It may appear that these rates do not align with the headline figures given in the LTE specifications. In turn this is able to provide a raw data rate of 108 Mbps as each symbol using 64QAM is able to represent six bits. This gives a 20 MHz bandwidth system a raw symbol rate of 18 Msps. To maintain orthogonality, this gives a symbol rate of 1 / 15 kHz = of 66.7 µs.Įach subcarrier is able to carry data at a maximum rate of 15 ksps (kilosymbols per second). the LTE subcarriers are spaced 15 kHz apart from each other. In addition to this the subcarriers spacing is 15 kHz, i.e. The channel bandwidths that have been chosen for LTE are: Obviously the greater the bandwidth, the greater the channel capacity. LTE defines a number of channel bandwidths.
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The available bandwidth influences a variety of decisions including the number of carriers that can be accommodated in the OFDM signal and in turn this influences elements including the symbol length and so forth. One of the key parameters associated with the use of OFDM within LTE is the choice of bandwidth. LTE channel bandwidths and characteristics In addition to this, OFDM can be used in both FDD and TDD formats. OFDM is also a modulation format that is very suitable for carrying high data rates - one of the key requirements for LTE. Also in recent years a considerable level of experience has been gained in its use from the various forms of broadcasting that use it along with Wi-Fi and WiMAX. However OFDM was chosen as the signal bearer format because it is very resilient to interference. mobile to the base station) as a result of the different requirements between the two directions and the equipment at either end. from base station to mobile) and the uplink (i.e. The actual implementation of the technology will be different between the downlink (i.e. Read more about OFDM, Orthogonal Frequency Division Multiplexing. The data to be transmitted is shared across all the carriers and this provides resilience against selective fading from multi-path effects. The close spaced signals would normally be expected to interfere with each other, but by making the signals orthogonal to each other there is no mutual interference. Orthogonal Frequency Division Multiplex, OFDM is a form of signal format that uses a large number of close spaced carriers that are each modulated with low rate data stream. However its use of multiple carrier each carrying a low data rate remains the same. While the basic concepts of OFDM are used, it has naturally been tailored to meet the exact requirements for LTE. The use of OFDM is a natural choice for LTE.
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Higher order modulation is used to achieve the higher data rates: the modulation order being determined by the signal quality. Within the basic LTE OFDM signal format a variety of modulation formats are used including PSK and QAM. Using multiple carriers, each carrying a low data rate, OFDM is ideal for high speed data transmission because it provides resilience against narrow band fading that occurs as a result of reflections and the general propagation properties at these frequencies. OFDM, Orthogonal Frequency Division Multiplex is the basic format used and this is modified to provide the multiple access scheme: OFDMA, orthogonal frequency division multiple access in the downlink and SC-FDMA, single channel orthogonal frequency division multiple access in the uplink.
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OFDM forms the basic signal format used within 4G LTE. LTE Advanced topics: LTE Advanced introduction Carrier aggregation Coordinated multipoint LTE relay Device to device, D2D What is LTE LTE OFDMA / SCFDMA MIMO LTE Duplex LTE frame & subframe LTE data channels LTE frequency bands LTE EARFCN UE categories / classes LTE-M (Machine to Machine) LTE-LAA / LTE-U VoLTE SRVCC LTE OFDM, OFDMA SC-FDMA & Modulation LTE uses OFDM as the basic signal format - OFDMA in the downlink and SC-FDMA in the uplink with various modulation formats.