11-05-2013, 03:29 PM
ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING
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ABSTRACT
OFDM (orthogonal frequency-division multiplexing) is one of the key digital communication technologies of the current decade. The first part of this paper presents the fundamentals of OFDM and its benefits in the presence of multipath propagation in a tutorial-like fashion. The second part details on some of the most important aspects of OFDM transceiver implementation: concept of receiver channel filtering and A/D conversion, radio impairment compensation (I/Q mismatch), and OFDM demodulator. In one part OFDMA is gone through by explaining the difference between OFDM and OFDMA. Use of FFT and IFFT is explained in modulation and demodulation. And application of OFDMA in Wi-Max wireless and LAN. Applications of OFDM and OFDMA are also discussed.
INTRODUCTION
MULTIPLE ACESS:
A channel-access or Multiple-acess is based on a multiplexing method, that allows several data streams or signals to share the same communication channel or physical medium. These are Fundamental types of channel access schemes
Frequency division multiple access(FDMA)
Time division multiple access (TDMA)
Code division multiple access (CDMA)/(SSMA)
Space division multiple access (SDMA)
Packet mode multiple access
TDMA:
Time Division Multiple Access or TDMA is a method used to enable multiple earth stations or VSAT terminals to transmit intermittently on the same frequency, but with the timing of their transmissions so arranged that the bursts do not overlay when they arrive at the satellite but arrive in sequence and thus are all successfully received by the teleport hub modem burst demodulator.
OFDM BASIC
Introduction
The basic principle of OFDM is to split a high-rate DataStream into a number of lower rate streams that are transmitted simultaneously over a number of sub carriers. Because the symbol duration increases for lower rate parallel sub carriers, the relative amount of dispersion in time caused by multipath delay spread is decreased. Intersymbol interference is eliminated almost completely
by introducing a guard time in every OFDM symbol. In the guard time, the symbol is cyclically extended to avoid inter carrier interference.
In OFDM design, a number of parameters are up for consideration, such as the number of sub carriers, guard time, symbol duration, subcarrier spacing, modulation type per sub carrier. The choice of parameters is influenced by
system requirements such as available bandwidth, required bit rate, tolerable delay spread, and Doppler values. Some requirement are conflicting. For instance, to get a good delay spread tolerance, a large number of sub carriers with small sub carrier spacing is desirable, but the opposite is true for a good tolerance against Doppler spread and phase noise.
Guard time and cyclic extension:
One of the most important reasons to do OFDM is the efficient way it deals with multipath delay spread. By dividing the input data stream in s N sub carriers, the symbol duration is made s N times smaller, which also reduces the relative multipath delay spread, relative to symbol time, by the same factor. To eliminate intersymbol interference almost completely, a guard time is introduced for each OFDM symbol. The guard time is chosen larger than the expected delay spread, such that multipath components from one symbol cannot interfere with the next
symbol. The guard time could consist of no signal at all. In that case, however, the problem of intercarrier (ICI) would arise. ICI is crosstalk between different subcarriers, which means they are no longer orthogonal. This effect is illustrated in figure in this example; a sub carrier 1 and a delayed sub carrier 2 are shown. When an OFDM receiver tries to demodulate the first sub carrier, it will encounter some interference from the second sub carrier, because within the FFT interval, there is no integer number of cycles difference between sub carrier 1and 2. At the same time, there will be crosstalk from the first to the second sub carrier for the same reason.
Orthogonal Frequency Division Multiple Access (OFDMA):
Orthogonal Frequency Division Multiplexing (OFDM) is a technique for transmitting large amounts of digital data over a radio wave The technology works by splitting the radio signal into multiple smaller sub-signals that are then transmitted simultaneously at different frequencies to the receiver. OFDM reduces the amount of crosstalk in signal transmissions.
OFDMA is a multi-user OFDM that allows multiple access on the same channel (a channel being a group of evenly spaced subcarriers).OFDMA distributes subcarriers among users so all users can transmit and receive at the same time within a single channel on what are called subchannels. What’s more, subcarrier-group subchannels can be matched to each user to provide the best performance, meaning the least problems with fading and interference based on the location and propagation characteristics of each user.OFDMA is a multiplexing technique that subdivides the bandwidth in to multiple frequency sub-carriers. Here the input data stream is divided in to several parallel sub-streams of reduced data rate and each substream is modulated and transmitted on a separate Orthogonal Subcarrier.
Conclusion
There is some debate as to whether multicarrier or single carrier modulation is better for ISI channels with delay spreads on the order of the symbol time.
It is claimed in that for some mobile radio applications, single carrier with equalization has roughly the same performance as multicarrier modulation with channel coding, frequency-domain interleaving, and weighted Maximum-likelihood decoding. Adaptive loading was not taken into account, which has the potential to significantly improve multicarrier. But there are other problems with multicarrier modulation that impair its performance, most significantly frequency offset and timing jitter, which degrade the orthogonally of the sub channels. In addition, the peak-to-average power ratio of multi carrier is significantly higher than that of single carrier systems, which is a serious problem when nonlinear amplifiers are used .Tradeoffs between multicarrier and single carrier block transmission systems with respect to these impairments are discussed. Despite these challenges, multicarrier techniques are common in high data rate wireless required for an equalizer with good performance in a high data rate system is typically large . Thus, these equalizers are highly complex. Weights for a large number of equalizer taps in a rapidly varying channel. For these reasons, most emerging high rate wireless systems use either multicarrier modulation or spread spectrum to eliminate ISI.
[attachment=53773]
ABSTRACT
OFDM (orthogonal frequency-division multiplexing) is one of the key digital communication technologies of the current decade. The first part of this paper presents the fundamentals of OFDM and its benefits in the presence of multipath propagation in a tutorial-like fashion. The second part details on some of the most important aspects of OFDM transceiver implementation: concept of receiver channel filtering and A/D conversion, radio impairment compensation (I/Q mismatch), and OFDM demodulator. In one part OFDMA is gone through by explaining the difference between OFDM and OFDMA. Use of FFT and IFFT is explained in modulation and demodulation. And application of OFDMA in Wi-Max wireless and LAN. Applications of OFDM and OFDMA are also discussed.
INTRODUCTION
MULTIPLE ACESS:
A channel-access or Multiple-acess is based on a multiplexing method, that allows several data streams or signals to share the same communication channel or physical medium. These are Fundamental types of channel access schemes
Frequency division multiple access(FDMA)
Time division multiple access (TDMA)
Code division multiple access (CDMA)/(SSMA)
Space division multiple access (SDMA)
Packet mode multiple access
TDMA:
Time Division Multiple Access or TDMA is a method used to enable multiple earth stations or VSAT terminals to transmit intermittently on the same frequency, but with the timing of their transmissions so arranged that the bursts do not overlay when they arrive at the satellite but arrive in sequence and thus are all successfully received by the teleport hub modem burst demodulator.
OFDM BASIC
Introduction
The basic principle of OFDM is to split a high-rate DataStream into a number of lower rate streams that are transmitted simultaneously over a number of sub carriers. Because the symbol duration increases for lower rate parallel sub carriers, the relative amount of dispersion in time caused by multipath delay spread is decreased. Intersymbol interference is eliminated almost completely
by introducing a guard time in every OFDM symbol. In the guard time, the symbol is cyclically extended to avoid inter carrier interference.
In OFDM design, a number of parameters are up for consideration, such as the number of sub carriers, guard time, symbol duration, subcarrier spacing, modulation type per sub carrier. The choice of parameters is influenced by
system requirements such as available bandwidth, required bit rate, tolerable delay spread, and Doppler values. Some requirement are conflicting. For instance, to get a good delay spread tolerance, a large number of sub carriers with small sub carrier spacing is desirable, but the opposite is true for a good tolerance against Doppler spread and phase noise.
Guard time and cyclic extension:
One of the most important reasons to do OFDM is the efficient way it deals with multipath delay spread. By dividing the input data stream in s N sub carriers, the symbol duration is made s N times smaller, which also reduces the relative multipath delay spread, relative to symbol time, by the same factor. To eliminate intersymbol interference almost completely, a guard time is introduced for each OFDM symbol. The guard time is chosen larger than the expected delay spread, such that multipath components from one symbol cannot interfere with the next
symbol. The guard time could consist of no signal at all. In that case, however, the problem of intercarrier (ICI) would arise. ICI is crosstalk between different subcarriers, which means they are no longer orthogonal. This effect is illustrated in figure in this example; a sub carrier 1 and a delayed sub carrier 2 are shown. When an OFDM receiver tries to demodulate the first sub carrier, it will encounter some interference from the second sub carrier, because within the FFT interval, there is no integer number of cycles difference between sub carrier 1and 2. At the same time, there will be crosstalk from the first to the second sub carrier for the same reason.
Orthogonal Frequency Division Multiple Access (OFDMA):
Orthogonal Frequency Division Multiplexing (OFDM) is a technique for transmitting large amounts of digital data over a radio wave The technology works by splitting the radio signal into multiple smaller sub-signals that are then transmitted simultaneously at different frequencies to the receiver. OFDM reduces the amount of crosstalk in signal transmissions.
OFDMA is a multi-user OFDM that allows multiple access on the same channel (a channel being a group of evenly spaced subcarriers).OFDMA distributes subcarriers among users so all users can transmit and receive at the same time within a single channel on what are called subchannels. What’s more, subcarrier-group subchannels can be matched to each user to provide the best performance, meaning the least problems with fading and interference based on the location and propagation characteristics of each user.OFDMA is a multiplexing technique that subdivides the bandwidth in to multiple frequency sub-carriers. Here the input data stream is divided in to several parallel sub-streams of reduced data rate and each substream is modulated and transmitted on a separate Orthogonal Subcarrier.
Conclusion
There is some debate as to whether multicarrier or single carrier modulation is better for ISI channels with delay spreads on the order of the symbol time.
It is claimed in that for some mobile radio applications, single carrier with equalization has roughly the same performance as multicarrier modulation with channel coding, frequency-domain interleaving, and weighted Maximum-likelihood decoding. Adaptive loading was not taken into account, which has the potential to significantly improve multicarrier. But there are other problems with multicarrier modulation that impair its performance, most significantly frequency offset and timing jitter, which degrade the orthogonally of the sub channels. In addition, the peak-to-average power ratio of multi carrier is significantly higher than that of single carrier systems, which is a serious problem when nonlinear amplifiers are used .Tradeoffs between multicarrier and single carrier block transmission systems with respect to these impairments are discussed. Despite these challenges, multicarrier techniques are common in high data rate wireless required for an equalizer with good performance in a high data rate system is typically large . Thus, these equalizers are highly complex. Weights for a large number of equalizer taps in a rapidly varying channel. For these reasons, most emerging high rate wireless systems use either multicarrier modulation or spread spectrum to eliminate ISI.