Electronics Engineering
Wireless Communication

Difference between ofdm dsss fhss?


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2011-09-12 21:47:11
2011-09-12 21:47:11

Orthogonal Frequency Division Multiplexing (OFDM) is a multiple-carrier (MC) modulation technique which creates frequency diversity. A high-speed data stream is converted into multiple low-speed data streams via Serial-to-Parallel (S/P) conversion. Each data stream is modulated by a subcarrier. That way, instead of having a frequency-selective fading wireless channel, where each frequency component of the signal is attenuated and phase-shifted in different amount, we have multiple flat-fading subchannels. In other words, instead of having a signal with symbol duration smaller than the channel delay (remember that high frequency means low duration because f = 1/T), we have may subsignals with duration larger than the channel delay (to simplify things, consider this: if the symbol duration is 1 s and the channel delay is 10 s, we will have interference between 10 symbols). That way, channel distortion is compensated. The OFDM symbol is the composite signal formed by the sum of the subcarriers, so the data rate is still the same as if we transmit the original high-speed signal, but as we said, the channel distortion is compensated. OFDM compensates the Inter-Symbol Interference (ISI) caused by the fact that different signals take different paths and arrive at the receiver with different delay (multipaht propagation distortion). OFDM subchannels are not separated by a guard band, but they overlap. However, they are orthogonal at the subcarrier frequencies, and that way they don't interfere with each other. We have very good utilization of the available bandwidth due to the overlapping of the subchannels. Moreover, each subcarrier can be modulated seperatelly (usually, QAM or QPSK modulation is used, depending on the channel conditions, which are measuer using channel estimation via pilot carriers). Also, we can use adaptive modulation and conding (AMC) at each subchannel in order to accomplish error-free communication with the highest data-rate possible. Due to the orthogonality principle, we don't need a bank of modulators at the transmiter and a bank of demodulators/detectors at the receiver, but simply a chip implementing the Inverse Discete Fourier Transform (IDFT) and the DFT respectively - and that can be done easy, effectively and with low-cost using a chip running the Fast Fourier Transform (FFT) algorithm. Timing errors/phase distortion must be controlled because they may create ISI between OFDM symbols and ICI (Inter-Carrier Interference) between subcarriers. We add a Cyclic Prefix (CP) to avoid ISI between OFDM symbols and synchronization methods to avoid ICI. Also, the inherent Peak-to-Average Power Ratio (PAPR) of the OFDM signal must be reduced because forces the power amplifier of the transmitter to operate on the non-linear region of its characteristic function.

Spread Spectrum (SS) techniques convert a low-speed data stream into a high-speed data stream. That way, the bandwidth of the modulated carrier becomes much larger than the minimum required transmission bandwidth. This is like Frequency Modulation (FM): we trade transmission bandwidth with Signal-to-Noise (S/N) ratio, meaning that we can have error-free communication transmiting lower-power signals. The signal is spreaded in a huge bandwidth. That way, instead of having the noise (which is like an interference signal) concetrated to some symbols and corrupting the signal, the noise is uniformly distributed over the signal bandwidth. Moreover, the signal is not easilly detectable by a third-party because it is hided in the background noise. Finally, it has anti-jamming characteristics. There are two techniques to create a SS signal: Direct-Sequence Spread Spectrum (DSSS) multiplies the data stream with a high-data rate sequence called chip sequence or Pseudo-Noise (PN) sequence, because due to its lenght is seems as a random signal, like the noise (but of course, it is a completely deterministic signal; that's why we use the Greek term "Pseudo-", which means "it appears to be, but it is not"). DSSS creates time diversity (a "variety" in the time domain). Frequency-Hopping Spread Specturm (FHSS) uses a chip sequence to conrol the frequency hops of the carrier. The resulting signal is like a progressive-FM signal. FHSS creates frequency diversity (a "variety" in the frequency domain). SS techniques give a Spreading Gain (SG) to the transmitted signal, which is simillar to the Coding Gain (CG) of the error-control codes [remember: in Forward-Error Correction (FEC) techniques, we add additional bits to correct errors; we use this rendudancy for error control. That is, we increase the transmitted bandwidth but we can decrease the transmitted power required to have an acceptable S/N at the receiver). Moreover, SS techniques can be combined with multiple access techniques (patterns for multiple users access a network by sharing the common channel). With Code Division Multiple Access (CDMA) a code sequence is used to give an identity to each user, which than we will transmit a signal spreaded by a PN sequence. So, each user can use the whole available bandwidth for all the time, but users do not interfere because they are separated in the code domain. The orthogonality of the codes (of the signals) must be maintained, because otherwise we will have interference between the users. Finally, a RAKE receiver can be used to resolve the multiple paths and compensated the ISI caused due to the multipath propagation.

OFDM and SS techniques can be combined (MC-CDMA). OFDM can also be combined with the Frequency Division Multiple Access (FDMA) -> OFDMA. Finally, OFDM can be combined with Multiple Input-Multiple Output (MIMO) techniques. In MIMO, we have multiple transmiting and receiving antennas. So, we have N parallel channels instead of a single channel, and this creates a signal which is N times faster (oversimplified, but basically true ...). In each channel we can use OFDM to avoid ISI and frequency-selectivity, while maintaining the high-data rate. Finally, MIMO can create diversity which enables the system to receive the best-quality signal.

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a. Direct sequence spread spectrum (DSSS). b. Frequency hopping spread spectrum (FHSS). c. Infrared d. Orthogonal frequency division multiplexing(OFDM)

Orthogonal frequency-division multiplexing

* FDM - hava a guard band , if the band width for the data is x * OFDM - no guard band , x/2 band with

To add a link back to your website, you need to add hyperlink or bb code(in forum), for eg. hyperlink: <a href="http://blog.ektel.com.np/2012/11/wlan-ieee-802-11a-ofdm-tutorial">OFDM tutorial</a> or [url=http://blog.ektel.com.np/2012/11/wlan-ieee-802-11a-ofdm-tutorial]OFDM Tutorial[/url] here http://blog.ektel.com.np/2012/11/wlan-ieee-802-11a-ofdm-tutorial is the link to your website and "OFDM Tutorial" is the title

OFDM uses 48 subchannels for data and 4 are used as Pilot Carriers.

In OFDM, sub-carrier spacing is maintained in such a way that the maximum of one sub-carrier occurs at the minimum of the successive sub-carrier, a loss of orthogonality results if this pattern is not achieved in the sub-carriers of OFDM transmission. Loss of orthogonality is due to ISI, ICS, Frequency offset amongst the sub-carriers of OFDM.

Orthogonal frequency-division multiplexing (OFDM) is a method of encoding digital. Pilot signals and training symbols (preambles) may also be used for time.


OFDM-based Broadband Wireless Networks covers the latest technological advances in digital broadcasting, wireless LAN, and mobile networks to achieve high spectral efficiency, and to meet peak requirements for multimedia traffic. The book emphasizes the OFDM modem, air-interface, medium access-control (MAC), radio link protocols, and radio network planning.

What is one disadvantage that 802.11a wireless has compared to 802.11g?A. Use of the 5GHz band requires much larger antennas.(Has to be this answer, others are just wrong.)B. The OFDM modulation technique results in a slower data rate.802.11a = OFDM 54 Mbps 802.11g = DSSS 11 Mbps or OFDM 54 MbpsC. There are fewer non-overlapping channels available to help reduce RF802.11a = 8 Channels 802.11g = 3 ChannelsD. The use of higher frequencies means that signals are more likely to be obstructed.802.11a = 5 GHz 802.11g = 2.4 GHzOther way around, the 2.4 GHz frequency band is crowded, and subject tointerference from other networking technologies such as microwave ovens,2.4GHz cordless phones (a huge market), and bluetooth. Also the higherthe frequency the better it is at penetrating walls and scattering orreflecting of walls and ceilings.

Because Eric sucked his dick like there's no tomorrow.

Inter Carrier interference can be resolved by the use of cyclic prefix :)

for fast calculations and also for sharp and narrow frequency band per symbol/subcarrier.

Because OFDM systems use orthogonal frequency(carrier),at the receiver the others copies from multipath has a different phases from the direct one, and by multiplying the generated carrier in the receiver with all received signal and integrated it over [-pi,+pi] only one the direct signal have (1) at the output and the others have(0) output.

Henrik Schulze has written: 'Theory and Applications of OFDM and CDMA' -- subject(s): Engineering, Nonfiction, Technology, OverDrive

Frequency division multiplexing (FDM Time division multiplexing (TDM) Orthogonal frequency division multiplexing (OFDM)

FDM and OFDM both have the same overlap!. In FDM the overlap is in the time domain. In OFDM the overlap is in the frequency domain. First (you may already know this) the relationship between the rectangular pulse and the sin(x)/x (sinc) function: A rectangular pulse in the time domain transforms to and from the sinc function in the frequency domain. A sinc function in the time domain transforms to and from a rectangular "brickwall" function in the frequency domain. In other words these two functions transform to each other by either FFT or IFFT. In both FDM and OFDM we are taking multiple carrier frequencies, modulating them, then combining them for transmission. For simplicity lets assume each carrier is on/off modulated. In idealized FDM, we modulate each carrier then send each though a brickwall filter before combining to the antenna. Say the carriers are separated by 500KHz, (say at 1GHz + 500KHz, 1GHz + 1MHz ...) Each carrier's 500KHz brickwall filter in the frequency domain cause a time domain spreading of its on/off pulses into time domain sinc function with zero crossings every 1us. Now, if we make the baud rate 1Mbps, each bit's ideal sampling point (center if eye) occurs at the zero crossing point of all of the potentially interfering sinc functions from previously received bits. In other words there is lots of ISI, but none at the critical moment when the bits are sampled. This is called "signalling at the Nyquist rate" and is related to but not the same as Nyquist sampling which you hear a lot about. (see en.wikipedia.org/wiki/Nyquist_ISI_criterion, apparently I am not allowed to include link because I am new to physicsforums). Of course brick wall filters are hard to make, so we use things like raised cosine filters that create the same beneficial sinc zero crossings. OFDM is analogous to FDM but with time and frequency domain reversed. We on/off modulate our carriers, but they are combined as unfiltered rectangular pulses and sent straight to the antenna (simplification of course). These time domain rectangular pulses become spread in the *frequency* domain as sinc functions. If we on/off modulate each carrier at 1Mbps (1us symbol time), and simultaneously maintain 1MHz carrier spacing ("orthogonal"), then the zero crossings of the sinc functions occur every 1MHz. Their positions are such that at each carrier frequency, all other carrier's smearing sinc functions have zero crossings. Thus each carrier frequency is free from interference. Again there is plenty of interference between these signals, but none at the critical frequencies where the carriers are located. Note the factor of 2 difference between the OFDM bandwidth and the FDM bandwith in my example. This is due to the convention of including negative frequency in the bandwidth in the OFDM case. Naturally, there is much more to it than this, but this is the basics. Hopefully you can figure out from this where the 50% comes from (look at the superimposed sinc functions).

we use IFFT in OFDM to convert the signal from frequency domain to time domain the idea in OFDM generation, the transmitter accepts a stream of data and converts them to symbols using modulation technique, for example QPSK. Then the S/P converter takes the output 4 symbols and mixes each one with one of the subcarrier, we now have 4 sine waves then add the 4 sine. Now we notice that S/P conversion stage the data represent as a function of frequency. After addition stage stage the data represent as a function of time. This conversion is actually a well-known computational technique called the inverse Fast Fourier transform.

The methods of ISI mitigation are 1- Adaptive Equalization 2- DS-Spread Spectrum 3- OFDM 4- Directional Antennas Hope this helps you. Regards Fahad

Yong Soo Cho has written: 'MIMO-OFDM wireless communications with MATLAB' -- subject(s): Orthogonal frequency division multiplexing, MIMO systems, MATLAB

Multi Carrier systems like OFDM has the problem of Inter Carrier Interference(ICI), which results from the loss of Orthogonality between the sub carriers.This happens when the FFT is considered over duration where the subcarrier is non integer number of cycles, which would be the case when multipath is present and the guard time has amplitude is zero.This is reduced by use of cyclic prefix, where we transmit a copy the last part of the symbol followed by the symbol itself.This ensures Orthogonality over the FFT period in case of delayed multipath. yup...that's all from me. Regards Girish .V.V There are many methods to reduce ICI not only CP like Frequency domain equalization,Time domain windowing,Pulse shaping,Maximum likelihood estimation,Extended kalman filtering and ICI self cancellation.And any one have their own advantages. Naser

OFDM (orthogonal frequency division multiplexing) is a technique for increasing the amount of information that can be carried over a wireless network. In frequency-division multiplexing, multiple signals, or carriers, are sent simultaneously over different frequencies between two points. However, FDM has an inherent problem: Wireless signals can travel multiple paths from transmitter to receiver (by bouncing off buildings, mountains and even passing airplanes); receivers can have trouble sorting all the resulting data out. Orthogonal FDM deals with this multipath problem by splitting carriers into smaller subcarriers, and then broadcasting those simultaneously. This reduces multipath distortion and reduces RF interference (a mathematical formula is used to ensure the subcarriers' specific frequencies are "orthogonal," or non-interfering, to each other), allowing for greater throughput.

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