U.S. patent application number 10/319722 was filed with the patent office on 2003-04-17 for packet transmissions over cellular radio.
Invention is credited to Maxemchuk, Nicholas Frank.
Application Number | 20030072295 10/319722 |
Document ID | / |
Family ID | 25529168 |
Filed Date | 2003-04-17 |
United States Patent
Application |
20030072295 |
Kind Code |
A1 |
Maxemchuk, Nicholas Frank |
April 17, 2003 |
Packet transmissions over cellular radio
Abstract
An improved system is achieved in a cellular arrangement where
mobile units employ a moveable slot TDM approach to send packets to
base stations, and where the base stations use a non-contention
approach. When a base station transmits information packets to
different mobile units, it merely queues the packets and transmits
over a given channel. In a corresponding inbound channel, the
mobile units transmit packets to the base station using the MSTDM
protocol. The base station also transmits information regarding
whether the inbound frequency is occupied by a signal that is being
transmitted to the base station, or whether a collision exists. A
collision on an inbound frequency occurs when more than one signal
is simultaneously transmitted on the inbound frequency. On an
additional control channel that is shared by all base stations, the
base station sends information in a TDM fashion.
Inventors: |
Maxemchuk, Nicholas Frank;
(Mountainside, NJ) |
Correspondence
Address: |
Henry T. Brendzel
P.O. Box 574
Springfield
NJ
07081
US
|
Family ID: |
25529168 |
Appl. No.: |
10/319722 |
Filed: |
December 13, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10319722 |
Dec 13, 2002 |
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08982450 |
Dec 2, 1997 |
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Current U.S.
Class: |
370/348 |
Current CPC
Class: |
H04W 74/08 20130101 |
Class at
Publication: |
370/348 |
International
Class: |
H04B 007/212 |
Claims
I claim:
1. A method for communicating comprising the steps of: a mobile
unit that wishes to transmit information in the form of packets,
over a first wireless, shared, inbound channel to a non-mobile base
station, determines, based on information derived from a second
wireless channel that is other than said first wireless channel and
logically distinct from a outbound channel by which said mobile
unit receives information packets from said non-mobile base,
whether the first wireless channel is busy, when the first wireless
channel is busy, the mobile unit refrains from transmitting, and
when the first wireless channel is not busy, the mobile unit
transmits a packet of information.
2. The method of claim 1 further comprising the steps of: when said
packet is a data packet, the mobile unit also determines, based on
information from said second wireless channel, whether a collision
has occurred with a transmission of a packet by some other mobile
unit, when said packet is a data packet and the mobile unit
determines the existence of said collision, the mobile unit stops
transmitting said packet, when said packet belongs to a voice
source and it is a first packet of said voice source, the mobile
unit determines, based on information from said second wireless
channel, whether a collision has occurred with a transmission of a
packet by some other mobile unit, when said packet belongs to said
voice source and it is a first packet of said voice source, and the
mobile unit determines the existence of said collision, the mobile
unit stops transmitting said packet, and when said packet belongs
to said voice source and it is other than said first packet of said
voice source, the mobile unit transmits the entire packet without
determining whether a collision has occurred with a transmission of
a packet by some other mobile unit.
3. The method of claim 1 where said information from said second
wireless channel is placed on said second wireless channel by said
base station.
4. The method of claim 3 where said second wireless channel is
embedded in said outbound channel.
5. The method of claim 4 where said information packets sent to
said mobile units are data packets, voice packets, or a mixture of
both.
6. The method of claim 3 where said information which enables said
mobile unit to determine whether said first wireless channel is
busy, comprises a first marker which indicates that said first
wireless channel becomes busy, and a second marker which indicates
that said first wireless channel becomes not busy.
7. The method of claim 3 where said information which enables said
mobile unit to determine whether said first wireless channel is
busy, comprises a first marker which indicates that said first
wireless channel becomes busy with a data packet, and a second
marker which indicates that said first wireless channel becomes
busy with a voice packet.
8. The method of claims 6 or 7 where said information which enables
said mobile unit to determine the existence of a collision
condition is a third marker.
9. The method of claim 8 where said markers comprise a data link
escape character followed by two bits of information that are
injected into a wireless channel that said base station employs for
sending information packets to said mobile unit.
10. The method of claim 2 where said mobile unit transmits said
data packet in one form of modulation, and transmits packets of
said voice source in another form of modulation.
11. The method of claim 10 where said one form of modulation is a
suppressed carrier form of modulation, and said another form of
modulation is a non-suppressed carrier form of modulation, or said
another form of modulation is a suppressed carrier form of
modulation, and said one form of modulation is a non-suppressed
carrier form of modulation.
12. The method of claim 2 where data packets are shorter than
packets from voice sources.
13. A cellular arrangement having mobile units and cells, where
each cell covers a roughly circular geographical area by means of a
base station at the center of the cell, and where a plurality of
cells are arranged to cover a larger geographical area, where each
base station comprises: a receiver, a transmitter, and means for
informing mobile units of simultaneous detection by said receiver
of packets in an inbound frequency channel, said informing being
carried out by causing said transmitter to transmit, over a
collision detection frequency channel that is distinct from said
inbound frequency channel and also distinct from outbound frequency
channels that are used by said transmitter to send information
packets to said mobile units.
14. The arrangement of claim 13 where the means for informing
mobile units also inform the mobile units as to whether the inbound
channel is busy.
15. The arrangement of claim 13 where said means for informing
mobile units comprises: apparatus associated with the receiver
which detects simultaneous reception over the inbound frequency
channel of data packets and voice packets, and apparatus associated
with the transmitter which signals a detection of said simultaneous
reception of data packets and voice packets over said collision
detection frequency channel.
16. The arrangement of claim 13 where said means for informing
mobile units includes apparatus in the receiver that detects
simultaneous reception of packets.
17. The arrangement of claim 16 where the apparatus that detects
simultaneous reception of packets checks error correcting codes
embedded in packets transmitted by mobile units.
18. The arrangement of claim 13 where said transmitter assigns the
frequency of the inbound channel, and where the inbound channel and
the outbound channel are in a first frequency band that is
substantially not overlapping with frequency bands that are
employed by transmitters of adjacent cells.
19. The arrangement of claim 18 where transmitters of adjacent
cells employ a second frequency band and a third frequency band
such that adjacent cells use a different one of said second
frequency band and third frequency band.
20. The arrangement of claim 13 where said transmitter also
transmits common control information over a frequency that is
shared by transmitters of all of said cells, in a time division
multiplexing.
21. The arrangement of claim 20 where the time division
multiplexing provides for three time channels, and a time channel
used by a base station is different from a time channel that is
used by an adjacent base station.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of application No.
08/982,450, filed Dec. 2, 1997.
[0002] This application is related to U.S. patent applications Ser.
No. 08/082,634 and Ser. No. 08/982,571 filed concurrently herewith,
entitled "Packet Switching Architecture In Cellular Radio" and
"Overload Control In A Packet-Switching Cellular Environment,"
respectively.
BACKGROUND
[0003] This invention relates to cellular telephony and, more
particularly, to the use of packet techniques in cellular
telephony.
[0004] The number of people that use cellular telephones is
continually increasing.
[0005] Because the available bandwidth is controlled by
governmental regulations, providers of cellular telephony are
meeting the increase in users by establishing smaller cell sizes.
Smaller cell sizes accommodate larger numbers of mobile units
within the same overall bandwidth because smaller cell sizes
effectively increase the rate of bandwidth re-use per unit area.
However, as cell sizes shrink, mobile units move between cells more
frequently. In a circuit switched system, each move requires that
one circuit be torn down and another one set up. Consequently, as
cell sizes decrease, the work associated with handing off users
between cells increases. In addition, when a mobile unit traverses
more cells during its connection, it is more likely that the mobile
unit will encounter a cell with more units than the bandwidth can
support.
[0006] Packet switching, as compared to circuit switching, reduces
the work required for hand off because addresses embedded within
the packets are used to route individual packets rather than
setting up and tearing down circuits. Packet switching was used in
early military cellular systems. Those networks were designed to be
rapidly deployed, were aimed primarily for wireless interconnection
between mobile units, and were not connected to a wired backbone
network.
[0007] Currently, the prevalent commercial cellular system in the
United States is a circuit switched arrangement that employs Time
Division Multiplexing (TDM). Another system, which is also a
circuit switched system, employs Code Division Multiple Access
(CDMA). These cellular systems can transmit data in the form of
packets, but that does not constitute "packet switching," either in
the sense employed in the aforementioned military system or in the
sense employed in this disclosure. Specifically, while the data may
have a packet format, the switching within the cellular environment
is not based on the explicit address information in the packets.
For example, in TDM the address is implicit in the frequency and
time slot at which the mobile unit operates.
[0008] The explicit addressing characteristic of packet switching
is more flexible than implicit addressing. With explicit
addressing, the capacity on the shared medium can be reassigned as
required and the destination can be changed without advance notice.
Because of that, it is beneficial to fashion a packet switching
approach for cellular communication that interfaces effectively
with a wired backbone network.
SUMMARY
[0009] An improved system is achieved in a cellular arrangement
where mobile units employ a moveable slot TDM approach to send
packets to base stations, and where the base stations use a
non-contention approach. More specifically, from among a specified
band of frequencies, the base station selects a frequency (an
outbound frequency) and uses it to transmit information packets to
a mobile unit within the cell covered by the base station. The
information packets are simply queued with information packets
destined to other mobile units, as necessary, and transmitted over
the outbound frequency. Corresponding to the outbound frequency
there is an inbound frequency that is used by the mobile unit to
transmit information packets to the base station.
[0010] The base station also transmits information about the status
of the inbound frequency; i.e., information that informs whether
the inbound frequency is occupied by a signal that is being
transmitted to the base station, or whether a collision exists. A
collision on an inbound frequency occurs when more than one signal
is simultaneously transmitted on the inbound frequency. This
information can be communicated over a separate channel. It can
also be embedded within the stream of bits on the outbound
frequency by appropriate injection of Data Link Escape (DLE)
sequences.
[0011] In addition to transmitting information packets and embedded
DLE sequences over the outbound frequencies, the base station
transmits general "control-channel" information, in a TDM fashion,
over a frequency that is shared by all base stations.
[0012] The mobile units, on the other hand, employ a movable slot
TDM (MSTDM) protocol in their transmissions to a base station.
Before it transmits, each mobile unit listens to a signal from the
base station that informs it of the status of the in-bound
frequency that the mobile unit is assigned to use for transmissions
to the base station. If the DLE sequences inform a mobile unit that
the inbound frequency is not busy (when such an approach is used),
the mobile unit is permitted to begin transmissions. When the
mobile unit wishes to transmit a data packet or a first voice
packet, the mobile unit is also sensitive to collision information
(e.g., delivered via the DLE sequences). When a collision is
detected, such a mobile unit stops transmitting and tries again
later. When the mobile unit wishes to transmit continuation voice
packets, it only listens for a non-busy inbound frequency before it
transmits. It does not stop transmitting in case of a collision.
Continuation packets include a short header that carries no
information, to insure against corruption of information in case of
a collision.
[0013] With the use of MSTDM for inbound traffic, the re-use of
frequencies is simplified in the system. Specifically, the only
requirement is that adjacent cells should not use the same
frequencies. Accordingly, the available band of frequencies (minus
the "control channel" frequency) is divided into three sub-bands,
and different sub-bands are assigned adjacent cells. The control
channel is broken up into time segments, and the time segments are
assigned to adjacent base stations in the same way that the
sub-bands are assigned.
BRIEF DESCRIPTION OF THE DRAWING
[0014] FIG. 1 presents a general block diagram of a system that
employs packet switching and switching agents;
[0015] FIG. 2 presents details about an illustrative topology of
the structure between the base stations and the switching
agents;
[0016] FIG. 3 shows frequency allocations for cells;
[0017] FIG. 4 shows the power considerations for transmissions by
mobile units;
[0018] FIG. 5 depicts the cell hysteresis that is created with
proper selection of base station and mobile unit transmission
power; and
[0019] FIG. 6 illustrates a portion of a mobile unit's structure
that allows graceful degradation in case of overload conditions at
a base station.
DETAILED DESCRIPTION
Structure
[0020] FIG. 1 depicts the general structure of a network that
includes a wired portion above dashed line 10 and a wireless
portion below dashed line 10. The wireless network comprises cells,
depicted in the form of hexagons, e.g., hexagons 11-18, which
completely cover a given collection of service areas. A service
area can span any convenient geography, such as a city, a city and
its suburbs, or an area into which people frequently commute. In
the center of each cell there is a base station, e.g., element 21,
that provides connection between the wireless network and the wired
network. Lines 31-36 diagrammatically show this connection. Each
switching center that is coupled to base stations connects the base
stations with the existing wide-area communications network
100--for instance, the conventional, circuit switched, telephone
network, or the Internet.
[0021] In accordance with the principles disclosed herein, the
coupling between the base stations and the service areas on one
side, and network 100 on the other side, is effected through
switching agents. Each registered mobile unit is represented by a
switching agent at the interface to network 100; for example,
agents 61-64. The agent is responsible for translating between the
formats that are used in network 100 and the service areas (if
necessary), and for all operations needed for mobility.
[0022] Service areas 40 and 50 are wired packet switched networks.
In addition to other functions, they serve as buffers to remove all
responsibility for mobility from network 100. A service area can
have any one of a number of different topologies, such as the
well-known star, tree, mesh, ring, bus, or regular mesh. All of
these networks can support packet switching. Of course, a service
area should employ a topology that allows for easy interconnection
with adjacent service areas. Moreover, interconnected adjacent
areas should be arranged to have a hysteresis at the areas'
boundaries. By hysteresis I mean that instead of moving a switching
agent to an adjacent service area as soon as the mobile unit
arrives at a cell of that adjacent service area, the switching
agent is kept in the original service area until the mobile unit
moves more deeply into the adjacent service area. This hysteresis
reduces processing associated with migration of a switching agent
from one service area to another, because at times the mobile unit
returns to the original service area. This hysteresis is depicted
in FIG. 1 by the overlap between service areas 40 and 50.
[0023] The specific architecture, or topology, of the service areas
is not important to the broad principles of this invention; but for
sake of completeness, it is useful to review the various topologies
that are implementable.
[0024] One such topology is the star topology, where lines from all
of the base stations in a service area terminate at one packet
switch (the "central office"). The switching agents are installed
between the "central office" of each service area and a switch on
the wide-area communications infrastructure 100, say in that
"central office". The "central offices" of the various service
areas are advantageously interconnected to allow for easy migration
of switching agents, e.g. via several lines running between
them.
[0025] As a mobile unit moves between cells within a service area,
the connection through the central office packet switch changes,
but the connection on infrastructure 100 remains fixed. A
disadvantage of this topology is that there is no redundancy in the
connection between the base stations and the central office.
[0026] A tree topology is similar to a CATV network, when the
"central office" is located at the root, and the cells are located
at the leaves of branches. For packets destined to the base
stations, routing decisions are made at each branch split in the
tree. For traffic destined to the head end, multiplexers combine
the packets and send them toward the "central office". An advantage
of this approach is that the CATV infrastructure is in place in
most parts of the United States, and packet multiplexers and
splitters are commercially available. The overlap between service
areas can be created by placing a splitter/multiplexer at the trunk
of the tree and using the multiplexer to switch a number of
connections to an adjacent tree. The principle disadvantage of this
architecture is its weak reliability. There are many locations
where the failure of a single line or component can disrupt
communications for a large number of cells.
[0027] A general mesh topology can be implemented by a network of
Internet routers between the central office and the base stations.
This type of network can be made as reliable as needed by
installing redundant lines and routers. Service areas can be
interconnected through the routers. The disadvantage of this
approach is the expense of locating a router at each cell site.
Routers may be used advantageously within the service areas, but a
simpler device should be associated with each cell.
[0028] Two possible distribution networks that are considerably
simpler than routers are the FDDI dual ring network, and the DQDB
dual bus network. Both of these networks can survive single
failures. The disadvantages of these networks are that it is
difficult to interconnect them to create overlapping service areas,
and the load per link increases linearly with the number of nodes
in the network. The latter characteristic constrains the number of
base stations that can be located on the same network.
[0029] The two disadvantages associated with FDDI and DQDB networks
are overcome by another regular topology, the Manhattan Street
Network (MSN), which was disclosed by me in U.S. Pat. No.
4,797,882, issued Jan. 10, 1989. Regular arrays of MSN's can be
interconnected into larger regular arrays to construct overlapping
service areas. The MSN can also be partitioned into
non-interfering, independent, communities of interest, which makes
it possible to support arbitrarily large numbers of base stations
that do not communicate with one another.
[0030] Actually, the disadvantages associated with specific
technologies are eliminated in the arrangement shown in FIG. 2 by
combining several technologies. In FIG. 2, the "central office"
switches in each service area, such as switches 41, 51, 61, and 71,
concentrate the connections from network 100 to a router. For
example, switch 41 has a number of logical connections to switch
101 on one side (by means of the various switching agents) and a
physical connection to router 81 on the other side. In the reverse
direction, switch 41 fans out the connections from the router to
switch 101. Router 81 is but one of a number of routers that make
up router network 80. Several service areas (and their associated
"central office" switches) are connected to router network 80, and
several other connections couple network 80 to distribution network
90. Those connections couple network 80 to neighborhoods of network
90, such as neighborhoods 95 and 96. The connection to each
neighborhood is, advantageously, a multiple connection. This
eliminates the problem of a single point of failure. Additionally,
the routers within network 80 are multiply interconnected for
increased connectivity and reliability. The distribution network in
FIG. 2 is an MSN network and, as indicated above, it comprises
neighborhoods. A service area can have several neighborhoods. As an
aside, the functionality of network 90 can subsume that of network
80. Network 80 is depicted to illustrate the fact that different
networks can be employed. Indeed, currently the components that
make up network 80 are readily available commercially, and the use
of network 80 allows network 90 to be smaller.
General Operation
[0031] The operation of the FIG. 2 network is quite effective. Each
mobile unit that is known to be present in the area (i.e., is
registered) has an associated switching agent--which is a software
module, or object--at a gateway between a service area and network
100. For convenience, the switching agent resides in a node within
a service area, and this disclosure refers to this node as a
"central office". Information that needs to be sent by network 100
to a particular mobile unit is transferred to that unit's switching
agent. From the switching agent, packets are sent to the mobile
unit via a path that comprises the service area's "central office"
where the switching agent resides, one or more routers in network
80, and one or more nodes in network 90. Packets that emanate from
a particular mobile unit are aimed at its associated switching
agent. That is, they conveniently contain an address that
identifies the "central office" and the switching agent. They also
contain the address of the base station that is to receive the
mobile unit's packets. The latter address allows the "central
office" to decide whether to migrate the switching agent to another
"central office" (thereby realizing the service area hysteresis
disclosed above). From the base station, the packets enter an MSN
(for example, MSN 95) and then they are passed to a router within
network 80, e.g., router 85. Network 80 routes the packets to the
switch with which the mobile unit's switching agent associates
(e.g., switch 41). All this is done based on the addresses
contained in the packets.
[0032] Typically, when the mobile unit moves to an adjacent cell,
there is no effect on operation other than the fact that the packet
enters the MSN network (e.g., network 95) at a different point. On
occasion, however, when the mobile unit moves from a cell in one
service area into a cell in an adjacent service area (and not into
an area where the two service areas overlap), the operation does
change. Specifically, the "central office" realizes that the base
station, which sent the packets, is far removed from the
geographical area that is normally handled by the "central office"
and the central office accordingly migrates the switching agent to
a new "central office".
[0033] In such an event, the connection with network 100 also needs
to be changed because network 100 needs to communicate with the
switching agent at its new location. When network 100 is a circuit
switched network, the existing circuit to the switching agent needs
to be torn down, and a new circuit needs to be established pointing
to the service area to which the switching agent was moved. When
network 100 is a packet network, e.g., Internet, then the
accounting for the moved switching agent must be carried out with
whatever particular protocol is employed in the network.
[0034] When a mobile unit wishes to register itself, it transmits a
packet without identifying a destination switching agent. The base
station accepts the packet and routes it to a central office that
is assigned to the base station. That is, the base station directs
the packets to a "central office" onto which it homes. Since the
packet does not identify a destination switching agent, the central
office creates one (after appropriate service provision tests have
been met) and responds to the mobile unit with the switching
agent's identity. When a mobile unit wishes to initiate a call, it
sends a control packet that causes the switching agent to
appropriately engage network 100 to establish the desired
connection.
[0035] When a call is initiated to a mobile unit that is not
registered, there is no switching agent available, and the calling
party receives a message to the effect that the mobile unit is not
found. When a call is initiated to a mobile unit that is inactive,
albeit registered and having an agent, the agent can establish
contact with the inactive mobile unit over a common control
channel.
[0036] Once a contact is established with a mobile unit, the
switching agent sends out encapsulated packets (i.e., each being a
packet within a packet) to the mobile unit. The outer packet is
addressed to a particular cell, or base station, while the inner
packet is addressed to a particular mobile unit. The agent needs to
change only the address of the outer packet when the mobile unit
moves from cell to cell. But, that is a lot less work--one
bookkeeping operation--than setting up and tearing down a
circuit-switched connection.
Communication to the Base Station
[0037] Current cellular systems are basically circuit switched
systems. Such systems inherently dedicate a channel to a particular
call, and the capacity of that channel is captured by that call
whether or not that call actually utilizes the captured capacity.
In a sense, this is an inefficiency. The FIGS. 1 and 2 systems are
packet systems that use random access techniques. Random access
techniques do not inherently assign a particular capacity to a call
and therefore have the potential for a more effective utilization
of the available bandwidth. However, pure random access
techniques--where a mobile unit is allowed to transmit at
will--also possess a characteristic that causes inefficiency.
Specifically, there is clearly a potential for collision of data
when two or more units are transmitting at the same time. Some
capacity is used by virtue of the means that are provided to
resolve contention over use of the inbound channel, and whatever
capacity is so used constitutes inefficiency.
[0038] What is interesting about cellular networks as they are
developing is the fact that they are shrinking in size. One
consequence of the shrinking size is a smaller propagation delay
within a cell. The smaller propagation delay makes it possible to
use efficient contention detection strategies, such as the Carrier
Sense Multiple Access (CSMA) protocol or CSMA with collision
detection (CSMA/CD). The latter is the protocol that is used on the
Ethernet.
[0039] In the CSMA protocol, a mobile unit listens to the
transmit-frequency before starting to transmit to determine whether
another mobile unit is already using the channel. When the channel
is not busy, the mobile unit stops listening and starts
transmitting. Because of propagation delays, however, it is
possible for different mobile units to find the channel not busy,
to start transmitting, and to thus create a collision condition.
CSMA/CD overcomes this problem by continuing to listen to the
channel even after the mobile unit begins transmitting. Collision
is detected by the mobile unit when it finds out that the channel
is carrying more than just its own transmission. When a collision
is detected, the unit stops transmitting, and tries to retransmit
at another (randomly selected) time. Another unit that caused the
collision also stops its transmission and also retries to
retransmit at a later time.
[0040] Identifying the presence of a collision condition requires
that a unit detect the presence of a signal from another unit while
it transmits on the same frequency and thus also receives its own
signal. In a cable environment, that is not too difficult because a
properly terminated cable does not produce echoes and therefore the
cable unit can easily subtract its own signal from the received
signal. Even when echoes exist, they are generally of small
magnitude and relatively constant with time, allowing conventional
echo cancellation techniques to be used effectively. Collision
detection in a radio network, however, is much more difficult
because unexpected echoes (reflections) can be much stronger than
the signal from other stations. Compensating for reflections in a
wireless system requires considerable processing and delay. This is
particularly true in a mobile environment where the echoes change
as a mobile unit moves from one location to another.
[0041] Contention systems can be used for voice communications.
However with such use it is difficult to provide the required
service guarantee. There are hybrid schemes that assign a channel
after a user successfully completes a contention protocol--for
example, "demand assigned multiple access" and "movable boundary"
protocols. These systems require both a contention and a circuit
allocation protocol.
[0042] A third alternative, which is a variant on CSMA/CD, is the
moveable slot TDM (MSTDM) protocol. In MSTDM, sources also contend
for a channel and then have a guaranteed rate until they relinquish
the channel. However, the MSTDM protocol is completely distributed,
and the assigned channels as well as the random access packets use
the same protocol to share all of the bandwidth.
[0043] In MSTDM, the notion is that there are data sources and
voice sources. Data sources always use CSMA/CD. The voice sources
use CSMA/CD only for the first packet of information, and use CSMA
for continuation packets. A continuation packet is transmitted a
fixed period after the successful transmission of the previous
packet in the same packet stream. If the channel is busy (e.g.,
because a data source grabbed the channel a moment earlier), the
continuation source waits and transmits as soon as the channel
becomes available. The CSMA protocol is viable for continuation
packets because the continuation voice packet includes a preempt
signal at the beginning of the packet. Consequently, a data source
which sees a non-busy channel, starts transmitting and then detects
a collision condition can stop transmitting before it interferes
with the voice source.
[0044] The length of data packets is constrained to be shorter than
continuation voice packets, so that a random access packet cannot
force a continuation voice packet that is waiting for the channel
to collide with the next scheduled voice packet. When a
continuation voice packet is delayed, all of the samples that
arrive while it is waiting are included in the packet. The next
packet is scheduled a standard delay after the channel is
successfully acquired, rather than after the channel should have
been acquired. With this protocol, voice sources never collide with
each other, even when the channel utilization factor approaches
one. Therefore, there is no distortion of the voice source and the
only voice delay is the packet assembly time.
[0045] While the MSTDM protocol allows mobile units that transmit
voice to operate mostly without the need to detect collisions,
there is still some collision detection that must be carried out
(for data packets and for the first voice packet). As mentioned
above, however, collision detection in a wireless environment is
difficult because of the echoes problem. I realized, however, that
a two-channel approach can be adopted for cellular transmission
which obviates the echoes problem, provides for easy detection of
collisions, and provides other benefits.
[0046] Specifically, in the two-channel collision detection
approach the mobile units send signals over one channel, and the
base station retransmits its received signal over another channel.
By performing the retransmission over a channel that is
non-interfering with the channel over which the mobile units
transmit to the base stations, e.g. over a different frequency,
avoidance of the echo problem becomes relatively easy. What the
mobile units receive over the second channel is precisely what the
base station has received. The strong echoes back to a transmitting
mobile unit are simply not seen on that second channel. By
listening to the "busy channel" over which the base station
retransmits its received signal, the mobile units can perform
collision detection and stop transmitting when appropriate.
[0047] Once a two-channel approach to collision detection is
settled upon, one can observe that the two-channel approach allows
the mobile units to transmit signals only as far as the base
station in the center of the cell, i.e., the longest distance is
the radius of the cell. By comparison, when the mobile units need
to listen to transmissions of other mobile units, the transmitted
signal must be capable of reaching from one point on the
circumference of the cell to a diametrically opposite point on the
circumference of the cell. This allows for a greater re-use of
frequencies. FIG. 3 shows the single-channel approach on the left,
and it requires seven different frequency bands. By comparison, the
two-channel approach is shown on the right, and it requires only
three different frequencies. In this arrangement, all cells
(hexagons) that are adjacent to a cell "A" have a different
frequency from the frequency of cell "A". The frequency of cell "A"
is repeated at cells whose centers are removed from the center of
cell "A" by a distance of 3D{square root}{square root over (3/2)}
distance units, where 2D is the distance between the center of cell
"A" and any adjacent cell.
[0048] FIG. 4 shows still another benefit of the two-channel
approach. Cell 201 uses frequency F1, cells 202, 204, and 206 use
frequency F2, and cells 203, 205, and 207 use frequency F3. Cell
208 re-uses frequency F1, and so the pattern repeats. A mobile unit
at the edge of cell 201 and communicating with the base station at
the center of cell 201 needs to transmit with only enough power to
reach the center of cell 201. This is depicted by circle 211 that
is centered about mobile unit 210. With that in mind, one might
realize that mobile unit 210 can transmit with substantially more
power before its signal would reach the center of cell 208 and
interfere with the operation of that cell. Specifically, it can
transmit with power that approaches the coverage of circle 212. Of
course, one would not want to operate this way with no guard area,
but it does suggest that both the power of the base station's
transmitter and the power of the mobile units may be increased.
Another way to view it is that the cell sizes may be increased
while keeping their centers constant. Such an arrangement creates
overlapping, non-interfering, cells, as shown in FIG. 5. The effect
of allowing the size of the cells to increase is dramatic. The area
that is blank within hexagon 201 of FIG. 5 is serviced by one of
the three frequency assignments. The areas that are striped are
serviced by two frequencies (in the group of three), and the areas
that are crosshatched can use all three frequencies. In effect, the
FIG.5 arrangement represents a planned hysteresis in the cells.
Communication from the Base Station
[0049] The base station communicates with the mobile units on three
levels: it transmits information from network 100, it transmits
"busy channel" information (for the MSTDM protocol), and it outputs
other control information over a control channel.
[0050] The outbound traffic of network 100 allows for a very simple
air interface. Since the base station is the sole signal source and
there is no question of collisions or interference, packets
destined to a number of mobile units are assigned a frequency,
queued as they arrive, and promptly transmitted over that
frequency. One needs to be concerned, of course, with voice
sources, where information must be sent at relatively regular
intervals. That concern has been put to rest in the prior art
through use of appropriate voice encoding and scheduling
techniques, which can be applied herein.
[0051] The information about the channel being busy or the channel
experiencing a collision can be sent over a separate channel, but
it does not need to be. The base station can easily differentiate
between a channel (i.e., a receiving frequency) being busy or not
busy. That information can be imparted by the base station simply
by transmitting information wherever the channel changes state.
[0052] Another way for the mobile units to receive the needed
information is for the base station to send information at the
instances when the channel becomes busy with voice packets or with
data packets. Since the length of the packets is known, the
intervals when the channel is not busy can be ascertained by the
mobile units themselves. Thus, the information that needs to be
sent by the base station over the second channel of the two-channel
collision detection approach requires very little capacity.
[0053] In addition to sending information that allows the mobile
units to determine when the channel is not busy, information needs
to be sent whenever a collision occurs. The latter will occur
fairly rarely in small cells, but it still can happen. What is
important in MSTDM is to detect collision with voice packets,
because transmission of continuation voice packets should not be
aborted. Since data packets are aborted when a collision occurs, it
is less important to detect collisions early. In fact, collision
for data packets can be detected by a base station when, after the
packet is received, the packet's error detection code indicates a
reception error. Although some capacity in the inbound channel
could have been saved by having an early detection of collision,
the overall loss in capacity caused by employing a separate channel
for re-transmitting to the mobile units the signal received by the
base station is not called for, in light of the low probability of
collisions in small cells.
[0054] As indicated above, however, it is important to detect
collision between data packets and voice packets as early as
possible. This may be achieved by incorporating a distinguishing
feature in the packets themselves; e.g. a given bit is 0 for voice,
and 1 for data. Alternatively, the distinguishing feature can be in
the mode of transmission that is employed. For example, data
packets can be transmitted by mobile units with a suppressed
carrier modulation scheme, whereas voice packets can be transmitted
with a non-suppressed carrier modulation scheme.
[0055] Transmitting busy/not busy/collision information in the
manner described above represents a very small amount of
information and, therefore, in the FIGS. 1 and 2 systems this
information is injected into the channel that carries the outbound
information packets. This is achieved by the base station injecting
a Data Link Escape character (DLE) into the bit stream followed by
two information bits, as shown by way of example in the table
below.
1 Bits following the DLE Option I Option II 00 Channel became not
busy Channel became busy with voice 01 Channel became busy Channel
became busy with data 10 A collision has occurred A collision has
occurred
[0056] In addition to the channel that transmits outgoing
information packets to the mobile units, each base station employs
a common control channel for sending control information to the
mobile units. Actually, since the amount of information that this
channel needs to carry is not great, all base stations employ a
common frequency for such transmissions. In order to avoid
interference between adjacent base stations, each base station is
assigned its own time slot on that frequency in such a way that
base stations that might interfere with one another do not transmit
at the same time. The interference between base stations using the
control channel in the time domain has the same constraints as the
interference in the frequency domain for communications from the
base station to the mobile units. Therefore, the pattern for
re-using time slots is the same as the pattern for re-using
frequencies. For example, in the arrangement of FIG. 4 time is
divided into three slots.
[0057] During its time slot a base station transmits a packet
containing:
[0058] the base station's identity,
[0059] the set of transmit and receive channels that it is
using,
[0060] its channel utilization,
[0061] broadcast requests from switching agents that are trying to
locate and activate mobile units, and
[0062] the list of mobile units that are currently registered to
receive packets in this cell and their allowed transmission rates
(e.g., all packets or only high-priority packets).
[0063] A packet on the control channel has a maximum length that is
constrained by the width of a time slot. If the complete list of
registered mobile units cannot be transmitted in one packet, it is
continued in the next packet, with an end-of-list identifier to
indicate when the list is complete.
[0064] The power transmitted in the control channel is sufficient
to guarantee that a mobile unit will always receive the signal from
at least one base station, but that power is lower than the power
used in the other channels. The power difference guarantees that a
mobile unit can receive data from any base station from which it
receives a control signal.
[0065] A mobile unit joins the list of active stations in a cell by
transmitting a data packet to a base station whose signal it
receives on the control channel. The base station notifies the
mobile unit's switching agent that all communications with the
mobile unit are to be addressed through this base station. If a
mobile unit receives a control signal from several base stations,
it can elect to join the base station with the lower
utilization.
[0066] Mobile units are removed from the list of registered units
in a cell whenever they leave the cell, stop transmitting, or
become disabled. Since a mobile unit cannot always notify the base
station when communication has ended, the list is maintained in the
base station as soft states. When the base station does not receive
a packet from a mobile unit for a period of time, the mobile unit
is removed from the list of registered units.
[0067] If a registered but inactive mobile unit receives its
identifier in the broadcast segment of the control slot, it means
that its agent is trying to establish a connection. If an active
mobile unit receives its identifier in this segment, it means that
its connection has been broken. In either instance, the mobile unit
sends a data packet to the base station in order to establish (or
re-establish) a connection.
[0068] To summarize the air interface between a base station and
mobile units, the base station has a band of frequencies that it
uses to transmit information packets to mobile units within the
cell. The information packets are simply queued as necessary and
transmitted over the base station transmit frequencies (outbound
frequencies). Corresponding to each outbound frequency there is a
frequency that is used by the mobile units to transmit information
packets to the base station (inbound frequency). Embedded within
the stream of bits on the outbound frequency which the base station
transmits are DLE sequences that inform the mobile unit of the
status of the inbound frequency. In addition, the base station
transmits control channel information, in a TDM fashion, over a
frequency that is shared by all base stations.
[0069] The mobile units, on the other hand, employ MSTDM protocol.
Each mobile unit listens before it transmits. If the DLE sequences
inform a mobile unit that the inbound frequency is not busy, the
mobile unit is permitted to begin transmissions. When the mobile
unit wishes to transmit a data packet or a first voice packet, the
mobile unit is also sensitive to collision information delivered
via the DLE sequences. When a collision is detected, such a mobile
unit stops transmitting and tries again later. When the mobile unit
wishes to transmit continuation voice packets, it only listens for
a non-busy inbound frequency before it transmits. It does not stop
transmitting in case of a collision. Continuation packets include a
short header that carries no information, to insure against
corruption of information in case of a collision.
Overload
[0070] The FIGS. 1 and 2 arrangement does not insure against
overload conditions. When a cell is heavily loaded, i.e., the
inbound channel or the outbound channel is close to being fully
loaded most of the time, service can be denied to a new mobile unit
that wishes to become active. However, that does not prevent
overload conditions because an active mobile unit can move into the
heavily loaded cell and cause an overload condition.
[0071] I realized that overload conditions can be accommodated with
the MSTDM protocol mostly without denying access to mobile units.
It helps when all of the periodic sources use the same packet rate.
Instead of treating a voice source as a single packet stream, the
voice source can be partitioned into two or more periodic streams.
For instance, one stream can contain the samples that are needed
for intelligible communications, while the second stream can
contain the samples that provide a higher quality connection.
Normally, a mobile unit acquires two periodic channels and sends
both packet streams. However, during an overload the mobile unit
can be instructed to only send one stream. The control channel
provides the mechanism for notifying the mobile units of how many
voice streams they may transmit. Of course, capacity in a cell can
also be increased for voice transmissions by asking the mobile
units to transmit only during the active speech intervals in a TASI
type of operation. TASI does tend to cut off a beginning portion of
an active speech interval. In this mode, therefore, when there are
more active speakers than available capacity, the beginning of
active intervals may be lost.
[0072] Another way to handle overload is to take advantage of the
hysterisis in the cells. As shown in FIG. 5, there can be
substantial areas within each cell that can be serviced by one or
two other adjacent cells. Taking advantage of this hysteresis is
applicable to both overload from active mobile units that come in
(and stay) in the cell as well as overload from inactive units
wishing to become active. The mobile unit selects the base station
with the strongest signal, which is not over-utilized, and directs
its packets to its switching agent via the selected base station.
The switching agent detects the identity of the base station from
which the packets come and accordingly adjusts the address of the
packets which the base station transmits when it wants to
communicate information to the mobile unit.
[0073] A combination of the above techniques is also possible. The
primary base station may constrain the mobile unit to send only
packets that are needed for comprehensive speech, and the mobile
unit may still be able to transmit the packets that can be used for
higher quality through another base station. In this instance, some
of the packets would arrive at the mobile unit's switching agent
through one base station and the remainder of the packets would
arrive at the mobile unit's switching agent through another base
station. The packets include a sequence number, if necessary, and
the agent is responsible for properly sequencing and spacing the
packets.
[0074] During severe overload, a protocol is needed to redistribute
mobile units. A hybrid protocol that couples independent operations
of the mobile units with the cooperative operations of the base
stations provides a means to be both responsive to short term
fluctuations and to level load imbalances over a large area. The
mobile unit can quickly shift its own load between overlapping
cells, while base stations must cooperate to redistribute the load
over a wider area.
[0075] The protocol to move mobile units can use different types of
information. A simple protocol could allow a heavily utilized base
station to use the control channel to move some mobile units to
overlapping, less heavily utilized cells. The base stations in the
adjacent cells could then move other mobile units to cells that are
further from the congested cell, making it possible for the
congested cell to move more units. In a more sophisticated
protocol, a base station could take into account the number of
units that adjacent cells can redistribute and any other congested
regions that may be near the adjacent cells.
[0076] With packet switching there is a possibility that packets
arrive out of order and that the inter-packet timing will not be
maintained, especially as a mobile unit changes base stations. To
overcome this potential problem, the packets in the arrangement
disclosed herein contain a sequence number and timing information
so that the switching agent can accurately reconstruct the signal
before transmitting it to the switched network. The RTP protocol,
used on the Internet, includes the necessary information.
[0077] FIG. 6 presents a general block diagram showing those
portions of a mobile unit that provide the capability to transmit
packets as described above. Receiver 304 receives signals from the
base station and derives from the control channel information about
overload. This information is applied to filter 300, coder 301, and
transmitter 303. The voice signals are applied to filter 300, and
appropriate signals are developed at the output of filter 300 and
applied to coders 301 and 302. Specifically, when a no-overload
condition is indicated, coder 301 receives the applied voice
signal, and coder 301 develops a stream of packets corresponding to
the applied voice signal. When an overload condition is indicated,
coder 301 receives only a portion of the voice signal that is
needed for intelligibility, and coder 301 develops a stream of
packets at a rate that is lower than the rate developed for a
no-overload condition. In system applications where a mobile unit
is directed to send some of its voice packets to a different base
station (when there is an overload at the base station with which
the mobile unit is communicating), transmitter 303 utilizes the
output packet stream of coder 302. Accordingly, coder 302 is
adapted to provide a packet stream in response to a signal that is
developed by filter 300. The signal developed by filter 300 and
applied to coder 302 is that portion of the applied voice signal
that complements the signal applied to coder 301 when an overload
condition exists. Illustratively, under normal conditions, filter
300 merely applies its incoming speech signal to coder 301. When a
control signal directs modified operation, filter 300 separates the
voice signal into a primary band and a secondary band. Both are
shifted to base-band, and then applied to coders 301 and 302.
[0078] It may be noted that the overlap depicted in FIG. 5, which
provides for cell hysteresis can be employed to advantage in more
than just overload situations. For example, cell hysteresis
eliminates the sometimes-occurring glitch in speech that comes
about from cell switching in the middle of an active speech
interval. Cell hysteresis allows a moving mobile unit to stay in
contact with the base station of the cell it has temporarily left,
so that when the moving unit returns to the cell, the process of
moving to a different base station and returning to the original
base station is eliminated. Lastly, cell hysteresis reduces the
surface area that loses service when a base station fails.
The Switching Agent
[0079] The switching agent must translate between the data format
that is used on network 100 and the packet format. For example,
when network 100 carries speech in 64 Kbps (i.e., 8 bit samples are
transmitted at the rate of 8000 samples per second) and the packets
carry 20 msec of speech each, the switching agent needs to assemble
20 msec worth of speech from network 100 in order to create a voice
packet. In the other direction, the switching agent needs to take
the 20 msec of speech delivered by a packet, create samples, and
evenly transmit them to network 100.
[0080] The switching agent also keeps track of the base station
that can transmit to its mobile unit. As a mobile unit moves from
cell to cell it notifies its agent. When an agent must locate an
inactive mobile unit, to place a phone call or to locate an active
unit that has lost contact, it broadcasts a message to all of the
base stations which is placed on their control channels. The hailed
mobile unit responds and thereby informs its switching agents of
its whereabouts.
[0081] The switching agent also maintains a connection on network
100 on behalf of the mobile unit. The agent breaks the connection
at the end of a communication session or when a failure occurs.
Since the switching agent is not always notified of a failure, it
maintains a soft state connection so that resources in network 100
are not tied up indefinitely. If the switching agent stops
receiving packets for a period of time, it first tries to contact
the last base station, and then tries a broadcast message to the
mobile unit. If communication with the mobile unit cannot be
re-established, the connection on network 100 is terminated.
[0082] For sake of completeness, it should be mentioned that the
FIGS. 1 and 2 arrangements do not, in and of themselves, overcome
the well-known privacy problem of cellular telephony. Packets that
are addressed to one mobile unit can be detected by another mobile
unit. The advantage of the packet system is that the data is
digital and can be encrypted or scrambled more easily. In other
words, the privacy problem is easily overcome with the disclosed
system by employing known encryption techniques, such as the one
disclosed by Reeds et al in U.S. Pat. No. 5,172,414, issued Dec.
15, 1992.
* * * * *