Nokia 2180 Service Manual ch3ovwps
Programme’s After Market Services
NHD–4 Series Transceivers
Chapter 3
System Overview
Original 11/97
NHD–4
System Overview
Technical Documentation
CONTENTS
Acronyms 3–4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cellular History 3–5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
AMPS Cellular Theory 3–6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Code Division Multiple Access (CDMA) 3–17. . . . . . . . . . . . . . . . . . . . . . . . . .
Quadrature Phase Shift Keying – QPSK 3–20. . . . . . . . . . . . . . . . . . . . . . .
The CDMA Signal 3–22. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Processing Gain 3–22. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The CDMA Forward Link 3–24. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
V ocoder 3–25. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Convolutional Encoder 3–25. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PAMS
Page No
Interleaver 3–26. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PN Code Generation 3–26. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Long Code Scrambling 3–28. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Walsh Code User Channelization 3–28. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Walsh Codes 3–29. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Orthogonal Functions 3–31. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Short Code Spreading 3–37. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Forward Link Channel Format 3–37. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CDMA Reverse Link 3–40. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Data Burst Randomizer 3–40. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reverse Link Error Protection 3–41. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
64–ary Modulation 3–41. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reverse Channel Long Code Spreading 3–41. . . . . . . . . . . . . . . . . . . . . . .
Reverse Channel Short Code Spreading 3–41. . . . . . . . . . . . . . . . . . . . . .
Mobile Phone Operation 3–42. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pilot Channel 3–42. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sync Channel 3–43. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Page 3–2
Paging Channel 3–43. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CDMA Call Initiation 3–44. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reverse Link Open Loop Power Control 3–44. . . . . . . . . . . . . . . . . . . . . . .
CDMA Call 3–45. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reverse Link Closed Loop Power Control 3–45. . . . . . . . . . . . . . . . . . . . .
CDMA Variable Rate Speech Coder 3–45. . . . . . . . . . . . . . . . . . . . . . . . . .
Mobile Power Bursting 3–45. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The Rake Receiver 3–46. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CDMA Hand–offs 3–47. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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System Overview
List of Figures
Figure 1. AMPS: BS/MS/MTX 3–6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 2. AMPS: Audio 3–6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 3. AMPS: Voice/RF 3–7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 4. AMPSTX/RX: 3–7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 5. FM Modulation 3–8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 6. AMPS: Specifications 3–8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 7. AMPS: Cellular Frequencies 3–9. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 8. AMP: Ch # & usage 3–9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 9. AMPS: Modulating signals 3–10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 10. AMPS: BS & Cell Set–up 3–11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 11. AMPS TX/RX registration 3–11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 12. AMPS: Call 3–13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 13. AMPS: Ch reuse & SAT Freq 3–14. . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 14. AMPS: Hand off 3–14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 15. AMPS: Mode Block Diagram 3–15. . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 16. AMPS: TDMA & CDMA Freq and time domain 3–17. . . . . . . . . . . .
Figure 17. CDMA Capacity gains 3–18. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 18. Analog, TDMA & CDMA Structure 3–19. . . . . . . . . . . . . . . . . . . . . . .
Figure 19. BPSK Modulator 3–20. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 20. I/Q Modulator 3–21. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 21. CDMA Waveforms 3–22. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 22. CDMA Forward Link 3–24. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 23. Convolutional encoder 3–25. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 24. Interleaver 3–26. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 25. PN Code generator 3–26. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 26. PN Code generator w/mask ckt. 3–27. . . . . . . . . . . . . . . . . . . . . . . . .
Figure 27. Mask offset example 3–28. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 28. CDMA Forward Link 3–28. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 29. Walsh code example 3–29. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 30. Orthogonal Functions. 3–31. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 31. Walsh Encoding Example 3–32. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 32. Walsh Decoding Example 3–33. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 33. Definition of orthonogonality 3–34. . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 34. Forward Link Channel Format 3–39. . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 35. CDMA Reverse Link 3–40. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 36. CDMA Pilot & Synch Channel Timing 3–42. . . . . . . . . . . . . . . . . . . .
Figure 37. Mobile Power Bursting 3–46. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 38. CDMA Hand–off 3–47. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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System Overview
Acronyms
PAMS
Technical Documentation
AMPS
BS
ББББББББ
CDMA
CTIA
DAMPS
ББББББББ
DTMF
FDMA
GSM
ББББББББ
HLR
ISDN
MS
ББББББББ
MSC
MTSO
MTX
ББББББББ
NADC
Advanced Mobile Phone System
Base Station
БББББББББББББББББББББ
Code Division Multiple Access
Cellular Telecommunications Industry Association
Digital Advanced Mobile Phone System
БББББББББББББББББББББ
Dual Tone Multi Frequency
Frequency Division Multiple Access
Global System for Mobile communications
БББББББББББББББББББББ
Home Location Register
Integrated Services Digital Network
Mobile Station (Cellular phone)
БББББББББББББББББББББ
Mobile Switching Center (see MTX also)
Mobile Telephone Switching Office
Mobile Telephone Exchange (see MSC also)
БББББББББББББББББББББ
North American Digital Communications (IS–54 DAMPS)
PCH
PN Code
ББББББББ
PSTN
RF
SAT
ББББББББ
ST
TCH
TS
ББББББББ
VLR
VOCODER
VOCODER
Paging Channel
Pseudo random Noise Code
БББББББББББББББББББББ
Public Switched Telephone Network
Radio Frequency
Supervisory Audio Tone (5970, 6000 and 6030 Hz)
БББББББББББББББББББББ
Signaling Tone (10 kHz)
Traffic CHannel
Time Slot
БББББББББББББББББББББ
Visitor Location Register
VOice COder DEcodeR
VOice CODER
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Cellular History
Mobile Radios have been in use for approximately 70 years and the cellular concept
was conceived in the 1940s. Public cellular mobile radio was not introduced in the
US until 1983.
In the beginning of the twentieth century, mobile radios were limited to shipboard
use due to the high power requirements and bulky tube radio technology.
Automotive systems in the 1920s operated on 6 volt batteries with a limited storage
capacity.
One of the first useful means of automotive mobile radio occurred in 1928 by the
Detroit police department. Transmission was broadcast from a central location and
could only be received by the mobile police radios.
Introduction of the first two way mobile application was delayed until 1933. This
simplex AM (Amplitude Modulation) push to talk system was introduced by the
police department in Bayonne, New Jersey. The first FM (Frequency Modulation)
mobile transmission (two frequency simplex) was used by the Connecticut State
Police at Hartford in 1940.
The first step towards mobile radio connection with the land line telephone network
was established in St. Louis in 1946. It was called an “urban” system and only
supported three channels.
In 1976, New York City had only 12 radio channels that supported 545 subscribers
with a waiting list of 3700.
In the 1970s, available cellular spectrum was constrained to frequencies above 800
MHz due to equipment design limitations and poor radio propagation characteristics
at frequencies above 1–GHz, this resulted in the allocation of the 825–890 MHz
region.
In 1974, 40 MHz of spectrum was allocated for cellular service and in 1986, an
additional 10 MHz of spectrum was added to facilitate expansion. The present
frequency assignments for the US Cellular system mobile phone is
824.040–848.970 MHz transmit and 869.040–893.970 MHz receive These bands
have been frequency divided (FDMA) into 30 kHz channels. This results in a
maximum capacity of 832 channels. These channels were then divided into two
groups with 416 channels assigned to each system.
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AMPS Cellular Theory
R. F. Communication
BS
MTX
AMPS_1
Figure 1. AMPS: BS/MS/MTX
PAMS
Technical Documentation
Phone
Land Line Comms
The main objective of a cellular system is to provide communications to many
mobile users. Communication between the Base Station and Mobile Phone is via a
Radio Frequency (RF) link. A Mobile Telephone Exchange (MTX) is the interface
between usually several base stations and Land line communications. The MTX
has a number of functions that include, controlling mobile phone transfers between
base stations, regulating mobile phone power output, establishing mobile phone
identity and billing for the air time.
300Hz–3000Hz
MIC
INFO
AMPS_2
Figure 2. AMPS: Audio
Audio signals
Cellular phones are designed to transmit audio signals in a frequency range of 300
Hz to 3000 Hz. This range of frequencies contains most of the intelligibility
necessary for one person to understand what another is saying. As the figure
above shows one person wants to talk to another who is some distance away.
Cellular phones allow communication between two or more people almost without
geographical restrictions.
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High frequency signals will radiate electromagnetic waves from an antenna.
Generally the higher the power the further the waves radiate. Our voice
information is at a low frequency, these frequencies will not radiate. The solution is
to put our information onto a high frequency carrier wave.
Phone
AMPS_3
Figure 3. AMPS: V oice/RF
These signals can then be radiated out an antenna and carry our information to the
receiver.
The function of placing information on a carrier wave is called MODULATION.
Carrier
Generation
800 MHz
Transmiter
Modulator
Audio
Amp
AMPS_4
RF
Amplifier
Receiver
Demodulator
Figure 4. AMPSTX/RX:
In AMPS mode information is placed on the carrier by changing the carrier’s
frequency. The modulating signal causes the carrier ’s frequency to increase and
decrease. This is called Frequency Modulation (FM), and changes to the carrier
frequency are known as deviation.
The receiver picks up RF signals, amplifies them and then retrieves (Demodulates)
information from the RF carrier. Demodulating changes in RF carrier frequency
recovers the original modulating audio frequencies.
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AMPS_5
Frequency Modulation
+8 kHz
Unmodulated Carrier
1 kHz
Mod
Tone
–8kHz
Figure 5. FM Modulation
The figure above represents the effect of a 1 kHz audio modulating tone on an RF
carrier. The RF carrier in Figure 5 has a deviation of 8 kHz. A carrier deviation of
8 kHz means the frequency swings from plus 8 kHz to minus 8 kHz about the center
frequency.
When using Frequency Modulation (FM) how far from the center frequency the
carrier is deviated relates to how strong (loud) the modulating signal is. For
example if you whispered at the microphone the carrier might deviate only 1 or 2
kHz. On the other hand someone shouting at the microphone of their cellular
phone might cause it to deviate the maximum amount of 12 kHz. The standard
convention is that the positive part of the modulating signal will cause the carrier to
deviate to a higher frequency. While a negative modulation signal will cause the
carrier to go lower in frequency.
The rate, how fast the carrier frequency changes from high to low frequency, is
determined by the modulating signal’s frequency.
Advanced Mobile Phone System ––– AMPS
Full Duplex Operation
TX to RX Spacing = 45 MHz
Channel Width = 30 kHz
832 Channels total for the 800 MHz band
AMPS_6
Figure 6. AMPS: Specifications
The above information describes what AMPS stands for along with a “Nuts and
Bolts” description of the 800 MHz AMPS cellular system
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AMPS_7
824.040MHz
Cellular Frequency Band
TX RX
RX
Phone
848.970MHz
Base Station
869.040MHz 893.970
TX
System Overview
MHz
Figure 7. AMPS: Cellular Frequencies
The 800 MHz cellular band consists of two 25 MHz wide frequency blocks. The
individual channel transmit and receive frequencies are spaced 45 MHz apart and
each channel is 30 kHz wide. Note that the transmit frequencies for a Base Station
are the receive frequencies for a cellular phone and vice–versa.
AMPS_7
824.040
Cellular Frequency Band
TX RX
Phone
848.970
869.040
893.970
MHz
RX
AMPS_8 A = A system provider channels
B = B system provider channels
MHz
Base Station
MHz
MHz
TX
Figure 8. AMP: Ch # & usage
The 800 MHz cellular phone band was divided into two parts by the FCC for
competitive reasons. When cellular phone service was first started channel
numbering was from 1 to 799. Later when the bottom 33 channels were added a
nonconsecutive channel numbering scheme was used. Cellular phones that were
in use when only 799 channels were available would have tuned to an incorrect
frequency if the channel numbering had been changed when the new channels were
added. Close examination will show that both “A” and “B” providers have an equal
number of channels.
When cellular phones were first introduced they scanned all available channels.
Present day cellular phone’s scan only the Control Channels. Most phones will
scan only the “A” or “B” control channels even though they are capable of scanning
all 42 channels. Air time is less expensive if only the phone’s Home channels are
used.
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Modulating Signals
SAT
5.97 kHz 6.0 kHz 6.03 kHz
To varify a constant RF
connection from BS & Phone
2 kHz
ST Signalling tone
10 kHz
Off–hook, On–hook
Hook Flash, Hand–off
8 kHz
Supervisory Audio Tone
Type:
Frequency:
Purpose:
Deviation:
Type:
Frequency:
Purpose:
Deviation:
AMPS_9
Data
(FSK) 10 kbps
Instructions & Information
between Base Station & Phone
8 kHz
V oice
300 Hz – 3 kHz
Person – Person
communication
12 kHz
Type:
Frequency:
Purpose:
Deviation:
Type:
Frequency:
Purpose:
Deviation:
Figure 9. AMPS: Modulating signals
As figure 9 illustrates an AMPS cellular phone can have four different types of
modulation.
DATA MODULATION
The first type of modulation a cellular phone uses when communicating with a Base
Station is Data. Data from the phone includes ESN, MIN, phone number to be
dialed and home system identification. Base Station data includes registration
conformation, notification of calls to the mobile, traffic channel assignment and
commands to adjust the mobile’s power output.
The data is Frequency Shift Keyed on the RF carrier. This is one way digital one’s
and zero’s can be modulated on an analog carrier. The 10 kHz data stream is
always transmitter by its self and not in combination with any other signal when in
the AMPS mode.
Supervisory Audio Tone (SAT)
SAT is used to insure that an RF link is being maintained between the mobile and
base station. The base station will transmit its assigned SAT frequency to the
mobile phone. When the mobile receives the SAT signal it will check to see that it is
the assigned frequency and then re–transmit SAT back to the base station. If either
the base station or the mobile fails to receive SAT the call will be terminated. SAT is
added to voice so a normal voice channel will have both voice and SAT. The user
does not hear SAT tones because they are filtered out with bandpass filters that only
allow voice frequencies to pass through.
Voice
Voice is transmitted in a range of frequencies of 300 Hz to 3 kHz. Remember that
SAT is also transmitted at the same time. The nominal deviation for Voice is 2.9
kHz and the maximum deviation for Voice is 12 kHz. Combining SAT and Voice
together will give a total maximum deviation of 14 kHz that the phone should never
exceed. If a mobile phone exceeds maximum deviation it will start interfering with
calls in adjacent channels.
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Signaling tone (ST)
Signaling tone is a plain 10 kHz tone. ST is used for signaling the base station
when the mobile phone is “off hook”, conversation is ended “on hook”, hook flash,
and handoff acknowledgment. When the mobile is being called,and is ringing, but
has not been taken “off hook” a continuous ST is transmitted to the base station.
When the mobile is taken “off hook”, answered, the ST is no longer sent. To
hang–up the “end” key is pressed, the mobile then sends out a 1.8 second burst of
ST. If during a conversation a mobile user wants “additional” service a
number/command is loaded into the mobile call memory and the “send” key is
pressed, the mobile will transmit ST for 400 ms. The base station responds will a
data acknowledgment, the mobile then sends its “additional” service request. When
a hand–off between base stations becomes necessary the MTX generates a
hand–off order and sends it to the mobile. The mobile stores this information and
sends a 50 ms burst of ST to the base station then changes to the new base station
and traffic channel. When the MTX detects SAT on the new channel the old base
station channel is shut down.
Base Station & Cell Set–up
X
Control Ch #
Traffic Ch #’s
Base Station
333
101 – 150
A4D7
ID
313
1 – 50
B9CE
320
201 – 250
8FB2
315
151 – 200
BC43
327
51 – 100
796F
Ch
333
313
329
327
330
315
322
AMPS_10
Info Rxd
A4D7
B9CE
–––––––
796F
–––––––
BC43
–––––––
Level
–100 dBm
–109 dBm
–120 dBm
–118 dBm
–120 dBm
–116 dBm
–120 dBm
Figure 10. AMPS: BS & Cell Set–up
When a cellular phone is first turned on it will scan the control channels. The phone
will record several different items of information, the channel number, Base Station
ID, System ID (SID) and the signal level of each Base Station received. In the
example Base Station A4D7 on control channel 333 was the strongest signal at a
level of –100 dBm. The mobile phone will then lock on control channel 333. Once
this is done the mobile phone is said to be “In Service”
TX Registration
BS
1.
2. Data ESN/MIN
3.
4.
Figure 11. AMPS TX/RX registration
Channel 333
Data A4D7 ”Service”
Data ESN/MIN OK
Data A4D7
AMPS_11
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Line 1 in figure 11 is the state the mobile phone was at in figure 10. The mobile has
found the strongest control channel and is “In service”.
In line 2 the mobile phone transmits it’s ESN/MIN to the Base Station (only if not in
its home area) using 10 kbytes/s data via the control channel.
Line 3 shows the Base Station sending the mobile phone a data transmission
confirming TX registration. At this time the Cellular Mobile Telephone Exchange
knows the mobile phone is ON and operating and which Base Station is currently in
contact with the mobile phone. This is important if the mobile phone gets a call
from another phone. The cellular system needs to know where each operating
mobile is in order to route calls to them.
Line 4 shows the Base Station back to continuously transmitting its ID. The mobile
phone stays on channel 333 and shows “In service”. Periodically the mobile will
rescan the control channels to make sure that it is still locked on the strongest
control channel.
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BS
1.
2. Data ESN/MIN/Dialed #
3.
4.
5. 1.8 sec (End Call) S.T.
CALL
Channel 333
Voice/SAT
AMPS_12
Data A4D7 ”Service”
Data ESN/MIN/OK
Goto 121/SAT=5970 Hz
System Overview
Figure 12. AMPS: Call
This is what happens when the mobile phone makes a “call”. As before the phone
is “In service” on control channel 333 with Base Station A4D7.
Line 2 shows the mobile phone sending a data stream to the Base Station that
contains the mobile’s ESN/MIN/phone # dialed.
In line 3 the Base Station confirms the mobiles data and instructs the mobile to go to
traffic channel 121 and to expect an SAT frequency of 5970 Hz.
In line 4 the Base Station and mobile are full duplex on traffic channel 121 with an
SAT frequency of 5970 Hz. Remember the SAT is used to confirm that a full duplex
(two way) link is operating between the Base Station and mobile phone.
Line 5 lists one of the four uses for Signaling Tone (ST). When the mobile phone
user finishes a call and presses “END” the phone sends out a 1.8 second burst of
ST that lets the Base Station know the call has ended. Another ST use is when the
mobile phone is being called. The phone sends a continuous ST signal back to the
Base Station until the mobile user answers the phone. As noted in the Modulating
Signals section the mobile also uses ST for “Hook Flash” and “Hand–off” signalling.
Original 11/97
Page 3–13
NHD–4
System Overview
333
101–150
A4D7
5970 Hz
320
201–250
8FB2
AMPS_13
Channel Reuse and SAT Frequencies
Reused Voice Channels
313
1–50
B9CE
315
151–200
BC43
Control Ch #
Traffic Ch’s
Base Station
ID
327
51–100
796F
Technical Documentation
Different SAT freq
prevents hook–up
to other cell
319
101–150
B8C3
6030 Hz
PAMS
Figure 13. AMPS: Ch reuse & SAT Freq
In this example Base Stations A4D7 and B8C3 are using the same traffic channels.
Base Stations are normally spaced far enough apart so that two stations using the
same traffic channels will not interfere with each other. However!!! Under some
conditions it is possible for two stations like A4D7 and B8C3 to be received by one
mobile phone. The cure for this problem is different SAT frequencies. When a
mobile is ordered to a traffic channel it is given a traffic channel number and an SAT
frequency. If the mobile should be getting its traffic from A4D7 it expects to see an
SAT frequency of 5970 Hz. If an SAT frequency of 6030 were received, the mobile
would drop the call and try listening for the traffic channel that had 5970 Hz for SAT.
When an AMPS mobile station (MS) travels from one Base Station (BS) cell to
another a “Hand–off” must occur. As the mobile travels from “A” to “B” the signal
strength will decrease in “A” and increase in “B”. This information is sent from the
BS to the Mobile Telephone Exchange (MTX). Each BS has a receiver that can
scan all channels. When a hand–off is imminent the MTX instructs BS “B” to listen
for the MS about to be handed–off in order to insure that signal strength is
adequate. When the MTX decides its time for a hand–off, the traffic channel from
BS “A” to the mobile is interrupted and hand–off instructions are sent to the mobile.
The mobile then sends a 50 msec burst of Signalling Tone to confirm receipt of
instructions, then changes to the new Base Station, traffic channel and ST. The
user does not hear any data because the ear piece is muted when the phone is not
receiving SAT.
Handoff
D
AMPS_14
X
X
A
MTX
B
C
Page 3–14
Figure 14. AMPS: Hand off
Original 11/97
PAMS
NHD–4
Technical Documentation
869–894
MHz
RX
Duplexer
TX
Power
Amplifier
& Driver
881.52
MHz
LNA
SAW Flter
BW 25 MHz
SAW
Filter
824–840
MHz
1st
Mixer
UHF VCO
914 – 939
MHz
Driver
Amps
45 MHz
IF
Amp
Analog
Modulation
VHF VCO
180 MHz
914–939 mhZ
SAW
Filter
824–840
MHz
2nd IF Amp & FM Det
Xtal
Filter
180 mhZ
TX Mix
455 kHz
2nd LO
44.545
MHz
2nd
IF Amp
TX gain
control
90 MHz
CDAGCT
Ceramic
Filter
455 kHz
2
System Overview
o
90
Ringing
Ckt
AMPS_15.DRW
FM
Det
Clipper
Amp
MIC
EAR
CODEC DSP CDSB CDRFI
To Analog Modulation
Figure 15. AMPS: Mode Block Diagram
The HD881 in AMPS mode is a dual conversion superheterodyne receiver. The
869.040 to 893.970 MHz receive band is passed through the receive side of the
duplex filter. The receive side of the duplexer bandpass filter is 25 MHz wide
centered at 881.52 MHz so that only cellular receive band frequencies will be
amplified by the LNA. After the LNA a second bandpass filter, a SAW (Surface
Acoustic Wave) filter is used to completely eliminate any frequencies outside the
receive band. The duplex and SAW filters also help eliminate any image
frequencies. Image frequencies are 45 MHz above the UHF VCO frequency. The
first mixer down converts the incoming signal by mixing the received frequency with
the UHF VCO, this produces a 45 MHz IF signal. A second mixer down converts
this 45 MHz IF to the second IF frequency of 455 kHz. The 455 kHz 2nd IF after
filtering is demodulated by the FM demodulator. Ten kbit/s data and SAT
(Supervisory Audio Tones) are separated in the CDRFI and these signals are sent to
the CDSB modem section where FSK data from the BS is demodulated. The RX
voice signal is converted to a digital bit stream in the CDRFI. Analog voice data is
sent to the AMPS Data Buffer in the CDSB. Analog voice data is sent from the
CDSB to the DSP for processing into a form that the Audio CODEC can use.
Analog voice data is converted to analog by the CODEC and then fed to the
earpiece.
Original 11/97
Page 3–15
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