Development of an Optical Repeater System
for Pagers and Cellular Phones
by Katsumi Sudo *, Koji Ando *, Ryuji Satake *,Nobuhiko Hattori *2 and Yousuke Tokunaga *2

An optical repeater system has been developed that provides pager and cellular

ABSTRACT phone services in an underground parking garage, where such services were unavailable. The transmitter uses the multi-coupler method which reduces the cost of equipment anddiminishes the number of fibers. Use of a low-noise, low-distortion 1.3-µm distributed feedback
laser provides high system performance.
Out- and indoor radio transmission design was carried out and the system's feasibility was tested
by installation in an underground parking garage. This paper reports on equipment design and radiotransmission design.

1.

280-MHzpager base station

INTRODUCTION
Outdoor antenna

There has been remarkable growth in Japan's mobile communications market, with the number of subscribers reaching some 45 million at the end of 1998.This has created aneed for services that extend to locations that were previously blind spots for pagers and cellular phones.
To address this need the advantages of systems using
analog optical transmission have been recognized in termsof flexibility in cable laying design, lower costs of installation, and broader service area. Furukawa Electric has
therefore developed an N-to-one system using the opticalmulti-coupler method, involving less equipment and lower
cost than conventional systems in which optical transmitters are connected one-to-one.

800-MHz cellular
phone base station
Coaxial cable

Extension station
Optical coupler

Indoor antenna
Cellular phone

Pager

Optical fiber
Coaxial cable

Services provided:
280-MHz pager
800-MHz analog cellular phone
800-MHz digital cellular phone
1.5-GHz digital cellular phone

Figure 12.

1.5-GHz cellular
phone base station

Center station

Schematic image of the system

SYSTEM CONFIGURATION

Figure 1 is a schematic image of the system, showing thenature of the services provided.
The system comprises an outdoor antenna, a central
station, extension stations and indoor antennas. It relays
all pager and cellular phone services, making them available within the underground parking garage.
In the downstream link of the system, the signals forpagers, 800-MHz cellular phones and 1.5-GHz cellular
phones that are transmitted from the base station are received by the outdoor antenna installed for each waveband
and converted into optical signals at the central station* Wireless Systems Development Project Team, Information & Electronics Lab., R & D Div.
* 2 Optical Systems NP Team, FITEL Systems Dept.

using direct intensity modulation. They are then transmitted through a single-mode (SM) optical fiber and optical
couplers to the extension stations installed in the parking
garage. The extension station then converts the opticalsignals into RF signals, amplifies them, and radiates themover the indoor antennas in the parking garage. The indoor antennas, like the outdoor antenna, are installed foreach frequency band.
In the upstream link, on the other hand, signals generated by cellular phones in the parking garage are received
by the indoor antennas and converted by the extension
stations into optical signals. The signals from each of theextension stations are combined by an optical coupler intoa single SM fiber. The central station converts the optical

79

Central repeater station
Outdoor antenna for
280-MHz pager

Opticalcoupler

Outdoor antenna for
800-MHz cellular phone
Outdoor antenna for
1.5-GHz cellular phone

DPX

BPF : Band pass filter
LNA : Low noise amplifier
HPA : High power amplifier
DPX : Duplexer
CMB : Combiner
DTR : Distributor

Extension station #1
Indoor antenna for
280-MHz pager

C
M
B

D
T
R

D
T
R

C
M
B

Indoor antenna for
800-MHz cellular phone
D
P
X

Indoor antenna for
1.5-GHz cellular phone

Extension station #2

SM fiber

Extension station #3

Extension station #4

Figure 2

System configuration

Table 1

Downstream
C/N (dB)a) Generation of distortionby 2 carriers

Figure 3b) Generation of distortion by multi-carriers

Relationship of plural carriers and distortion

IMD3 (dBc)
UpstreamC/N (dB)
IMD3 (dBc)signals from the extension stations into RF signals, amplifies them and sends them from the outdoor antenna to
the base station of each cellular phone service.
Figure 2 shows the configuration of the pager systemand of the cellular phone system, the optical transmitter ofwhich multiplexes the 800-MHz and 1.5-GHz signals.
The optical transmitter uses a multi-coupler method
which uses an optical coupler for branching or combining.
The lasers are low-noise, low-distortion 1.3-µm distributed
feedback (DFB) laser diodes, and PIN photodiodes areused as photodetectors because of their good linearity.
In the upstream links particularly, care is taken that thedifference in wavelength between the laser diodes in eachof the extension stations does not overlap with the RFband of the cellular phones, and by providing stable control, a single optical fiber can be connected to up to four
extension stations.
At the input of both the downstream and upstream links,
a low-noise amplifier (LNA) is used to inhibit the generation of noise and amplify to the level required by the electrooptical converters (E/O). A Class A high-power amplifier(HPA) having a high intercept point (ICP) is used at theantenna output to inhibit the generation of thirdintermodulation distortion (IMD3).

80

Distribution of C/N and IMD3
Low noiseamplifierOpticaltransmitter

High power
amplifier

Total

67

62

70

60

-110

-80

-85

-75

80

70

70

66

-70

-64

-64

-56

3 . GENERAL DESCRIPTION OF THE
EQUIPMENT
3.1 Characteristics Distribution
In the distribution of characteristics to the LNA, opticalconverter (E/O, O/E) and HPA, C/N distribution was doneusing

C/N total =10log{10 –C/N opt
C/N LNA–
10 +1010
HPA
C/N+10 – 10 } –1 [dB]..... (1)and IMD3 distribution using

IMD3 total =20log{10 (IMD3 LNA
IMD3 opt) +10 ()
20
20
IMD3 HPA )+10 ([dBc]..... (2)20

In the system design adopted here, the number of carriers aligned at equal frequency spacing was 60 for downstream links and 8 for upstream.
As Figure 3 shows, composite triple beat (CTB) distortion generated by multi-carriers produces more distortion
than the IMD3 generated by two carriers and distortion
output is also greater. When 60 carriers are transmitted at
equal-frequency spacing in the downstream link, the CTBlevel becomes 37.1 dB larger than IMD3 on the condition

Furukawa Review, No. 18. 1999

that output per carrier is constant and irrelevant to thephase of distortion. Similarly, the increase in the upstream
links is 17.8 dB1). Table 3 shows the characteristics distribution for cellular phone service in this system, where distortion is converted from CTB to IMD3.
3.2 Optical Transmitter Section
This system makes use of: 1) both subcarrier transmission technology, in which frequency-multiplexed RF signals undergo direct intensity modulation by the laser diode of the E/O converter and are demodulated again tothe original RF signal by the photodetector of the O/E converter; and 2) the optical multi-coupler method, in which
optical signals are branched and combined by the optical
coupler. In subcarrier transmission, no RF-band modem
is required, and the optical multi-coupler system makes
possible a reduction in the number of E/O and O/E converters, and in the number of optical fibers used. The combination of the two technologies improves efficiency andlowers cost.
For cellular phone service, the 800-MHz and 1.5-GHz
signals are multiplexed for transmission.
To achieve the multi-carrier transmission characteristics
described in Section 3.1 above, the E/O converters in the
upstream and downstream links have 1.3-µm DFB laser
diodes suited to analog transmission having low distortion characteristics.
The C/N of the optical transmitter may be found by(m.S.P) 2
C/N opt =10log . 2 . . .
[dB]
2 {Ith +2 e S P+(S.P) 2.RIN}.B............... (3)where:
m is the modulation depth (true value),
Ith is the equivalent input noise current density (A/Hz1/2),
120

100

P is the optical receiving level of the photodiode(W),
S is the optical sensitivity of the photodiode (A/W),
B is the noise bandsidth (Hz) ande is the quantum of electricity.
The IMD3, for its part, is found by

IMD3 opt =IMD3 LD +2 . 20log(m opt /m LD )[dBc]..... (4)where: mLD and IMD3LD are the modulation depth
and distortion of the laser diode, and mopt and IMD3opt
are the modulation depth and distortion in this system.
The C/N and IMD3 characteristics of the optical transmitter are in a tradeoff relationship with optical modulation
depth m. As an illustration, Figure 4 plots C/N and IMD3
against modulation depth for downstream links.
To inhibit the generation of distortion of the signal itself,a region must be used in which optical modulation depth
is not overmodulated by multi-carrier transmission.
From these conditions, a modulation depth mopt of 2.6
for downstream links and 21.0% for upstream links wasused respectively.
When, on the other hand, optical signal is multiplexed indownstream links, beat noise occurs at the frequency ofthe difference between laser diode wavelengths. If the original RF signal is being modulated, chirping will occur andthe beat noise band will become broader. In this system
when these beat noises overlap the RF signal band forcellular phones, nominal RIN deteriorates, that is to say
the possibility of deterioration in system C/N arises. It is
known that if beat noise is separated to some degree fromthe signal band its influence on RIN virtually disappears2).
In this system this point was fully considered in selecting
the lasing wavelength of the laser diodes, and automatic
power control (APC) and automatic temperature control(ATC) circuits are provided to stabilize the lasing wavelength.
By this means it is possible to connect up to four extension stations per optical fiber.

110

90

C/N (left scale)100

C/N (dB)

80

70

90

60

80

IMD3 (dBc)

IMD3 (right scale)

3.3 Amplifier Section
In both the upstream and downstream links, a low-noise
amplifier (LNA) is used for the first-stage amplifier that
amplifies the RF signal received by the antenna to thelevel required for optical modulation, and a high-power
amplifier (HPA) having a high ICP is used for the amplifierthat amplifies the RF signal demodulated from the optical
AGC for excessive output
power suppression

50

1

5

10

20

m (%)

Figure 4

C/N and IMD3 vs. modulation depth

AGC for temperature vs.
gain slope suppression

70
ATT for gain
control

ATT for AGC

Figure 5

Amplifier

RF coupler

Block diagram of HPA

81

signal to the level required for radiation via the antenna. To
inhibit generation of IMD3, all amplifiers use a region ofgood linearity in A-class operation and are provided witha variable attenuator for gain control.
The relationship among the ICP (dBm), signal output
power Pout (dBm) and IMD3 (dBc) for the amplifiers may
be expressed by

IMD3
+Pout
.................... (5)2allowing the ICP required for the amplifier to be selected.
ICP=

Figure 5 is a block diagram showing the structure of the
HPA, which comprises an automatic gain control (AGC)
for output power suppression and an AGC for temperature compensation, configured using the same feedback
circuit. The output power suppression AGC acts when input is excessive to produce a constant output, and is also
provided with the function of interrupting the amplifier power
supply when further excessive input occurs, preventing
the radiation of spurious radio waves.
Figure 6 shows the relationship between ambient temperature and gain. The AGC has resulted in stable gain
and has stabilized the output.
3.4 Alarm Section
Alarms between the central station and the extension stations are transmitted on a twisted pair with RS-485. The
extension stations are polled from the central station, andonce data on the amplifier faults, optical device faults,
power supply faults, etc. for each of the central station's
extension stations has been gathered, it is connected byrelay contacts to the fire prevention center within the building.

4.

Photo 1

Table 265

Gain (dB)64

System specifications
Specification

Item

Pager
Frequency band

Upstream

Downstream—

276~288MHz

No. of transmit
carriers—

Device output—

0 dBm/carrier

915~940MHz
940~958MHz

810~828MHz
860~885MHz

No. of transmit
carriers

8

104

Device output

16 dBm/carrier

7 dBm/carrier

800-MHz cellular phone
Frequency band

1.5-GHz cellular phone
Frequency band

TRANSMISSION CHARACTERISTICS

Table 2 shows the system specifications, which defines
spurious emissions including IMD3. Photo 1 shows the

Appearance of central repeater station

70

1,429~1,453MHz 1,477~1,501MHz

No. of transmit
carriers

860

Device output

16 dBm/carrier

7 dBm/carrier

C/N

60 dB or better(signal bandwidth 32 kHz)

Spurious radiation

25 W or less

No. of extension
stationMax. 4

Transmission range

Max. 1 km

Operating ambienttemp.

0~40˚C

Overall dimensions(W x H x D)

1,140x1,625x550 mm(central and extension stations)

63

62

0

25

50

Temperature (˚C)

Figure 682

appearance of the central repeater station. Extension stations have a similar appearance. Figure 7 shows the temperature dependence of C/N and IMD3 for 1.5-GHz-band
system, as representative of the temperature characteristics of the system. All transmission characteristics, including these, meet the desired quality levels.

Gain stability of HPA for 1.5-GHz band

Furukawa Review, No. 18. 1999

Photo 2

Outdoor antennas for base station (from top inorder: sleeve antenna for 280-MHz pager,Yagi
antenna with radome for 800 MHz,Yagi antenna
with radome for 1.5 GHz, 2-tier stackedYagi
antenna with radome for 800 MHz band)

86

C/N (dB)

68

82

66

80
78

64

IMD3 (dBc)

84

IMD3

C/N
62

76

60

74

0

20

40

Temperature (˚C)

Figure 75.

Temperature dependence of C/N and IMD3 for
1.5-GHz-band system

RADIO TRANSMISSION DESIGN

In installing the system in the underground parking garage a process of radio transmission design was undertaken.
Photo 2 shows the disposition of the antennas for thebase station. Taking into account the location and distanceof each service and antenna directivity, it was possible toreceive from and transmit to 10 services with these fourantennas. This represents a significant saving in terms of

Photo 3

Indoor antennas for mobile stations (from front
in order: monopole antenna with radome for 800
MHz band, monopole antenna with radome for
1.5 GHz)the number of antennas compared to dedicating one toeach service and does not detract from the appearanceof the building.
Photo 3 shows the indoor antennas for cellular phones
mounted in the parking garage.
Analysis of signal propagation in the parking garage was
performed using free space propagation loss. The garage
contained a variety of obstacles that increased losses,
however, such as partitions, corners and parked vehicles.
Losses were standardized based on experimental dataon these increases, and a calculation of propagation losses
was performed. Taking the requisite reception level for eachservice as 30 dBµV/m for pagers3) and -98 dBm for cellular phones4), and using the radiated power of the antennas and the above calculation of propagation loss, it was
decided to install three indoor antennas on each floor toassure reception of all services throughout the parking
garage.
Table 3 shows the reception level for cellular phones at
the numbered points within the parking garage shown in
Figure 8. Measured values obtained were substantially thesame as the calculated values, and although, due to theeffect of the surroundings, some were as much as 10 dBlower, the reception level for the mobile stations was foundto be adequate at all points measured.
Thus superior reception quality in the underground parking garage was confirmed.

83

Indoor antenna

46 m

Extension
stationIndoor antenna

Indoor antenna

58 m

Figure 8

Table 3

Measurement points in underground parking garage

Received power of dipole antenna in underground parking garage (in dBm) at numbered
points
800-MHz band

1.5-GHz band

Measurement Calculated Measured Calculated Measured
point
value
value
value
value
(see Figure 8)
[dBm]
[dBm]
[dBm]
[dBm]

6.

CONCLUSIONS

Through the use of optical multi-couplers it has been possible to develop a repeater system for pagers and cellular
phones in an underground parking garage that has lower
equipment costs and reduces the number of optical fibers.
Radio transmission design was carried out, enabling inthe-field use of all pagers and cellular phones.
It is intended to proceed with the development of compact wall-mounted equipment.

1

-56

-59

-61

-62

2

-38

-46

-43

-51

3

-56

-59

-61

-71

4

-56

-58

-60

-64

5

-73

-78

-78

-80

6

-56

-57

-60

-59

7

-38

-55

-43

-51

8

-72

-73

-76

-83

9

-83

-82

-88

-89

10

-81

-77

-86

-87

11

-38

-42

-43

-46

12

-60

-64

-64

-71

1) Iwata, Matsunaga; Interference in Mobile Wireless Communication, NTT Research & Development Report, Vol.11, No.10,(1962), 1759. (in Japanese)2) Shinoda et al.; Relationship between beat noise and wavelength
spacings in optical passive network, Proceedings of the 1994
IEICE Spring Conference, B-1123, (1994), 4-189. (in Japanese)3) Radio System Development Center; Standard Regulations for
Advanced Pager Systems, RCR STD-43, (1995) (in Japanese)4) Omatsuzawa et al.; Station Placement Design in Mobile Communications, NTT DoCoMo Technical Journal, Vol.2, No.4,(1995), 35. (in Japanese)

13

-60

-50

-64

-62

Manuscript received on July 7, 1998.

REFERENCES

84

Furukawa Review, No. 18. 1999