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The basic difference between OFDMA and SC-FDMA is that, there there is an additional block of N point DFT before the sub-channel mapping in SC-FDMA.
This will effectively reduce the PAPR, thus the power amplifier will have a lesser linear range in SC-FDMA when compared with OFDMA
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What is the difference between FDM and OFDMA?
* FDM - hava a guard band , if the band width for the data is x * OFDM - no guard band , x/2 band with
What is traffic load sector?
ofdma frequency range
What is the meaning of fft in ofdma?
Fast Fourier Transform
What is OFDMA?
Orthogonal Frequency Division Multiple Access. It has something to do with the way cell phones transmit signals.
What is the meaning of inter cell interference?
due to OFDMA some interference occurs which limits extention service of mobile
What type of modulation is used in GSM and CDMA?
I think OFDMA is used for cellphone communications via satellites. As far as I know, it is the best technique available which addresses all challenges like multipath fading, error correction, effective bandwidth utilization, security issues etc. Here, harmonics of a base carrier are considered for modulation. These signals will be orthogonal to each other. Hence can be overlapped on each other for transmission, without the signals getting collided. If we use N such orthogonal carriers, the BW requirement will be N times lesser than the classical modulation schemes (such as ASK, BPSK etc.) Practically, using N signal generators to produce precise orthogonal signals is difficult. Hence we do this in digital domain, where IFFT operation performs the same. (producing & adding orthoganal carrier signals). I'm Shreyas. Setting up an OFDM based Tx-Rx model was my academic project during my bachelor studies under Telecommunications. Hope I have answered in a simple way, with enough details :)
What is peak rate?
Long Term Evolution (LTE) is the project name of a new air interface for wireless access being developed by the Third Generation Partnership Project (3GPP). LTE is the evolution of 3GPP's Universal Mobile Telecommunication System (UMTS) towards an all-IP network. The LTE specifications provide a framework for increasing capacity, improving spectrum efficiency, improving cell-edge performance, and reducing latency. Many of the targets for LTE are similar to those for the continuing development of High Speed Packet Access (HSPA) - generally known as HSPA+ - although LTE has some specific additional capabilities such as flexible channel bandwidths and the advantages of Orthogonal Frequency Division Multiple Access (OFDMA). LTE is being developed in Releases 8 and 9 of the 3GPP specifications.To meet the demand for ever-higher data rates, LTE offers a 100 Mbps download rate and 50 Mbps upload rate for every 20 MHz of spectrum. Support is intended for even higher rates, to 326.4 Mbps in the downlink, using multiple antenna configurations. To allow the use of both new and existing frequency bands, LTE provides scalable bandwidth from 1.4 MHz to 20 MHz in both the downlink and the uplink. LTE is optimized for low speeds (0 - 15 km/h) but will still provide high performance to 120 km/h with support for mobility maintained up to 350 km/h. 3GPP are considering support for even higher speeds up to 500 km/h.Downlink Peak Data Rates(64QAM)Antenna configurationSISO2x2 MIMO4x4 MIMOPeak data rateMbps100172.8326.4Uplink Peak Data Rates(single antenna)Modulation depthQPSK16QAM64QAMPeak data rateMbps5057.686.4Figure 1: Peak data rate targets for LTE1
Difference between ofdm dsss fhss?
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.
What is the meaning 4G?
4G phone means that it is enabled to run on a 4G network and can handle the highspeed internet available over the 4G network. If you are thinking what's a 4G Network then it means a network that can offers not only phone connectivity but also a HI SPEED data connectivity with speeds similar to a DSL or Broadband network. It is in other others a big WI-FI network. 4G enabled phones will be as good for browing an internet as a normal computer and will even try to replace computers for everyday surfing and otherwise uses.
