1. Introduction
In a single carrier communication system, the symbol period must be much greater than the delay time in order to avoid inter-symbol interference (ISI) [1]. Since data rate is inversely proportional to symbol period, having long symbol periods means low data rate and communication inefficiency. A multicarrier system, such as FDM (aka: Frequency Division Multiplexing), divides the total available bandwidth in the spectrum into sub-bands for multiple carriers to transmit in parallel [2]. An overall high data rate can be achieved by placing carriers closely in the spectrum. However, inter-carrier interference (ICI) will occur due to lack of spacing to separate the carriers. To avoid inter-carrier interference, guard bands will need to be placed in between any adjacent carriers, which results in lowered data rate. OFDM (aka: Orthogonal Frequency Division Multiplexing) is a multicarrier digital communication scheme to solve both issues. It combines a large number of low data rate carriers to construct a composite high data rate communication system. Orthogonality gives the carriers a valid reason to be closely spaced, even overlapped, without inter-carrier interference. Low data rate of each carrier implies long symbol periods, which greatly diminishes inter-symbol interference [3].
2.1 – OFDM Basics
In digital communications, information is expressed in the form of bits. The term symbol refers to a collection, in various sizes, of bits [6]. OFDM data are generated by taking symbols in the spectral space using M-PSK, QAM, etc, and convert the spectra to time domain by taking the Inverse Discrete Fourier Transform (IDFT). Since Inverse Fast Fourier Transform (IFFT) is more cost effective to implement, it is usually used instead [3]. Once the OFDM data are modulated to time signal, all carriers transmit in parallel to fully occupy the available frequency bandwidth [7]. During modulation, OFDM symbols are typically divided into frames, so that the data will be modulated frame by frame in order for the received signal be in sync with the receiver. Long symbol periods diminish the probability of having inter-symbol interference, but could not eliminate it. To make ISI nearly eliminated, a cyclic extension (or cyclic prefix) is added to each symbol period. An exact copy of a fraction of the cycle, typically 25% of the cycle, taken from the end is added to the front. This allows the demodulator to capture the symbol period with an uncertainty of up to the length of a cyclic extension and still obtain the correct information for the entire symbol period. As shown in Figure 1 [8], a guard period, another name for the cyclic extension, is the amount of uncertainty allowed for the receiver to capture the starting point of a symbol period, such that the result of FFT still has the correct information.
In Figure 2 [9], a comparison between a precisely detected symbol period and a delayed detection illustrates the effectiveness of the cyclic extension.
OFDM Parameters and Characteristics
The number of carriers in an OFDM system is not only limited by the
available spectral bandwidth, but also by the IFFT size (the relationship is described
by:
number of carriers
which is determined by the complexity of the
system [10]. The more complex (also more costly) the OFDM system is, the higher IFFT size it has; thus a higher number of carriers can be used, and higher data transmission rate achieved. The choice of M-PSK modulation varies the data rate and Bit Error Rate (BER). The higher order of PSK leads to larger symbol size, thus less number of symbols needed to be transmitted, and higher data rate is achieved. But this results in a higher BER since the range of 0-360 degrees of phases will be divided into more sub-regions, and the smaller size of sub-regions is required, thereby received phases have higher chances to be decoded incorrectly. OFDM signals have high peak to-average ratio, therefore it has a relatively high tolerance of peak power clipping due to transmission limitations.
Orthogonality
The key to OFDM is maintaining orthogonality of the carriers. If the integral of the product of two signals is zero over a time period, then these two signals are said to be orthogonal to each other. Two sinusoids with frequencies that are integer multiples of a common frequency can satisfy this criterion. Therefore, orthogonality is defined by:
where n and m are two unequal integers; fo is the fundamental frequency; T is the period over which the integration is taken. For OFDM, T is one symbol period and fo set to to 1 /T for optimal effectiveness [11 and 12].
2.2 – Overview of This OFDM Simulation Project
Chapter 3 fading channels
Chapter 4 – DESIGN and IMPLEMENTATION
Orthogonal Frequency-Division Multiplexing (OFDM)
OFDM uses A large number of closely spaced orthogonal sub-carrier signals to carry data. The data is divided into several parallel data streams or channels, one for each sub-carrier. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase-shift keying) at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth.
Fig:1 OFDM orthogonal sub-carrier
The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions (for example, attenuation of high frequencies in a long copper wire, narrowband interference and frequency-selective fading due to multipath) without complex equalization filters. Channel equalization is simplified because OFDM may be viewed as using many slowly modulated narrowband signals rather than one rapidly modulated wideband signal. The low symbol rate makes the use of a guard interval between symbols affordable, making it possible to eliminate intersymbol interference (ISI) and utilize echoes and time-spreading (that shows up as ghosting on analogue TV) to achieve a diversity gain, i.e. a signal-to-noise ratio improvement. This mechanism also facilitates the design of single frequency networks (SFNs), where several adjacent transmitters send the same signal simultaneously at the same frequency, as the signals from multiple distant transmitters may be combined constructively, rather than interfering as would typically occur in a traditional single-carrier system.
Fig: 2 OFDM Modem using IFFT and FFT
Summary of advantages
• Can easily adapt to severe channel conditions without complex time-domain equalization.
• Robust against narrow-band co-channel interference.
• Robust against intersymbol interference (ISI) and fading caused by multipath propagation.
• High spectral efficiency as compared to conventional modulation schemes, spread spectrum, etc.
• Efficient implementation using Fast Fourier Transform (FFT).
• Low sensitivity to time synchronization errors.
• Tuned sub-channel receiver filters are not required (unlike conventional FDM).
• Facilitates single frequency networks (SFNs); i.e., transmitter macrodiversity.
Summary of disadvantages
• Sensitive to Doppler shift.
• Sensitive to frequency synchronization problems.
• High peak-to-average-power ratio (PAPR), requiring linear transmitter circuitry, which suffers from poor power efficiency.
• Loss of efficiency caused by cyclic prefix/guard interval
Example of OFDM applications
The following list is a summary of existing OFDM based standards and products :
Cable
• ADSL and VDSL broadband access via POTS copper wiring.
• DVB-C2, an enhanced version of the DVB-C digital cable TV standard.
• ITU-T G.hn, a standard which provides high-speed local area networking of existing home wiring (phone lines and coaxial cables).
• Multimedia over Coax Alliance (MoCA) home networking.
Wireless
• The wireless LAN (WLAN) radio interfaces IEEE 802.11a, g, n and HIPERLAN/2.
• The digital radio systems DAB/EUREKA 147, DAB+, Digital Radio Mondiale, HD Radio, T-DMB and ISDB-TSB.
• The terrestrial digital TV systems DVB-T and ISDB-T.
• The terrestrial mobile TV systems DVB-H, T-DMB, ISDB-T and MediaFLO forward link.
• The wireless personal area network (PAN) ultra-wideband (UWB) IEEE 802.15.3a implementation suggested by WiMedia Alliance.
The OFDM based multiple access technology OFDMA is also used in several 4G and pre-4G cellular networks and mobile broadband standards:
o The mobility mode of the wireless MAN/broadband wireless access (BWA) standard IEEE 802.16e (or Mobile-WiMAX).
o The mobile broadband wireless access (MBWA) standard IEEE 802.20.
o The downlink of the 3GPPLong Term Evolution (LTE) fourth generation mobile broadband standard. The radio interface was formerly named High Speed OFDM Packet Access (HSOPA), now named Evolved UMTS Terrestrial Radio Access (E-UTRA).