RussellHarlan B Abstract To respond to dynamic channel conditions caused by fading, shadowing, and other time-varying disturbances, orthogonal frequency division multiplexing OFDM packet radio systems should adapt transmission parameters on a packet-by-packet basis to maintain or improve performance over the channel. For this to be possible, there are three key ideas that must be addressed: In this thesis, we propose a procedure for determining relative subchannel quality without using any traditional channel measurements.
FFT techniques can be used to reduce the number of multiplications for an FIR filter -based time-domain equalizer to a number comparable with OFDM, at the cost of delay between reception and decoding which also becomes comparable with OFDM.
If differential modulation such as DPSK or DQPSK is applied to each sub-carrier, equalization can be completely Ofdm thesis 2013, since these non-coherent schemes are insensitive to slowly changing amplitude and phase distortion.
In a sense, improvements in FIR equalization using FFTs or partial FFTs leads mathematically closer to OFDM,[ citation needed ] but the OFDM technique is easier to understand and implement, and the sub-channels can be independently adapted in other ways than varying equalization coefficients, such as switching between different QAM constellation patterns and error-correction schemes to match individual sub-channel noise and interference characteristics.
Pilot signals and training symbols preambles may also be used for time synchronization to avoid intersymbol interference, ISI and frequency synchronization to avoid inter-carrier interference, ICI, caused by Doppler shift.
OFDM was initially used for wired and stationary wireless communications. However, with an increasing number of Ofdm thesis 2013 operating in highly mobile environments, the effect of dispersive fading caused by a combination of multi-path propagation and doppler shift is more significant.
Over the last decade, research has been done on how to equalize OFDM transmission over doubly selective channels. Frequency subcarrier interleaving increases resistance to frequency-selective channel conditions such as fading.
For example, when a part of the channel bandwidth fades, frequency interleaving ensures that the bit errors that would result from those subcarriers in the faded part of the bandwidth are spread out in the bit-stream rather than being concentrated.
Similarly, time interleaving ensures that bits that are originally close together in the bit-stream are transmitted far apart in time, thus mitigating against severe fading as would happen when travelling at high speed. However, time interleaving is of little benefit in slowly fading channels, such as for stationary reception, and frequency interleaving offers little to no benefit for narrowband channels that suffer from flat-fading where the whole channel bandwidth fades at the same time.
The reason why interleaving is used on OFDM is to attempt to spread the errors out in the bit-stream that is presented to the error correction decoder, because when such decoders are presented with a high concentration of errors the decoder is unable to correct all the bit errors, and a burst of uncorrected errors occurs.
A similar design of audio data encoding makes compact disc CD playback robust. A classical type of error correction coding used with OFDM-based systems is convolutional codingoften concatenated with Reed-Solomon coding.
Usually, additional interleaving on top of the time and frequency interleaving mentioned above in between the two layers of coding is implemented. The choice for Reed-Solomon coding as the outer error correction code is based on the observation that the Viterbi decoder used for inner convolutional decoding produces short error bursts when there is a high concentration of errors, and Reed-Solomon codes are inherently well suited to correcting bursts of errors.
Newer systems, however, usually now adopt near-optimal types of error correction codes that use the turbo decoding principle, where the decoder iterates towards the desired solution.
Some systems that have implemented these codes have concatenated them with either Reed-Solomon for example on the MediaFLO system or BCH codes on the DVB-S2 system to improve upon an error floor inherent to these codes at high signal-to-noise ratios.
Based on this feedback information, adaptive modulationchannel coding and power allocation may be applied across all sub-carriers, or individually to each sub-carrier. In the latter case, if a particular range of frequencies suffers from interference or attenuation, the carriers within that range can be disabled or made to run slower by applying more robust modulation or error coding to those sub-carriers.
The term discrete multitone modulation DMT denotes OFDM-based communication systems that adapt the transmission to the channel conditions individually for each sub-carrier, by means of so-called bit-loading. The upstream and downstream speeds can be varied by allocating either more or fewer carriers for each purpose.
Some forms of rate-adaptive DSL use this feature in real time, so that the bitrate is adapted to the co-channel interference and bandwidth is allocated to whichever subscriber needs it most. OFDM extended with multiple access[ edit ] OFDM in its primary form is considered as a digital modulation technique, and not a multi-user channel access methodsince it is used for transferring one bit stream over one communication channel using one sequence of OFDM symbols.
However, OFDM can be combined with multiple access using time, frequency or coding separation of the users.
OFDMA supports differentiated quality of service by assigning different number of sub-carriers to different users in a similar fashion as in CDMAand thus complex packet scheduling or Media Access Control schemes can be avoided.
OFDMA is used in: Co-channel interference can be mitigated, meaning that manual fixed channel allocation FCA frequency planning is simplified, or complex dynamic channel allocation DCA schemes are avoided.
Space diversity[ edit ] In OFDM-based wide-area broadcasting, receivers can benefit from receiving signals from several spatially dispersed transmitters simultaneously, since transmitters will only destructively interfere with each other on a limited number of sub-carriers, whereas in general they will actually reinforce coverage over a wide area.
This is very beneficial in many countries, as it permits the operation of national single-frequency networks SFNwhere many transmitters send the same signal simultaneously over the same channel frequency.
SFNs use the available spectrum more effectively than conventional multi-frequency broadcast networks MFNwhere program content is replicated on different carrier frequencies.
SFNs also result in a diversity gain in receivers situated midway between the transmitters.
|Find a copy online||Xianbin Wang Abstract Wireless communications is widely employed in modern society and plays an increasingly important role in people's daily life.|
|OFDM Trasnmitter and Receiver (Matlab Code) - File Exchange - MATLAB Central||Huseyin Arslan Abstract Orthogonal frequency-division multiplexing OFDM is communication technique that is robust against multipath spread, efficient in equalization implementation, and flexible in dynamic spectrum usage.|
|"Self-interference in OFDM-Based Systems: Identification and Separation" by Ertugrul Guvenkaya||Simulations and experiments are presented to analyze the performance of both systems. Advisor ; Donald Ucci, Dr.|
|Orthogonal frequency-division multiplexing - Wikipedia||Download Kb Preview Abstract Multi-input and multi-output MIMO and orthogonal frequency division multiplexing OFDM have attracted significant attention, and become promising techniques for high data rate wireless communication systems.|
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The coverage area is increased and the outage probability decreased in comparison to an MFN, due to increased received signal strength averaged over all sub-carriers.Basic OFDM. version ( KB) by Vinay Jonnakuti. Vinay Jonnakuti (view profile) 1 file; 1 Nov i need matlab codes for self cancellation with and without ICI cancellation using bpsk and qpsk modulation for diffrent values of frequency offsets in ofdm system i am doing thesis topics name OFDM.
Can any one help me OFDM. This is to certify that the work in the thesis entitled FPGA Implementation of Circularly Shifted PTS Technique for PAPR Reduction in OFDM by Seemanjali Sahoo is a record of an original research work carried out by her during - under my.
Rethinking Wireless: Building Next-Generation Networks by Eugene Songyou Chai A dissertation submitted in partial ful llment of the requirements for the degree of. OFDM code with QAM modulation. Can be easily Understood and modified.
Nice work. I am writing a thesis on implementing radix 4 FFT algorithm for OFDM. I am new to simulink. 16 Dec can you please send me the matlab code for ofdm using qpsk..
to my id [email protected] P. Wang, X. Zhang, M. Song, Doppler compensation based optimal resource allocation for QoS guarantees in underwater MIMO-OFDM acoustic wireless relay networks, in: MILCOM – IEEE Military Communications Conference, POWER ALLOCATION IN OFDM-BASED COGNITIVE RADIO SYSTEMS by Shibiao Zhao metin2sell.com, Jilin University, Changchun, metin2sell.com, A thesis Presented to the Yeates School of Graduate Studies at.