Re: Space time...
vkekk said:
1. What is the difference between Space time and space time coding?
2. what is temporal coding?
3. advantages of QAM compared with QPSK.. what effect if increase 4QAM To 16QAM...
1. In wireless communication, space-time refers to space and time coordinate of the wireless channel. As you might know, wireless channels have three dimensions: time, frequency, and space. Time dimension is well known. Frequency dimension is due to multipath fading (frequency selective channel). Space dimension occurs when you have multiple antennas at the transmitter or/and the receiver. This three dimensions of wireless channels may be exploited by communication systems to improve the quality of transmission. In simplest form, to improve the quality you may transmit a signal in several copies. This multiple copies may be transmitted in multiple time (repetition in time) or multiple space (antenna), or in case of space-time coding this multiple copies are transmitted both in multiple space and time based in specific way. Hopefully, one or few of this copies are in good quality so the receiver can decode the transmitted signal perfectly.
2. Temporal coding in wireless communication refers to coding only in time coordinate. This is the old kind of coding in wireless communication. Reed-Solomon and convolutional coding is a kind of temporal coding.
3. Actually the difference between 4QAM and QPSK is only in rotation of the signal constellation. QSPK signals are at +1, -1, +j, -j, whereas 4QAM signal are at \[e^{j \pi /4}\], \[-e^{j \pi /4}\], \[j e^{j \pi /4}\], and \[-j e^{j \pi /4}\], i.e. QPSK signals rotated by \[\pi /4\]. Rectangular QAM constellation are, in general, sub-optimal in the sense that they do not maximally space the constellation points for a given energy. However, they have the considerable advantage that they may be easily transmitted as two pulse amplitude modulation (PAM) signals on quadrature carriers, and can be easily demodulated.
If you increase to higher order modulation scheme, you will have more bits per trasmission of a symbol. This is a good strategy to transmit higher order modulation schemes if SNR is high. But if SNR is low then transmitting higher order modulation may introduce higher bit errors.
May it helps.
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