We propose a link adaptation technique for IEEE 802.11n MIMO-OFDM systems with convolutional coding. The technique dynamically selects the best modulation and coding scheme (MCS) based on channel conditions and on the medium access control (MAC) layer performance (coded BER or PER) requested by applications. We show that the coded performance at decoder output can be modeled as a function of decoder-input uncoded BER and the code rate. For MIMO-OFDM systems with different modulation types for different spatial streams at each subcarrier, we propose a simple and accurate method for computing the BER for each spatial stream mixed from different subcarriers. We also discuss approaches for searching and selecting the MCS.

Adaptive Modulation and Coding for IEEE 802.11n

In future wireless mobile networks, data rate and quality of service are expected to be comparable to those of wired deployments. To achieve this target, novel architectures must be adopted, successfully countering the disadvantages of the wireless transmission. Inspired by that, cooperative relaying was proposed because of the various gains it introduces to the network. In this work we propose a scheme consisting of multi-mode decode and forward relays facilitating the communication between a base station and a user terminal (UT). By equipping the relays with two interfaces, we can exploit the plethora of the available wireless protocols. Also, instead of performing multi-relay transmissions, we adopt an opportunistic relaying scheme due to its simplicity and outage-optimality. Additionally, we incorporate successive transmissions to improve the spectral efficiency, thus recovering the half-duplex loss in capacity due to the two-hop transmission. However, as inter-relay interference arises from successive transmissions, we propose mitigation techniques through interference cancellation and out-band transmissions using the multi-mode relays. At the same time, an energy-aware mechanism is implemented in the selected relay's transmission, opting for power reduction, as the channel state information is acquired prior to the signal's forwarding to the UT. Finally, we give numerical results by comparing the proposed energy-aware multi-mode relaying (EA-MMR) scheme, with two other schemes in terms of average end-to-end capacity, outage probability, delay distribution and power gain.

Capacity Maximization through Energy-Aware Multi-Mode Relaying

Modulation classification is to identify the modulation type of a received signal automatically. It is a promising technique that can increase the throughput of adaptive modulation systems such as IEEE 802.11 and HIPERLAN systems. While a variety of approaches have been proposed for modulation classification, most of them rely on the unrealistic assumption that the channel is additive white Gaussian noise channel. This paper presents an algorithm using hybrid likelihood ratio test for BPSK and QPSK classification in multipath fading environment. The unknown channel coefficients are first estimated blindly using the EM algorithm. The estimates are then used in likelihood ratio test for classification. Simulations show that the proposed classifier has high accuracy and is superior to the classifier that does not take the channel distortion into account. An added advantage of the proposed algorithm is that the maximum likelihood estimate of the channel coefficients can be used in equalization and data demodulation.

Maximum likelihood BPSK and QPSK classifier in fading environment using the EM algorithm

Link adaption varies channel parameters to match dynamic changes in wireless link in order to achieve an optimal throughput performance. It is a very important research issue in Wireless LANs(WLANs), but there is few description about that in 802.11 specifications. In this paper, we propose an Enhanced Minstrel Link Adaption (EMLA) scheme, which is partly based on Minstrel and includes three parts of SNR Calibration, MCS Protection and Guard interval(Gi) Adaption. SNR Calibration is a mechanism to estimate the downlink SNR from transmitter side via probing. The more accurate of the SNR estimation brings the more rational of the MCS selection, thus the transmitting error ratio decreases.MCS Protection provides a MCS range according to the SNR estimation and only MCS in the range is permitted to select on transmitter, which prevent the MCS from misadjusting, then PHY_rate of transmitting is protected. Another improvement comes from the Gi adaption. By choosing the Gi mode dynamically, the transmitter can adapt to the channel state and extend effective transmission time. The experiment and simulation shows, in contrast to Minstrel, EMLA can cope with the worsening of the throughput caused by collision, interference and time varying effectively, therefore network performance enhanced.

http://www.sersc.org/journals/IJFGCN/vol9_no1/8.pdf

An Enhanced Minstrel Link Adaption Scheme for IEEE 802.11 WLANs

Cooperative relaying with Distributed Space-Time Coding (DSTC) and cooperative Opportunistic Relaying (OR) are the two main relaying techniques that are typically employed in current wireless systems. However, OR is less complex and proven to be outage optimal compared to DSTC. Thus, in this paper we propose a Multi-Mode Relaying (MMR) technique that is based on OR and combined with the Successive Relaying (SR) technique in order to deal with the half-duplex constraint of the relay nodes. MMR, by employing Multi-Mode Decode-and-Forward (MMDF) relay nodes and incorporating two interference mitigation algorithms, not only combines the merits of OR and SR but also is able to alleviate the increased inter-relay interference caused by SR. The performance of MMR is evaluated through simulation and the results show that it is able to increase the average end-to-end capacity of the wireless path as well as the percentage of high quality packets received at the terminal of the user.

A spectral efficient multi-mode relaying technique

Adaptive modulation has been shown to be effective in performance enhancement for multi-carrier wireless communication. In this paper we present an adaptive modulation framework for the OFDM based IEEE 802.11a wireless LAN system. The goal is to increase the data transmission rate subject to fixed transmission power and the upper bound constraint of packet error rate. Two adaptive modulation schemes are proposed and both use the estimated subband SNR values to adjust the modulation scheme in a blockwise manner. The simulation results indicate both schemes can effectively increase the average data rate up to 147% under the 802.11a model. The performance improvement is even prominent for severe frequency selective fading environments.

Block-wise adaptive modulation for OFDM WLAN systems

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Block-wise adaptive modulation for OFDM WLAN systems