How does QAM affect LTE latency and throughput?

1 What is QAM?

QAM is a technique that combines two orthogonal signals, called in-phase and quadrature, to encode data bits into symbols that are transmitted over a radio channel. By varying the amplitude and phase of the two signals, QAM can create different symbol shapes that represent different combinations of bits. For example, QPSK, or quadrature phase shift keying, is a type of QAM that uses four symbols, each corresponding to two bits. QAM can increase the number of symbols by increasing the amplitude levels and the phase shifts, creating more complex constellations. For example, 16-QAM uses 16 symbols, each corresponding to four bits.

2 How does QAM increase throughput?

The main advantage of using higher-order QAM is that it can increase the throughput, or the amount of data that can be transmitted in a given time, by packing more bits into each symbol. For example, if the symbol rate, or the number of symbols transmitted per second, is fixed, then using 16-QAM instead of QPSK can double the throughput, since each symbol carries twice as many bits. Similarly, using 64-QAM instead of 16-QAM can increase the throughput by another factor of two. LTE supports up to 256-QAM, which can encode eight bits per symbol.

3 How does QAM affect latency?

Latency, or the time it takes for a data packet to travel from the sender to the receiver, is also influenced by the modulation scheme. Higher-order QAM can reduce the latency by transmitting the same amount of data in fewer symbols, thus reducing the transmission time. For example, if a packet of 1,000 bits needs to be sent over a channel with a symbol rate of 1,000 symbols per second, then using QPSK would take one second, while using 256-QAM would take only 0.125 seconds. However, this latency reduction comes at a cost of increased sensitivity to noise and interference.

4 What are the challenges of using higher-order QAM?

Higher-order QAM requires a higher signal-to-noise ratio (SNR), or the ratio of the signal power to the noise power, to maintain a reliable communication. This is because the symbols in higher-order QAM are closer together in the constellation, making them more susceptible to distortion and errors caused by noise and interference. If the SNR is too low, the receiver might not be able to distinguish between the symbols correctly, leading to a higher bit error rate (BER), or the fraction of bits that are received incorrectly. To cope with this challenge, LTE uses adaptive modulation and coding (AMC), which adjusts the modulation scheme and the error correction code according to the channel conditions and the feedback from the receiver. For example, if the channel quality is good, the transmitter can use 256-QAM with a low code rate, which means less redundancy and more data bits. If the channel quality is poor, the transmitter can switch to QPSK with a high code rate, which means more redundancy and less data bits, but more robustness against errors.