The exact LTE modulation format is chosen depending upon the actual conditions. Within the OFDM signal it is possible to choose between four types of modulation for the LTE signal: In this way all mobiles will be able to talk to any base station. Although it is mandatory for the mobiles to have capability to be able to receive all 2048 sub-carriers, not all need to be transmitted by the base station which only needs to be able to support the transmission of 72 sub-carriers. The OFDM signal used in LTE comprises a maximum of 2048 different sub-carriers having a spacing of 15 kHz. If some of the carriers are lost due to multi-path effects, then the data can be reconstructed by using error correction techniques. By carrying the data at a lower rate across all carriers, the effects of reflections and inter-symbol interference are also overcome. By making the signal orthogonal to each other, the signals would not interfere with other signals and thus mutual interference is avoided. Orthogonal frequency-division multiplexing (OFDM) is a method for encoding digital data transmission which uses a large number of closely spaced carriers that are modulated with low rate data stream. LTE is based on Orthogonal Frequency Division Multiple Access (OFDM), and achieves high data rates by combining large bandwidths, higher order modulation and spatial multiplexing. There are multi path fading problems in UMTS so LTE uses OFDM in the downlink to overcome such problems. As a result, higher versions of QAM are used only when there is a sufficiently high signal to noise ratio(S/N).ฤก.1 Orthogonal Frequency Division Multiplexing (OFDM) However the disadvantage is that it is more susceptible to noise. The most common forms of QAM are 16-QAM, 64-QAM and 256-QAM. By using higher order modulation formats, it is possible to transmit more bits per symbol. The advantage of moving to the higher order formats is that it is possible to transmit more bits per symbol. Quadrature amplitude modulation is used in radio communication as it is able to carry higher data rates when compared to other modulation techniques. Lets discuss only the QAM which is widely used in radio communication especially in LTE. The most fundamental digital modulation techniques are Phase-shift keying (PSK), Frequency-shift keying (FSK), Amplitude-shift keying (ASK) and Quadrature Amplitude Modulation (QAM). The device which does both modulation and demodulation is called as the MODEM. A modulator does the modulation of a signal and a demodulator does exactly the reverse of modulation. Modulation is the process of varying the properties of a carrier signal such as frequency, phase and amplitude with a modulating signal that contains the actual information to be transmitted.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |