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04-01-2010, 02:36 PM

III/IV Btech, ECE.
P.Karuna Sree.


This paper gives a brief idea of how the mobile communication has evolved,its generations. Different generations have been developing since then each generation coming out overcoming previous generation drawbacks.One such technology in fourth generation is OFDM. OFDM based technique looks more promising as a 4G standard surpassing the 3G standards .So,a complete review of OFDM is provided explaining its spectrum utilization,transmitter,receiver,its mathematical description,advantages and disadvantages.This paper presents how signals are multiplexed using OFDM and transmitted using MIMO technology.We discussed MIMO technology,its transmission,advantages and disadvantages. Different multiple access techniques like TDMA,FDMA,CDMA,SDMA are discussed.and finally the various fields that are being explored to improve the utility of 4G systems.
Introduction to mobile communication:
There is tremendous growth in mobile communication since 1980. Digital services started replacing the analog services ones. The evolution began with the first radio ph one service which was introduced in the 1940s. It is not efficient for data Transfer applications like mobile communication because of the low data rates provided by 2G technology.The data rates worsen at inappropriate weather conditions.Due to this disadvantage new technologies which have come into picture have increased speed. The first is 2.5G (GPRS) technology it allows data transfer at a faster rate than GSM . Further enhancements to GSM networks are provided by Enhanced Data rates for GSM Evolution (EDGE) technology. EDGE provides up to three times the data capacity of GPRS.This technology is being launched in india for the first time by vodofone corporation for its users .There after 3G (WCDMA/UMTS) technology has come into existence. The maximum data transfer which can be done with 3G technology is 2Mbps. The 4G technology promises data transfer of up to 100Mbps to 1Gbps .
Popping of generations:
THE future always comes too fast," Alvin Toffler, an eminent futurologist, once said this is evident from the fast changes taking place in the telecommunication market.
First generation:
This first generation (1G) analog system for mobile communications had seen key improvements during the 1970s:. This was due to the invention of microprocessor and even the digitization of the control link between the mobile phone and the cell site.
Second generation (2G):
By the end of the year 1980 the digital cellular systems were developed. These systems digitized the control link and the voice signal. These systems provided better voice quality with higher capacity which even costed low.
2.5 Generation
CDMA2000's 1xRTT is the first technology for the evolution of cdmaOne 2G networks to 2.5G networks.GPRS represents the first packet-based technology for evolution from 2G GSM networks to 2.5G networks
Third generation (3G) :
These systems provide faster communication services which include: voice ,fax ,internet.Momentous capacity , Broadband capabilities to support greater numbers of voice and data customers .
Why Did 3G fail
1.High input fees for the 3G service licenses;
2.Current high debt of many telecommunication companies;
3.Challenge to build the necessary infrastructure for 3 G
4. Expense and bulk of 3G phones
5.Lack of coverage because it is still new service;
6.High prices of 3G mobile services in some countries
Fourth generation: Interactive services like Video Conferencing (with more than 2 sites simultaneous ly), Wireless Internet,etc.
2.The bandwidth would be much wider (100 MHz) and data would be transferred at much higher rates. 3. The cost of the data transfer would be comparatively very less and global mobility would be possible. 4. The networks will be all IP networks based on IPv6.
5.The antennas will be much smarter and improved access technologies like OFDM and MC-CDMA (Multi Carrier CDMA) will be used.
6.Higher bandwidths would be available which would make cheap data transfer possible.
7. The network security would be much tighter . The other great advantage of 4G will be its speed. While 3G networks provide 2 megabytes per second, 4G can reach anywhere between 20 and 100 megabytes per second
8.Due to 4G it will be possible to use several applications like videoconference or picture playback simultaneously all through the mobile phone with maximum resolution. Some of the standards which pave the way for 4G systems are :
3. 3GPP
3G Vs 4G: between some key parameters of 3G Vs possible 4G systems.
Frequency Band 1.8 - 2.5 GHz 2 - 8 GHz
Bandwidth 5-20 MHz 5-20 MHz
D data rate Up to 2Mbps Up to 20 Mbps
Access Wideband DMA
Multi-carrier - CDMA or
FEC Turbo-codes Concatenated codes
Mobile top
200 kmph


OFDM uses the technique in which the data is spread over number of carriers which are at specific predefined frequencies. Orthogonal means the perpendicular nature of the signal, where in the modulate frequencies are perpendicular to each other. This reduces or eliminates the cross talk .But if the transmitter or the receiver is in motion, or in a vehicle there is one problem of intersymbol interference, since the frequency changes as per the motion between the transmitter and the receiver. Thus the performance is poor due to this interference OFDM spectrum utilization: -
Using ODFM, it is possible to exploit the time domain, the space domain, the frequency domain and even the code domain to optimize radio channel usage. It ensures very robust transmission in multi-path environments with reduced receiver complexity.In order to reduce this Intersymbol interference a technique of introducing a guard band is inserted between two symbols of the OFDM.

An OFDM system treats the source symbols (e.g., the QPSK or QAM symbols that would be presentin a single carrier system) at the transmitter as though they are in the frequency - domain. These symbols are used as the inputs to an IFFT block that brings the signal into the timedomain.The IFFT takes in N symbols at a time where N is the number of subcarriers in the system. Each of these N input symbols has a symbol period of T seconds. The basis functions for an IFFT are N orthogonal sinusoids. These sinusoids each have a different frequency and the lowest frequency is DC. Each input symbol acts like a complex weight for the corresponding sinusoidal basis function. Since the input symbols are complex, the value of the symbol determines both the amplitudeamplitude and phase of the sinusoid for that subcarrier. The IFFT output is the summation of all N sinusoids. Thus, the IFFT block provides a simple way to modulate data onto N orthogonal subcarriers. The block of N output samples from the IFFT make up a single OFDM symbol.
Ofdm transmitter:
The whole chunk of data to be transmitted which is in serial form is converted to parallel Ëœnâ„¢ number of data streams using a demultiplexer

In the OFDM carrier signal, each of the sub - carriers in a particular frequency spectrum are modulated separately. Now these modulated sub carriers are then passed through a inverse Fourier Transform module. In inverse FFT the complex time domain signal is generated with real and imaginary parts. At the transmitter we need analog signal to transmit, hence each of these parts which are in digital form are then converted to analog signal using a Digital to Analog converter.
Ofdm receiver:
At the receiver the exact opposite things take place, first of all the received signal is mixed with the locally generated carrier signal which has the same frequency as it was at the transmitter. Thus base band signals are separated from the carrier, but during this process the higher frequency components of the carrier are generated, thus low pass filter is used to block these additional unwanted signals. To get the digital data, the signals are passed through the Analog to Digital converter. Now the result is the real and imaginary part of the complex signal which is similar as created at the transmitter. Now these time domain signals are converted to frequency domain using the forward Fourier transform .

Now we have Ëœnâ„¢ number of parallel symbol streams. These streams are then passed through a symbol detection technique, which gives the original binary streams. All the individual streams are then sent to a multiplexer where the they are converted from parallel to serial form, thus representing the original chunk of data which was there at the transmitter.
Mathematical description:
If N sub-carriers are used, and each sub-carrier is modulated using M alternative symbols, the OFDM symbol alphabet consists of MN combined symbols. The low-pass equivalent OFDM signal is expressed as: where {Xk} are the data symbols, N is the number of sub - carriers, and T is the OFDM symbol time. The sub -carrier spacing of 1 / T makes them orthogonal over each symbol period; this property is expressed as:
where denotes the complex conjugate operator and is the Kronecker delta.. To avoid inter symbol interference in multipath fading channels, a guard interval of length Tg is inserted prior to the OFDM block. During this interval, a cyclic prefix is transmitted such that the signal in the interval equals the signal in the interval . The OFDM signal with cyclic prefix is thus: The low-pass signal above can be either real or complex - valued. Real-valued low-pass equivalent signals are typically transmitted at baseband”wireline applications such as DSL use this approach. For wireless applications, the low-pass signal is typically complex-valued; in which case, the transmitted signal is up-converted to a carrier frequency fc. In general, the transmitted signal can be represented as:
Advantages of OFDM:-
High efficiency of the spectrum FFT can be implemented efficiently Eliminates Inter-symbol interference and fading caused by multipath propagation Eliminates narrow band co-channel interference Less sensitive to time synchronization errors
Applications of OFDM:
DAB - OFDM forms the basis for the Digital Audio Broadcasting (DAB) standard in the European market. ADSL - OFDM forms the basis for the global ADSL (asymmetric digital subscriber line) standard. Wireless Local Area Networks - development is ongoing for wireless point-to-point and point-to-multipoint configurations using OFDM technology
Disadvantages of OFDM: -
Highly sensitive to frequency synchronization errors Peak to average power ratio (PARA) is high High power transmitter amplifiers are required to have linear transmission Power and Capacity is wasted due to the guard band Guard band can consume up to 20% of transmitted power and bandwidth


MIMO means multiple input and multiple output transmission. MIMO is a method in which multiple antennas are used for wireless communication over the channel. This is a technology in which migrating the negative effect s of the wireless channel, providing better link quality and /or higher data rate without consumin g extra bandwidth or transmitting power. Multiple-input multiple-output (MIMO) channels or Vector channels, these represent a wide range of applications. In some special cases they also include MISO (Multiple -Input Single-Output), SISO (Single-Input Single-Output), and SIMO (Single -Input Multiple-Output) channels, but most of the time MIMO is associated with multiple antenna systems. These technologies are mostly used in multi-user communications. Figure below shows things in a very abstract form. 5.2 Principle of multiple accesses to a common channel : Here Ni ----inputs and NO----outputs and here the term channel is not limited to the physical transmission medium, which is a radio channel but has a general meanin g and it also includes parts of digital communication systems.
If we consider a transmission where t antennas simultaneously transmit one signal each and r antennas receive these signals considering the Gaussian noise only, the channel input and output relationship is
y = H x + z
matrix which describes the fading gain, is the Gaussian noise.The difference between single - user and multi-user communications is, in a single -user, the multiple inputs and outputs of a vector channel may be correspond to be different i.e. transmitting and receiving antennas, carrier frequencies and time slots. Due to the fact that the data stems from a single user, intelligent signaling at the transmitter can be performed. Multiple antennas can also be employed for increasing the system diversity degree and therefore they enhance the link performance. The reliability of the link can also be improved by beam-forming, this enlarges the signal to noise ratio. Due to this several data streams can be multiplexed over spatially separated channels in order to multiply data rate without increasing the bandwidth.
6. Multiple Access Techniques:
It can be seen that transmission of multiple data streams which share a common medium are separated/or managed by multiplexing techniques in both single -user or multiple access techniques in multi -user communications. To ensure reliable communication, most of the systems try to avoid interference by choosing orthogonal acces s schemes so that there is no multiple access interference (MAI) or disturbance in the transmission. However, in many cases, orthogonality cannot be maintained due to the influence of the mobile channel.

Time Division Multiplexing (TDM) and Multiple Ac cess (TDMA) :
Data to be transmitted is divided into packets and each data packet is assigned to a slot, users can also occupy several slots. Defined number N slots are build in a frame, which are periodically repeated. So each user has access to the sha red medium in periodical manner. A guard interval of length T is inserted in the slots to avoid interference between them.
Frequency Division Multiplexing (FDM) and Multiple Access (FDMA) :
In this frequency axis is divided into Nf sub-bands each of width B as shown in the Figure.The data packets are distributed on different frequency bands .In mobile environments, the signal bandwidth is spread by the Doppler Effect, so the gaps of an appropriate width of are obtained. This effect is done at the expense of reduced spectral efficiency which is required for Frequency division multiple accesses (FDMA).
Code Division Multiplexing (CDM) and Multiple Accesses (CDMA) This contrast for both the preceding schemes, CDMA allows simultaneous access on a single channel in the same frequency range. The basic principle is to spectrally spread the data streams with a specific sequence called spreading codes (Spread Spectrum technique). The signals can be distinguished by assigning them individually which opens a third dimension, as seen in the below Figure. This would lead us to orthogonal codes, ensuring a parallel transmission for different users.
Space Division Multiplexing (SDM) and Multiple Access (SDMA) This scheme exploits the resources in space. Data streams can simultaneously access the channel in the same frequency band, provided the location of transmit and receive Principle of space division multiple access antennas are appropriately chosen. This requirement is sometimes difficult to fulfill in mobile environment, as the users change their position during the connection.. Mutual interference is likely to occur in Space division multiple access (SDMA) systems. The well-known (GSM) Communications and (DCS) standards both combined with TDMA and FDMA. In UMTS or IMT-2000 systems, CDMA is used in connection with TDMA and FDMA. While TDMA, FDMA, and CDMA have already been used for a fairly long time, SDMA and rather recent in comparison it is yet to be exploited in practical purposes.
The upcoming challenges ahead:
High Speed and large capacity wireless transmission techniques :

Frequency Refarming

1. Advanced adaptive techniques to increase spectral efficiency
2. MIMO techniques for exploiting spatial multiplexing
3. Multicarrier techniques
4. Interference and fading mitigat ing

Network Technologies :
1. Radio Access networking techniques
2. Robust networks
3. Adhoc networks
Mobile terminal technologies :
1. Circuit and component technology
2. Battery technology
3. Human interface Mobile system technologies :
4. Quality of service
5. Mobility control
6. Mobility multicast techniques

This paper envisages how the fourth generation has mended itself to overcome the issues related to the previous generaions of communication systems and evolved to be the most promising system . The robustness against frequency selective feeding is vital feature in OFDM which supports high speed data transmission. 4G base stations will use smart antennas to directly transmit or receive radio-beam patterns to and from individual users, which will make possible more reliable calls at greater distances from base stations. Greater DSP power will enable better ameliorations of fading and interference from multipath reflections and from other cell phones, producing better quality audio and video. 4g technology enables biometric security features like thumbprint readers and location - centric (GPS and more) capabilities as well. The OFDM technology makes sure the efficient use of the available spectrum of the carrier signal . Such a development in the world of wireless te chnology would mean that compatibility with multiple radio systems could be achieved in software alone, enabling the development of simple terminals that can communicate from anywhere in the world. Users could adapt communications according to end use, wit h complete freedom to select their own style of services irrespective of network or operator, bringing the ultimate dream of software -defined radio to reality.

8.Current work in progress :
The Future technology for universal radio environment (FuTURE) is a government driven research project and implimentation in china, the plans and goals of FuTURE project and implimentation until 2010 are as shown below :
The long term goal is to put china into competitive R&D position once the standardization and development activities on 4G are started on a global scale. Standardization of the trials and precommercial systems The FuTURE project and implimentation is divided into following sub - project and implimentations : B3G radio access techniques Wireless LAN and Adhoc networks Ipv6 based mobile core networks European funded 4G research cooperation project and implimentations are focusing on FP6(Sixth Framework Program)which is having certain objectives : Encourage its international competitiveness while promoting research activities in support and other EU policies.

1. Ramjee, P. (2004). 4G Roadmap and Emerging Communication Technologies . : PHS.
2. Smith, C., & Collins, D. (2005). 3G Networks. : .
3. 4G. (n.d.). Retrieved October 18, 2006, from
4. 3G. (n.d.). Retrieved October 23, 20 06, from
5. The principles of OFDM by Louis Litwin and Michael Pugel
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