OFDMA as an access method resembles general spread spectrum techniques such as code division multiple access (CDMA) and users can achieve different data rates by assigning a different code spreading factor or a different number of spreading codes to each user.
Adaptive user-to-subcarrier assignment is decided only after a close examination of the radio environment. If the assignment is done sufficiently fast, this further improves the OFDM robustness to fast fading and narrow-band co channel interference, and makes it possible to achieve even better system spectral efficiency.
The assigned number of subcarriers per user varies, and depends on the predefined Quality of Service and related operator policies.
OFDMA can be seen as an alternative to combining OFDM with time division multiple access (TDMA) or time-domain statistical multiplexing, i.e. packet mode communication. Low data-rate users can send continuously with low transmission power instead of using a "pulsed" high-power carrier
OFDMA can also be viewed as a combination of frequency domain and time domain multiple accesses, where the resources are partitioned in the time-frequency space and slots are assigned along the OFDM symbol index as well as OFDM subcarrier index.
Single-carrier FDMA (SC-FDMA)
SC-FDMA is frequency division multiple access scheme and a multi-user version of Single-carrier frequency domain equalization (SC-FDE) modulation. SC-FDE can be viewed as a linearly pre-coded OFDM scheme (LP-OFDMA) or as a single carrier multiple access scheme.
One prominent advantage over conventional OFDM and OFDMA is that the SC-FDE and LP-OFDMA/SC-FDMA signals have lower peak-to- average power ratio (PAPR) because of its inherent single carrier structure
OFDM applies the fast Fourier transform (FFT) on the receiver side of each block of symbols, and the inverse FFT (IFFT) on the transmitter side. SC-FDE, applies both the FFT and IFFT on the receiver side, but not on the transmitter side.
In SC-FDMA, both FFT and IFFT are applied on the transmitter side, and also on the receiver side.
OFDM , SC-FDE and SC-FDMA, achieve equalization on the receiver side after the FFT calculation, by multiplying each Fourier coefficient by a complex number thus managing frequency-selective fading and phase distortion. In SC-FDMA, multiple access is achieved by inserting silent Fourier-coefficients on the transmitter side before the IFFT, and removing them on the receiver side before the IFFT. Different users are assigned to different Fourier coefficients (subcarriers).
The use of OFDM, a system where the available spectrum is divided into many thin carriers, each on a different frequency, each carrying a part of the signal, enables E-UTRA to be much more flexible in its use of spectrum than the older CDMA based systems that dominated 3G. CDMA networks require large blocks of spectrum to be allocated to each carrier, to maintain high chip rates, and thus maximize efficiency.
Downlink
LTE uses OFDM for the downlink – that is, from the base station to the terminal. OFDM meets the LTE requirement for spectrum flexibility and enables cost efficient solutions for very wide carriers with high peak rates. It is a well-established technology, for example in standards such as IEEE 802.11a/b/g, 802.16, HIPERLAN-2, DVB and DAB.
The downlink radio frame is 10 ms long and consists of 10 sub frames of 1 ms each. Every sub frame consists of 2 slots where each slot is 0.5 ms. The subcarrier spacing in the frequency domain is 15 kHz. Twelve of these subcarriers together (per slot) make up a resource block of 180 kHz. Six Resource blocks fit in a carrier of 1.4 MHz and 100 resource blocks fit in a carrier of 20 MHz. Supported modulation formats on the downlink data channels are QPSK, 16QAM and 64QAM.
For MIMO operation, a distinction is made between single user MIMO, for enhancing one user's data throughput, and multi user MIMO for enhancing the cell throughput. In MIMO systems, a transmitter sends multiple streams by multiple transmit antennas.
The transmit streams go through a matrix channel which consists of multiple paths between multiple transmit antennas at the transmitter and multiple receive antennas at the receiver. Effective reception of the transmitted signal vectors is accomplished by multiple antennas; the received signal vectors are then decoded into the original information.
Uplink
The LTE uplink invokes a pre-coded version of OFDM called Single Carrier Frequency Division Multiple Access (SC-FDMA). This is to compensate for a drawback with normal OFDM, which has a veryhigh Peak to Average Power Ratio (PAPR). HighPAPR requires expensive and inefficient power amplifiers with high requirements on linearity, which increases the cost of the terminal and drains the battery faster. SC-FDMA solves this problem by grouping together the resource blocks in such a way reducing the need for linearity and power amplifier power consumption.
A low PAPR also improves coverage and the cell-edge performance.
Supported modulation formats on the uplink data channels are QPSK, 16 QAM and 64 QAM.I f virtual MIMO / Spatial division multiple access(SDMA) is introduced the uplink data rate can be increased depending on the base station antenna number. With this technology more than one mobile can reuse the same resources.
In conclusion, OFDMA is considered highly suitable for broadband wireless networks, because of its scalability and MIMO compatibility, and ability to take advantage of channel frequency selectivity.
Antonis Hontzeas - Antonis Hontzeas has been in the forefront of the Telecommunications industry for over twenty years. Tenures include positions ranging ...
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