Author Topic: Implementation of Adaptive Modulation and Coding Technique using  (Read 2526 times)

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Author : Sami H. O. SALIH, Mamoun M. A. SULIMAN
International Journal of Scientific & Engineering Research, Volume 2, Issue 5, May-2011
ISSN 2229-5518
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AbstractóDifferent order modulations combined with different coding schemes, allow sending more bits per symbol, thus achieving higher throughputs and better spectral efficiencies. However, it must also be noted that when using a modulation technique such as 64-QAM with less overhead bits, better signal-to-noise ratios (SNRs) are needed to overcome any Intersymbol Interference (ISI) and maintain a certain bit error ratio (BER). The use of adaptive modulation allows wireless technologies to yielding higher throughputs while also covering long distances. The aim of this paper is to implement an Adaptive Modulation and Coding (AMC) features of the WiMAX and LTE access layer using SDR technologies in Matlab. This papper focusing on the physical layer design (i.e. Modulation), here the various used modulation type will be implemented in a single Matlab function that can be called with the appropriate coefficients. A comparison with the hardware approaches will be made in terms of SNR vs. BER relation.
Index Termsó. Adaptive Modulation and Coding (AMC), Cognitive Radio (CR), LTE, Software Defined Radio (SDR), WiMAX.

1   INTRODUCTION                                                                      
The growth in the use of the information networks lead to the need for new communication networks with higher data rates. The telecommunication industry is also changing, with a demand for a greater range of services, such as video conferences, or applications with multimedia contents. The increased reliance on computer networking and the Internet has resulted in a wider demand for connectivity to be provided "any where, any time", leading to a rise in the requirements for higher capacity and high reliability broadband wireless access Broadband wireless Access (BWA) telecommunication systems.
BWA intensively focused in the last few years. Thus, various new technologies with high transmission abilities have been designed. The BWA has become the best way to meet escalating business demand for rapid Internet connection and integrated "triple play" services. That is the very base of the HSPA, WiMAX, and LTE concept: a wireless transmission infrastructure that allows a fast deployment as well as low maintenance costs.
The emergent demand of all types of services, not only voice and data but also multimedia services, aims for the design of increasingly more intelligent and agile communication systems, capable of providing spectrally efficient and flexible data rate access. These systems are able to adapt and adjust the transmission parameters based on the link quality, improving the spectrum efficiency of the system, and reaching, in this way, the capacity limits of the underlying wireless channel.
Link adaptation techniques, often referred to as adap-tive modulation and coding (AMC), are a good way for reaching the cited requirements. They are designed to track the channel variations, thus changing the modula-tion and coding scheme to yield a higher throughput by transmitting with high information rates under favorable channel conditions and reducing the information rate in response to channel degradation.
2.1 Preface
The current WiMAX revision is based upon IEEE802.16e-2005, approved in December 2005. It is a supplement to the IEEE802.16-2004. [1] Thus, IEEE 802.16e-2005 improves by:
   Adding support for mobility
   Scaling of the Fast Fourier transform (FFT) to the channel bandwidth in order to keep the carrier spacing constant across different channel band-widths (typically 1.25 MHz, 5 MHz, 10 MHz or 20 MHz
   Advanced antenna diversity schemes, and hybrid automatic repeat-request (HARQ)
   Adaptive Antenna Systems (AAS) and MIMO technology
   Denser sub-channelization, thereby improving indoor penetration
   Introducing Turbo Coding and Low-Density Parity Check (LDPC)
   Introducing downlink sub-channelization, allowing administrators to trade coverage for capacity or vice versa
   Adding an extra QoS class for real time applica-tions
In the other hand, Long Term Evolution (LTE) is the latest standard in the 3rd Generation Partnership Project (3GPP), mobile network technology tree that produced the GSM/EDGE and UMTS/HSPA network technologies.[1][2]
The LTE specification provides downlink peak rates of at least 100 Mbps, an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms. LTE supports scalable carrier bandwidths, from 1.4 MHz to 20 MHz and supports both frequency division duplexing (FDD) and time division duplexing (TDD).
The main advantages with LTE are high throughput, low latency, plug and play, FDD and TDD in the same platform, an improved end-user experience and a simple architecture resulting in low operating costs. LTE will also support seamless passing to cell towers with older network technology such as GSM, cdmaOne, UMTS, and CDMA2000. The next step for LTE evolution is LTE Advanced and is currently being standardized in 3GPP Release 10. [3]

The most important similarity between LTE and Wi-MAX is orthogonal frequency division multiplex (OFDM) signaling. Both technologies also employ Viterbi and turbo accelerators for forward error correction. From a chip designer's perspective, that makes the extensive reuse of gates highly likely if one had to support both schemes in the same chip or chip-set. From a software defined radio (SDR) perspective, the opportunity is even more enticing. Flexibility, gate reuse and programmability seem to be the answers to the WiMAX-LTE multimode challenge.
2.2 Hypothesis of AMC
In traditional communication systems, the transmission is designed for the "worst case" channel scenario thus, coping with the channel variations and still delivering an error rate below a specific limit. Adaptive transmission schemes, how-ever, are designed to track the channel quality by adapting the channel throughput to the actual channel state. These techniques take advantage of the time-varying nature of the wireless channel to vary the transmitted power level, symbol rate, coding scheme, constellation size, or any combination of these parameters, with the purpose of improving the link average spectral efficiency (bits/s/Hz).

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