Editing Quadrature amplitude modulation (section)

== Digital QAM ==
[[File:16-QAM Demonstration 3.gif|alt=Digital 16-QAM with example symbols|thumb|Digital 16-QAM with example symbols]]
[[File:Rectangular constellation for QAM.svg|thumb|Constellation points for 4-QAM, 16-QAM, 32-QAM, and 64-QAM overlapped]]
As in many digital modulation schemes, the [[constellation diagram]] is useful for QAM. In QAM, the constellation points are usually arranged in a square grid with equal vertical and horizontal spacing, although other configurations are possible (e.g. a hexagonal or triangular grid). In digital [[telecommunications]] the data is usually [[Binary numeral system|binary]], so the number of points in the grid is typically a power of 2 (2, 4, 8, …), corresponding to the number of bits per symbol. The simplest and most commonly used QAM constellations consist of points arranged in a square, i.e. 16-QAM, 64-QAM and 256-QAM (even powers of two). Non-square constellations, such as Cross-QAM, can offer greater efficiency but are rarely used because of the cost of increased modem complexity.

By moving to a higher-order constellation, it is possible to transmit more [[bit]]s per [[Symbol (data)|symbol]]. However, if the mean energy of the constellation is to remain the same (by way of making a fair comparison), the points must be closer together and are thus more susceptible to [[noise]] and other corruption; this results in a higher [[bit error rate]] and so higher-order QAM can deliver more data less reliably than lower-order QAM, for constant mean constellation energy. Using higher-order QAM without increasing the bit error rate requires a higher [[signal-to-noise ratio]] (SNR) by increasing signal energy, reducing noise, or both.

If data rates beyond those offered by 8-[[Phase-shift keying|PSK]] are required, it is more usual to move to QAM since it achieves a greater distance between adjacent points in the I-Q plane by distributing the points more evenly. The complicating factor is that the points are no longer all the same amplitude and so the [[demodulator]] must now correctly detect both [[Phase (waves)|phase]] and [[amplitude]], rather than just phase.

64-QAM and 256-QAM are often used in [[digital cable]] television and [[cable modem]] applications. In the United States, 64-QAM and 256-QAM are the mandated modulation schemes for [[digital cable]] (see [[QAM tuner]]) as standardised by the [[SCTE]] in the standard [https://web.archive.org/web/20140817034950/http://www.scte.org/FileDownload.aspx?A=3445 ANSI/SCTE 07 2013]. In the UK, 64-QAM is used for [[digital terrestrial television]] ([[Freeview (UK)|Freeview]]) whilst 256-QAM is used for Freeview-HD.

[[File:ADSL spectrum Fritz Box Fon WLAN.png|thumb|Bit-loading (bits per QAM constellation) on an ADSL line]]

Communication systems designed to achieve very high levels of [[spectral efficiency]] usually employ very dense QAM constellations. For example is [[ADSL]] technology for copper twisted pairs, whose constellation size goes up to 32768-QAM (in ADSL terminology this is referred to as bit-loading, or bit per tone, 32768-QAM being equivalent to 15 bits per tone).<ref>{{Cite web |others=Constellation mapper - maximum number of bits per constellation BIMAX ≤ 15 |title=G.992.3 : Asymmetric digital subscriber line transceivers 2 (ADSL2) |url=https://www.itu.int/rec/T-REC-G.992.3-200904-I |access-date=2024-10-09 |website=www.itu.int}}</ref>

Ultra-high capacity microwave backhaul systems also use 1024-QAM.<ref name="auto">{{Cite web |date= |title=TrangoLink Apex Orion - Trango Systems |url=http://www.trangosys.com/products/point-to-point-wireless-backhaul/licensed-wireless/trangolink-apex-orion.shtml |archive-url=https://web.archive.org/web/20120315235455/http://www.trangosys.com/products/point-to-point-wireless-backhaul/licensed-wireless/trangolink-apex-orion.shtml |archive-date=2012-03-15 |access-date= |website=www.trangosys.com}}</ref> With 1024-QAM, [[adaptive coding and modulation]] (ACM) and [[XPIC]], vendors can obtain gigabit capacity in a single 56&nbsp;MHz channel.<ref name="auto"/>