1. Adaptive modulation and coding (AMC)
Dalya A.Alrahim

2. Table of content Modulation Adaptive modulation and coding (AMC)
Modulation techniques
QAM noise margin
Adaptive modulation vs fixed modulation
The goal of Adaptive Modulation
Advantage
Challenges
Hybrid automatic repeat request (hybrid ARQ or HARQ)
WiMAX
Using AMC and HARQ to Optimize System Capacity and
Application Delays in WiMAX Networks
Link adaptation techniques
Long Term Evolution (LTE)
Channel Coding and Link Adaptation
Link adaptation in LTE

3. Modulation
In telecommunications, modulation is the process of conveying a message signal, for example a digital bit stream or an analog audio signal, inside another signal that can be physically transmitted. Modulation of a sine waveform transforms a baseband message signal into a passband signal.
A modulator is a device that performs modulation. A demodulator (sometimes detector or demod) is a device that performs demodulation, the inverse of modulation. A modem (from modulator–demodulator) can perform both operations.
Modulation is the addition of information to an electronic or optical carrier signal. A carrier signal is one with a steady waveform -- constant height (amplitude) and frequency. Information can be added to the carrier by varying its amplitude, frequency, phase, polarization (for optical signals), and even quantum-level phenomena like spin.
analog modulation
digital modulation

5. analog modulation

6. digital modulation

7. Adaptive modulation and coding (AMC)
Link adaptation, or adaptive modulation and coding (AMC), is a term used in wireless communications to denote the matching of the modulation, coding and other signal and protocol parameters to the conditions on the radio link.
For example, WiMAX uses a rate adaptation algorithm that adapts the modulation and coding scheme (MCS) according to the quality of the radio channel, and thus the bit rate and robustness of data transmission.
The process of link adaptation is a dynamic one and the signal and protocol parameters change as the radio link conditions change.

8. Adaptive modulation systems invariably require some channel state information at the transmitter. This could be acquired in time division duplex systems by assuming the channel from the transmitter to the receiver is approximately the same as the channel from the receiver to the transmitter. Alternatively, the channel knowledge can also be directly measured at the receiver, and fed back to the transmitter.

9. Adaptive modulation and coding block diagram.

10. Adaptive Modulation and Coding

13. Modulation techniques
Both QAM and QPSK are modulation
techniques used in IEEE (Wi-Fi*), IEEE
(WiMAX*) and 3G (WCDMA/HSDPA)
wireless technologies. The modulated signals are
then demodulated at the receiver where the
original digital message can be recovered. The
use of adaptive modulation allows wireless
technologies to optimize throughput, yielding
higher throughputs while also covering long
distances.

14. QPSK
Quadrature Phase Shift Keying (QPSK) is a form of Phase Shift Keying in which two bits are modulated at once, selecting one of four possible carrier phase shifts (0, 90, 180, or 270 degrees). QPSK allows the signal to carry twice as much information as ordinary PSK using the same bandwidth. QPSK is used for satellite transmission of MPEG2 video, cable modems, videoconferencing, cellular phone systems, and other forms of digital communication over an RF carrier.

15. Quadrature Phase Shift Keying (QPSK)

16. QAM
QAM (quadrature amplitude modulation) is a method of combining two amplitude-modulated (AM) signals into a single channel, thereby doubling the effective bandwidth. QAM is used with pulse amplitude modulation (PAM) in digital systems, especially in wireless applications.

17. Quadrature Amplitude Modulation (16-QAM)

18. In the Figure showing 16-QAM, each
symbol can now represent four bits instead of just the two bits per symbol with QPSK. Each point indicates a unique amplitude and phase of the wave (for example, point (1, 1) indicates 90 degrees and amplitude of 1).
A variety of forms of QAM are available and some of the more common forms include 16 QAM, 32 QAM, 64 QAM, 128 QAM, and 256 QAM.

19. 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.

20. 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.

21. Adaptive modulation vs fixed modulation
Adaptive modulation techniques in wireless communications are reactive ways designed in communication systems to thrive in unpredictable channel environments. The attractive use of adaptive communications will prove to bring more robustness and flexibility compared to fixed modulation schemes. In order for adaptive modulation to work correctly, it requires an accurate estimation of the channel condition at the receivers’ end to make decisions and take action. Channel state information (CSI) has several of other uses in wireless communication systems. Accordingly, a communication link which adapts the degree of modulation scheme according to the estimated signal-to-noise ratio (SNR) values is proposed. The system estimates the current channel condition in the form of CSI and feedback to the transmitter. Hence, the objective of the adaptive system is to stay opportunistic in favourable circumstances while achieving acceptable quality margin in a time-varying communication link.

22. Major functions in an adaptive modulation system.

23. Block diagram of link adaptation mechanism

24. The goal of Adaptive Modulation
The goal of Adaptive Modulation is to improve the operational efficiency of Microwave links by increasing network capacity over the existing infrastructure - while reducing sensitivity to environmental interferences. Adaptive Modulation means dynamically varying the modulation in an errorless manner in order to maximize the throughput under momentary propagation conditions. In other words, a system can operate at its maximum throughput under clear sky conditions, and decrease it gradually under rain fade.

25. Advantage
Adaptive modulation systems improve rate of transmission, and/or bit error rates, by exploiting the channel state information that is present at the transmitter. Especially over fading channels which model wireless propagation environments, adaptive modulation systems exhibit great performance enhancements compared to systems that do not exploit channel knowledge at the transmitter.

26. Challenges
The implementation of AMC offers several challenges. First, AMC is sensitive to measurement error and delay.In order to select the appropriate modulation, the scheduler must be aware of the channel quality. Errors in the channel estimate will cause the scheduler to select the wrong data rate and either transmit at too high a power,
wasting system capacity, or too low a power, raising the block error rate. Delay in reporting channel measurements also reduces the reliability of the channel quality estimate due to the constantly varying mobile channel. Furthermore changes in the interference add to the measurement errors. Hybrid ARQ (HARQ) enables
the implementation of AMC by reducing the number of required MCS levels and the sensitivity to measurement error and traffic fluctuations.

27. Hybrid automatic repeat request (hybrid ARQ or HARQ)
Hybrid automatic repeat request (hybrid ARQ or HARQ) is a combination of high-rate forward error-correcting coding and ARQ error-control. In standard ARQ, redundant bits are added to data to be transmitted using an error-detecting (ED) code such as a cyclic redundancy check (CRC). Receivers detecting a corrupted message will request a new message from the sender. In Hybrid ARQ, the original data is encoded with a forward error correction (FEC) code, and the parity bits are either immediately sent along with the message or only transmitted upon request when a receiver detects an erroneous message. The ED code may be omitted when a code is used that can perform both forward error correction (FEC) in addition to error detection, such as a Reed-Solomon code. The FEC code is chosen to correct an expected subset of all errors that may occur, while the ARQ method is used as a fall-back to correct errors that are uncorrectable using only the redundancy sent in the initial transmission. As a result, hybrid ARQ performs better than ordinary ARQ in poor signal conditions, but in its simplest form this comes at the expense of significantly lower throughput in good signal conditions. There is typically a signal quality cross-over point below which simple hybrid ARQ is better, and above which basic ARQ is better.

28. WiMAX
The IEEE technology (WiMAX)(Worldwide Interoperability for Microwave Access) is a promising technology for providing last-mile connectivity by radio link due to its high speed data rates, low cost of deployment, and large coverage area.
WiMAX is a wireless communications standard designed for creating metropolitan area networks (MANs). It is similar to the Wi-Fi standard, but supports a far greater range of coverage. While a Wi-Fi signal can cover a radius of several hundred feet, a fixed WiMAX station can cover a range of up to 30 miles. Mobile WiMAX stations can broadcast up to 10 miles.
While Wi-Fi is a good wireless Internet solution for home networks and coffee shops, it is impractical for larger areas. In order to cover a large area, multiple Wi-Fi repeaters must be set up at consistent intervals. For areas that span several miles, this is a rather inefficient method to provide wireless access and typically requires lots of maintenance. WiMAX, on the other hand, can cover several miles using a single station. This makes it much easier to maintain and offers more reliable coverage.

30. Using AMC and HARQ to Optimize System Capacity and Application Delays in WiMAX Networks
Since we do not want to lose the advantages of AMC A (less system capacity usage), but at the same time, we do not want to cause TCP retransmissions that significantly degrade application performance, so that we decided to use AMC with HARQ , Using the HARQ can give dual advantages of SNR gain and fast retransmissions.
By using the HARQ with AMC A, radio resources usage had reduced back while average delay were also small, so that both capacity and application delay criteria have now been optimized

31. Link adaptation techniques
The ever increasing demand of all types of services, voice,
data and above all, multimedia services, requires the design of
increasingly more intelligent and nimble communication
systems, which are capable of providing access to spectrally
efficient and flexible data rate. These systems are able to adapt
and adjust the transmission parameters based on the link
quality, hence improving the spectrum efficiency of the
system, and in this way, reaching the capacity limits of the
underlying wireless channel. Link adaptation techniques, often
referred to as adaptive modulation and coding (AMC), are a
good way for reaching the cited requirements. They are so
designed to track the channel variations, thus changing the
modulation and coding scheme to yield a higher throughput by
transmitting with high information rates under favourable
channel conditions and reducing the information rate in
response to degradation effects of the channel.

32. 1 - Variable-Rate Techniques
2-Variable-Power Techniques
3-Variable Error Probabilit
4-Variable Coding Techniques
5- Hybrid Techniques

33. Long Term Evolution (LTE)
Is a standard for wireless communication of high-speed data for mobile phones and data terminals.It is based on the GSM/EDGE and UMTS/HSPA network technologies, increasing the capacity and speed using a different radio interface together with core network improvements. The standard is developed by the 3GPP (3rd Generation Partnership Project)
LTE is referred to as the next generation network beyond 3G, with the capacity to support a high demand for connectivity from new consumer devices tailored to new mobile applications. In an LTE live air demo, Web browsing, HD video, and telecommunications are demonstrated simultaneously inside a single computer moving within a vehicle at 108 kilometers per hour.
3GPP engineers named the technology "Long Term Evolution" because it represents the next step (4G) in a progression from GSM, a 2G standard, to UMTS, the 3G technologies based upon GSM.
In 2010, many well-known U.S. and global wireless service providers/manufacturers began using LTE.

34. Channel Coding and Link Adaptation
which are important issues in modern digital communication systems. With channel coding, errors caused by distortion during transmission are detected and/or corrected. In LTE both convolutional and Turbo codes are used.

35. Channel coding is one of the most important aspects in digital communication systems, which can be considered as the main difference between analog and digital systems making error detection and correction possible. Error correction exists in two main forms: ARQ (Automatic Repeat Request) and FEC (Forward Error Correction).
With ARQ the receiver requests retransmission of data packets, if errors are detected, using some error detection mechanism. In FEC some redundancy bits are added to the data bits, which is done either blockwise (so-called block coding) or convolutional, where the coded bit depends not only on the current data bit but also on the previous bits.

36. In LTE both block codes and convolutional codes are used
In LTE both block codes and convolutional codes are used. There is also an enhanced coding technique used in LTE, called Turbo code, which has performances within a few tenth of a dB from the Shannons limit.

37. Another feature of LTE, which is considered here, is link adaptation
Another feature of LTE, which is considered here, is link adaptation. Link adaptation is referred to a mechanism matching automatically transmission parameters to the channel. As an example for older systems of link adaptation the early versions of UMTS (Universal Mobile Telecommunication System) can be mentioned, where fast closed-loop power control used to support an almost constant data rate. In UMTS the UE (User Equipment) transmitter adjusts its output power in accordance with one or more Transmit Power Control (TPC) commands received in the downlink, in order to keep the received uplink Signal-to-Interference Ratio (SIR) at a given SIR target.
In HSPA (High Speed Packet Service Access) and LTE the transmitted information data rate is adjusted dynamically to use the channel capacity efficiently.

38. Link adaptation in LTE
In LTE, link adaptation is based on the Adaptive Modulation and Coding (AMC). AMC can adapt modulation scheme and code rate in the following way:
• Modulation scheme: if the SINR (Signal-to-Interference plus Noise Ratio) is sufficiently high, higher-order modulation schemes with higher spectral efficiency (hence with higher bit rates) like 64QAM are used. In the case of poor SINR a lower-order modulation scheme like QPSK, which is more robust against transmission errors but has a lower spectral efficiency, is used.
• Code rate: for a given modulation scheme, an appropriate code rate can be chosen depending on the channel quality. The better the channel quality, the higher the code rate is used and of course the higher the data rate.

39. In LTE for data channels a Turbo encoder with a mother code rate of 1/3 is used. There is a Rate Matching (RM) module following the Turbo encoder, which makes it possible to get other code rates, if desired. Increasing and decreasing the code rate is done via puncturing and
repetition, respectively. Both, puncturing and repetition are integrated in the Rate Matching module.
In the Fig. the whole signal generation chain of the LTEs physical layer with Turbo coding and modulation modules can be seen, which are parts of the link adaptation system.

40. Signal generation chain in LTE

41. LTE uses Adaptive Modulation and Coding (AMC) as the link adaptation technique to adapt transmission parameters, modulation scheme and code rate dynamically to the channel. The higher the channel quality, the higher is the used modulation order and code rate. Channel quality in downlink is measured in UE using the reference symbols.

42. References
Shami, Abdallah; Maier, Martin; Assi, Chadi ( ). Broadband Access Networks: Technologies and Deployments. Springer Science & Business Media. p ISBN
Sauter, Martin ( ). From GSM to LTE: An Introduction to Mobile Networks and Mobile Broadband. John Wiley & Sons. p ISBN
Adaptive Modulation and Coding (AMC)Stockholm, Sweden, 20th-24th Oct 2000 , Agenda Item: Adhoc#24, HSDPA ,Source: Motorola
REVIEW OF VARIOUS ADAPTIVE MODULATION AND CODING
TECHNIQUES IN WIRELESS NETWORK , Lily Mishra1, M.H.Patwardhan2
Channel Coding and Link Adaptation , Shahram Zarei , 16. December 2009
Using AMC and HARQ to Optimize System Capacity and Application Delays in WiMAX Networks, Iwan Adhicandra

43. Adaptive Modulation (QPSK, QAM),Rao Farhat Masood, Member IEEE, MIE (Pak), PEC,National University of Sciences and Technology, Pakistan.