Cross-Layer Combining of Adaptive Modulation and

Coding with Truncated ARQ over Wireless Links Review Article Prepared by: Raheel Muzzammel

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Cross-Layer Combining of Adaptive Modulation and Coding with Truncated ARQ over Wireless Links

In wireless communication networks, the demand for high data rates and quality of service is increasing.

Performance of wireless links is going down due to channel fading in comparison to wired links. Studies and

research has been conducted to enhance through put and reliability at the physical layers by the application of

adaptive modulation and coding (AMC) and small size constellations or error control codes respectively. Channel

fading is also mitigated by automatic repeat request (ARQ) for retransmission of only packets containing errors at

the data link layer and is more effective than forward error coding (FEC). Truncated ARQ is employed to minimize

buffering and delays by imposing limitations on retransmission. In this research, cross layer design of combination

of AMC at the physical layer and ARQ at the data link layer is implemented to increase spectral efficiency and

throughput. It has been found that joint design of AMC and ARQ‘s performance is far better than performance of

AMC and ARQ separately.

AMC selects the modes of transmission from uncoded and coded QAM modulation modes at transmitter and

receiver via feedback channel. ARQ generator and controller are connected via feedback channel for retransmission

in case of errors in packets. However, both ARQ and AMC related parameters are sent back via same physical

feedback channel. This model is implemented with few assumptions i-e channel is frequency flat and remains

constant per frame but can be varied from frame to frame, channel state information should be perfect between

transmitter and receiver i-e, feedback channel is error free and has no latency, error detection codes should be

perfect. Nakagami – m model is adopted to describe the channel quality in terms of SNR.

Cross layer design with AMC and truncated ARQ is implemented with following delay constraint i-e,

retransmissions should be bounded and probability of packet losses should not be larger than bit-error rate range.

With this bounded retransmission constraint, delay and performance should be satisfied at the physical layer.

Therefore, AMC should be designed to maximize spectral efficiency in a way that it satisfies the condition of packet

error rate in terms of bit error rate. Minimum SNR for transmission mode should be required to achieve target

packets.

Performance comparison between cross layer design and truncated ARQ is made with the assumption that fading

channel coefficients are independent identically distributed random variable. It has been found that the packet loss

condition is satisfied only with the adaptive instantaneous SNR to achieve targeted packets. It is possible only in the

cross layer design.

Numerical analysis based on number of retransmissions give the following result that the spectral efficiency gain of

joint design of AMC with ARQ exceeds that of only AMC scheme for both uncoded and coded modes of

transmission. Spectral efficiency can also be increased by increasing number of retransmissions as error correcting

capability of truncated ARQ increases and hence physical layer is relieved of stringent error correction

requirements.

Numerical analysis based on Nakagami fading factor gives the following result that Nakagami fading channel

provide a diversity order and increase in diversity order results in improving spectral efficiency with AMC at

physical layer. However, increment of spectral efficiency degrades quickly in a same way as in number of

retransmissions.

Combined results of number of retransmissions and diversity order provide the benefits of implementing joint design

than only AMC.

Numerical comparison with truncated ARQ only has given the conclusion that spectral efficiency can be further

increased by combining truncated ARQ with AMC with fixed transmission and exploitation of channel knowledge

at transmitter.

Future contribution to this work could be the design and analysis of cross layer design with imperfect channel state

information. Multi dimensional problems could also be implemented by employing multi antennas for receiver and

transmission.

Declaration: This article is prepared from the paper “Cross-Layer Combining of Adaptive Modulation and Coding

with Truncated ARQ over Wireless Links written by Qingwen Liu, Shengli Zhou and Georgios B. Giannakis.