QAM, quadrature amplitude modulation provides some significant benefits for data transmission. As 16QAM transitions to 64QAM, 64QAM to 256 QAM and so forth, higher data rates can be achieved, but at the cost of the noise margin.

Many data transmission systems migrate between the different orders of QAM, 16QAM, 32QAM, etc., dependent upon the link conditions. If there is a good margin, higher orders of QAM can be used to gain a faster data rate, but if the link deteriorates, lower orders are used to preserve the noise margin and ensure that a low bit error rate is preserved.

As the QAM order increases, so the distance between the different points on the constellation diagram decreases and there is a higher possibility of data errors being introduced. To utilise the high order QAM formats, the link must have a very good E_b/N_o otherwise data errors will be present.When the E_b/N_o deteriorates, then other the power level must be increased, or the QAM order reduced if the bit error rate is to be preserved.

Accordingly there is a balance to be made between the data rate and QAM modulation order, power and the acceptable bit error rate. Whilst further error correction can be introduced to mitigate any deterioration in link quality, this will also decrease the data throughput.

Bit sequence mapping for a 16QAM signal

QAM formats and applications

QAM is in many radio communications and data delivery applications. However some specific variants of QAM are used in some specific applications and standards.

There is a balance between data throughput and signal to noise ratio required. As the order of the QAM signal is increased, i.e. progressing from 16QAM to 64QAM, etc. the data throughput achievable under ideal conditions increases. However the downside is that a better signal to noise ratio is required to achieve this.

For some systems the order of the modulation format is fixed, but in others where there is a two way link, it is possible to adapt the order of the modulation to obtain the best throughput for the given link conditions. The level of error correction used is also altered. In this way, changing the modulation order, and the error correction, the data speed can be optimised whilst maintaining the required error rate.

For domestic broadcast applications for example, 64 QAM and 256 QAM are often used in digital cable television and cable modem applications. The order of the QAM modulation has to be set at the transmitter, because the transmission is only one way, and in addition to this, there are thousands of receivers, making it impossible to have a dynamically adaptive form of modulation.

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 standardised by the SCTE in the standard ANSI/SCTE 07 2000.

For the many forms of wireless and cellular technology it is possible to dynamically alter the order of QAM modulation and error correction according to the link conditions between the two ends.

As data rates have risen and the demands on spectrum efficiency have increased, so too has the complexity of the link adaptation technology. Data channels are carried on the cellular radio signal to enable fast adaptation of the link to meet the prevailing link quality and ensure the optimum data throughput, balancing transmitter power, QAM order, and forward error correction, etc.

Constellation diagrams for QAM

The constellation diagrams show the different positions for the states within different forms of QAM, quadrature amplitude modulation. As the order of the modulation increases, so does the number of points on the QAM constellation diagram.

The diagrams below show constellation diagrams for a variety of formats of modulation:

16QAM constellation32QAM constellation64QAM constellation

It can be seen from these few QAM constellation diagrams, that as the modulation order increases, so the distance between the points on the constellation decreases. Accordingly small amounts of noise can cause greater issues.

As the level of noise increases due to low signal strengths, so the area covered by a point on the constellation increases. If it becomes too large, then the receiver is unable to determine which position on the constellation the transmitted signal was meant to be, and this results in errors.

It is also found that the higher the order of modulation for the QAM signal, the greater the amount of amplitude variation is present on the transmitted signal. For transmitter RF amplifiers for everything from Wi-Fi to cellular and more, it means that linear amplifiers are required. As linear amplifiers are less efficient than those that can be run in saturation, it means that techniques like Doherty amplifiers and envelope tracking may be needed.

Also as the amplitude variation increases, so the level of efficiency falls. This is very important for mobile equipment battery efficiency, and base station power efficiency.

QAM bits per symbol

The advantage of using QAM is that it is a higher order form of modulation and as a result it is able to carry more bits of information per symbol. By selecting a higher order format of QAM, the data rate of a link can be increased.

The table below gives a summary of the bit rates of different forms of QAM and PSK.

Bit mapping for a 16QAM signal

QAM FORMATS & BIT RATES COMPARISON
MODULATION	BITS PER SYMBOL	SYMBOL RATE
BPSK	1	1 x bit rate
QPSK	2	1/2 bit rate
8PSK	3	1/3 bit rate
16QAM	4	1/4 bit rate
32QAM	5	1/5 bit rate
64QAM	6	1/6 bit rate

The power spectrum and bandwidth efficiency of QAM modulation is identical to M-ary PSK modulation, in other words for the same order phase shift keying, the power spectrum and bandwidth efficiency levels are exactly the same whether quadrature amplitude modulation or phase shift keying is used.

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.

QAM FORMATS & NOISE PERFORMANCE
MODULATION	ηB	EB / NO FOR BER = 1 IN 106
16QAM	2	10.5
64QAM	3	18.5
256QAM	4	24
1024QAM	5	28

Selecting the right order of QAM modulation for any given situation, and having he ability to dynamically adapt it can enable the optimum throughput to be obtained for the link conditions for that moment. Reducing the order of the QAM modulation enables lower bit error rates to be achieved and this reduces the amount of error correction required. In this way the throughput can be maximised for the prevailing link quality.