RESEARCH Open AccessMAC and baseband processors for RF-MIMOWLANZoran Stamenkovic 1* , Klaus Tittelbach-Helmrich 1 , Milos Krstic 1 , Jesus Ibanez 2 , Victor Elvira 2 and Ignacio Santamaria 2AbstractThe article describes hardware solutions for the IEEE 802.11 medium access control (MAC) layer and IEEE 802.11adigital baseband in an RF-MIMO WLAN transceiver that performs the signal combining in the analogue domain.Architecture and implementation details of the MAC processor including a hardware accelerator and a 16-bit MAC-physical layer (PHY) interface are presented. The proposed hardware solution is tested and verified using a PHY linkemulator. Architecture, design, implementation, and test of a reconfigurable digital baseband processor aredescribed too. Description includes the baseband algorithms (the main blocks being MIMO channel estimation andTx-Rx analogue beamforming), their FPGA-based implementation, baseband printed-circuit-board, and real-timetests.Keywords: baseband, MAC, MIMO, processor1. IntroductionCurrent multiple-input multiple-output (MIMO) wire-less systems perform the combining and processing ofthe complex antenna signal in the digital baseband.Since complete transmitter and receiver are required foreach path, the resulting power consumption and costsof the conventional MIMO approaches [1] limit applica-tions for ubiquitous networks. A low-power and low-cost RF-MIMO (MIMAX) system for maximum reliabil-ity and performance (Figure 1) compliant to the IEEEStandard 802.11a [2] has recently been proposed [2-4].It significantly decreases the hardware complexity byperforming the adaptive weighting and combining of theantenna signals in the RF front-end [5-8].Multiple antennas are used to increase the transmis-sion reliability through spatial diversity. Redesigns havemostly been done in the physical medium-dependent(PMD) layer. They demand for changes in the physicallayer convergence (PLC) and medium access control(MAC) protocols to optimally exploit the benefits of thenew RF front-end [9-13]. The PLCP pursues mappingMAC protocol data units in PMD layer compliant frameformats. This task is common for all communicationschemes defined by the IEEE Standard 802.11.Furthermore, the spatial diversity must be exploited,possible impairments in the RF spatial processing haveto be compensated and the MIMO channel has to beestimated. Particularly, these tasks are not needed in theIEEE802.11a scheme, which is specified for SISOcommunication.There are several differences between the MIMAXapproach and the full multiplexing MIMO approach. InMIMAX, the same weight is used for all subcarriers inOFDM transmissions, whereas it is possible to weighteach subcarrier independently from the others in thefull MIMO transmission scheme.Integrating the signal processing in analogue circuits islimited in the maximum achievable resolution becauseof noise processes, process variations or nonlinear beha-viour of the devices. Therefore, the signal processing hasto be calibrated by the baseband to adapt to the RFimpairments. This mainly considers the correlationbetween real and imaginary parts of the vector modula-tor approach. Compensation is achieved by a calibrationperformed by the RF control unit in Figure 1. The char-acteristics of the vector modulator are analysed by thismodule and stored in an internal memory. The weightsprovided by the baseband are then transferred into cor-responding values of the vector modulator using thepreviously determined relationship and these newweights control the vector modulator. Integrating* Correspondence: stamenko@ihp-microelectronics.com1 IHP, Im Technologiepark 25, 15236 Frankfurt (Oder), GermanyFull list of author information is available at the end of the articleStamenkovic et al. EURASIP Journal on Wireless Communicationsand Networking 2011, 2011:207http://jwcn.eurasipjournals.com/content/2011/1/207© 2011 Stamenkovic et al; licensee Springer. This is an Open Access article distributed under the terms of the Creative CommonsAttribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction inany medium, provided the original work is properly cited.
