High Performance Wireless Telecommunications Modulation
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The primary goal of the project is to analyze of OFDM system and to assess the suitability of OFDM as a modulation technique for wireless communications. In the part of project is covered two leading successfully implementation of OFDM based technologies are Digital Video Broadcasting (DVB-T and DVB-H) and Long Term Evolution (LTE advanced for 4G).
Wireless communications is an emerging field, which has seen enormous growth in the last several years. The huge uptake rate of mobile phone technology, Wireless Local Area Networks (WLAN) and the exponential growth of the Internet have resulted in an increased demand for new methods of obtaining high capacity wireless networks. For cellular mobile applications, we will see in the near future a complete convergence of mobile phone technology, computing, Internet access, and potentially many multimedia applications such as video and high quality audio.
In fact, some may argue that this convergence has already largely occurred, with the advent of being able to send and receive data using a notebook computer and a mobile phone. The goal of third and fourth generation mobile networks is to provide users with a high data rate, and to provide a wider range of services, such as voice communications, videophones, and high speed Internet access. The higher data rate of future mobile networks will be achieved by increasing the amount of spectrum allocated to the service and by improvements in the spectral efficiency. OFDM is a potential candidate for the physical layer of fourth generation mobile systems.
Basic Principles of OFDM
The Orthogonal Frequency Division Multiplexing (OFDM) is a modulation technique where multiple low data rate carriers are combined by a transmitter to form a composite high data rate transmission. The first commercial use of OFDM in the communication field was in the 1980s, and it was later widely used in the broadcast audio and video field in the 1990s in such areas as, ADSL, VHDSL, ETSI standard digital audio broadcast (DAB), digital video broadcast (DVB), and high-definition digital TV (HDTV).
Digital signal processing makes OFDM possible. To implement the multiple carrier scheme using a bank of parallel modulators would not be very efficient in analog hardware. However, in the digital domain, multi-carrier modulation can be done efficiently with currently available DSP hardware and software. Not only can it be done, but it can also be made very flexible and programmable. This allows OFDM to make maximum use of available bandwidth and to be able to adapt to changing system requirements.
Figure 1 is illustrated, Instead of separate modulators; the outgoing waveform is created by executing a high-speed inverse DFT on a set of time-samples of the transmitted data (post modulation). The output of the DFT can be directly modulated onto the outgoing carrier, without requiring any other components. Each carrier in an OFDM system is a sinusoid with a frequency that is an integer multiple of a base or fundamental sinusoid frequency. Therefore, each carrier is like a Fourier series component of the composite signal. In fact, it will be shown later that an OFDM signal is created in the frequency domain, and then transformed into the time domain via the Discrete Fourier Transform (DFT).
Two periodic signals are orthogonal when the integral of their product, over one period, is equal to zero. This is true of certain sinusoids as illustrated in Equation 1.
Definition of Orthogonal
The carriers of an OFDM system are sinusoids that meet this requirement because each one is a multiple of a fundamental frequency. Each one has an integer number of cycles in the fundamental period. [2, 145-153; 6]
The importantance of being orthogonal
The main concept in OFDM is orthogonality of the sub-carriers.Since the carriers are all sine/cosine wave, we know that area under one period of a sine or a cosine wave is zero. Let's take a sine wave of frequency m and multiply it by a sinusoid (sine or a cosine) of a frequency n, where both m and n are integers. The integral or the area under this product is given by
These two components are each a sinusoid, so the integral is equal to zero over one period.
When we multiply a sinusoid of frequency n by a sinusoid of frequency m/n the area under the product is zero. In general for all integers n and m , sin(mx), cos(mx), cos(nx) , sin(nx) are all orthogonal to each other. These frequencies are called harmonics. Making the subcarriers mathematically orthogonal was a breakthrough for OFDM because it enables OFDM receivers to separate the subcarriers via an FFT and eliminate the guard bands.
As figure 3 shows, OFDM subcarriers can overlap to make full use of the spectrum, but at the peak of each subcarrier spectrum, the power in all the other subcarriers is zero. OFDM therefore offers higher data capacity in a given spectrum while allowing a simpler system design. Creating orthogonal subcarriers in the transmitter is easy using an inverse FFT. To ensure that this orthogonality is maintained at the receiver (so that the subcarriers are not misaligned), the system must keep the transmitter and receiver clocks closely synchronized--within 2 parts per million in 802.11a systems. The 802.11a standard therefore dedicates four of its 52 subcarriers as pilots that enable phase-lock loops in the receiver to track the phase and frequency of the incoming signal.
The 802.11a standard therefore dedicates four of its 52 subcarriers as pilots that enable phase-lock loops in the receiver to track the phase and frequency of the incoming signal. This method also eliminates low-frequency phase noise.Separating the subcarriers via an FFT require about an order of magnitude fewer multiply-accumulate operations than individually filtering each carrier. In general, an FFT implementation is much simpler than the RAKE receivers used for CDMA and the decision-feedback equalizers for TDMA.This idea are key to understanding OFDM. The orthogonality allows simultaneously transmission on a lot of sub- carriers in a tight frequency space without interference form each other. In essence this is similar to CDMA, where codes are used to make data sequences independent (also orthogonal) which allows many independent users to transmitin same space successfully.[2, 153-154; 6 ; 7]
When the DFT (Discrete Fourier Transform) of a time signal is taken, the frequency domain results are a function of the time sampling period and the number of samples as shown in Figure 4. The fundamental frequency of the DFT is equal to 1/NT (1/total sample time). Each frequency represented in the DFT is an integer multiple of the fundamental frequency.
Parameter Mapping from Time to Frequency for the DFT
The maximum frequency that can be represented by a time signal sampled at rate 1/T is fmax = 1/2T as given by the Nyquist sampling theorem. This frequency is located in the center of the DFT points. All frequencies beyond that point are images of the representative frequencies. The maximum frequency bin of the DFT is equal to the sampling frequency (1/T) minus one fundamental (1/NT).The IDFT (Inverse Discrete Fourier Transform) performs the opposite operation to the DFT. It takes a signal defined by frequency components and converts them to a time signal.
The parameter mapping is the same as for the DFT. The time duration of the IDFT time signal is equal to the number of DFT bins (N) times the sampling period (T).It is perfectly valid to generate a signal in the frequency domain, and convert it to a time domain equivalent for practical use (The frequency domain is a mathematical tool used for analysis. Anything usable by the real world must be converted into a real, time domain signal). This is how modulation is applied in OFDM. In practice the Fast Fourier Transform (FFT) and IFFT are used in place of the DFT and IDFT, so all further references will be to FFT and IFFT.[1 ,118 ; 4]
Definition of Carriers
The maximum number of carriers used by OFDM is limited by the size of the IFFT. This is determined as follows in Equation 2.
OFDM Carrier Count
In order to generate a real-valued time signal, OFDM (frequency) carriers must be defined in complex conjugate pairs, which are symmetric about the Nyquist frequency (fmax). This puts the number of potential carriers equal to the IFFT size/2. The Nyquist frequency is the symmetry point, so it cannot be part of a complex conjugate pair. The DC component also has no complex conjugate. These two points cannot be used as carriers so they are subtracted from the total available.
If the carriers are not defined in conjugate pairs, then the IFFT will result in a time domain signal that has imaginary components. This must be a viable option as there are OFDM systems defined with carrier counts that exceed the limit for real-valued time signals given in Equation 2.In general, a system with IFFT size 256 and carrier count 216. This design must result in a complex time waveform. Further processing would require some sort of quadrature technique (use of parallel sine and cosine processing paths). In this report, only real-value time signals will be treated, but in order to obtain maximum bandwidth efficiency from OFDM, the complex time signal may be preferred (possibly an analogous situation to QPSK vs. BPSK). Equation 2, for the complex time waveform, has all IFFT bins available as carriers except the DC bin.
Both IFFT size and assignment (selection) of carriers can be dynamic. The transmitter and receiver just have to use the same parameters. This is one of the advantages of OFDM. Its bandwidth usage (and bit rate) can be varied according to varying user requirements. A simple control message from a base station can change a mobile unit's IFFT size and carrier selection.[2,199-206; 4]
Binary data from a memory device or from a digital processing stream is used as the modulating (baseband) signal. The following steps may be carried out in order to apply modulation to the carriers in OFDM:
- combine the binary data into symbols according to the number of bits/symbol selected
- convert the serial symbol stream into parallel segments according to the number of carriers, and form carrier symbol sequences
- apply differential coding to each carrier symbol sequence
- convert each symbol into a complex phase representation
- assign each carrier sequence to the appropriate IFFT bin, including the complex conjugates
- take the IFFT of the result
OFDM modulation is applied in the frequency domain. Figure 5 and Figure 6 give an example of modulated OFDM carriers for one symbol period, prior to IFFT.
OFDM Carrier Magnitude prior to IFFT
For this example, there are 4 carriers, the IFFT bin size is 64, and there is only 1 bit per symbol. The magnitude of each carrier is 1, but it could be scaled to any value. The phase for each carrier is either 0 or 180 degrees, according to the symbol being sent. The phase determines the value of the symbol (binary in this case, either a 1 or a 0). In the example, the first 3 bits (the first 3 carriers) are 0, and the 4th bit (4th carrier) is a 1.
OFDM Carrier Phase prior to IFFT
Note that the modulated OFDM signal is nothing more than a group of delta (impulse) functions, each with a phase determined by the modulating symbol. In addition, note that the frequency separation between each delta is proportional to 1/N where N is the number of IFFT bins.
The frequency domain representation of the OFDM is described in Equation 3.
OFDM Frequency Domain Representation (one symbol period)
After the modulation is applied, an IFFT is performed to generate one symbol period in the time domain. The IFFT result is shown in 7. It is clear that the OFDM signal has varying amplitude. It is very important that the amplitude variations be kept intact as they define the content of the signal. If the amplitude is clipped or modified, then an FFT of the signal would no longer result in the original frequency characteristics, and the modulation may be lost.
This is one of the drawbacks of OFDM, the fact that it requires linear amplification. In addition, very large amplitude peaks may occur depending on how the sinusoids line up, so the peak-to-average power ratio is high. This means that the linear amplifier has to have a large dynamic range to avoid distorting the peaks. The result is a linear amplifier with a constant, high bias current resulting in very poor power efficiency.
OFDM Signal, 1 Symbol Period
Figure 8 is provided to illustrate the time components of the OFDM signal. The IFFT transforms each complex conjugate pair of delta functions (each carrier) into a real-valued, pure sinusoid. Figure 8 shows the separate sinusoids that make up the composite OFDM waveform given in Figure 7. The one sinusoid with 180 phase shift is clearly visible as is the frequency difference between each of the 4 sinusoids.
The key to the uniqueness and desirability of OFDM is the relationship between the carrier frequencies and the symbol rate. Each carrier frequency is separated by a multiple of 1/NT (Hz). The symbol rate (R) for each carrier is 1/NT (symbols/sec). The effect of the symbol rate on each OFDM carrier is to add a sin(x)/x shape to each carrier's spectrum. The nulls of the sin(x)/x (for each carrier) are at integer multiples of 1/NT. The peak (for each carrier) is at the carrier frequency k/NT. Therefore, each carrier frequency is located at the nulls for all the other carriers. This means that none of the carriers will interfere with each other during transmission, although their spectrums overlap. The ability to space carriers so closely together is very bandwidth efficient.
OFDM Time Waveform
Figure 9 shows the OFDM time waveform for the same signal. There are 100 symbol periods in the signal. Each symbol period is 64 samples long (100 x 64 = 6400 total samples). Each symbol period contains 4 carriers each of which carries 1 symbol. Each symbol carries 1 bit. Note that Figure 9 again illustrates the large dynamic range of the OFDM waveform envelope.
Figure 10 shows the spectrum for of an OFDM signal with the following characteristics:
- 1 bit / symbol
- 100 symbols / carrier (i.e. a sequence of 100 symbol periods)
- 4 carriers
- 64 IFFT bins
- spectrum averaged for every 20 symbols (100/20 = 5 averages)
Red diamonds mark all of the available carrier frequencies. Note that the nulls of the spectrums line up with the unused frequencies. The four active carriers each have peaks at carrier frequencies. It is clear that the active carriers have nulls in their spectrums at each of the unused frequencies (otherwise, the nulls would not exist). Although it cannot be seen in the figure, the active frequencies also have spectral nulls at the adjacent active frequencies. It is not currently practical to generate the OFDM signal directly at RF rates, so it must be up converted for transmission. To remain in the discrete domain, the OFDM could be upsampled and added to a discrete carrier frequency. This carrier could be an intermediate frequency whose sample rate is handled by current technology. It could then be converted to analog and increased to the final transmit frequency using analog frequency conversion methods. Alternatively, the OFDM modulation could be immediately converted to analog and directly increased to the desired RF transmits frequency. Either way, the selected technique would have to involve some form of linear AM (possibly implemented with a mixer). [1, 122-125; 6]
Reception and Demodulation
The received OFDM signal is down converted (in frequency) and taken from analog to digital. Demodulation is done in the frequency domain (just as modulation was). The following steps may be taken to demodulate the OFDM:
- partition the input stream into vectors representing each symbol period
- take the FFT of each symbol period vector
- extract the carrier FFT bins and calculate the phase of each
- calculate the phase difference, from one symbol period to the next, for each carrier
- decode each phase into binary data
- sort the data into the appropriate order
OFDM Carrier Magnitude following FFT
Figure 11 and Figure 12 show the magnitude and spectrum of the FFT for one received OFDM symbol period. For this example, there are 4 carriers, the IFFT bin size is 64, there is 1 bit per symbol, and the signal was sent through a channel with AWGN having an SNR of 8 dB. The figures show that, under these conditions, the modulated symbols are very easy to recover.
OFDM Carrier Phase following FFT
In Figure 12 that the unused frequency bins contain widely varying phase values. These bins are not decoded, so it does not matter, but the result is of interest. Even if the noise is removed from the channel, these phase variations still occur. It must be a result of the IFFT/FFT operations generating very small complex values (very close to 0) for the unused carriers. The phases are a result of these values. [1, 125 -128; 3]
OFDM signals are typically generated digitally due to the difficulty in creating large banks of phase lock oscillators and receivers in the analog domain. Figure 13 shows the block diagram of a typical OFDM transceiver. The transmitter section converts digital data to be transmitted, into a mapping of subcarrier amplitude and phase. It then transforms this spectral representation of the data into the time domain using an Inverse Discrete Fourier Transform (IDFT). The Inverse Fast Fourier Transform (IFFT) performs the same operations as an IDFT, except that it is much more computationally efficiency, and so is used in all practical systems. In order to transmit the OFDM signal the calculated time domain signal is then mixed up to the required frequency.
Block diagram showing a basic OFDM transceiver 
The receiver performs the reverse operation of the transmitter, mixing the RF signal to base band for processing, then using a Fast Fourier Transform (FFT) to analyze the signal in the frequency domain. The amplitude and phase of the subcarriers is then picked out and converted back to digital data. The IFFT and the FFT are complementary function and the most appropriate term depends on whether the signal is being received or generated. In cases where the Signal is independent of this distinction then the term FFT and IFFT is used interchangeably. [1, 125 -128, 3]
Analysis of OFDM characteristics
OFDM demodulation must be synchronized with the start and end of the transmitted symbol period. If it is not, then ISI will occur (since information will be decoded and combined for 2 adjacent symbol periods). ICI will also occur because orthogonality will be lost (integrals of the carrier products will no longer be zero over the integration period),
To help solve this problem, a guard interval is added to each OFDM symbol period. The first thought of how to do this might be to simply make the symbol period longer, so that the demodulator does not have to be so precise in picking the period beginning and end, and decoding is always done inside a single period. This would fix the ISI problem, but not the ICI problem. If a complete period is not integrated (via FFT), orthogonality will be lost.
The effect of ISI on an OFDM signal can be further improved by the addition of a guard period to the start of each symbol. This guard period is a cyclic copy that extends the length of the symbol waveform. Each subcarrier, in the data section of the symbol, (i.e. the OFDM symbol with no guard period added, which is equal to the length of the IFFT size used to generate the signal) has an integer number of cycles. Because of this, placing copies of the symbol end-to-end results in a continuous signal, with no discontinuities at the joins. Thus by copying the end of a symbol and appending this to the start results in a longer symbol time.
Addition of a guard period to an OFDM signal 
In Figure 14, The total length of the symbol is Ts=TG + TFFT, where Ts is the total length of the symbol in samples, TG is the length of the guard period in samples, and TFFT is the size of the IFFT used to generate the OFDM signal. In addition to protecting the OFDM from ISI, the guard period also provides protection against time-offset errors in the receiver.
For an OFDM system that has the same sample rate for both the transmitter and receiver, it must use the same FFT size at both the receiver and transmitted signal in order to maintain subcarrier orthogonality. Each received symbol has TG + TFFT samples due to the added guard period. The receiver only needs TFFT samples of the received symbol to decode the signal. The remaining TG samples are redundant and are not needed.
For an ideal channel with no delay spread the receiver can pick any time offset, up to the length of the guard period, and still get the correct number of samples, without crossing a symbol boundary.
Function of the guard period for protecting against ISI 
Figure 15 shows this effect. Adding a guard period allows time for the transient part of the signal to decay, so that the FFT is taken from a steady state portion of the symbol. This eliminates the effect of ISI provided that the guard period is longer than the delay spread of the radio channel. The remaining effects caused by the multipath, such as amplitude scaling and phase rotation are corrected for by channel equalization.
In order to avoid ISI and ICI, the guard period must be formed by a cyclic extension of the symbol period. This is done by taking symbol period samples from the end of the period and appending them to the front of the period. The concept of being able to do this, and what it means, comes from the nature of the IFFT/FFT process. When the IFFT is taken for a symbol period (during OFDM modulation), the resulting time sample sequence is technically periodic. This is because the IFFT/FFT is an extension of the Fourier Transform which is an extension of the Fourier Series for periodic waveforms. All of these transforms operate on signals with either real or manufactured periodicity. For the IFFT/FFT, the period is the number of samples used.
Guard Period via Cyclic Extension
With the cyclic extension, the symbol period is longer, but it represents the exact same frequency spectrum. As long as the correct number of samples are taken for the decode, they may be taken anywhere within the extended symbol. Since a complete period is integrated, orthogonality is maintained. Therefore, both ISI and ICI are eliminated. Note that some bandwidth efficiency is lost with the addition of the guard period (symbol period is increased and symbol rate is decreased) [2,154-160, 3]
The OFDM signal is made up of a series of IFFTs that are concatenated to each other. At each symbol period boundary, there is a signal discontinuity due to the differences between the end of one period and the start of the next. These discontinuities can cause high frequency spectral noise to be generated (because they look like very fast transitions of the time waveform). To avoid this, a window function (Hamming, Hanning, Blackman, ...) may be applied to each symbol period. The window function would attenuate the time waveform at the start and the end of each period, so that the discontinuities are smaller, and the high frequency noise is reduced. However, this attenuation distorts the signal and some of the desired frequency content is lost.[1, 121;2 154]
OFDM avoids frequency selective fading and ISI by providing relatively long symbol periods for a given data rate. This is illustrated in Figure 17. For a given transmission channel and a given source data rate, OFDM can provide better multipath characteristics than a single carrier.
OFDM vs. Single Carrier, Multipath Characteristic Comparison
However, since the OFDM carriers are spread over a frequency range, there still may be some frequency selective attenuation on a time-varying basis. A deep fade on a particular frequency may cause the loss of data on that frequency for a given time, but the use of Forward Error Coding can fix it. If a single carrier experienced a deep fade, too many consecutive symbols may be lost and correction coding may be ineffective. 
A comparison of RF transmits bandwidth between OFDM and a single carrier is shown in Figure 18 (using the same example parameters as in Figure 17).
OFDM Bandwidth Efficiency
In Figure 18, the calculations show that OFDM is more bandwidth efficient than a single carrier. Note that another efficient aspect of OFDM is that a single transmitter's bandwidth can be increased incrementally by addition of more adjacent carriers. In addition, no bandwidth buffers are needed between transmit bandwidths of separate transmitters as long as orthogonality can be maintained between all the carriers.[2, 161-163; 8; 9]
Since OFDM is carried out in the digital domain, there are many ways it can be implemented. Some options are provided in the following list. Each of these options should be viable given current technology:
- ASIC (Application Specific Integrated Circuit)
- ASICs are the fastest, smallest, and lowest power way to implement OFDM
- Cannot change the ASIC after it is built without designing a new chip
- PowerPC 7400 or other processor capable of fast vector operations
- Highly programmable
- Needs memory and other peripheral chips
- Uses the most power and space, and would be the slowest
- An FPGA combines the speed, power, and density attributes of an ASIC with the programmability of a general purpose processor.
- An FPGA could be reprogrammed for new functions by a base station to meet future (currently unknown requirements).This should be the best choice.
OFDM uses in DVB (Digital Video Broadcasting)
DVB (Digital Video Broadcast) is a set of standards for the digital transmission of video and audio streams, and also data transmission. The DVB standards are maintained by the DVB Project, which is an industry-led consortium of over 260 broadcasters, manufacturers, network operators, software developers, regulatory bodies and others in over 35 countries. DVB has been implemented over satellite (DVB-S, DVB-S2), cable (DVB-C), terrestrial broadcasting (DVB-T), and handheld terminals (DVB-H). the DVB standard following the logical progression of signal processing steps, as well as source and channel coding, COFDM modulation, MPEG compression and multiplexing methods, conditional access and set-top box Technology. In this project is presented an investigation of two OFDM based DVB standards, DVB-T and DVB-H.
DVB-T (Digital Video Broadcasting - Terrestrial)
The first Terrestrial Digital Video Broadcasting pilot transmissions were started in the late 90's, and the first commercial system was established in Great Britain. In the next few years the digital broadcasting system has been set up in many countries, and the boom of the digital terrestrial transmission is estimated in the next few years, while the analogue transmission will be cancelled within about 15 years. The greatest advantage of the digital system is the effective use of the frequency spectrum and its lower radiated power in comparison with the analogue transmission, while the covered area remains the same.
Another key feature is the possibility of designing a so-called Single Frequency Network (SFN), which means that the neighboring broadcast stations use the same frequency and the adjacent signals don't get interfered. The digital system transmits a data stream, which means that not only television signals but data communication (e.g. Internet service) may be used according to the demands. The data stream consists of an MPEG-2 bit stream, which means a compression is used, enabling the transfer of even 4 or 5 television via the standard 8 MHz wide TV channel. For the viewer, the main advantages are the perfect, noise-free picture, CD quality sound, and easier handling, as well as services like Super Teletext, Electronic Programme Guide, interactivity and mobility.[11, 251-253]
Modulation technique in DVB-T
The DVB-T Orthogonal Frequency Division Multiplexing (OFDM) modulation system uses multi-carrier transmission. There are 2 modes, the so-called 2k and 8k modes, using 1705 and 6817 carriers respectively, with each carrier modulated separately and transmitted in the 8 MHz TV channel. The common modulation for the carriers is typically QPSK, 16-QAM or 64-QAM. Each signal can be divided into two, so-called „In Phase" (I) and „Quadrature Phase" components, being a 90° phase shift between them. The constellation diagram and the bit allocation is shown in bellow
16-QAM constellation diagram and bit allocation 
This modulation can be demonstrated in the constellation diagram, where the 2 axes represent the 2 components (I and Q). In case of using 16-QAM modulation, the number of states is 16, so 1 symbol represents 4 bits. [11, 255; 6; 14]
If we simulate all the carriers in the constellation diagram we get not just 1 discrete point, but many points, forming a „cloud" and representing each state. In case of additive noise the „cloud" gets bigger and the receiver may decide incorrectly, resulting in bit errors. Figure 2 shows the measured constellation diagram without and with additive noise.
Measured 16-QAM constellation diagram a) without additive noise b) with additive noise 
To ensure perfect picture quality, the DVB-T system uses a 2 level error correction (Reed-Solomon and Viterbi). This corrects the bad bits at an even 10-4 Bit Error Rate (BER) and enables error-free data transmission. [13, 32-36]
The multi-carrier structure
The structure of carriers can be illustrated also in the function of time (Figure 20). The horizontal axis is the frequency and the vertical axis is the time. The 8 MHz channel consists of many carriers, placed 4462 Hz or 1116 Hz far from each other according to the modulation mode (2k or 8k).
Structure of OFDM carriers 
There are some reserved, so-called Transmission Parameter Signalling (TPS) carriers that do not transfer payload, just provide transmission mode information for the receiver, so the total number of "useful" carriers is 1512 and 6048 respectively in the two transmission modes, and the resultant bit rate is between 4,97 and 31,66 Mbit/s, depending on the modulation (QPSK, 16-QAM or 64-QAM), the transmission mode (2k or 8k), the Code Rate (CR) used for error correction and the selected Guard Interval (GI). This guard interval means that there is a small time gap between each symbol, so the transmission is not continuous. This guarding time enables perfect reception by eliminating the errors caused by multipath propagation.[4, 79-90; 13]
In 2k mode, 1705 carriers are modulated in the 8 MHz TV channel, so each carrier is 4462 Hz far from its neighbor, while in 8k mode this distance is 1116 Hz. In digital broadcasting, there are no vision and sound carriers, so the power for each carrier is the same. This means the amplitude of the frequency spectrum of the DVB-T signal is constant in the TV channel, and the radiated power is smaller than in the case of analogue broadcasting. A measured spectrum is shown in figure 4, where there are three TV channels, two of them digital (DVB-T), and one of them analogue (PAL signal).[14, 34-28]
Channel estimation for OFDM
The method of channel estimation implied by the frame structure of DVB-T is channel estimation via interpolation. The basic principle is depicted in figure 23.
Principle of channel estimation via interpolation 
Embedded into the OFDM data stream are training symbols (depicted as arrows) that can be used to obtain samples of the channel transfer function. The values of the channel in between the samples can then be obtained via a interpolation procedure. Generally we have a two dimensional interpolation problem.
Fortunately the problem can be separated into an interpolation in time and in frequency. The most critical task is the design of the interpolation filters used. Both interpolations must agree with the sampling theorem:
- The interpolation in time is band limited by the time-variant behavior of the channel. This is cause by a movement of the receiver and by uncompensated synchronization errors. The maximum allowable bandwidth of these disturbances is determined by the number of training symbols in one subcarrier.
- Due to the duality of time and frequency the interpolation in frequency is bandlimited by the length of the cir. The maximum allowable cir-length thus is not only determined by the length of the guard interval but also by the number of training symbols in one OFDM symbol. If we use fixed filters for implementation where the maximum dispersion to be assumed is given by the length of the guard interval this implies that for short guard intervals the channel can be estimated with a higher accuracy than with a larger guard interval.
For interpolation in frequency a interpolation filter optimized according to the Wiener filter theory is used. For interpolation in time a linear interpolation is sufficient. [14, 28-38]
Tasks of the inner receiver and receiver structure
As mentioned before in order for a digital transmission system to work, receiver and transmitter have to be synchronized. This involves the following tasks:
- Timing synchronization: Since it is unknown to the receiver, to which exact (absolute) time instant the symbol has been transmitted and how long the dispersion of the channel is, one essential task is to find the 'beginning' of a received OFDM symbol. Thus the time scales of transmitter and receiver are synchronized and the removal of the guard interval can be done with the required accuracy.
- Frequency synchronization: The signal is usually not transmitted in baseband but modulated with a radio carrier at a frequency assigned by the standard. Though this frequency is known to the receiver the tolerance of the RF components usually applied is so large that there will be a frequency-deviation. In many cases this deviation will be too large for a reliable data transmission. It therefore must be estimated and compensated at the receiver.
- Sampling-clock synchronization: The signal produced by the FFT will be converted into an analog signal assuming a certain span of time between two values. At the receiver the down converted RF signal is sampled in order to obtain a discrete time signal for further (digital) processing. The sampling times assumed in the receiver must match very accurately in order to avoid a degradation of the performance. A possible deviation between transmitter and receiver must again be estimated and compensated.
- Channel estimation: If a coherent modulation scheme is used (which must not be necessarily the case) the channel transfer function must be estimated and compensated.
A receiver structure that allows to estimate and compensate all parameters required is depicted in figure 24 .
structure for a DVB-T receiver 
In addition to the elementary tasks found in single carrier receivers too for the receiver under consideration here two further tasks can be identified:
- TPS detection: So called TPS (transmission parameter signaling) data is provided in DVB-T to inform the receiver about the modulation and coding scheme used. This information is provided via selected subcarriers that are modulated in a robust differential BPSK.
- CPE detection (and correction): The common phase error (CPE) is a phenomena that results from imperfections of the oscillators used for modulation and demodulation. Instead of providing a stable frequency real oscillators tend to provide a frequency that is slowly changing in time. This change in time leads to an additional modulation of the OFDM signal which in some cases must be estimated and compensated. For the constellations used in DVB-T it can be shown that due to other reasons the quality of the oscillators must be so high that this effect can be neglected.
The Orthogonal Frequency Division Multiplexing used in Terrestrial Digital Video Broadcasting is a new and a very complex modulation form. This is a multi-carrier system, where each carrier is modulated digitally. Due to the error correction, the Guard Interval, and to the fact that the carriers are dispersed in the frequency spectrum, OFDM is a very robust and noise-resistant modulation system. The transmitted data stream can be used not just for video broadcasting, but if only TV channels are transmitted, due to the MPEG compression 4-5 TV programs can be transferred in 1 TV channel, so the frequency spectrum can be allocated more effectively. And even the mobile reception is possible, which means that the viewer sits in his up-to-date solar cell car and watches digital broadcast channels, while receiving better picture quality and has access to more services, so the time is right to change progressively the present analogue broadcasting systems and make a step forward.
Mobile TV DVB-H
DVB-H or Digital Video Broadcast - Handheld, is one of the major systems to be used for mobile video and television for cellular phones and handsets. DVB-H has been developed from the DVB-T (Terrestrial) television standard that is used in many countries around the globe including much of Europe including the UK, and also other countries including the USA. The DVB-T standard has been shown to be very robust and in view of its widespread acceptance it forms a good platform for further development for handheld applications.
DVB-H development requirements
The environment for handheld devices is considerably different to that experienced by most televisions. Normally domestic televisions have good directional antenna systems and in addition to this the reception conditions are fairly constant. Additionally most televisions receiving DVB-T will be powered by mains supplies. As a result current consumption is not a major issue.
The conditions for handheld receivers are very different. In the first instance the antennas will be particularly poor because they will need to be small, and integrated into the handset in such a way that they either appear fashionable, or they are not visible. Additionally they will obviously be mobile, and this will entail receiving signals in a variety locations, many of which will not be particularly suitable for video reception. Not only will be signal be subject to considerable signal variations and multi-path effects, but it may also experience high levels of interference. Also some difficulties are presented by the fact that the handset could be in a vehicle and actually on the move. The operation of DVB-H has to be sufficiently robust to accommodate all these requirements.
While DVB-T proved to be remarkably robust under many circumstances, one of the major problems was that of current consumption. Battery life for handsets is a major concern where users anticipated the life between charges will be several days.
Operation of DVB-H
DVB-H (Digital Video Broadcast Handheld) is based on the very successful DVB-T (Digital Video Broadcast Terrestrial) standard that is now used in many countries for domestic digital television broadcasts. DVB-H has taken the basic standard and adapted so that it is suitable for use in a mobile environment, particularly with the electronics incorporated into a mobile phone.
The DVB-H standard like DVB-T uses a form of transmission called Orthogonal Frequency Division Multiplex (OFDM). This has been adopted because of its high data capacity and suitability for applications such as broadcasting. It also offers a high resilience to interference can tolerate multi-path effects and is able to offer the possibility of a single frequency network, SFN.
There are a variety of modes in which the DVB-H signal can be configured. These are conform to the same concepts as those used by DVB-T. These are 2K, 4K, and 8K modes, each having a different number of carriers as defined in the table below. The 4K mode is a further introduction beyond that which is available for DVB-T.
The different modes balance the different requirements for network design, trading mobility for single frequency network size, with the 4K mode being that which is expected to be most widely used.
The standard will support a variety of different types of modulation within the OFDM signal. QPSK (Quadrature Phase Shift Keying), 16QAM (16 point Quadrature Amplitude Modulation), and 64QAM (64 point Quadrature Amplitude Modulation) will all be supported, chipsets being able to detect the modulation and receive the incoming signal. The choice of modulation is again a balance, QPSK offering the best reception under low signal and high noise conditions, but offering the lowest data rate. 64QAM offers the highest data rate, but requires the highest signal level to provide sufficiently error free reception.
Conceptual structure of a DVB-H receiver
An example of using DVB-H for transmission of IP-services is given in figure 25 and figure 26. In this example, both traditional MPEG-2 services and time-sliced "DVB-H services" are carried over the same multiplex. The handheld terminal decodes/uses IP-services only.[10 ;11]
One of the key requirements for any mobile TV system is that it should not give rise to undue battery drain. Mobile handset users are used to battery life times extending over several days, and although battery technology is improving, the basic mobile TV technology should ensure that battery drain is minimized.
There is a module within the standard and hence the software that enables the receiver to decode only the required service and shut off during the other service bits. It operates in such a way that it enables the receiver power consumption to be reduced while also offering an uninterrupted service for the required functions.
The time slicing elements of DVB-H enable the power consumption of the mobile TV receiver to be reduced by 90% when compared to a system not using this technique. Although the receiver will add some additional power drain on the battery, this will not be nearly as much as it would have been had the TV reception scheme not employed the time slicing techniques.[1,7 ; 2, 2-3]
Interleaving is a technique where sequential data words or packets are spread across several transmitted data bursts. In this way, if one transmitted burst or group is lost as a result of noise or some other drop-out, then only a small proportion of the data in each original word or packet is lost and it can be reconstructed using the error detection and correction techniques employed.
Further levels of interleaving have been introduced into DVB-H beyond those used for DVB-T. The basic mode of interleaving used on DVB-T and which is also available for DVB-H is a native interleave that interleaves bits over one OFDM symbol. However DVB-H provides a more in-depth interleave that interleaves bits over two OFDM symbols (for the 4K mode) and four bits (for the 2K mode).
Comparison of the OFDM modes and interleaving schemes in the case of mobile reception as a function of Doppler frequency 
Using the in-depth interleave enables the noise resilience performance of the 2K and 4K modes to be brought up to the performance of the 8K mode and it also improves the robustness of the reception of the transmissions in a mobile environment.[1, 7; 3, 129-138 ; 11]
In view of the particularly difficult reception conditions that may occur in the mobile environment, further error correction schemes are included. A scheme known as MPE-FEC provides additional error correction to that applied in the physical layer by the interleaving.
In Figure 28 shows The MPE-FEC frame structure. The FEC frame consists of a maximum of 1024 rows and a constant number of 255 columns; every frame cell corresponds to one byte, the maximum frame size is approx. 2 Mbit.
MPE-FEC frame structure 
The frame is separated into two parts, the application data table on the left (191 columns) and the RS data table on the right (64 columns). The application data table is filled with the IP packets of the service to be protected. After applying the RS(255,191) code to the application data row-by row, the RS data table contains the parity bytes of the RS code. After the coding, the IP packets are read out of the application data table and are encapsulated in IP sections in a way which is well known from the MPE method. These application data are followed by the parity data which are read out of the RS data table column-by-column and are encapsulated in separate FEC sections. The FEC frame structure also contains a "virtual" block interleaving effect in addition to the coding. Writing to and reading from the FEC frame is performed in column direction. This is a forward error correction scheme that is applied to the transmitted data and after reception and demodulation, allows the errors to be detected and corrected. [11;13]
Compatibility with DVB-T
DVB-H is a development of DVB-T and as a result it shares many common components. It has also been designed so that it can be used in 6, 7, and 8 MHz channel schemes although the 8MHz scheme will be the most widely used. There is also a 5MHz option that may be used for non-broadcast applications.
In view of the similarities between DVB-H and DVB-T it is possible for both forms of transmission to exist together on the same multiplex. In this way a broadcaster may choose to run two DVB-T services and one DVB-H service on the same multiplex. This feature may be particularly attractive in the early days of DVB-H when separate spectrum is not available
DVB-H has been used in a number of trials and appear to perform well. It ahs support from a number of the major industry players and is likely to achieve a considerable degree of acceptance world-wide. Accordingly it is likely to be one of the major standards, if not the major standard used for mobile video.[11; 12 , 49-55]
The suitability of OFDM in LTE
LTE (Long Term Evolution)
LTE is the next major step in mobile radio communications, and is being introduced in 3rd Generation Partnership Project (3GPP) Release 8.The aim of this 3GPP project is to improve the Universal Mobile Telecommunications System (UMTS) mobile phone standard and provide an enhanced user experience and simplified technology for next generation mobile broadband. Researchers and development engineers' worldwide - representing more than 60 operators, vendors and research institutes - are participating in the joint LTE radio access standardization effort.
One of the key elements of LTE is the use of OFDM (Orthogonal Frequency Division Multiplex) as the signal bearer and the associated access schemes, OFDMA (Orthogonal Frequency Division Multiplex) and SC-FDMA (Single Frequency Division Multiple Access). In view of its advantages, the use of ODFM and the associated access technologies, OFDMA and SC-FDMA are natural choices for the new LTE cellular standard.The use of OFDM is a natural choice for LTE. While the basic concepts of OFDM are used, it has naturally been tailored to meet the exact requirements for LTE. However its use of multiple carrier each carrying a low data rate remains the same.
OFDM was chosen as the signal bearer format because it is very resilient to interference. Also in recent years a considerable level of experience has been gained in its use from the various forms of broadcasting that use it along with Wi-Fi and WiMAX. OFDM is also a modulation format that is very suitable for carrying high data rates - one of the key requirements for LTE. In addition to this, OFDM can be used in both FDD and TDD formats. This becomes an additional advantage.[2 341-246; 18 ]
LTE channel bandwidths and characteristics
One of the key parameters associated with the use of OFDM within LTE is the choice of bandwidth. The available bandwidth influences a variety of decisions including the number of carriers that can be accommodated in the OFDM signal and in turn this influences elements including the symbol length and so forth.LTE defines a number of channel bandwidths. Obviously the greater the bandwidth, the greater the channel capacity.The channel bandwidths that have been chosen for LTE are respectively 1.4MHz, 3MHz , 5MHz, 10 MHz, 15MHz and 20MHz.In addition to this the subcarriers are spaced 15 kHz apart from each other. To maintain orthogonality, this gives a symbol rate of 1 / 15 kHz = of 66.7 µs.
Each subcarrier is able to carry data at a maximum rate of 15 ksps (kilosymbols per second). This gives a 20 MHz bandwidth system a raw symbol rate of 18 Msps. In turn this is able to provide a raw data rate of 108 Mbps as each symbol using 64QAM is able to represent six bits. It may appear that these rates do not align with the headline figures given in the LTE specifications. The reason for this is that actual peak data rates are derived by first subtracting the coding and control overheads. Then there are gains arising from elements such as the spatial multiplexing, etc. 
LTE OFDM cyclic prefix, CP
One of the primary reasons for using OFDM as a modulation format within LTE (and many other wireless systems for that matter) is its resilience to multipath delays and spread. However it is still necessary to implement methods of adding resilience to the system. This helps overcome the inter-symbol interference (ISI) that results from this.
In areas where inter-symbol interference is expected, it can be avoided by inserting a guard period into the timing at the beginning of each data symbol. It is then possible to copy a section from the end of the symbol to the beginning. This is known as the cyclic prefix, CP. The receiver can then sample the waveform at the optimum time and avoid any inter-symbol interference caused by reflections that are delayed by times up to the length of the cyclic prefix, CP.
The length of the cyclic prefix, CP is important. If it is not long enough then it will not counteract the multipath reflection delay spread. If it is too long, then it will reduce the data throughput capacity. For LTE, the standard length of the cyclic prefix has been chosen to be 4.69 µs. This enables the system to accommodate path variations of up to 1.4 km. With the symbol length in LTE set to 66.7 µs.The symbol length is defined by the fact that for OFDM systems the symbol length is equal to the reciprocal of the carrier spacing so that orthogonality is achieved. With a carrier spacing of 15 kHz, this gives the symbol length of 66.7 µs.[15, 127-130 ; 16, 6-9]
LTE OFDMA in the downlink
The OFDM signal used in LTE comprises a maximum of 2048 different sub-carriers having a spacing of 15 kHz. Although it is mandatory for the mobiles to have capability to be able to receive all 2048 sub-carriers, not all need to be transmitted by the base station which only needs to be able to support the transmission of 72 sub-carriers. In this way all mobiles will be able to talk to any base station.
Within the OFDM signal it is possible to choose between three types of modulation:
- QPSK (= 4QAM) 2 bits per symbol
- 16QAM 4 bits per symbol
- 64QAM 6 bits per symbol
The exact format is chosen depending upon the prevailing conditions. The lower forms of modulation, (QPSK) do not require such a large signal to noise ratio but are not able to send the data as fast. Only when there is a sufficient signal to noise ratio can the higher order modulation format be used.
The LTE downlink physical resource based on OFDM 
Figure 30 shows In the downlink, the subcarriers are split into resource blocks. This enables the system to be able to compartmentalize the data across standard numbers of subcarriers.Resource blocks comprise 12 subcarriers, regardless of the overall LTE signal bandwidth. They also cover one slot in the time frame. This means that different LTE signal bandwidths will have different numbers of resource blocks. [16, 346-347]
LTE SC-FDMA in the uplink
For the LTE uplink, a different concept is used for the access technique. Although still using a form of OFDMA technology, the implementation is called Single Carrier Frequency Division Multiple Access (SC-FDMA). One of the key parameters that affects all mobiles is that of battery life. Even though battery performance is improving all the time, it is still necessary to ensure that the mobiles use as little battery power as possible. With the RF power amplifier that transmits the radio frequency signal via the antenna to the base station being the highest power item within the mobile, it is necessary that it operates in as efficient mode as possible. This can be significantly affected by the form of radio frequency modulation and signal format.
Signals that have a high peak to average ratio and require linear amplification do not lend themselves to the use of efficient RF power amplifiers. As a result it is necessary to employ a mode of transmission that has as near a constant power level when operating. Unfortunately OFDM has a high peak to average ratio. While this is not a problem for the base station where power is not a particular problem, it is unacceptable for the mobile. As a result, LTE uses a modulation scheme known as SC-FDMA - Single Carrier Frequency Division Multiplex which is a hybrid format. This combines the low peak to average ratio offered by single-carrier systems with the multipath interference resilience and flexible subcarrier frequency allocation that OFDM provides.[16,127-353]
LTE Advanced for IMT 4G
LTE is slated to become part of IMT-2000, the 3G "standard", so in a strict standards committee environment, LTE would be called a 3G technology. An updated version, 3GPP release 10, which might be called LTE-Advanced, is expected to be submitted to the IMT-Advanced standards committee, which would cause those standards committee members to declare it officially a 4G standard. Everyone else will refer to LTE as 4G from the start. From the wireless operator's perspective, 4G systems are vastly more efficient at using valuable wireless spectrum. These spectral efficiency improvements support new high-speed services, as well as larger numbers of users. Key objectives of 4G LTE networks are to support higher data rates, improve spectral efficiency, reduce network latency, support flexible channel bandwidths, and simplify or flatten the network by utilizing an all packet (Ethernet/IP) architecture.
LTE wireless networks offer vastly more bandwidth than traditional GSM 2G and UMTS 3G networks. While these enhanced speeds are very impressive, there has been a great deal of hype within the industry on the theoretical performance as compared to the more typical or realistic bandwidths that will be needed at LTE cell sites The amount of bandwidth on a wireless network is ultimately constrained by two factors: the spectral Efficiency of the wireless interface and the amount of licensed spectrum a carrier owns. Spectral efficiency is a fancy way of saying how much information can be transmitted over a given radio channel (i.e., Hz). Spectral efficiency is measured as the amount of data (bps) that can be transmitted for every Hz of spectrum; the higher the number (bps/Hz) the better. Newer technologies, such as LTE, use advanced modulation scheme (OFDM) that support higher spectral efficiencies and higher data rates than 2G and 3G wireless networks.
Spectral efficiency decreases with the distance from the cell tower, due to lower received power and higher noise levels. To accommodate for users at varying distances within a cell sector, wireless networks adjust their modulation scheme for each user. Those closest to the tower, with the highest received power, operate with the most advanced modulation technique resulting in the highest data rates. Further away from the tower, simpler modulation techniques are utilized which result in lower spectral efficiencies and slower data rates.
The maximum amount of bandwidth required at a cell site is simply the amount of licensed spectrum (i.e., channel size) owned by the wireless operator, multiplied by the spectral efficiency of the air interface. As an example, the table 2 illustrates typical cell site bandwidths required by the following four scenarios:
- GSM 2G voice 1.2 MHz
- GSM/EDGE 2.75G 3.5 MHz
- UMTS/HSPDA 3G 5.0 MHz
- LTE 4G 5.0 MHz
Wireless Capacity Requirements 
There are a number of key technologies that will enable LTE Advanced to achieve the high data throughput rates that are required. MIMO and OFDM are two of the base technologies that will be enablers. Along with these there are a number of other techniques and technologies that will be employed. The full specification for LTE Advanced the new 4G technology also referred to as IMT Advanced is still some while away. However many of the features and technologies have been trialed and are ready to be incorporated into the standard. Yet despite this it will take many months after the finalization of the standard for LTE Advanced before equipment is available and networks start to be deployed.
In comparison with traditional TDMA, FDMA and CDMA, OFDM displays its unique advantages
When used in the mobile communication field. First, OFDM allows the arrangement of user information in partial superimposition in the frequency domain, which avoids protection belts in the traditional communication system and allows the use of the spectrum resource to the maximum extent. Second, OFDM can allocate information to N sub-carriers for transmission, and thus, converting broadband transmission into narrowband transmission on multiple sub-carriers. Moreover, the channel on each sub-carrier can be seen as a horizontal attenuation channel, hence, reducing the complexity of the equalizer in the receiver. Theoretically, it also avoids the multi -path equalization of single-carrier signals in the traditional system, and effectively solves the problem related with the complexity of time domain equalization that increases sharply as bandwidth increases. This also provides further proof that OFDM is more suitable for the use in the wireless broadband field.
Finally, OFDM allows the flexible allocation of frequency resources through the division of subcarriers. This flexibility of resource allocation can help solve problems that might arise in wireless transmission. For example, by adjusting the number o f sub -carriers, OFDM can readily expand the bandwidth, which is less probable with traditional single-carrier technology. In addition, a terminal can adjust the allocation of sub-carriers depending on its own service conditions, utilizing small power amplifier. A base station can adjust the positions of subcarriers based on the conditions of the channels of different subscribers, thus, avoiding a of frequency and narrowband interference. Furthermore, different bandwidth resources can be allocated to different subscribers depending on their service requirements.
However, OFDM also has a few intrinsic weaknesses. In comparison with the single-carrier system, the OFDM system outputs multiple superimposed independent sub-carrier signals, therefore, combined signals may cause or lead to a great peak to average ratio (PAR). A high PAR imposes a high requirement on the linearity of the radio frequency power amplifier of the transmitter, and also causes a reduction in the power efficiency of the transmitter.
OFDM has opened the door for future wireless communication technologies, as it continues to make improvements on technologies developed in previous generations, such as broadband, time domain equalization, and spectrum efficiency. But OFDM alone is not able to fully satisfy the development of subsequent wireless communications. For example, the study of technologies such as multi-antenna processing, wireless resource allocation, adaptive modulation of codes (AMC), channel assessment, and adaptive frequency hopping in relation to OFDM technology, have currently become hot topics, and appear to be the general direction for future development.
The traditional cell division mode is not a very economical one, since it creates the necessity for more base stations to be constructed. Multi-antenna technology makes use of space-time processing, and can help improve the quality of subscribers' transport channels, or can increase diversity gain, enhancing spectrum efficiency. However, for a single site, the equipment cost may be higher, but because of better coverage and bigger capacity, the construction cost of the entire network will be reduced, and the investment costs will also be considerably reduced. Because OFDM itself imposes a lower requirement on physical layer processing, then multi-antenna technology can be integrated with OFDM, exerting very little influence on the overall complexity of the system. It is expected therefore, the application of OFDM with multi-antenna technology will become the inevitable mode for emerging wireless communication technologies.
The current status of the research is that OFDM appears to be a suitable technique as a Modulation technique for high performance wireless telecommunications. An OFDM link has been confirmed to work by using computer simulations, and some practical tests performed on a low bandwidth base-band signal. So far only four main performance criteria have been tested, which are OFDM's tolerance to multipath delay spread, channel noise, peak power clipping and start time error. Several other important factors affecting the performance of OFDM have only been partly measured. These include the effect of frequency stability errors on OFDM and impulse noise effects.
One important major area, which hasn't been investigated, is the problems that may be encountered when OFDM is used in a multiuser environment. One possible problem is that the receiver may require a very large dynamic range in order to handle the large signal strength variation between users.
The goal of the project is to analysis of OFDM techniques and to assess the suitability of OFDM as a modulation technique for wireless applications which is achieved in this project .This thesis has concentrated on OFDM, however most practical system would use forward error correction to improve the system performance. Thus more work needs tobe done on studying forward error correction schemes that would be suitable for telephony applications, and data transmission.
Several modulation techniques for OFDM is investigated in this thesis including BPSK, QPSK, 16PSK and 256PSK, however possible system performance gains may be possible by dynamically choosing the modulation technique based on the type of data being transmitted. More work could be done on investigating suitable techniques for doing this. OFDM promises to be a suitable modulation technique for high capacity wireless communications and will become increasing important in the future as wireless networks become more relied on.
- Ramjee Prasad, OF
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