WLAN Interface for a Wireless EEG System

E. Velarde-Reyes and F. Martin-Gonzalez

Cuban Neuroscience Center, Havana, Cuba

Keywords: WLAN, Interface, Wireless, EEG, Holter.

Abstract: A WLAN interface for a Wireless EEG System is presented in this paper. Selection of broadcasting band,

available hardware, and connection algorithm to use are discussed before making a choice. Two alternatives

were explored: Wireless EEG Device (Holter) and its Server communicate with each other within the same

physical network, and from a complex network like the Internet. Results of experimental tests carried out on

the prototype demonstrate the functionality of the implemented interface.

1 INTRODUCTION

Electroencephalography (EEG) studies are

increasingly important in numerous applications,

from clinical diagnosis of different brain pathologies

to research on cognitive processes and to the

development of brain-computer interfaces and

neurofeedback. In some of these applications long-

lasting recordings are required and desk

electroencephalographs with personal computer

(PC) wired interfaces as USB, are not the best

solution since they restrict patient movements. Such

is the case of study and diagnosis of epilepsy whose

studies can last up to three continuous days. This

has brought attention to the need of developing

wireless interfaces in order to add telemetry

capability to EEG recorders.

Other requirement for EEG long-lasting recordings

is the portability of the recorder. Portable devices

with telemetry capability and possibility to record

medical data in an ambulatory way may receive the

generic name of Holter monitors. Holter monitor,

the PC where the doctors process EEG studies

(Server) and the possible communications

infrastructure conform a Wireless EEG System

(WES).

Moreover, WLAN is the protocol commonly used in

PC-based wireless networks. Some hospitals use

this telecommunication technology in automation of

processes. Doctors can immediately access medical

records and patients’ special medication by means of

this technology (Goldman, 2008). If our WES

supports WLAN, we can guarantee monitoring of

patients at all times and in all places in the hospital,

by adapting into existing WLANs or installing new

ones. This makes WLAN a very convenient option.

The aim of this paper is to describe the design and

implementation of a WLAN interface for a Wireless

EEG System.

2 DESIGNING THE WLAN

INTERFACE

2.1 Broadcasting Band

In order to ensure its compatibility with installed

systems, broadcasting band of our WES must be

recognized as a license-free band in the majority of

countries around the world. Therefore, we selected

the frequency band between 2400–2483.5 MHz.

This band is frequently used by numerous devices

such as microwave ovens, wireless phones, RFID

units, and wireless local area networks (WLANs),

causing a potential source of strong interference

between devices within a domestic or hospital

environment. There exist various techniques for

decreasing the interference impact. The advantages

of the selected broadcasting band include the short

length of antennas, the existence of relatively cheap

radio frequency transceivers and certified modules

that work in the band, the universal acceptance of

the band, and the better propagation characteristics

over other world-wide accepted bands such as 5727–

5875 MHz.

Velarde-Reyes E. and Martin-Gonzalez F..

WLAN Interface for a Wireless EEG System.

DOI: 10.5220/0005067900890093

In Proceedings of the 2nd International Congress on Neurotechnology, Electronics and Informatics (NEUROTECHNIX-2014), pages 89-93

ISBN: 978-989-758-056-7

 2014 SCITEPRESS (Science and Technology Publications, Lda.)

2.2 Bit Rate Demands of the WES

In EEG recording is recommended a sampling rate

of at least 200 Hz and a resolution of 12 bits. Even

so, higher sampling rates and resolutions are

preferable.

An EEG Holter monitor which could record data of

40 channels in simultaneous mode using a sampling

rate of 400 Hz and coding the samples with 24 bits

needs a data throughput of 384 kbps. That data

throughput is achievable in any WLAN network as

discussed later on.

2.3 WLAN Standard

One of the fundamental advantages of WLAN over

other standards is the ability to connect to a local

area network (LAN) using a wireless Access Point

(AP), which allows patient monitoring in a most

extense area and the development of future

applications of telemedicine.

WLAN or IEEE 802.11 is a family of specifications.

Among them, IEEE 802.11b and IEEE 802.11g are

the most ubiquitous, and is very convenient that our

WLAN interface supports them both. Both

specifications of the WLAN work in the 2400–

2483.5 MHz band and implement modulation

techniques against interferences, as DSSS, FHSS

and OFDM (Proakis, 2008). Also, WLAN allows

retransmission in case of occurring errors in

transmission or reception and supports throughputs

from 1 to 54 Mbps in the case of IEEE 802.11b and

IEEE 802.11g specifications.

2.4 WLAN Interface Implementation

Our WLAN Interface must be integrated, in its more

simplified form by: one antenna for the transmission

and reception of RF signals, one RF transceiver and

one base band modulator with an 802.11 MAC

Layer implementation.

In the short term, among the options to develop a

WLAN Interface using integrated circuits and to

acquire a certified WLAN integrated module, the

most economic one is the second. There exist

WLAN integrated modules having a

firmware/hardware implementation of a TCP/IP

(transmission control protocol/internet protocol)

stack, that results essential for an adequate

communication into a LAN network.

In summary it is very important that the selected

WLAN integrated module has the following

characteristics: low power consumption (Holter

monitor necessarily works with battery) and a proper

implementation of a TCP/IP stack.

3 WES MERGED IN THE

COMMUNICATIONS

INFRASTRUCTURE

Our Wireless EEG System could face two situations

using WLAN standard:

1. The Server is in the same physical network.

2. The Server is in an infrastructure network.

Our WES must be authenticated and associated in a

WLAN for transmitting data, it is an unavoidable

requirement. The WLAN can be an infrastructure or

ad hoc network. In the first case, authentication and

association processes are executed by Holter

monitor and an AP, while in the second case the

processes are executed by Holter monitor and a

WLAN module embedded in a PC. Authentication

and association are indispensable conditions for a

successful data interchange, but it is also necessary

to know the destination and source internet protocol

(IP) addresses and to possess a suitable TCP/IP

stack. Moreover, source and destination devices

must implement a network layer protocol for IP

address dynamic assignation (e.g., Dynamic Host

Configuration Protocol, DHCP). Since WLAN is a

network-oriented standard, it would be convenient

for our WES to include some protocols of the

transport layer (e.g., TCP and UDP). There are

some IEEE 802.11 embedded modules that, besides

physically implementing the communication,

incorporate a complete stack of TCP/IP protocols.

In accordance with the type of situation, our

Wireless EEG System must employ a different

technique or algorithm in order to connect to its

server.

3.1 The Server Is in the Same Physical

Network

In the case that Server is in the same physical

network of the Holter monitor (Client from now on),

it is appropriate that they implement a connection

algorithm that resolves the Server’s IP address and

enables medical data transmission through a

transport layer protocol. This algorithm must ensure

connectivity and good performance in any network

topology. It is convenient that it be simple, with low

program processing times, and therefore easy for

programmable devices to implement.

NEUROTECHNIX2014-InternationalCongressonNeurotechnology,ElectronicsandInformatics

We propose a new version of algorithm presented in

Velarde-Reyes et al (2008). This algorithm (Figure

1) ensures a fast and reliable data interchange and

has four stages: annunciation, acceptation, TCP

connection establishment and reconnection. In the

annunciation stage, the Client must begin the

communication as soon as it has been authenticated

and associated in a WLAN. Communication is

initiated by the transmission of a command sequence

that travels within UDP datagrams addressed to the

broadcast address and to a specific registered port.

Commands consist in annunciation’s indicatives that

will reach all hosts inside the same physical

network. Inside annunciation’s indicatives are the

Client IP address and the registered port opened by

the Client. Annunciation’s indicatives are

repetitively transmitted until the Client receives the

Server’s acceptation commands. The application

running on the Server opens its specific registered

port and receives the annunciation’s indicatives. As

soon as the Server obtains that information, it

permits data transmission to the Client. This new

stage is known as acceptation, and it is executed by

the Server in automatic or manual (by the Server’s

user) form. The Server can simultaneously allow

various Clients to pass to the next stage:

Acceptation.

Acceptation is accomplished by sending commands

to the Client IP address. These commands travel in

UDP datagrams and consist in acceptation’s

indicatives that contain the Server IP address and the

TCP port opened for the next connection.

After the acceptation stage is TCP connection

establishment. In this stage, the Server opens the

TCP registered port indicated in the acceptation

command. Then it starts a passive opening and

waits for a TCP connection establishment. After the

TCP connection establishment stage, the Server send

to Client a TCP Connection Established Indicator

(“Go”) every 3 seconds announcing that it is

prepared to receive all the data sent by the Client.

Client must response to that indicator with a “Go”

ack in case that it is not transmitting medical data.

If Client could not receive “Go” Indicators, it will

initiate the reconnection stage, establishing again a

TCP Connection with previous Server. Also, if

Server could not receive data or “Go” acks, it will

initiate too the reconnection stage, waiting for a TCP

connection establishment by its open TCP port.

3.2 The Server Is in an Infrastructure

Network

In this section we named Infrastructure Network to

the networks that connect different network devices

which are located in different physical networks. In

this case, data travels through different network

nodes (switches, routers, bridges, or others). A

group of interconnected LANs and WANs (wide

area networks) that conforms to the Internet

infrastructure are examples of an Infrastructure

Network.

Our Client could not connect to its Server in this

kind of network using the algorithm proposed in the

previous section. The difficulty is that the broadcast

UDP messages cannot travel out of its physical

networks because most routers in their default state

do not have that configuration. Therefore, it is

indispensable to use a new connection mode in order

to establish a TCP connection between our Client

and its Server when they are in different physical

networks.

The new proposed connection mode is by means of

the use of the Domain Name System (DNS) protocol

services. DNS is a mechanism that implements a

machine name hierarchy for computers, services, or

any resource connected to any network. It associates

several information with domain names assigned to

each of the resources in the network. Its most

important function is to translate domain names

meaningfully to humans into the numerical

addresses associated with networking equipment (or

any resource) for the purpose of locating and

addressing these devices worldwide.

DNS uses a hierarchical naming scheme known as

domain names. A domain name consists of a

sequence of subnames separated by a delimiter

character, the period. Thus, the domain name

electron.cneuro.edu contains three subnames:

electron, cneuro, and edu. Any subname in a domain

name is also called a domain. In the above example

the lowest level domain is electron.cneuro.edu, (the

domain name for the Electronic Design Department

at the Cuban Neuroscience Center), the second level

domain is cneuro.edu (the domain name for the

Cuban Neuroscience Center), and the top level

domain is edu (the domain name for educational

institutions). DNS protocol makes it possible to

assign domain names to users or groups (e.g.,

Internet users) in a meaningful way, independent of

each user’s physical location. Because of this,

Internet contact information can remain consistent

and constant even if the current Internet routing

arrangements change.

Using the DNS protocol, our Client can establish a

TCP connection with its Server if the Server has a

domain name (e.g., epilepticserver.electron.edu.cu).

It will not matter if the Server IP address is unknown

WLANInterfaceforaWirelessEEGSystem

Figure 1: Connection Algorithm.

to it. To use the DNS protocol our Client must

implement a complete stack of TCP/IP protocols.

4 INTERFACE VALIDATION

For validating the wireless communication interface

we built a prototype integrated by an OWS451

module from ConnectBlue and a microcontroller.

The broadcasting power selected for the module was

+17 dBm. The microcontroller was used for

executing the tasks assigned to the Client in the

connection algorithm. A PC was used as Server.

Two experiments were selected for modelling the

two principal scenarios that could face a WES in its

same physical network. They were:

 Experiment 1 consisted of broadcasting data

inside a room from a fixed position using

multipath trajectories. This situation modelled

a hospital room scenario due to the geometry of

the room and the materials of the walls and

furniture (Schäfer et al, 2005).

 Experiment 2 consisted of broadcasting data

inside a corridor using line of sight (LOS)

trajectories. The corridor selected is similar to

other modern hospital corridors, with similar

dimensions and materials (Schäfer et al, 2005).

In both experiments the Packet Error Rate (PER)

was measured and the faults in the connection

algorithm were counted. In Velarde-Reyes et al

(2008) are depicted in detail the two experiments.

4.1 Considerations about the

Experimental Results

The experimental results show that the proposed

connection algorithm guarantees the WES

communication and it works satisfactorily in multi-

path and LOS trajectories.

5 CONCLUSIONS

The main result of this work was the description of

the design and implementation of a WLAN interface

for a Wireless EEG System.

NEUROTECHNIX2014-InternationalCongressonNeurotechnology,ElectronicsandInformatics

REFERENCES

Goldman, J., 2008. Wireless Hospital: Orlando Regional

Healthcare. In Wi-Fi Planet, March 13, 2008, accessed

30 April 2014, <http://www.wi-fiplanet.com/columns/

article.php/3734011> .

Proakis, J., 2008. Digital Communications, Mc Graw -

Hill. New York, 5

edition.

Velarde-Reyes, E., Marante-Rizo, F., Morgalo-Santos, B,

Garrote-Jorge, J., and Martin-Gonzalez, F., 2010.

Wireless communication interface for EEG/PSG

Holter monitor. In Journal of Medical Engineering &

Technology, vol. 34, pp. 172-177, April 2010.

Schäfer, T., Maurer, J., Hagen, J., and Wiesbeck, W..

2005. Experimental Characterization of Radio Wave

Propagation in Hospitals. In IEEE Transactions on

Electromagnetic Compatibility, vol. 47, pp. 304-311,

May 2005.

WLANInterfaceforaWirelessEEGSystem