To increase the data rate, a modulation method is used in LTE, which can transmit a plurality of bits per signal – the so-called Quadrature Amplitude Modulation (QAM).
This is a transmission technology in the long applied modulation method, which guarantees high transmission density. This amplitude modulation and phase modulation are combined. The method has different levels, and it can have multiple bits in groups of 4, 8, 16, 32 or 64 bits are combined – correspondingly increases, the transmitted data rate. The method with 64-bit, 64 short is called QAM, however, it is only possible in the vicinity of the transmission mast, because the higher the modulation, the more susceptible to interference. Thus, in LTE systems it uses in further transmission distance 4 QAM, 16 QAM in a medium distance.
In practice, therefore, the data rate close to the LTE radio mast will be higher than in a clear distance. The announced top speeds with LTE are therefore also dependent on how far away the nearest transmission tower, where the customer is staying.
QAM is in many radio communications and data delivery applications. However, some specific variants of QAM are used in some specific applications and standards.
For domestic broadcast applications for example, 64 QAM and 256 QAM are often used in digital cable television and cable modem applications. In the UK, 16 QAM and 64 QAM are currently used for digital terrestrial television using DVB – Digital Video Broadcasting. In the US, 64 QAM and 256 QAM are the mandated modulation schemes for digital cable as standardized by the SCTE in the standard ANSI/SCTE 07 2000.
In addition to this, variants of QAM are also used for many wireless and cellular technology applications.
QAM noise margin
While higher order modulation rates are able to offer much faster data rates and higher levels of spectral efficiency for the radio communications system, this comes at a price. The higher order modulation schemes are considerably less resilient to noise and interference.
As a result of this, many radio communications systems now use dynamic adaptive modulation techniques. They sense the channel conditions and adapt the modulation scheme to obtain the highest data rate for the given conditions. As signal to noise ratios decrease errors will increase along with re-sends of the data, thereby slowing throughput. By reverting to a lower order modulation scheme the link can be made more reliable with fewer data errors and re-sends.