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Editor Responsable: Mario Chauca Saavedra (IIITEC, PERU)

Coordinador: Miguel Ángel Gómez Martínez MEXICO

International Institute of Innovation and Technology E.I.R.L. Avenida Los Gorriones 373 E-402 Chorrillos Lima 09, Perú Lima, Noviembre del 2011 Producido en Lima (PERU)

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IIITEC-Perú Avenida Los Gorriones 373 E-402 Chorrillos Lima-9 Perú

Teléfono (+51)12520782 isit2011@iiitec.org iiitec@iiitec.org www.iiitec.org

"Centennial Year of Machu Picchu to the World" Lima, August 30, 2011. Letter-30082011_27IAF Ph.D. Zoran Stamenkovic

IHP - Frankfurt

GERMANY

On behalf of Technical Program Committee of the 2nd Symposium IIITEC International Symposium on Innovation and Technology ISIT2011, we thank you for accept our invitation to participate as a reviewer, member of Technical Committee and as Speaker in Special Session. It is noteworthy that our Organizing Committee invited specialists of recognized professional and academic, who according to the criteria established by the Committee ISIT 2011 to choose the most representative. In addition to his valuable participation in the Conferences, take this opportunity to invite you to attend the final competition of papers as Chair Session, where the finalists will make an oral presentation of their work. The session papers contest will take place from November 28 to 30 November this year in the city of Lima, Peru, where he has scheduled oral presentation assisted by a chair session appointed by the Committee ISIT 2011. We are aware of the level and area of their work considering these qualities and others for the benefit of students and general public attendees, so we believe it is an ideal person to participate in this event and would be happy to have you. We hope your presence in the ISIT2011. Sincerely Mario Chauca General Chair ISIT2011 IIITEC

978-612-45917-1-6 © 2011 IIITEC 12 ISIT2011

802.11

Data Link LLC

MAC

MIMAX

PHY PLCP

PMD

802.11

Data Link LLC

MAC

802.11a

PHY PLCP

PMD

CIRCUITS AND SYSTEMS FOR RF-MIMO WLAN

Z. Stamenkovic*, K. Tittelbach-Helmrich*, R. Kraemer*, S. Ruiz**, O. Gago**, J. Ibanez

à, I. Santamaria

à,

M. Wickertàà

, and R. Eickhoffàà

Abstract - This paper describes the concept of a MIMO RF

front-end based wireless LAN system (called MIMAX), the

ver ification of the individual subsystems and the results of the

final tests. The MAC and baseband processors have been

designed, implemented, and ver ified. The transmitter and

receiver of the analog front-end have been designed, fabr icated,

and separately ver ified. The system demonstrator in laptop

form factor has been presented.

Index Terms Î Communications, Microelectronics,

Design, Test.

1 INTRODUCTION

We introduce a concept based on the multiple-input,

multiple-output (MIMO) communication and the signal

combining near the transmitting and receiving antennas in

the analog front-end [1]-[3]. Therefore, synergies between

front-end, and antenna array are described. Finally, the

integration and test aspects are discussed and the complete

system demonstrator is presented.

2 SYSTEM ARCHITECTURE MIMAX operates compatibly to the WLAN 802.11a and

amends some parts of this standard such as the physical and

data link layer. Figure 1 shows the communication layers

investigated and developed in MIMAX. The MIMAX

physical layer consists of a new analog front-end (AFE),

which performs weighting of multiple transmit/receive

signals by means of analog circuits. It is controlled by an

IEEE802.11a baseband which is extended for MIMAX

specific operation. Likewise, the medium access control

(MAC) ensures data and control flow between the new

baseband and the link layer control. Finally, the

compatibility to IEEE802 is achieved by a compliant link

layer control.

multiple paths can be exploited and several components can

be saved in comparison to the approaches used in the

WLAN standard 802.11n, WiMAX or LTE. Consequently,

more compact wireless radios can be used, that also results

in consuming less power. As a result of this paradigm shift,

customized integrated circuits and systems need to be

developed that comply with the new requirements in relation

to the front-end processing concept. Our RF-MIMO

MIMAX device

Wireless channel

802.11a device

approach called MIMAX (MIMO for MAXimum reliability

and performance) is compatible with existing WLAN

standards (IEEE802.11a) and able to achieve faster and

more energy efficient data transmission. The goal of

MIMAX is to develop a system demonstrator to test and

validate the MIMAX concept in real scenarios. The

demonstrator will consist of a communication link between

two laptops which are both using MIMAX devices. The

improvement in performance will be certified according to

the reliability of communication, achieved distance, and data

throughput.

The paper gives a brief overview of the MIMAX system

architecture. Then subsystems and individual components

such as the MAC processor, baseband processor, analog

* IHP, Im Technologiepark 25, 15236 Frankfurt (Oder), Germany (e-mail:

stamenko,tittelbach,kraemer@ihp-microelectronics.com).

** TTI Norte, Calle Albert Einstein 14, 39011 Santander, Spain (e-mail:

sruiz,ogago@ttinorte.es).

à University of Cantabria, Avda Los Castros s/n, 39005 Santander, Spain

(e-mail: jesus,nacho@gtas.dicom.unican.es).

àà Dresden University of Technology, Helmholtzstraße 18, 01062 Dresden,

Germany (e-mail: michael.wickert,ralf.eickhoff@tu-dresden.de).

Figure 1: Communication layers addressed in MIMAX

Adjustments of the layers are performed in such a way

that a MIMAX device can operate with other MIMAX and

IEEE802.11a devices simultaneously without affecting

neither the performance enhancements of the first nor the

standard performance of the second. Modifications in the

physical layer convergence protocol (PLCP) and MAC of

the MIMAX device have to be provided for ensuring

compatibility to the IEEE802.11a standard. The physical

medium dependent (PMD) sublayer is not affected by

amendments because each receive and transmit path is

designed for an IEEE802.11a link budget.

The system functionalities are integrated into separated

subsystems that are eventually assembled into a system

demonstrator for verification and testing. These subsystems

comprise the MAC processor, the baseband processor and

RF front-end control unit (RFCU), the analog transceiver

(separated in the transmitter and receiver chips), and the

antenna array [4]. The complete system architecture is

shown in Figure 2. The subsystems have been developed,

integrated and tested. Finally, their interaction is verified by

simulations, measurement, and field tests.

� � � � � � � �� � � �

978-612-45917-1-6 © 2011 IIITEC 14 ISIT2011

feedback to the baseband processing. These techniques are

based on look-up tables. The vector modulator is connected

to the RF control unit by a serial peripheral interface (SPI).

For the MIMAX data link layer, the standard IEEE802.2

link layer control is used on top of the IEEE802.11 medium

access control protocol. The new functionalities of the

MIMAX baseband processor impose some changes on the

MAC processor. It is necessary to know the configuration of

the transceiver including the number of antennas at

transmitter and receiver side and to maintain a database of

active and available users (MAC addresses, last optimum

weights, etc.). These tasks and the storage are related to

MAC because no memory is available at the baseband

processor. The MAC processor has to control the data flow and

specific instructions to the baseband processor depend-

ing on the communication scheme. It also initiates the MIMAX

channel estimation procedure, which normally consists

of a pair of training frames, exchanged between two stations.

Another important MIMAX feature is the transmission

of an RTS frame before each regular 802.11 frame. This is to

allow the receiver to identify the transmitter of the frame and

to set its receiver antenna configuration to the optimum weights

for that connection before receiving the regular frame. There is

a single RTS frame before each multicast or broadcast

frame, and a RTC / CTS pair before each unicast frame.

The MAC layer has been developed using the state

machine approach and the Specification and Description

Language (SDL). Functionally complex or rarely used parts of

the MAC protocol have been implemented in software. Time

critical or software inefficient functions are swapped to dedicated

hardware [10]. The MAC processor consists of a 32-bit RISC

processor and a hardware accelerator. The hardware

accelerator functionality for the transmit direction includes

a buffer for the next frame, cyclic redundancy check

(CRC) and an optional encryption. In the receive

direction, it performs CRC checking, frame address filtering,

generation of acknowledgements and CTS frames, and

optionally decryption. Furthermore, tracking channel state

(busy/idle) including back-off for sending frames, 6 timers

(32bit), a system timer (64 bit) and several interfaces are

provided as hardware modules. The interfaces include a 16-

bit interface to the physical layer (MIPP), a CardBus

interface to the host computer, a serial RS232 interface and

general purpose I/Os. A simplified architecture of the MAC

processor is shown in Figure 2.

3 COMPONENTS AND CIRCUITS The developed algorithms, components, and subsystems have been implemented in hardware (except the MAC pro-

cessor, which is a hardware/software co-design). The

MIMAX modem prototype assembles the following hardware:

" An antenna array made up of 4 multiband micro- strip-fed planar F-shaped monopoles,

" An analog front-end that uses vector modulators as an analog weighting circuitry,

" An OFDM baseband processor that supports the new analog front-end, and

" An enhanced MAC processor for MIMAX specific tasks and compatibility to 802.11a.

3.1 Antenna Array

The MIMAX antenna array consists of four multiband

radiating elements [17], [18] that provide dual band

performance (2.4 GHz and 5 GHz bands operations). The

considered form factor is determined by the laptop cover of

14.1Ñ in size. The multiband antenna is based on a micro-

strip-fed planar F-shaped monopole, printed on a FR4

substrate of thickness h=0.5 mm. The overall size of the

antenna is 35.9 mm x 11.17 mm. Two slots have been cut in

the upper part of the antenna to obtain the desired resonance

electric lengths without exceeding in size. In particular, the

lower branch of the F-shaped antenna controls the resonance

at 5.6 GHz, while the upper part determines the antenna

resonance at 2.4 GHz. Both branches are designed to gene-

rate a resonant path for currents, which results to be less than

a quarter of the free-space wavelength. It is important to

observe that there is the need to obtain two そ/4 resonant

paths at the two operating frequencies without exceeding in

the size. Therefore, a curl path was designed in the upper

part of the antenna, as shown in Figure 3. This solution

provides a control of the resonance at the lower frequency.

Consequently, this antenna can be used for multi-standards

such as 802.11a/b/g/n. The MIMAX antenna array has been

designed using four multiband radiating elements. In order

to keep low correlation between antennas, each radiator is

placed in orthogonal position between them. In this way,

polarization diversity is achieved [19].

Figure 3: Multiband antenna

3.2 Analog Front-End

The analog front-end is designed to support the transmitted

and received signal weighting in RF domain [14]-[16]. The

transceiver is fabricated in the 250 nm SiGe BiCMOS

technology [12]. The photos of the transmitter and receiver

RF integrated circuit (RFIC) chips are shown in Figure 4 and

Figure 5.

The incoming standard-conform signals from the

antennas have a signal power of each -82 to -30 dBm and are

filtered by a bandpass filter (BPF) and amplified by a low

noise amplifier (LNA) before they are eventually led into the

weighting module, namely a vector modulator. This module

alters phase and amplitude of the incoming RF signals in a

way so that, after adding them up, the summing signal has an

improved signal quality leading to higher data rates and/or

higher link quality. Because the MIMAX receiver was

intended to be fully backwards compatible to 802.11a and to

enable a fair comparison with existing WLAN receivers, it is

possible to switch off complete branches and let the front-

end operate as a single-input single-output (SISO) device.

Therefore, the following down-conversion path has to handle

an approximately 12 dB increased effective dynamic range.

978-612-45917-1-6 © 2011 IIITEC 16 ISIT2011

" VGA monitor output: to be connected to the standard

computer monitor for debugging purposes.

" Some components for future extension: user flash, RS

232 interface, temperature sensor, etc.

" Power supply, clock generator, flash storing the

Virtex FPGA configuration, etc.

Figure 6: Baseband board

3.4 MAC Processor

The MAC hardware platform [11] is based on two major

digital chips located on a printed-circuit board in CardBus

form factor (Figure 7):

" The MAC processor is an ASIC fabricated in IHP's

250 nm CMOS technology [12]. It provides a MIPS

4KEp 32-bit microprocessor core and the CardBus

(or PCMCIA) interface to a host PC.

" A Xilinx Virtex-5 LX50 FPGA implements the MAC

hardware accelerator and the MACÎPHY interface

(MIPP).

Figure 7: MAC board in CardBus form factor

The board contains many other components such as 4

MByte of processor flash memory and 1 MByte of RAM,

clock generator, power supply for FPGA, configuration flash

for FPGA, debugging and programming interfaces (UART /

RS232 for MAC firmware download, logic analyzer inter-

face), status LEDs, etc.

The MIMAX MAC protocol is fully compatible with the

IEEE 802.11a standard (5 GHz band). Although there are

some special MIMAX features such as training frames and

request-to-send (RTS) frames being transmitted before

multicast frames, these are compatible with the standard and

do not disturb normal WLAN traffic. Special MAC features

for coexistence with the IEEE 802.11n standard will be

considered. The features of the IEEE 802.11h standard such

as the dynamic frequency selection (DFS) and the transmit

power control (TPC) are included in the required extent.

4 SYSTEM TESTING

This section describes the subsystem integration and testing

of different components and circuits composing the final

MIMAX demonstrator: the antenna array, the analog front-

end, the baseband board with the integrated RF control unit,

and the MAC board. The components and subsystems of the

final MIMAX demonstrator have been tested and their

functionalities have been verified.

4.1 Analog Front-End Testing

The integrated analog front-end chips providing the weight

adjustment properties required for the MIMAX principle are

subdivided into the transmitter and receiver blocks. The

main part of each block is an RFIC chip. The receiver chip

has been assembled on a test board which is optimized for

measurement purposes (Figure 8).

Figure 8: Chip measurements on a printed-circuit board

Figure 9: Weighting characteristic of the 4-antenna-Rx-chain

The measurement results offer the possibility to verify

the weighting characteristics after the individual branches.

The measurements have been compared with the combined

constellation diagrams. After down-conversion, one can

evaluate the signals obtained at I and Q outputs of the mixer

and compare those weighting performance with the RF

curves. Figure 9 shows the weighting behavior after down-

conversion for all 4 branches of one MIMAX receiver at 1

MHz signal frequency. The limited measurement equipment

978-612-45917-1-6 © 2011 IIITEC 17 ISIT2011

did not allow a correct evaluation of phase and amplitude in

close proximity around zero. That is why the X and Y axis

are surrounded by an area with no data points. This

measurement uses a 6-bit stepping in the real and imaginary

direction, which can be extended up to 8-bit. It is visible that

a reasonable square shaped area can be accessed by each

antenna with a good homogeneity. The different orientation

angle comes from the different physical path lengths in

receiver. Those effects are transferred into the channel

transfer function and are compensated by the baseband

algorithms.

For the final integration in the MIMAX system, a

customized printed-circuit board containing those chips has

been developed. The highest integration level is offered by

the final demonstrator which employs both developed

integrated front-ends (Tx and Rx). Both RFIC chips are

assembled as single chips on the demonstrator board. The

separated chip solution is favored to prevent strong coupling

between the Tx and the Rx paths. Because of the high Rx

sensitivity, a good isolation between both branches is

required that can be achieved with less effort by using two

chips instead of a single chip solution. Besides RFIC chips,

the supplementary components to initialize the board and

control the data flow between the analog front-end and the

digital baseband are assembled.

4.2 Baseband Testing

For testing the MIMAX baseband board and its processor,

we have built a test setup consisting of two stations.

Bypassing the analog front-ends and the wireless channel,

both baseband boards are connected by a cross-cable,

connecting each DAC output to the respective ADC input on

the other board. Hence, this scenario uses a perfect wired

channel.

With this test setup, we have verified the correct reading,

changing, and re-reading of a few configuration parameters.

Afterwards, we sent beacons, data and training frames and

checked the display at the receiving station. For short frames

(< 50 bytes), the transfer was successful even for the 64-

QAM modulation. For longer frames, minor problems

occurred, which are probably due to inadequate timing

(insufficient speed) of the USB interface. We also expect

robustness and stability issues for larger amounts of data.

Figure 10: Calculated optimal weights

In order to check the real-time behavior of the specific

MIMAX blocks, we have used a different setup consisting of

a baseband board in reception mode and a vector signal

generator providing the I and Q baseband signals of training

frames affected by different known 4x4 MIMO channel. In

an intensive test procedure we have used different flat and

also frequency-selective MIMO channels. In all cases, a very

good agreement between the optimal weights calculated by

the FPGA and those obtained in simulation was observed.

Figure 10 shows an example of the calculated weights on the

baseband board compared to those provided by simulation.

4.3 MAC Testing

The IEEE 802.11a protocol and the modifications due to

MIMAX were implemented by using the Specification and

Description Language (SDL). The SDL model was

simulated thoroughly to verify the functional correctness of the

MAC design including hardware-software partitioning. Then,

the real-time MAC implementation was derived by gener-

ating the MAC firmware by automatic code conversion from

SDL to C code and compiling it for the selected hardware

platform. The hardware accelerator was designed in VHDL,

tested in VHDL simulations, and finally ported to the FPGA.

For MAC testing under real-time conditions and

evaluating the performance of the MAC implementation

including host drivers, we have developed a special test

setup called "Link Emulator". This is a small FPGA board,

which provides communication between up to three MAC

stations via a wired (emulated) PHY link. The

Link Emulator is designed in VHDL. It implements the

exact PHY frame timing including collision modeling,

supports frame transfer on two independent (emulated)

radio channels, and provides a lot of debug cap-

abilities. The interface to the MAC board is the original

MAC-PHY (or MIPP) interface. Later, the PHY emulator

can be replaced with the real PHY / baseband processor

without any change in the MAC layer or driver software.

To run a test, the MAC firmware was downloaded via a

MIPS UART port to the MAC processorÓs flash memory,

and the VHDL design of hardware accelerator was

downloaded via a JTAG port to the Xilinx flashes. Two

MAC boards were attached to laptop computers via their

CardBus interfaces. On both PCs, the drivers and a terminal

program were started. These have been specifically designed

for integrating the MIMAX WLAN module as network

device into the TCP/IP stack of the computer's operating

system (Linux or Windows). The terminal program allows to

interactively send any command to the WLAN module and to

display any incoming response. Some useful commands are:

" Scanning for available WLAN cells,

" Starting a new WLAN cell,

" Joining an existing WLAN cell,

" Authentication / Deauthentication,

" Association / Disassociation,

" Reading or setting MAC and PHY parameters,

" Sending and receiving single test data frames, and

" Sending and receiving MIMAX training frames.

978-612-45917-1-6 © 2011 IIITEC 18 ISIT2011

When a WLAN connection has been set up, it is possible

to activate a TAP device to integrate the WLAN link as

network connection into the operating system's network

stack. Then, the remote PC can be reached with all normal

network commands like ping, sftp, ssh, etc. Our standard test

application was the transfer of a video stream from one PC

to the other.

4.4 Demonstrator Testing

A laptop demonstrator for trying out the complete MIMAX

system (antenna array, analog front-end board, baseband

board, MAC board, and WLAN driver) has finally been

assembled. The purpose is to debug, improve, and verify the

system in real-time conditions and to prove its better

performance (as well as, the compatibility) in comparison

with the standard 802.11a WLAN. The baseband and analog

front-end boards have been mounted into the housing which

was developed to make the MIMAX demonstrator laptop-

compact and small. A photo of the system test setup is

shown in Figure 11.

Figure 11: Photo of the demonstrator test setup

The monitor displays a good example of the QPSK

constellation diagram. Next to the antenna, the front-end and

baseband boards mounted into the black MIMAX housing

are visible. In the foreground right is the MAC board. Right

on the screen, the MIMAX traffic monitor can be seen. The

lower bar with many blue stripes shows the transmitted

frames and the upper bar with short green stripes shows the

received frames. Single red lines in between indicate the

training frames. The orange and yellow bars on top visualize

the weight settings (I and Q components) for three antennas.

This setup has been used in two configurations: one made of

two MIMAX stations to evaluate a pure MIMAX WLAN

with the MIMAX specific features (in particular, handling of

training frames), and the other made of a MIMAX station

and a commercial 5 GHz WLAN access point to verify the

MIMAX compatibility with the standard 802.11a WLAN. In

second configuration, the MIMAX features (RTS/CTS and

training frames) are disabled since the access point cannot

correctly handle them.

5 CONCLUSION

MIMAX technology integrates the concept for short range

communication, in particular for an IEEE802.11a wireless

local area network, introducing MIMO communication

modes but still offering compatibility to legacy devices. The

project has developed a system prototype for a proof-of-

concept, testing and performance evaluation. The prototype

contains the customised hardware and software systems

which enable MIMO modes and ensure compatibility.

System architecture, design, implementation, integration

and verification are presented. The MIMAX technology is

evaluated with respect to the competing wireless radio

technologies such as SISO systems. The improvement in

performance is shown in terms of the user show-cases

(transmission of video streams).

Acknowledgments

The research leading to these results has received funding

from the European CommunityÓs Seventh Framework

Programme FP7 (2007 - 2013) under the grant agreement

no. 213952 also referred as MIMAX.

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