Cypress CY62167EV18 Manual de usuario descargar pdf (Pagina 20)

US

8,121,078

B2

19

The

wireless

camera

can

include

a

high-bandwidth

radio

transceiver,

which

can

operate

under

a

steady

state

commu

nication

condition.

For

example,

the

wireless

camera

media

access

control

(MAC)

for

the

high-bandwidth

radio

can

be

programmed

to

setup/tear

down

connections

as

determined

by

the

Transmission

Store/Control

Block. This

allows

the

high-bandwidth

bulk

data

transmission

radio

to

power

down

completely

for

extended

periods

of

time.

When

the radio

is

switched

on

it

can be

instantly

assumed

to

be

logically

linked

with

the

base

station.

A

primitive

MAC

layer

can

be

used,

but

this

may

not

be

the

preferred

imple

mentation.

Thus,

the radio

can

avoid

the

usual

discovery

period,

and

advance

to

the

authentication

request

and

reply,

followed

by

the

associated

request

and

reply

messages

in

a

three-way

handshaking

process.

This

differs

from

the

regular

beacon

behavior

of

802.11

when

operating

in

a

rendezvous

mode.

Discovery

sequences

can

be

suppressed

except

during

initialization/

installation

conditions.

A

very

light

OS

can

run

on

the

wireless

camera

to

bring

up

the

MAC

with

the

minimal

con?guration.

This

can

reduce

the

need

for

the

power

and

time

consuming

mechanisms

associated

with

current

wireless

link

technologies.

In

certain

implementations,

the

MAC

layer

can

almost

be

entirely

eliminated

from

the

camera

and

a

rudimentary

slave

response

can

be

implemented

which

responds

to

control

signals

received

from

a

secondary,

low

power,

low-bandwidth

radio

channel.

The

algorithm

for

the

burst

transmission

processing

is

a

timing

loop

where

data

is

transmitted

based

on

the

data

rate

used

and

the

available

channel

characteristics.

A

calculation

is

done

to

determine

the

optimum

timing

for

the

burst

trans

mission

and

the

system

is

then

set

up

to

match

this

as

closely

as

possible.

During

non-transmission

periods

the

high-band

width

radio

can

be

completely

powered

down.

This

can

be

different

from

“doze”

or

“standby”

modes

often

provided

by

commercial

integrated

circuits.

These

modes

often

dissipate

energy

at

levels

that

can

defeat the

possibility

of

extremely

long

term

battery

life.

During

this

non

transmission

time

the

high-bandwidth

radio

can

use

less

than

tens

of micro

watts

of

power.

The

timing

to

transmit

for

the

burst

transmission

is

based

on

the

following

parameters:

Average

Maximum

Channel

Bandwidth

is

represented

by

Bm

in

M

bits

per

second

(Mbps).

Channel

bandwidth

is

the

average

bandwidth

that

can

be

achieved

by

the

high-bandwidth

link.

Average

sustained

Data

Rate

is

represented

by Bs

in

Mbps, which

is

the data

rate

of

captured

audio/video

data.

The

higher

the

rate,

the

better

the

?delity

and

frame

rate

of

the

transmitted

information.

FIG.

4

is

a

diagram

showing

the

burst

data

transmission,

according

to

some

implementations.

To

take

advantage

of

the

fact

that

the

sustained

data

rate

Bs

is

much

smaller

than

the

capability

of

the

bulk

radio;

the

transmission

will

be

on

for

a

brief

period

of

time

to

burst

the

data.

This period

can

be

designated

by

Tx

(sec),

and

the

time

period

between

bursts

can

be

represented

by Tc

(sec).

Hence

TcIBs

Referring

to

the

bottom

of

FIG.

4,

there

can

be

a

time

associated

with

setting

up

the

link

and

terminating

the

link.

For

example,

the

time

to

set

up

link

is

represented

by

Tsu

(see),

and

the

time

to

tear

down

link

is

represented

by

Ttd

(sec).

Therefore

the

aggregate

time

to

set-up

and

tear

down

link

Tw:Tsu+Ttd

(sec).

To

obtain

maximum

power

saving

ef?ciency

on

the

bulk,

high-bandwidth

radio,

ideally

the

ratio

of

the

transmit

time

Tx

to

power

down

time

should

be

equal

to

the

ratio

between

Bs

and

Bm.

20

25

30

35

40

45

50

55

60

65

20

During

the

Tx

period,

the

power

drawn

by

the

high-band

width

radio

can

be

very

high

relative

to

the

power

down

periods.

For

example,

the

wireless

camera

in

the

802.1

in

transmitter

which

is

operating

using

diversity

or

multiple

transmitters

can

use

between

100

mW

to

1.5

W

during

the

Tx

period

instead

of

a

few

hundred

microwatts

in

other

periods.

This

level

of

power

consumption

during

the

transmission

of

data

can

be

a

distinguishing

feature

of

this

system

compared

to

existing

low

power

remote

sensor

systems.

In

the

image

transmission

operation,

various

battery

oper

ated

camera

systems

which

transmit

data

intermittently,

can

have

a

transmitter-off

to

transmitter-on

ratio

of 10

or

less.

As

such,

the

transmitter

in

these

wireless

camera

systems

is

on

mo

st

of

the

time.

In

contrast,

of

the

transmitter

in

the

present

systems

can be

designed

to

have

a

high-bandwidth

radio

for

transmission

and

such

a

high-bandwidth-ratio

transmitter

is

on

only

for

a

short

period

of

time.

In

this

manner,

the

burst

transmission

of

the current

wireless

cameras

systems

can

have

a

transmitter-off

to

transmitter-on

ratio

of

much

greater

than

10

and

thus

provide

signi?cant

saving

in

power

con

sumption.

However,

the

system

timing

needs

to

take

into

account

the

“wasted”

time

necessary

to

setup

and

tear

down

the

link

during

which

the radio

is

active,

which

is

Tw.

In

order

to

approach

the

ideal

ef?ciency,

period

Tw

needs

to

be

amor

tized

across

a

relatively

long

period

of

active

data

transmis

sion

time

(Tx).

This

means

that

the

time in-between

bursting

the

radio,

as

represented

by

Tc,

can be

extended

as

Tw

increases

to

maintain

the

same

ef?ciency

level.

Hence

the

ef?ciency

(E,

in

percentage)

can

be

determined

by

E

=

—

-100%

(Tx+

Tw)

Given

the

above,

the

average

optimum

time

between

trans

mission

of

the

burst

of

audio/video

(Tc)

data

for

a

given

ef?ciency

E,

can

be

determined

as

follows:

Bm E

The

following

example

can

better

illustrate

the

equation

above:

Tw:3

ms

(highly

optimized

system)

Bin:54

M

bits/

sec

(ideal

802.11g

data

rate)

Bs:192 k

bits/sec

(5

frames/

sec

with

0.5

bits/pixel

at

320x

240,

no

audio)

Then

the best

cycle

time

to

set-up

and

burst

transmission

is,

Tc:2.53

seconds.

System

latency

(or lag)

can be

greater

than

or

equal

to

Tc.

If

latency

is

too

high

an

unacceptable

lag

can

occur

between

the

capturing

of

audio/video

information

to

its

availability

to

serve

a

surveillance

application.

To

reduce

latency

without

negatively

impacting

energy

consumption,

signi?cant

opti

mizations

need

be

made

to

the

MAC

behavior

in

order

to

reduce

Tw.

In

order

to

reduce

time

period

Tw

during

steady

state

conditions

(i.e.

not

during

discovery

or

initialization

states)

certain

modi?cations

can

be

made.

For

example,

a

modi?cation

to

the

regular

beacon

behavior

of

802.11

can

be

made.

When

the

high-bandwidth

radio

is

switched

on

for

transmission,

it

can

be

assumed

to

be

synchronized

with

the

base

station.

Thus,

the

usual

discovery

period

can

be

avoided

and

the

high-bandwidth

radio

can

advance

immediately

to

the

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