Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR OPTICAL PROCESSING
. 5 OF A RADIO FREQUENCY (RF) SIGNAL
Field of the Invention
The present invention relates, in general, to
wireless communication systems and, more particularly,
to a method and apparatus for optical processing of a
radio frequency signal.
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Background of the Invention
Wireless communication systems, particularly
'- cellular systems, typically consist of a plurality of
base sites. Each base site will contain, inter alia, RF
receivers, RF transmitters, Base Site Controllers ~BSC),
scanning receivers, and voice channel controllers. :
; Because of the size of this equipment, it is difficult,
and expensive, to find suitable locations. As the
demand for cellular use increases, it is envisioned that
the cell sizes will decrease to satisfy the added
demand. This increase in demand will continue to add to
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the equipment loci~tion problem.
One solution to this problem is~to place the
minimal amount of equipment at the antenna site and l~
route the signals to a central location. However, in
many cases, before any benefit can be derived, the base
site must be moved a substantial distance from the cell
~- (antenna) site. This can cause a degradation in service
due to the signal loss~in transmission.
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'One solution to this problem is to use analog
fiberoptic links between the base site and the cell
sites. This is suggested in U.S. Patent No. 4,916,460
(April 10, 1990) issued to Alan J. Powell. However, the
use of an analog laser limits the attainable
InterModulation (IM) and the Signal-to-Noise (S/N)
ratio. In addition, analog lasers are relatively
expensive.
Therefore, there exists in the industry a need for
a more economical solution to the problem of quickly and
accurately transferring RF signals between cell sites
and base sites.
Summary of the Invention
An apparatus is provided for optical processing of
a Radio Frequency (RF) signal. The apparatus for
processing a received RF signal consists of a filter for
; anti-aliasing the received RF signal. After filtering,
an Analog-to-Digital (A/D) converter converts the
received RF signal to a digital signal. The digital
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signal is then converted, in an optical converter, to an
optical signal.
An apparatus for optical processing of a received
;25 optical, digital signal to an RF signal consists of a ;~
converter for converting the optical, digital signal to
an electrical, digital signal. The electrical, digital
signal is then converted to an RF signal in a Digital-
to-Analog converter (D/A). The RF signal is then l~
filtered.
' A method for optical processing of an RF signal
iconsists of anti-aliasing the RF signal. The signal is
then converted from an analog signal to a digital signal
,'and then converted to an optical, digital signal.
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A method for optical processing of an RF signal
also consists of converting a received optical, digital
signal to a electrical, digital signal. The electrical,
digital signal is then converted to an RF signal which
is then filtered.
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Brief Description of the Drawings
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FIG. 1 is a block diagram of a cell site embodying
the present invention;
FIG. 2 is a block diagram of a base site embodying
the present invention;
EIG. 3 is a flow chart illustrating a process
embodying the present invention for converting an RF
signal to an optical signali and
FIG. 4 is a flow chart illustrating a process
embodying the present invention for converting an
optical signal to an RF signal.
Detailed Description of the Drawings
Referring initially to FIG. 1, a block diagram of a
cell (antenna) site, generally designated 10, embodying
; the present invention is illustrated wherein blocks
shown in dashed form represent optional functions. Cell
site 10, on the transmit side, consists essentially of a
converter 11, a digital-to-analog (D/A) converter 12,
and a filter 13. In operation, an optical, digital
signal is received from a base site, FIG. 2, at l~
converter 11. Converter 11 is a photo detector or the
like and converts the optical signal into a electrical,
~ digital signal.
The electrical, digital signal is then forwarded to
D/A converter 12 where it is converted to an analog
signal. The analog signal is filtered by filter 13 to
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remove any spurious signals generated in the converting
processes. The filtered signal is transmitted to a
duplexer 14. Duplexer 14 is provided since the same
antenna, antenna 15, is used to transmit and receive RF
signals. The signal received form filter 13 is then
transmitted out antenna 15.
. In addition to the basic elements, the transmission
side of the cell site may contain one or more of the
' following devices. An amplifier 16 is provided to
amplify the electrical, digital signal before performing
the D/A conversion on the signal. A synchronizer 17 may
be used to obtain a timing signal from the incoming data
stream. Since it is important to maintain
' synchronization with the base site, this is a convenient
way to transfer timing information. The timing signal
from synchronizer 17 is then used to drive a clock 18.
' Clock 18, in turn, drives a synthesizer 19.
,. The local oscillator output from synthesizer 19 is
provided to a mixer 20. Mixer 20 is required if the
i'; 20 analog signal being processed is processed at base band
rather than its transmission frequency. Mixer 20 is
provided to up convert the analog signal to its
transmission frequency.
Prior to being transmitted to duplexer 14, the ~;
25 analog signal can be amplified using linear power -
amplifier 21.
On the receive side of cell site 10, an RF signal
.~ is received by antenna 15 and passed through duplexer 14
-- to a filter 24. Filter 24 is an anti-aliasing type of
~'- 30 filter used to prevent an image of the desired signal
from being mixed down into the desired signal.
The filtered signal is then converted to an
electrical, digital signal in analog-to-digital (A/D)
converter 25. The output of A/D converter 25 is
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converted to an optical, digital signal in converter 26.Converter 26 is typically a digital laser diode.
By converting the signal to a digital signal, a
digital laser can be utilized to transmit the optical
signal. In addition to being more economical than
analog lasers, using a digital laser provides improved
signal-to-noise (S/N) ratios than is presently
attainable.
' As with the transmit side, there are also
additional devices which may be provided on the receive
side. A preselector 27 can be used at the output from
duplexer 14 to filter any interfering signals. A mixer
28 may be used to down convert the signal to a base
band. Mixer 28 also has an input to receive a local
oscillator generated by a synthesizer 29 which is
clocked by the signal from clock 18.
In addition, a frame synchronizer 30 may be
provided at the output of A/D converter 25 to maintain
the proper timing of the output bit stream. The digital
,~ 20 bit stream may then be amplified, in amplifier 31,
before entering converter 26. The necessity of
!, amplifier 31 would depend upon the input signal
requirements of convertex 26. -
In FIG. 2, a base site, generally designated 40, is
illustrated which couples the cellular system to the
public switched telephone network tPSTN) 41. Upon
receipt of an optical signal from cell site 10, the
optical slgnal is converted into an electrical, digital
signal in converter 42. This digital signal is then
converted to an analog signal in a D/A convertex 43.
The analog signal is filtered in filter 44 to remove any
spurious signals. The filtered RP signal is then
i provided to base station 45.
Optionally, the electrical digital signal from
converter 42 may be amplified in amplifier 46. The
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digital signal may also be synchronized in synchronizer
47 before being converted to an analog signal. An
optional mixer 48 is also shown receiving a reference
input from receive reference 49. Mixer 48 serves to up
convert the received signal to the receive band required
by base station 45.
When a signal is received by base station 45 from
PSTN 41, it is first filtered in anti-aliasing filter
51. The filtered signal is then converted to a digital
signal in A/D converter 52. The digital signal is then
converted to an optical signal in a converter 53. The
resulting optical, digital signal is transmitted to cell
site 10.
Optionally, if the incoming signal is not received
at base band, mixer 54 is provided with transmit
reference 55 to down convert the RF signal from the base
station transmit band prior to filtering. A frame
synchronizer 56 may also be used to synchronize the
transmission of the digital bits. Finally, an amplifier
57 is provided if the requirements of converter 57 so
require.
A particular application of the invention can be
illustrated using present day cellular systems. In the
United States, 25 MegaHertz (MHz) of channels are used ;~
around the spectrum at 880 MHz. These signals will
; typically be mixed down to a baseband of 0 Hz through 25
MHz. The signal is sampled at a rate of at least 50 MHz
and converted to a digital bit stream through the A~D
converter.
. 30 A practical issue in digitizing the RF signal is
how many bits are required to adequately represent the
- RF signal. If the system requirements are set to 60 dB
(deciBel) spurious response, then 10 bits would normally
.~ be required. However, in digitizing the RF spectrum,
quantizing noise is produced. W1th a single channel
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-width of 30 kHz (kiloHertz) and the quantizing noise
spread across the entire 25 MHz spectrum, the noise is
reduced by -29 dB, or about 5 bits, as illustrated in
equation ~1).
10 log (30k/25M) = -29 dB, or about 5 bits (1)
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Because of the bandwidth reduction, this noise is less
than what would normally be expected. Another factor to
be considered is the degradation caused by processing
more than one signal. Since there are several channels
in each cell, there will be times when the signals add
up. To keep from clipping the A/D, the signal must be
reduced, which reduces the dynamic range. Assuming a
. 15 load of 20 channels on a cell, the amount of degradation
would be a m~;mllm of 26 dB, as shown in equation (2
and a average of 13 dB, as shown in equation (3~
20 log (20) = 26 dB (2)
~20 log ( ~ ) = 13 dB (3)
iHowever, the problem with using one of these
figures is that the 26 dB degradation for the peak casè
will rarely occuri and the 13 dB for the average will
occur fairly often. Therefore, a general rule-of-thumb
is to use a number 10 dB above the averagej or 23 dB in
this case, which corresponds to approximately 4 bits.
r'The final result is that the number of bits !'
required is 9 bits, as shown in equation (4).
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' 10 - 5 + 4 = 9 bits (4)
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An additional requirement of the system is the
linearity of the A/D and D/A converters. In order to
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obtain a 60 dB IM performance, a 10 bit device would be
needed. Since there is no improvement gained with
bandwidth reductions, as above, the linearity
specification will determine the required bit size of
the converters. In a preferred embodiment, a 10 bit, 50
MHz or faster device is required. This device will
provide a 500 Mbps (Megabits per second) or more output
at the laser.
Referring now to FIG. 3, a flo~ chart of a process,
generally designated 60, illustrating the present
invention is shown, wherein blocks shown in dashed form
are optional steps. Process 60 commences when an RF ~ :
signal is received, step 61. The signal is filtered to ~ ~ ;
remove interfering signals, step 62; and mixed down to ~ '~
base band, step 63.
The resulting signal is then filtered by anti-
aliasing the RF signal, step 64; and the resulting
: filtered signal is converted to a digital signal, step
65. Once digitized, the signal is synchronized, step
66; and amplified, step 67. The digital signal is then
converted to an optical signal, step 68, for
transmission, step 69.
In FIG. 4, a flow chart of a process, generally
designated 80, for converting an optical signal to an RF
signal is illustrated. Again, the steps illustrated by
dashed boxes in FIG. 4 are optional steps in process 80.
. Process 80 commences upon receipt of an optical signal,
step 81. The optical signal is converted to an
electrical, digital signal, step 82. Depending upon the
requirements of the system, the electrical, digital
signal is amplified, step 83; and synchronized, step 84.
The resulting digital signal is converted to an
analog signal, step 85, and filtered, step 86, to
provide an RF signal. If the resulting RF signal is not
at the appropriate transmission frequency, the RF signal
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is then mixed up from base band to the transmission
frequency, step 87. The signal is then amplified, step
88; and transmitted, step 89.
As a result, a method and apparatus are provided
which do not limit the attainable IM and S/N ratios and
which is more economical to implement.
Thus, it will be apparent to one skilled in the art
that there has been provided in accordance with the
invention, a method and apparatus for optical processing
of an RF signal that fully satisfies the objects, aims,
- and advantages set forth above.
While the invention has been described in ;
' conjunction with specific embodiments thereof, it is
;- evident that many alterations, modifications, and
~ 15 variations will be apparent to those skilled in the art
; in light of the foregoing description. Accordingly, it
is intended to embrace all such alterations,
modifications, and variations in the appended claims.
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