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Description  |
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CROSS REFERENCE TO RELATED APPLICATION
This application contains subject matter which is related to U.S. patent
application Ser. No. 07/955,591, now U.S. Pat. No. 5,353,332, entitled
"Method and Apparatus for Communication Control in a Radiotelephone
System," filed on Oct. 2, 1992, to U.S. patent application Ser. No.
07/956,640, now U.S. Pat. No. 5,404,355, entitled "Digital Control
Channel," filed on Oct. 5, 1992, and to co-pending U.S. patent application
Ser. No. 08/047,452, entitled "Layer 2 Protocol for the Random Access
Channel and the Access Response Channel," filed on Apr. 19, 1993. These
three co-pending applications are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to wireless communication systems, and more
particularly, to a method and apparatus for communicating information in
wireless communications systems including, for example, a cellular radio
system.
2. History of the Prior Art
Cellular Telephone Service
Cellular mobile telephony is one of the fastest growing segments in the
worldwide telecommunications market. Between 1984 and 1992, for example,
the number of mobile telephone subscribers in the United States grew from
around 25,000 to over 10 million. It is estimated that the number of
subscribers will rise to nearly 22 million by year end 1995 and to 90
million by the year 2000.
Cellular telephone service operates much like the fixed, wireline telephone
service in homes and offices, except that radio frequencies rather than
telephone wires are used to connect telephone calls to and from the mobile
subscribers. Each mobile subscriber is assigned a private (10 digit)
directory telephone number and is billed based on the amount of "airtime"
he or she spends talking on the cellular telephone each month. Many of the
service features available to landline telephone users, e.g., call
waiting, call forwarding, three-way calling, etc., are also generally
available to mobile subscribers.
In the United States, cellular licenses are awarded by the Federal
Communications Commission (FCC) pursuant to a licensing scheme which
divides the country into geographic service markets defined according to
the 1980 Census. Only two cellular licenses are awarded for each market.
The two cellular systems in each market are commonly referred to as the
"A" system and "B" system, respectively. Each of the two systems is
allocated a different frequency block in the 800 MHz band (called the
A-band and B-band, respectively). To date, the FCC has released a total of
50 Mhz for cellular services (25 MHz per system).
Mobile subscribers have the freedom to subscribe to service from either the
A-system or the B-system operator (or both). The local system from which
service is subscribed is called the "home" system. When travelling
("roaming") outside the home system, a mobile subscriber may be able to
obtain service in a distant system if there is a roaming agreement between
the operators of the home and "visited" systems.
The Cellular System
In a typical cellular radio system, a geographical area, e.g., a
metropolitan area, is divided into several smaller, contiguous radio
coverage areas called "cells." The cells are served by a series of fixed
radio stations called "base stations." The base stations are connected to
and controlled by a mobile services switching center (MSC). The MSC, in
turn, connected to the landline (wireline) public switched telephone
network (PSTN). The telephone users (mobile subscribers) in the cellular
radio system are provided with portable (hand-held), transportable
(hand-carried) or mobile (car-mounted) telephone units (mobile stations)
which communicate voice and/or data with the MSC through a nearby base
station. The MSC switches calls between and among wireline and mobile
subscribers, controls signalling to the mobile stations, compiles billing
statistics, and provides for the operation, maintenance and testing of the
system.
FIG. 1 illustrates the architecture of a conventional cellular radio system
built according to the Advanced Mobile Phone Service (AMPS) standard. In
FIG. 1, an arbitrary geographic area may be seen divided into a plurality
of contiguous radio coverage areas, or cells, C1-C10. While the system of
FIG. 1 is, for illustration purposes, shown to include only ten cells, the
number of cells may be much larger in practice. Associated with and
located in each of the cells C1-C10 is a base station designated as a
corresponding one of a plurality of base stations B1-B10. Each of the base
stations B1-B10 includes a plurality of channel units, each comprising a
transmitter, a receiver and a controller, as is well known in the art.
In FIG. 1, the base stations B1-B10 are located at the center of the cells
C1-C10, respectively, and are equipped with omni-directional antennas
transmitting equally in all directions. In this case, all the channel
units in each of the base stations B1-B10 are connected to one antenna.
However, in other configurations of the cellular radio system, the base
stations B1-B10 may be located near the periphery, or otherwise away from
the centers of the cells C1-C10 and may illuminate the cells C1-C10 with
radio signals directionally. For example, the base station may be equipped
with three directional antennas, each one covering a 120 degrees sector
cell as shown in FIG. 2. In this case, some channel units will be
connected to one antenna covering one sector cell, other channel units
will be connected to another antenna covering another sector cell, and the
remaining channel units will be connected to the remaining antenna
covering the remaining sector cell. In FIG. 2, therefore, the base station
serves three sector cells. However, it is not always necessary for three
sector cells to exist and only one sector cell needs to be used to cover,
for example, a road or a highway.
Returning to FIG. 1, each of the base stations B1-B10 is connected by voice
and data links to a mobile switching center (MSC) 20 which is, in turn,
connected to a central office (not shown) in the public switching
telephone network (PSTN), or a similar facility, e.g., an integrated
system digital network (ISDN). The relevant connections and transmission
modes between the mobile switching center MSC 20 and the base stations
B1-B10, or between the mobile switching center MSC 20 and the PSTN or
ISDN, are well known to those of ordinary skill in the art and may include
twisted wire pairs, coaxial cables, fiber optic cables or microwave radio
channels operating in either analog or digital mode. Further, the voice
and data links may either be provided by the operator or leased from a
telephone company (telco).
With continuing reference to FIG. 1, a plurality of mobile stations M1-M10
may be found within the cells C1-C10. Again, while only ten mobile
stations are shown in FIG. 1, the actual number of mobile stations may be
much larger in practice and will generally exceed the number of base
stations. Moreover, while none of the mobile stations M1-M10 may be found
in some of the cells C1-C10, the presence or absence of the mobile
stations M1-M10 in any particular one of the cells C1-C10 depends on the
individual desires of each of the mobile subscribers who may travel from
one location in a cell to another or from one cell to an adjacent or
neighboring cell.
Each of the mobile stations M1-M10 includes a transmitter, a receiver, a
controller and a user interface, e.g., a telephone handset, as is well
known in the art. Each of the mobile stations M1-M10 is assigned a mobile
identification number (MIN) which, in the United States, is a digital
representation of the telephone directory number of the mobile subscriber.
The MIN defines the subscription of the mobile subscriber on the radio
path and is sent from the mobile station to the MSC 20 at call origination
and from the MSC 20 to the mobile station at call termination. Each of the
mobile stations M1-M10 is also identified by an electronic serial number
(ESN) which is a factory-set, "unchangeable" number designed to protect
against the unauthorized use of the mobile station. At call origination,
for example, the mobile station will send the ESN to the MSC 20. The MSC
20 will compare the received ESN to a "blacklist" of the ESNs of mobile
stations which have been reported to be stolen. If a match is found, the
stolen mobile station will be denied access.
Each of the cells C1-C10 is allocated a subset of the radio frequency (RF)
channels assigned to the entire cellular system by the concerned
government authority, e.g., the Federal Communications Commission (FCC) in
the United States. Each subset of RF channels is divided into several
voice or speech channels which are used to carry voice conversations, and
at least one paging/access or control channel which is used to carry
supervisory data messages, between each of the base stations B1-B10 and
the mobile stations M1-M10 in its coverage area. Each RF channel comprises
a duplex channel (bidirectional radio transmission path) between the base
station and the mobile station. The RF channel consists of a pair of
separate frequencies, one for transmission by the base station (reception
by the mobile station) and one for transmission by the mobile station
(reception by the base station). Each channel unit in the base stations
B1-B10 normally operates on a preselected one of the radio channels
allocated to the corresponding cell, i.e., the transmitter (TX) and
receiver (RX) of the channel unit are tuned to a pair of transmit and
receive frequencies, respectively, which is not changed. The transceiver
(TX/RX) of each mobile station M1-M10, however, may tune to any of the
radio channels specified in the system.
Depending on capacity needs, one cell may have 15 voice channels, while
another may have over a 100 voice channels, and corresponding channel
units. Generally speaking, however, there is only one control channel (CC)
in each omnidirectional or sector cell served by a base station, i.e., a
base station serving an omnidirectional cell (FIG. 1) will have one
control channel unit while a base station serving three sectors cells
(FIG. 2) will have three control channel units. The RF (control and voice)
channels allocated to any given cell may be reallocated to a distant cell
in accordance with a frequency reuse pattern as is well known in the art.
To avoid radio interference, all radio channels in the same cell will
operate on different frequencies and, furthermore, the radio channels in
any one cell will operate on a set of frequencies which is different from
that used in any neighboring cell.
When in the idle state (turned on but not in use), each of the mobile
stations M1-M10 tunes to and then continuously monitors the strongest
control channel (generally, the control channel of the cell in which the
mobile station is located at that moment) and may receive or initiate a
telephone call through the corresponding one of the base stations B1-B10
which is connected to the mobile switching center MSC 20. When moving
between cells while in the idle state, the mobile station will eventually
"lose" radio connection on the control channel of the "old" cell and tune
to the control channel of the "new" cell. The initial tuning to, and the
change of, control channel are both accomplished automatically by scanning
all the control channels in operation in the cellular system to find the
"best" control channel (in the United States, there are 21 "dedicated"
control channels in each AMPS system, i.e., their TX/RX frequencies are
predefined and cannot be changed, which means that the mobile station has
to scan a maximum number of 21 channels). When a control channel with good
reception quality is found, the mobile station remains tuned to this
channel until the quality deteriorates again. In this manner, all mobile
stations are always "in touch" with the system.
While in the idle (standby) state, each of the mobile stations M1-M10
continuously determines whether a page message addressed to it has been
received over the control channel. When, for example, an ordinary
(landline) subscriber calls one of the mobile subscribers, the call is
directed from the PSTN to the MSC 20 where the dialed number is analyzed.
If the dialed number is validated, the MSC 20 requests some or all of the
base stations B1-B10 to page the called mobile station throughout their
corresponding cells C1-C10. Each of the base stations B1-B10 which receive
the request from the MSC 20 will then transmit over the control channel of
the corresponding cell a page message containing the MIN of the called
mobile station. Each of the idle mobile stations M1-M10 will compare the
MIN in the page message received over the control channel being monitored
with the MIN stored in the mobile station. The called mobile station with
the matching MIN will automatically transmit a page response over the
control channel to the base station which forwards the page response to
the MSC 20.
Upon receiving the page response, the MSC 20 selects an available voice
channel in the cell from which the page response was received, turns the
selected voice channel transceiver on, and requests the base station in
that cell to order the mobile station via the control channel to tune to
the selected voice channel (the MSC keeps a list of all of the channels in
its service area and their status, i.e., free, busy, blocked, etc., at any
time). A through-connection is established once the mobile station has
tuned to the selected voice channel.
When, on the other hand, a mobile subscriber initiates a call, e.g., by
dialing the telephone number of an ordinary subscriber and pressing the
"send" button on the telephone handset in the mobile station, the MIN and
ESN of the mobile station and the dialed number are sent over the control
channel to the base station and forwarded to the MSC 20 which validates
the mobile station, assigns a voice channel and establishes a
through-connection for the conversation as before.
If the mobile station moves between cells while in the conversation state,
the MSC will perform a "handoff" of the call from the old base station to
the new base station. The MSC selects an available voice channel in the
new cell and then orders the old base station to send to the mobile
station on the current voice channel in the old cell a handoff message
which informs the mobile station to tune to the selected voice channel in
the new cell. The handoff message is sent in a "blank and burst" mode
which causes a short but hardly noticeable break in the conversation. Upon
receipt of the handoff message, the mobile station tunes to the new voice
channel and a through-connection is established by the MSC via the new
cell. The old voice channel in the old cell is marked idle in the MSC and
may be used for another conversation.
In addition to call originations and page responses, an AMPS mobile station
may access the cellular system for registrations. Two types of
registrations are possible in AMPS: (i) periodic registration which is
based on time or, more specifically, on the REGID value ("current time")
and REGINCR value ("registration period") transmitted by the base station
and the NXTREG value ("wake-up time") stored in the mobile station, and
(ii) system area registration which is based on location or, more
specifically, on the system identification (SID) transmitted in the
serving cellular system. Periodic registration may be used to determine
whether a mobile station is active (within radio range and switched on) or
not in a cellular system. System area registration may be used to
determine when a mobile station has crossed the border from one cellular
system to another.
Upon receipt of a REGID message on the forward control channel (base
station to mobile station), if registration is enabled in the serving
cellular system, the mobile station compares the REGID value to the NXTREG
value and compares the last received SID value with the value of the SID
of the cellular system in which the mobile station last registered. If
either the value of REGID is greater or equal to the value of NXTREG
indicating that periodic registration is due, or the value of the last
received SID is different than the value of the last stored SID indicating
that the mobile station has travelled from one cellular system to another
since the last successful registration, the mobile station will
automatically send a registration access message over the reverse control
channel (mobile station to base station) and will update the NXTREG value
with the sum of the last received REGID value and REGINCR value, after
receipt of a registration acknowledgement message on the forward control
channel (the mobile station also updates the NXTREG value after each call
origination or page response).
Radio Transmission Format
From its inception, the radio transmission format in cellular systems has
been analog frequency modulation (FM). In each cell, the voice (analog)
signals and data (digital) signals form the input signals to a transmitter
(in the base station or the mobile station) which generates a sinusoidal
carrier wave having a constant frequency corresponding to one of the
frequencies allocated to the cell. With FM, the frequency of the carrier
wave is modulated (varied) in proportion to the instantaneous amplitude of
the input signal. The modulated carrier occupies a relatively narrow
region of the spectrum about a nominal center frequency (the unmodulated
carrier frequency). The resulting deviation of the modulated carrier wave
frequency about the unmodulated (center) frequency is normally limited (by
the use of bandpass filters) within a certain bandwidth, e.g., 30 KHz in
the U.S., to avoid overlapping adjacent RF channels and causing adjacent
channel interference. Each analog voice signal, therefore, occupies 30 KHz
of spectrum, and a voice conversation requires 60 KHz.
In the conventional AMPS system, therefore, an analog speech signal
modulates the carrier wave used for transmission over the RF channel. The
AMPS system uses analog frequency modulation (FM) and is a
single-channel-per-carrier (SCPC) system, i.e., one voice circuit
(telephone conversation) per RF channel. The radio channel access scheme
in the AMPS system is frequency division multiple access (FDMA) in which
multiple users have access to the same set of RF channels, each user is
assigned one of the available RF channels on demand, and different users
are assigned different RF channels.
The Migration from Analog to Digital
Recent developments have ushered in a new digital era for cellular
communications. The main driving force behind the switch to digital has
been the desire to increase spectrum efficiency to meet the
ever-increasing demands on system capacity. As each cellular system is
allocated a finite amount of radio spectrum, capacity may be increased by
reducing the amount of bandwidth required for each voice channel or,
conversely, by sharing each RF channel among several voice conversations.
This is made possible with the use of digital technology.
By encoding (digitizing and compressing) speech from several voice circuits
prior to modulation and transmission, a single RF voice channel may be
shared by several digital speech channels instead of being occupied by
only one analog speech channel (one voice conversation). In this manner,
the channel capacity and, consequently, the overall system capacity, may
be increased dramatically without increasing the bandwidth of the voice
channel. As a corollary, the cellular radio system is able to serve a
substantially greater number of mobile stations at a significantly lower
cost, e.g., a smaller number of channel units (transceivers) required in
the base stations. Furthermore, the digital format facilitates integration
of the cellular system with the emerging digital network.
In the United States, the migration from analog to digital has been
spearheaded by the Electronics Industries Association (EIA) and the
Telecommunication Industry Association (TIA). The EIA/TIA have undertaken
the task of formulating a common air interface standard to meet industry
requirements for the next generation digital cellular systems. To date,
the EIA/TIA has published two separate air interface standards which are
based on different radio channel multiple access schemes. The first
EIA/TIA interim standard (IS) is based on a time division multiple access
(TDMA) scheme and is known as the "Dual-Mode Mobile Station-Base Station
Compatibility Standard" (IS-54B). The second standard is based on a code
division multiple access scheme (CDMA) and is known as "Mobile
Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread
Spectrum Cellular System" (PN-3118 to be published as IS-95). These
standards are incorporated by reference herein (copies of the various
revisions of IS-54B and PN-3118 may be obtained from the Electronics
Industries Association; 2001 Pennsylvania Avenue, N.W.; Washington, D.C.
20006).
The term "dual-mode" in these two standards refers to the capability of the
system to operate in either an analog or a digital mode. The analog mode
of operation uses analog FM and draws on the older EIA/TIA-553 standard
which is based on the AMPS standard. The digital mode of operation uses
TDMA (IS-54B) or CDMA (PN-3118). The dual-mode capability facilitates the
deployment of digital systems through a gradual reduction in analog
capacity, i.e., the removal of RF channels from analog FM service to
provide digital service. This was deemed desirable to ease the transition
from analog to digital and to provide so-called "backward" compatibility
with the existing analog system. Although the analog and digital modes of
operation can exist alone, the goal is for them to coexist, at least in
the short term, in order to allow roaming in existing systems which have
not deployed the new digital technology. In the transition phase, existing
analog-only mobile stations will continue to be served while the use of
digital-capable mobile stations and base stations becomes more widespread.
A mobile station which complies with the defined specifications (IS-54B or
PN-3118) can obtain service from an analog-only base station, a
digital-only base station or an analog-digital (dual-mode) base station.
The type of system serving the mobile station will depend on the
availability of digital service (TDMA or CDMA) in the geographic area of
the mobile station and the preference of the mobile subscriber. At call
set-up or handoff, a dual-mode mobile stations can access either an analog
voice channel (AVC) or, alternatively, a digital traffic channel (DTC). In
analog-only or a digital-only mobile station, however, can only be
assigned an AVC or DTC, respectively.
TDMA Systems
TDMA is a multiple access scheme which is based on time division
multiplexing (TDM) techniques long used in the land-line telephone network
to carry multiple telephone conversations simultaneously over one physical
channel. In the wire-line telephone network, analog speech signals
transmitted by local telephone subscribers over separate lines (subscriber
loops) to the local te | | |
