasting, Digital Subscriber
Lines (HDSL),Very High Speed Digital Subscriber Lines (VHDSL), Asymmetrical
Digital Subscriber Lines (ADSL), Digital television and Digital Audio Broadcasting
(DAB) (Sari et al., 1995; Chow
et al., 1991a, b; Hara
et al., 1996; Hara and Prasad, 1997). Rappaport
(2002) outlined the advantages of Multicarrier Communication and the advancement
in VLSI technology in realizing the OFDM system on "Wireless Communication"
(Pandey et al., 2002) describes the VLSI implementation
In 1993, a group of researchers Hara and Prasad (1997)
proposed a new hybrid combination by integrating Code Division Multiple Access
(CDMA) and OFDM. Some of the techniques proposed were Multicarrier CDMA (MC-CDMA),
Multitone CDMA (MT-CDMA) and MC-DS-CDMA (Fazel, 1993;
DaSilva and Sousa, 1993; Nithyanandan
and Dananjayan, 2006). Direct Sequence CDMA is a spread-spectrum communication
technique (Yang and Hanzo, 2003; Proakis,
2000). DS-CDMA systems (Viterbi, 1995; Lee
and Miller, 1998) are capable of supporting a multiplicity of users within
the same bandwidth by assigning different typically unique user-specific codes
to different users for their communications, in order to be able to distinguish
their signals from each other at the receiver. Thenmozhi
et al. (2011) enumerated Space Time Frequency coded (STF) OFDM for
broadband wireless communication systems.
Spread-spectrum techniques were developed originally for military guidance
and communication systems (Scholtz, 1982). During late
1970s, the employment of spread-spectrum techniques was proposed for efficient
cellular communication (Cooper and Nettleton, 1978).
The MC-CDMA technique is believed to be able to outperform the other techniques
such as DS-CDMA mainly due to the reduction in complexity of the receiver while
at the same time providing equal and even better performance. A detailed study
on Multicarrier Code Division Multiple Access (MC-CDMA) had been carried out
and the description of MC-CDMA system was given by (Zou and
Wu, 1995). 3G wireless communication explored to meet the demand of wireless
communications was suggested by Meng et al. (2008),
Hemalatha et al. (2009) and Wu
et al. (2010a), even this could be used in Wi-Fi (Hemalatha
et al., 2011).
Since there is no such survey or review has been made on the existing spread spectrum use in secure communication. An optimistic maiden effort has been taken, to study the progress on OFDM, CDMA, MC-CDMA and spread spectrum based steganography for secure communication.
OFDM SYSTEM DESCRIPTION
OFDM (Orthogonal Frequency Division Multiplexing) Akansu
et al. (1998) is a technique of mapping the information utilizing
the bandwidth sharing concept. So often its a combination of modulation
and multiplexing techniques (Chang and Gibby, 1968).
In OFDM first the signal splits into independent channels (Saltzberg,
1967) and is modulated by information and then demultiplexed to create OFDM
carrier. Its even a special form of FDM (Frequency Division Multiplexing)
with subcarriers (Casas and Leung, 1991) being orthogonal
to each other.
|| Block diagram of OFDM system
|| Parameters of OFDM
Its a special case of multi-carrier modulation with high data rate capability
(Weinstein and Ebert, 1971). OFDM systems diagram
has been presented in Fig. 1.
In OFDM the serial data stream is initially formatted based on the required
transmission, (1 bit/word for BPSK and 2 bits/word for QPSK and 3 bits/word
for QAM ) and converted into a parallel format by assigning each data with one
carrier. It increases the symbol duration (Chuang, 1987)
by dividing the entire channel bandwidth into many narrow orthogonal sub-channels
(Doelz et al., 1957). Hence the symbol duration
becomes much larger than the channel impulse response which will greatly mitigate
the effects of intersymbol interference (ISI) in multipath channel (Zhaogan
et al., 2007). Based on the modulation employed the data on each
symbol is mapped to a phase angle (Hirosaki, 1981).
Applying IFFT to the sequence results in frequency to time domain conversion.
To each OFDM symbol a guard period is added at the beginning of each symbol
(Kalet, 1989), cyclic prefix is a just the cyclic extension
of the transmitted symbol. It is inserted between each OFDM symbol which eliminates
ISI almost completely (Haque et al., 2008). Since
it is larger than the maximum time delay it maintains the orthogonality between
sub-carriers (Rappaport, 2002). Parameters of OFDM has
been presented in (Table 1).
The symbol duration is Ttotal = TFFT+TGI+Twin. The insertion of guard interval will reduce data throughput so it is usually less than T/4 (Fig. 2).
The symbols are converted into a serial time waveform. The base band signal of the OFDM is then applied to a channel model which is normally AWGN.
The kth OFDM signal can be written as:
|| Guard interval in OFDM
xik-Symbol modulated on ith subcarrier on kth OFDM symbol, fc-centre
frequency of the spectrum, TFFT-FFT time, Twin-window
interval, TG-guard time. F-frequency spacing between adjacent subcarriers,
T-time between two OFDM symbols, K-index on transmitted symbol, I-index on subcarriers,
The receiver will perform the reverse operation of the transmitter. Initially
the guard period is detached from each symbol (Cimini et
al., 1996) and FFT is applied to find the transmitted spectrum. The
phase angle of each carrier is then converted back to the data by demodulating
the received phase. The data words are combined again to form the original data.
CDMA system description: In data communication several transmitters
are allowed to send information simultaneously (Casas and
Leung, 1991) over a single communication channel which allows several users
to share the bandwidth called as multiple access scheme (Venkatesan
and Ravichandran, 2007). CDMA is a special form of Spread Spectrum (SS)
coding technology (Cooper and Nettleton, 1978) where
each transmitter is assigned a code allowing multiple users to be multiplexed
over the same channel (Britto and Sankaranarayanan, 2006).
Yue (1983) summarised spread spectrum mobile radio systems.
Normally SS make use of large number of frequency hopped symbols which results
in minimum mutual interference. The receiver should always synchronize with
the code to recover the data which ensures multiple users to access the network
at the same time. There are three specific ways in spreading the bandwidth namely
frequency hopping, time hopping and direct sequence (Prasad
and Hara, 1996).
The PN sequence must hold the following properties (Yang
and Hanzo, 2003):
||Deterministic and random
||The cross-correlation between any two PN codes must be very small
||The PN code must have a long time period
The narrow band data in CDMA is multiplied by a large bandwidth signal usually
a pseudo random noise code (PN code) (Wu et al.,
2010b). All users in a CDMA system (Fig. 3) transmit simultaneously
making use of the same frequency band. By correlating the received signal with
the same PN code used at the transmitter, the original data is recovered (Arnstein,
1979). PN sequence has a chip rate higher then the data bit rate. PN code
is a sequence of ones and zeros normally referred as chips which alternates
in a random fashion. Mingxin et al. (2008) proposes
scrambling codes using PN sequences provides excellent autocorrelation and cross
correlation properties. The modulation is performed by modulo 2 addition of
data with PN sequence. Since each user retains their permanent unique code,
there is no need for channel switching or address changing (Viterbi,
1985) as the user moves from cell to cell the desired signal is then filtered
by removing the wide spread interference and noise signals. The interference
limit of the CDMA system is always limited by the tolerated total interference
(Malarkkan and Ravichandran, 2006). CDMA systems using
QPSK modulation was studied and simulated by Guo et al.
(2010). The effect of multipath fading on high data rate transmission will
be combated by spread spectrum techniques was proposed by Turin
|| CDMA system
|| CDMA transmission and reception
Properties that made CDMA system more secured and work in presence of jamming:
||Non interference and interference rejection (Sivakumar
et al., 2006)
||Diversity in multipath fading channels (Sourour and
||Implementation cost is low
||Robust to channel conditions
||Built-in addressing and security
The main disadvantage with SS system is power control due to the near far problem
(Kavehrad and McLane, 1987). The performance evaluation
of any transmission system is studied and verified based on the Bit Error Rate
(BER) and Signal to Noise Ratio (SNR). SS systems always aims at maximum BER
Steps in CDMA transmission and reception (Fig. 4):
||Initially a PN code is generated and EX-ored with user data
for each transmitter
||The input data modulates and spreads the PN code
||The spreaded signal modulates the carrier (Hara and
||Then it is amplified and passed through the channel for broadcasting
||At the receiver the received carrier is amplified
||Then it is mixed with a local carrier to recover the spreaded data
||Receiver receives the correct code
||The received signal is correlated with the code generated and the information
|| MC-CDMA system model
Process gain of CDMA: By the spreading and despreading process the gain
or signal to noise improvement is referred to as process gain (Kennedy,
1982). It is the ratio of spread spectrum bandwidth to the original data
RF bandwidth refers to the transmitted bandwidth after spreading the data and
information bandwidth is the bandwidth of the information (Alper
and Arnbak, 1980). Direct sequence CDMA uses rake receivers which applies
spreading sequences in time domain. High chip rate, high clock rate and high
power consumptions are required to reduce the multipath fading (Lionti
et al., 2001).
To overcome these impairments CDMA is combined with Multi carrier modulation
called OFDM to produce MC-CDMA. Radio channel is usually time invariant and
frequency selective. In spread spectrum transmission, signal occupies an excess
bandwidth than the required one and spreading is accomplished by a code independent
of the input (Pickholtz et al., 1982, 1984,
Multi carrier (MC)-code division multiple access (CDMA): MC-CDMA that
combines Multi Carrier (MC) and Code Division Multiple Access (CDMA) deals with
frequency-selective fading (Sourour and Nakagawa, 1996;
Hartmann et al., 2003) providing multiple-access
facility. In MC-CDMA, normally a symbol is spread into multiple spreading sequences.
Then each spreaded sequence is transmitted over a subchannel and be distributed
along the time or frequency (Hara and Prasad, 1996).
Multiple users transmit at the same time and frequency and the symbols from
each user are detected by using the difference of spreading codes for different
users McCormick and Al-Susa (2002) and Yang
and Hanzo (2003). MC-CDMA system model is presented in Fig.
Combination of CDMA schemes and OFDM signalling can be called as spreading
assisted OFDM (Chouly et al., 1993). In MC-CDMA
system spreading sequences are applied in frequency domain rather than in time
domain. This maps different spreading codes to the OFDM subcarriers (Hara
and Prasad, 1997). Hence, each OFDM subcarrier has a data rate identical
to the original input data rate and the multicarrier system has an increased
rate due to spreading in a wider frequency band (Kondo and
Milstein, 1995). Spreading sequences are applied to each OFDM subcarrier
which has a data rate identical to the input rate (Wang
and Giannakis, 2000). The spreading chips are applied to the serial to parallel
converter and IFFT is imposed on these parallel data. Spreading sequences are
orthogonal to each other which separate the desired signal from other users.
Walsh codes and Gold codes are the well known orthogonal codes (DaSilva
and Sousa, 1993) which provides zero correlation and is best suited for
The MC-CDMA transmitter spreads the original data stream over øÛh subcarriers using a given spreading code of Ck , Ck , Ck ,
.Ck [Np-1] 19ê in the frequency domain. By spreading each data bit across all of the øÛh subcarriers the fading effects of multipath channels is mitigated.
The Kth MC-CDMA users transmitted signal is given by:
bk (t) represents the binary data of the kth user.
Ts is the symbol duration, Δ is the minimum spacing between two adjacent
subcarriers where Δ = 1/Ts:
Ck , Ck , Ck ,
[Np-1] be the spreading sequences
Ts represents the symbol duration.
fn denotes the subcarrier frequencies where, n = 0, 1, 2
Np-1 where, Np denotes the subcarriers. P be the transmitted power.
In MC-CDMA, spreading sequences are orthogonal and hence separates the desired
signal from other users (McCormick and Al-Susa, 2002).
High speed operations are required at the output of the spreader in order to
carry out the chip-related operations. At the receiver end, the corresponding
chip sequence is recovered using FFT after sampling at a rate of N/T samples/sec
and the recovered chip sequence is correlated with the desired users spreading
code in order to recover the original binary information.
The aim of MC-CDMA is to support high data rate services and frequency diversity
in remote wireless environments and also prevents the annihilation of certain
subcarriers by deep fades. This is achieved by spreading each subcarriers
signal with a spreading code and thereby increasing the achievable error-resilience,
since corrupting a few chips of a spreading code the subcarrier signal may still
be recovered. MC-CDMA provides better multiple access which reduces multiple
access interference and intersymbol interference which improves the bit error
rate (Venkatesan and Ravichandran, 2007).
One of the main disadvantages with OFDM based Multicarrier CDMA systems is
the Peak to Average Power Ratio (PAPR) of the transmitted signal. Whenever,
the peak transmitted power is limited by implementation Constraints, it reduces
the average power of the transmitter and limits the range of transmissions.
Multicarrier signal exhibits a high amplitude variation subjected to nonlinear
distortions resulting in out-of-band emissions and co-channel interference causing
a significant degradation in the systems performance.
SPREAD SPECTRUM AND STEGANOGRAPHY (SSS)
Since early 1990s researchers and industries concentrates on Cryptography
(Schneier, 2007) and there has been a rapid interest in
Steganography since the late 90s (Cheddad et al.,
2010). It concentrates on embedding secret message in a cover message associated
with a key to produce stego image (Stefan and Fabin, 2000;
Rabah, 2004). The cover can be a Plaintext, Still image
(Janakiraman et al., 2012; Hmood
et al., 2010; Amirtharajan and Balaguru, 2009;
Padmaa et al., 2011), Audio (Ahmed
et al., 2010) or Video (Al-Frajat et al.,
2010). In todays communication broadcasting industries and publishing
medias are interested in information hiding (Bender
et al., 1996), copyright marks in books, digital films, multimedia
and audio recordings. It also includes automatic monitoring of radio advertisements,
medical safety by using Digital Imaging and Communications in Medicine (DICOM)
which is a standard for handling, printing and storing information about the
doctor and the patient in medical imaging ensuring safety (Anderson
and Petitcolas, 1998) and for telemedicine application (Thenmozhi
and Prithiviraj, 2008). Due to the rapid growth of multimedia communications
high data rate and protection against data piracy is required. OFDM ensures
high data rate and Steganography takes care of embedding additional confidential
information in it. Cox et al. (1997) and Xie
et al. (2007) proposed a secured Spread Spectrum (SS) based watermarking
for multimedia images which provides robustness to compression, filtering and
digital to analog conversions.
Here, a message is embedded in the digital image by the encoder (Stefan
and Fabin, 2000) which utilizes a key results in stego-image. Then it is
transmitted over a channel. At the receiver it is processed by the decoder using
the same key used for transmission to provide the message output. Classification
of information hiding is presented by Petitcolas et al.
(1999) in their review paper as covert channel, steganography, anonymity
and watermarking. Even though this paper intention has to give an overview about
stego implementation in spread spectrum for more clarity, a simple discrimination
among three has been given in Table 2. A simple classification
in steganography is spatial domain method (Thanikaiselvan
et al., 2011b) and transforms domain method (Thanikaiselvan
et al., 2011a). General block diagram of stego system is presented
in Fig. 6.
Banoci et al. (2009) proposed a codebook based
secret data embedding in CDMA system. Banoci et al.
(2010) extended their work by making use of error control codes in CDMA
systems. Another CDMA based embedding has been analysed by Amirtharajan
and Balaguru (2011) where they divide the secret message in to four parts
and then embed it in the four quadrants of the cover.
|| Discrimination between cryptography, steganography and water
|| General block diagram of stego system
|| Spread spectrum based image steganography
|| Constellation table for BPSK
|IPCBE: In-phase component before embedding, IPCAE: In-phase
component after embedding, QPCBE: Quadrature phase component before embedding,
QPCAE: Quadrature phase component after embedding
In this study we concentrate on Spread Spectrum Image Steganography (SSIS)
introduced by Marvel et al. (1998) which embeds
a secret message on a digital image. The embedded secret data can be retrieved
with the help of keys. Lin et al. (2006) proposes
a new embedding scheme in which datas are embedded in the physical layer
of OFDM networks and extended in Kumar et al. (2011).
Tie-sheng et al. (2008) presented a Quantization
Index Modulation (QIM) in OFDM networks where embedding of the secret data is
done by selecting the quantizer based on space domain which operates on gray
scale images of 0 to 255 levels to represent the pixels.
Embedding the secret information can also be done at the signal mapper block
utilizing BPSK/QPSK /QAM as shown in Fig. 7. Here embedding
is done making use of BPSK technique and the constellation and phasor diagrams
and their related equations are presented in (Fig. 8, 9,
BPSK before embedding:
||Phasor diagrams, where embedding is done in signal mapper
using BPSK modulation scheme
||Constellation diagram where embedding is done in signal mapper
using BPSK modulation scheme
BPSK after embedding:
Classification on SSS and its future trend: The future trend on spread spectrum steganography is outlined in Fig. 10.
The confidential information could be embedded in any one of the following ways:
|| Classification of spread spectrum steganography (SSS)
The first two always offers good cryptic effect and the later four are really useful for covert communication.
Wireless communication must cope up with performance limiting challenges such as frequency selective fading and ISI for high data rate transmission. OFDM as a multicarrier modulation technique is effective for supporting high speed transmission as well as combating multipath fading and frequency selective fading in broad band wireless communication. This review presents, basic OFDM System implementation, CDMA and MC-CDMA. Later, how steganography could be used to send additional confidential information using signal mapper and other possible ways are discussed in detail.
Hmood, A.K., B.B. Zaidan, A.A. Zaidan and H.A. Jalab, 2010.
An overview on hiding information technique in images. J. Applied Sci., 10: 2094-2100.CrossRef | Direct Link |
Akansu, A.N., P. Duhamel, X.M. Lin and M. de Courville, 1998.
Orthogonal transmultiplexers in communication: A review. IEEE Trans. Signal. Process., 46: 979-995.CrossRef | Direct Link |
Bell, A.G., 1876.
Improvement in telegraphy. U.S. Patent No. 174465. http://www2.iath.virginia.edu/albell/bpat.1.html.
Al-Frajat, A.K., H.A. Jalab, Z.M. Kasirun, A.A. Zaidan and B.B. Zaidan, 2010.
Hiding data in video file: An overview. J. Applied Sci., 10: 1644-1649.CrossRef | Direct Link |
Alper, A. and J. Arnbak, 1980.
Capacity allocation and reservation in common-user satellite communications systems with a reconfigurable multiple-beam antenna and a nonlinear repeater. IEEE Trans. Commun., 28: 1681-1692.CrossRef | Direct Link |
Amirtharajan, R. and R.J.B. Balaguru, 2009.
Tri-layer stego for enhanced security-a keyless random approach. Proceedings of the IEEE International Conference on Internet Multimedia Services Architecture and Applications, December 9-11, 2009, Bangalore, India, pp: 1-6CrossRef | Direct Link |
Amirtharajan, R. and R.J.B. Balaguru, 2011.
Covered CDMA multi-user writing on spatially divided image. Proceedings of the 2nd International Conference on Vehicular Technology, Information Theory and Aerospace and Electronic Systems Technology, February 28-March 3, 2011, Chennai, India, pp: 1-5CrossRef |
Anderson, R.J. and F.A. Petitcolas, 1998.
On the limits of steganography. IEEE J. Selected Areas Commun., 16: 474-481.CrossRef | Direct Link |
Arnstein, D., 1979.
Power division in spread spectrum systems with limiting. Trans. Commun., 27: 574-582.CrossRef | Direct Link |
Pandey, A., S.R. Agrawalla and S. Manivannan, 2002.
VLSI implementation of OFDM modem. http://www.feng.pucrs.br/~decastro/pdf/VLSI_implementation_of_an_OFDM_modem-WiProTechnologies.pdf.
Banoci, V., G. Bugar and D. Levicky, 2009.
Steganography systems by using CDMA techniques. Proceedings of the 19th International Conference on Radioelektronika, April 22-23, 2009, Bratislava Slovakia, pp: 183-186CrossRef |
Banoci, V., G. Bugar and D. Levicky, 2010.
Information hiding using pseudo-random number sequences with error correction. Proceedings of the 20th International Conference on Radioelektronika, April 19-21, 2010, Brno, Czech Republic, pp: 1-4CrossRef |
Bender, W., D. Gruhl, N. Morimoto and A. Lu, 1996.
Techniques for data hiding. IBM Syst. J., 35: 313-336.CrossRef | Direct Link |
Bingham, J.A.C., 1990.
Multicarrier modulation for data transmission. An idea whose time has come. IEEE Commun. Mage., 28: 5-14.CrossRef | Direct Link |
Bidet, E., D. Castelain, C. Joanblanq and P. Senn, 1995.
A fast single-chip implementation of 8192 complex point FFT. IEEE J. Solid. State Circuits, 30: 300-305.CrossRef | Direct Link |
Casas, E.F. and C. Leung, 1991.
OFDM for data communication over mobile radio FM channels. I. Analysis and experimental results. IEEE Trans. Commun., 39: 783-793.CrossRef | Direct Link |
Chang, R.W., 1970.
Orthogonal frequency division multiplexing. U.S. Patent No. 3488445.
Cheddad, A., J. Condell, K. Curran and P. McKevitt, 2010.
Digital image steganography: Survey and analysis of current methods. Signal Process., 90: 727-752.CrossRef | Direct Link |
Chouly, A., A. Brajal and S. Jourdan, 1993.
Orthogonal multicarrier techniques applied to direct sequence spread spectrum CDMA systems. Proceedings of the Global Telecommunications Conference, November 29-December 2, 1993, Houston, TX., USA., pp: 1723-1728CrossRef |
Chang, R. and R. Gibby, 1968.
A theoretical study of performance of an orthogonal multiplexing data transmission scheme. IEEE Trans. Commun. Technol., 16: 529-540.CrossRef | Direct Link |
Lin, C., J.S. Pan, C.S. Shieh and P. Shi, 2006.
An information hiding scheme for OFDM wireless networks. Proceedings of the International Conference on Intelligent Information Hiding and Multimedia Signal Processing, December 18-20, 2006, Pasadena, CA., USA., pp: 51-54CrossRef |
Chuang, J., 1987.
The effects of time delay spread on portable radio communications channels with digital modulation. IEEE J. Selected Areas Commun., 5: 879-889.CrossRef | Direct Link |
Chow, P.S., J.C. Tu and J.M. Cioffi, 1991.
Performance evaluation of a multichannel transceiver system for ADSL and VHDSL services. IEEE J. Selected Areas Commun., 9: 909-919.CrossRef | Direct Link |
Chow, J.S., J.C. Tu and J.M. Cioffi, 1991.
A discrete multitone transceiver system for HDSL applications. IEEE J. Selected Areas Commun., 9: 895-908.CrossRef |
Cimini, L.J., 1985.
Analysis and simulation of a digital mobile channel using orthogonal frequency division multiplexing. IEEE Trans. Commun., 33: 665-675.CrossRef | Direct Link |
Cimini Jr., L.J., B. Daneshrad and N.R. Sollenberger, 1996.
Clustered OFDM with transmitter diversity and coding. IEEE Global Telecommun. Conf., 1: 703-707.CrossRef |
Cooper, G.R. and R.W. Nettleton, 1978.
A spread-spectrum technique for high-capacity mobile communications. IEEE Trans. Veh. Technol., 27: 264-275.CrossRef | Direct Link |
Cox, I.J., J. Kilian, F.T. Leighton and T. Shamoon, 1997.
Secure spread spectrum watermarking for multimedia. IEEE Trans. Image Process., 6: 1673-1687.CrossRef | Direct Link |
DaSilva, V. and E.S. Sousa, 1993.
Performance of orthogonal CDMA codes for quasi-synchronous communication systems. Proc. IEEE Int. Conf. Universal Personal Commun., 2: 995-999.CrossRef |
Doelz, M.L., E.T. Heald and D.L. Martin, 1957.
Binary data transmission techniques for linear systems. Proc. IRE, 45: 656-661.CrossRef | Direct Link |
Tie-sheng, F., X. Jian-sheng, X. Wei-hong and Q. Da-peng, 2008.
OFDM information hiding method by preposition embedded QIM. Int. Conf. Comput. Intell. Modell. Control. Autom., 1: 19-24.Direct Link |
Fazel, K., 1993.
Performance of CDMA/OFDM for mobile communication system. Proc. IEEE Int. Conf. Universal Personal Commun., 2: 975-979.CrossRef |
Guo, Z., Z. Xu, F. Wang and B. Huang, 2010.
Empirical mode decomposition for BER improvement in cellular network. Inform. Technol. J., 9: 146-151.CrossRef | Direct Link |
Haque, M.D., S.E. Ullah and M.R. Ahmed, 2008.
Performance evaluation of a wireless orthogonal frequency division multiplexing system under various concatenated FEC channel-coding schemes. Proceedings of the 11th International Conference on Computer and Information Technology, December, 24-27, 2008, Bangladesh, pp: 94-97
Hara, S. and R. Prasad, 1997.
Overview of multicarrier CDMA. IEEE Commun. Magaz., 35: 126-133.CrossRef | Direct Link |
Hara, S. and R. Prasad, 1996.
DS-CDMA, MC-CDMA and MT-CDMA for mobile multi-media communications. Proc. IEEE Vehicular Technol. Conf., 2: 1106-1110.CrossRef |
Hara, S., M. Mouri, M. Okada and N. Morinaga, 1996.
Transmission performance analysis of multi-carrier modulation in frequency selective fast rayleigh fading channel. Wireless Personnel Commun., 2: 335-356.CrossRef |
Hartmann, M.M., G. Malz and D. Schafhuber, 2003.
Theory and design of multipulse multicarrier systems for wireless communications. IEEE Asilomar Conf. Signals Syst. Comput., 1: 492-496.CrossRef |
Hemalatha, M., K. Thenmozhi, V. Prithiviraj, D. Bharadwaj and R. Vignesh, 2009.
Diversity reception for CDMA based mobile communication systems. Proceedings of the 1st International Conference on Wireless Communication, Vehicular Technology, Information Theory and Aerospace and Electronic Systems Technology, May 17-20, 2009, Aalborg, pp: 660-664CrossRef |
Hemalatha, M., V. Prithivraj, S. Jayalalitha and K. Thenmozhii, 2011.
Diversity analysis in WiFi system. J. Theor. Applied Inform. Technol., 33: 111-118.Direct Link |
Hirosaki, B., 1981.
An orthogonally multiplexed QAM system using the discrete fourier transform. IEEE Trans. Commun., 29: 982-989.Direct Link |
Kalet, I., 1989.
The multitone channel. IEEE Trans. Commun., 37: 119-124.CrossRef | Direct Link |
Keasler, W.E., D.L. Bitzer and P.T. Tucker, 1980.
High speed modem suitable for operating with a switched network. U.S. Patent No. 4206320. http://www.patents.com/us-4206320.html.
Kondo, S. and L.B. Milstein, 1995.
On the performance of multicarrier DS CDMA systems. IEEE Trans. Commun., 43: 3101-3101.CrossRef | Direct Link |
Yang, L.L. and L. Hanzo, 2003.
Multicarrier DS-CDMA: A multiple access scheme for ubiquitous broadband wireless communications. IEEE Commun. Mag., 41: 116-124.CrossRef |
Lionti, R., D. Noguet, N. Daniele and D. Chagnot-Auclair, 2001.
Wireless transmissions in multipath environments benefit from direct sequence spread spectrum techniques. Proceedings of the 8th IEEE International Conference on Emerging Technologies and Factory Automation, October 15-18, 2001, Antibes-Juan les Pins, France, pp: 169-178CrossRef | Direct Link |
Marvel, L.M., C.T. Retter and C.G. Jr. Boncelet, 1998.
A methodology for data hiding using images. Proceedings of the IEEE on Military Communications Conference, October 18-21, 1998, Boston, MA., USA., pp: 1044-1047CrossRef |
McCormick, A.C. and E.A. Al-Susa, 2002.
Multicarrier CDMA for future generation mobile communication. Electron. Commun. Eng. J., 14: 52-60.CrossRef | Direct Link |
Lee, J.S. and L.E. Miller, 1998.
CDMA Systems Engineering Handbook. Artech House, London, ISBN: 9780890069905, Pages: 1228
Mosier, R.R. and R.G. Clabaugh, 1958.
Kineplex, a bandwidth efficient binary transmission system. Am. Inst. Electron. Eng. Trans., 76: 723-728.
Ahmed, M.A., M.L.M. Kiah, B.B. Zaidan and A.A. Zaidan, 2010.
A novel embedding method to increase capacity and robustness of low-bit encoding audio steganography technique using noise gate software logic algorithm. J. Applied Sci., 10: 59-64.CrossRef | Direct Link |
Nithyanandan, L. and P. Dananjayan, 2006.
Modified frequency and frame synchronization algorithms for MC-CDMA system. Inform. Technol. J., 5: 273-280.CrossRef | Direct Link |
Yue, O.C., 1983.
Spread spectrum mobile radio, 1977-1982. IEEE Trans. Veh. Technol., 32: 98-105.CrossRef | Direct Link |
Padmaa, M., Y. Venkataramani and R. Amirtharajan, 2011.
Stego on 2n
: 1 Platform for users and embedding. Inform. Technol. J., 10: 1896-1907.CrossRef | Direct Link |
Prasad, R. and S. Hara, 1996.
An overview of multi-carrier CDMA. Proceedings of the IEEE 4th International Symposium on Spread Spectrum Techniques and Applications, September 22-25, 1996, Mainz, Germany, pp: 107-114CrossRef |
Petitcolas, F.A.P., R.J. Anderson and M.G. Kuhn, 1999.
Information hiding-a survey. Proc. IEEE, 87: 1062-1078.CrossRef | Direct Link |
Pickholtz, R.L., D.L. Schilling and L.B. Milstein, 1982.
Theory of spread-spectrum communications--a tutorial. IEEE Trans. Commun., 30: 855-884.CrossRef | Direct Link |
Pickholtz, R.L., D.L. Schilling and L.B. Milstein, 1984.
Revisions to Theory of spread-spectrum communications-a tutorial. IEEE Trans. Communi., 32: 211-212.CrossRef | Direct Link |
Pickholtz, R.L., L.B. Milstein and D.L. Schilling, 1991.
Spread spectrum for mobile communications. IEEE Trans. Vehicular Technol., 40: 313-322.CrossRef | Direct Link |
Venkatesan, G.K.D.P. and V.C. Ravichandran, 2007.
Performance analysis of MC-CDMA for wide band channels. Inform. Technol. J., 6: 267-270.CrossRef | Direct Link |
Rabah, K., 2004.
Steganography-the art of hiding data. Inform. Technol. J., 3: 245-269.CrossRef | Direct Link |
Schneier, B., 2007.
Applied Cryptography: Protocols, Algorithms and Source Code in C. 2nd Edn., John Wiley and Sons, New Delhi, India, ISBN-13: 9788126513680, Pages: 784
Janakiraman, S., R. Amirtharajan, K. Thenmozhi and J.B.B. Rayappan, 2012.
Pixel forefinger for gray in color: A layer by layer stego. Inform. Technol. J., 11: 9-19.CrossRef | Direct Link |
Scholtz, R.A., 1982.
The origins of spread-spectrum communications. IEEE Trans. Commun., 30: 822-854.CrossRef | Direct Link |
Stefan, K. and A. Fabin, 2000.
Information Hiding Techniques for Steganography and Digital Watermarking. Artech House, London, UK
Thanikaiselvan, V., P. Arulmozhivarman, R. Amirtharajan and J.B.B. Rayappan, 2011.
Wave (let) decide choosy pixel embedding for stego. Proceedings of the International Conference on Computer, Communication and Electrical Technology, March 18-19, 2011, India, pp: 157-162CrossRef | Direct Link |
Thanikaiselvan, V., S. Kumar, N. Neelima and R. Amirtharajan, 2011.
Data battle on the digital field between horse cavalry and interlopers. J. Theor. Applied Inform. Technol., 29: 85-91.Direct Link |
Turin, G.L., 1980.
Introduction to spread-spectrum anti multipath techniques and their application to urban digital radio. Proc. IEEE., 68: 328-353.CrossRef | Direct Link |
Kennedy, M.D., 1982.
Regulatory aspects of spread spectrum communications. Progress Spread Spectrum Commun. Military Commun. Conf., 2: 34-38.
Kavehrad, M. and P. McLane, 1987.
Spread spectrum for indoor digital radio. IEEE Commun. Mag., 25: 32-40.CrossRef | Direct Link |
Kumar, P.P., R. Amirtharajan, K. Thenmozhi and J.B.B. Rayappan, 2011.
Steg-OFDM blend for highly secure multi-user communication. Proceedings of the 2nd International Conference on Vehicular Technology, Information Theory and Aerospace and Electronic Systems Technology, February 28-March 3, 2011, Chennai, India, pp: 1-5CrossRef |
Meng, L., J. Hua, Y. Zhang, Z. Xu and G. Li, 2008.
A novel algorithm for initial frame synchronization in TD-SCDMA downlink. Inform. Technol. J., 7: 545-548.CrossRef | Direct Link |
Zhaogan, L., W. Liejun, Z. Taiyi and R. Yun, 2007.
A new steiner channel estimation method in MIMO OFDM systems. Inform. Technol. J., 6: 1238-1244.CrossRef | Direct Link |
Malarkkan, S. and V.C. Ravichandran, 2006.
On the performance analysis of call admission control with SIR guard margin in WCDMA systems with multi class, non-uniform traffic distribution. Inform. Technol. J., 5: 937-943.CrossRef | Direct Link |
Proakis, J., 2000.
Digital Communication. McGraw-Hill, New York., USA., ISBN-13: 978-0072321111
Rappaport, T.S., A. Annamalai, R.M. Buehrer and W.H. Tranter, 2002.
Wireless communications: Past events and a future perspective. IEEE Commun. Maga., 40: 148-161.CrossRef | Direct Link |
Xie, R., K. Wu, J. Du and C. Li, 2007.
Survey of public key digital watermarking systems. Proc. 8th ACIS Int. Conf. Software Eng, Artificial Intelli. Networking Parallel/Distributed Comput., 2: 439-443.CrossRef | Direct Link |
Saltzberg, B., 1967.
Performance of an efficient parallel data transmission system. IEEE Trans. Commun. Technol., 15: 805-811.CrossRef | Direct Link |
Sari, H., G. Karam and I. Jeanclaude, 1995.
Transmission techniques for digital terrestrial TV broadcasting. IEEE Commun. Mag., 33: 100-109.CrossRef | Direct Link |
Britto, K.S.S. and P.E. Sankaranarayanan, 2006.
CDMA based optical lan. Inform. Technol. J., 5: 673-678.CrossRef | Direct Link |
Sivakumar, B., S. Subha Rani, V.S.V. Ravi Kiran and P. Abhishek, 2006.
Adaptive filter approaches for interference suppression in CDMA systems. Inform. Technol. J., 5: 1098-1101.CrossRef | Direct Link |
Schwartz, M., 2008.
The origins of carrier multiplexing: Major george owen squier and AT and T. IEEE Commun. Mag., 46: 20-24.CrossRef | Direct Link |
Sourour, E.A. and M. Nakagawa, 1996.
Performance of orthogonal multicarrier CDMA in a multipath fading channel. IEEE Trans. Commun., 44: 356-367.CrossRef | Direct Link |
Speth, M., S.A. Fechtel, G. Fock and H. Meyr, 1999.
Optimum receiver design for wireless broad-band systems using OFDM. I. IEEE Trans. Commun., 47: 1668-1677.CrossRef | Direct Link |
Thenmozhi, K., V.K. Konakalla, S.P.P. Vabbilisetty and R. Amirtharajan, 2011.
Space Time Frequency coded (STF) OFDM for broadband wireless communication systems. J. Theor. Applied Inform. Technol., 3: 53-59.Direct Link |
Thenmozhi, K. and V. Prithiviraj, 2008.
Suitability of Coded Orthogonal Frequency Division Multiplexing (COFDM) for multimedia data transmission in wireless telemedicine applications. Conf. Comput. Intelli. Multimedia Appl., 4: 288-292.CrossRef |
Vahlin, A. and N. Holte, 1994.
Use of a guard interval in OFDM on multipath channels. Electron. Lett., 30: 2015-2016.CrossRef | Direct Link |
Van Nee, R. and R. Prasad, 2000.
OFDM for Wireless Multimedia Communications. Artech House, Norwell, MA., USA., ISBN-13: 9780890065303, Pages: 260
Viterbi, A., 1985.
When not to spread spectrum--A sequel. IEEE Commun., Mag., 23: 12-17.CrossRef | Direct Link |
Viterbi, A.J., 1995.
CDMA: Principles of Spread Spectrum Communication. Addison-Wesley, USA., ISBN-13: 978-0201633740, Pages: 245
Weinstein, S.B., 2009.
The history of orthogonal frequency-division multiplexing [history of communications]. IEEE Commun. Mage., 47: 26-35.CrossRef | Direct Link |
Wu, Z., N. Zhao, G. Ren and T. Quan, 2010.
Anti-interference strategies review of unified spread spectrum telemetry tracking and control system. Inform. Technol. J., 9: 979-983.CrossRef | Direct Link |
Wang, Z. and G.B. Giannakis, 2000.
Wireless multicarrier communications. IEEE Signal Process. Mag., 17: 29-48.CrossRef | Direct Link |
Mingxin, Z., R. Wenhui, X. Feng, Z. Yatong, J. Shen and Z. Yaling, 2008.
A novel CDMA-BLAST space-time code scheme. Inform. Technol. J., 7: 1067-1071.CrossRef | Direct Link |
Zimmerman, M. and A. Kirsch, 1967.
The AN/GSC-10 (KATHRYN) variable rate data modem for HF radio. IEEE Trans. Commun. Technol., 15: 197-204.CrossRef | Direct Link |
Zou, W.Y. and Y. Wu, 1995.
COFDM: An overview. IEEE Trans. Broadcast, 41: 1-8.CrossRef | Direct Link |
Weinstein, S. and P. Ebert, 1971.
Data transmission by frequency-division multiplexing using the discrete fourier transform. IEEE. Trans. Commun., 19: 628-634.CrossRef | Direct Link |
Wu, Z., Y. Kuang, N. Zhao and Y. Zhao, 2010.
A hybrid CDMA multiuser detector with ant colony optimization and code filtering system. Inform. Technol. J., 9: 818-824.CrossRef | Direct Link |