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Claims  |
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We claim:
1. A code division multiple access radio transmitter unit comprising
means for receiving a digital bitstream from a user at one of a plurality
of source bit rates, wherein said plurality of source bit rates includes a
basic bit rate R and at least one other bit rate which is a multiple M of
the basic bit rate R, where R and M are positive integers of at least 1, a
user input selecting a particular user source bit rate by identifying a
basic bit rate multiple M,
adjustable coding means, responsive to said user input, for spreading and
transmitting the user digital bit stream received at the selected bit rate
to a channel bit rate which at least equals the highest bit rate of said
plurality of source bit rates,
wherein said adjustable coding means includes
a serial-to-parallel means, responsive to a user's input identifying a
basic bit rate multiple M, for converting a received user bit stream,
which is M times a basic bit rate R, into M basic bit rate streams,
M encoder means for spreading each of the M basic bit rate streams, using a
different spread code C, into M channel bit rate signals, and
means for combining the M channel bit rate signals into one channel bit
rate signal and modulating the channel signal onto a carrier signal for
transmission from said transmitter unit.
2. The radio transmitter of claim 1 wherein the user can dynamically change
the selected bit rate during a transmission.
3. The radio transmitter unit of claim 1 which communicates with a base
station over a facility and which further comprises
means for receiving an uplink control signal over the facility,
means, responsive to the uplink control signal from the base station, for
making a probabilistic determination of the success of the radio
transmitter unit transmitting at one or more multiples M of the basic bit
rate R, and wherein
said adjustable coding means is responsive to a determined multiple M for
transmitting the user's digital bit stream at M times the basic bit rate R
over the facility.
4. The radio transmitter unit of claim 1 communicates with a base station
over a facility and which further comprises
means for receiving a signal from the base station indicating a probability
of success for transmissions at different multiples M and wherein
said adjustable coding means determines which multiple M of the basic bit
rate R at which to transmit the user's digital bit stream over the
facility.
5. The radio transmitter unit of claim 1 wherein
the user's digital bit stream is received as burst data and wherein
said adjustable coding means transmits the burst data at a first bit rate
and maintains a sub-rate signaling during a silent interval between bursts
of data.
6. A code division multiple access radio transmitter unit comprising
means for receiving a digital bitstream from a user at one of a plurality
of source bit rates, wherein said plurality of source bit rates includes a
basic bit rate R and at least one other bit rate which is a multiple M of
the basic bit rate R, where R and M are positive integers of at least 1, a
user input selecting a particular user source bit rate by identifying a
basic bit rate multiple M,
adjustable coding means, responsive to said user input, for spreading and
transmitting the user digital bit stream received at the selected bit rate
to a channel bit rate which at least equals the highest bit rate of said
plurality of source bit rates,
wherein said adjustable coding means includes
first coding means for adjusting its coding in response to said user
identified base bit rate multiple M,
serial-to-parallel means, responsive to said multiple M, for converting a
signal from said first coding means into M basic bit rate streams,
M encoder means for spreading each of the M basic bit rate streams, using a
different spread code C, into M channel bit rate signals, and
means for combining the M channel bit rate signals into one channel bit
rate signal and modulating the channel signal onto a carrier signal for
transmission from said transmitter unit.
7. A code division multiple access radio transmitter unit comprising
means for receiving a digital bitstream from a user at one of a plurality
of source bit rates, wherein said plurality of source bit rates includes a
basic bit rate R and at least one other bit rate which is a multiple M of
the basic bit rate R, where R and M are positive integers of at least 1, a
user input selecting a particular user source bit rate by identifying a
basic bit rate multiple M,
adjustable coding means, responsive to said user input, for spreading and
transmitting the user digital bit stream received at the selected bit rate
to a channel bit rate which at least equals the highest bit rate of said
plurality of source bit rates,
wherein said adjustable coding means includes
repeater means, responsive to a user's input identifying a basic bit rate
multiple M, for generating a chip bit rate signal by repeating a packet of
user data from said user's digital bit stream C/M times, where C is the
ratio of a chip bit rate to basic bit rate R,
first encoder means for encoding the C/M user's data packets in said chip
bit rate signal to form an encoded chip bit rate signal,
selector means for selecting one of the encoded user's data packets from
said encoded chip bit rate signal,
serial-to-parallel means, responsive to said multiple M, for converting
said encoded chip bit rate signal into M basic bit rate streams,
M encoder means for spreading each of the M basic bit rate streams, using a
different spread code C, into M channel bit rate signals, and
means for combining the M channel bit rate signals into one channel bit
rate signal and modulating the channel signal onto a carrier signal for
transmission from said transmitter unit.
8. The radio transmitter unit of claim 7 wherein said chip bit rate is the
same as said channel bit rate or a submultiple of said channel bit rate.
9. A code division multiple access radio transmitter unit comprising
means for receiving a digital bitstream from a user at one of a plurality
of source bit rates, wherein said plurality of source bit rates includes a
basic bit rate R and at least one other bit rate which is a multiple M of
the basic bit rate R, where R and M are positive integers of at least 1, a
user input selecting a particular user source bit rate by identifying a
basic bit rate multiple M,
adjustable coding means, responsive to said user input, for spreading and
transmitting the user digital bit stream received at the selected bit rate
to a channel bit rate which at least equals the highest bit rate of said
plurality of source bit rates,
the radio transmitter unit further comprising
first means for requesting a communication connection over a common access
channel of a communication facility,
means for receiving a primary code C.sub.1, over a broadcast channel of
said facility, said primary code enabling said transmitter unit to
transmit at the basic bit rate R,
second means for requesting over said facility, in response to the user
input, a change in transmission rate from the basic bit rate R to a rate
which is multiple M of the basic bit rate R, and wherein
said adjustable coding means changes its transmission bit rate in response
to a control signal received over said facility identifying a multiple M'
(M'.ltoreq.M) times the basic bit rate R at which said radio transmitter
unit can transmit.
10. The radio transmitter unit of claim 9 further comprising
means for generating codes C.sub.2 through C.sub.M, using said primary code
C.sub.1, and wherein
said adjustable coding means is responsive to the codes C.sub.2 through
C.sub.M, to enable transmitting the user digital bit stream at a multiple
M' times the basic bit rate R.
11. The radio transmitter unit of claim 10 wherein one of said codes
C.sub.1 through C.sub.M, has sub-rate capability.
12. The radio transmitter unit of claim 10 arranged to transmit at a
carrier frequency to a base station, and wherein said codes C.sub.1
through C.sub.M, used by said radio transmitter unit are different from
the codes used by a second radio transmitter unit which transmits to the
base station at said carrier frequency.
13. A method of providing code division multiple access for a radio
transmitter unit, comprising the steps of
receiving a digital bit stream from a user at one of a plurality of source
bit rates, wherein said plurality of source bit rates includes a basic bit
rate R and at least one other bit rate which is a multiple M of the basic
bit rate, where R and M are positive integers of at least 1, and
in response to a user input, selecting a bit rate multiple M,
converting a received serial user bit stream, which is M times a basic bit
rate R, into M parallel basic bit rate streams,
spreading each of the M basic bit rate streams, using a different spread
code C, into M channel bit rate signals, and
combining the M channel bit rate signals into one channel bit rate signal,
and
transmitting the user digital bit stream at the channel bit rate modulated
onto a predefined carrier frequency.
14. A radio communication system comprising
a base station for receiving a channel bit rate signal modulated onto a
predefined carrier frequency,
at least two radio transmitter units, each radio transmitter unit including
means for receiving a digital bitstream from a user at one of a plurality
of source bit rates, wherein said plurality of source bit rates includes a
basic bit rate R and at least one other bit rate which is a multiple M of
the basic bit rate R, where R and M are positive integers of at least 1, a
user input selecting a particular user source bit rate by identifying a
basic bit rate multiple M,
adjustable coding means, responsive to said user input, for spreading and
transmitting the user digital bit stream received at the selected bit rate
to a channel bit rate which at least equals the highest bit rate of said
plurality of source bit rates,
wherein said adjustable coding means includes
a serial-to-parallel means, responsive to a user's input identifying a
basic bit rate multiple M, for converting a received user bit stream,
which is M times a basic bit rate R, into M basic bit rate streams,
M encoder means for spreading each of the M basic bit rate streams, using a
different spread code C, into M channel bit rate signals,
means for combining the M channel bit rate signals into one channel bit
rate signal, and
means for transmitting the user digital bit stream at the channel bit rate
by modulating it onto the predefined carrier frequency. |
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Claims |
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Description |
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FIELD OF THE INVENTION
This invention relates to code division multiple access (CDMA) systems and,
more particularly, to a CDMA system for providing a user with variable and
dynamic bandwidth capacity access.
BACKGROUND OF THE INVENTION
In future wireless networks, a large variety of services, such as
voice/video/data/image, are expected. The most precious resource in most
wireless systems is the radio spectrum. To maximize its effective use,
packet switched wireless access using code division multiple access (CDMA)
has been pursued and offers increased service quality and transmission
bandwidth. These CDMA systems provide reduced multiple path distortion and
co-channel interference, and avoid the need for frequency planning that is
common with frequency division multiple access (FDMA) and time division
multiple access (TDMA) systems.
In a CDMA system, a unique binary spreading sequence (a code) is assigned
for each call to each user. Multiplied by the assigned code, the user's
signal is "spread" onto a channel bandwidth much wider than the user
signal bandwidth. The ratio of the system channel bandwidth to the user's
bandwidth is commonly called "the spreading gain." All active users share
the same system channel bandwidth frequency spectrum at the same time.
Given a required signal-to-interference (S/I), the equivalent system
capacity is proportional to the spreading gain. The signal of each user is
separated from the others at the receiver by using a correlator keyed with
the associated code sequence to "de-spread" the desired signal.
In these CDMA systems, there is a continuing need to increase the
performance of the system by accommodating users having different source
rates.
SUMMARY OF THE INVENTION
In accordance with the present invention, a multi-code CDMA system allow a
user at a radio transmitter unit to dynamically change its source data
rate. In response to a user input selecting one of said plurality of
source bit rates, an adjustable coding means spreads and transmits the
user's digital information received at the selected bit rate to a channel
bit rate which at least equals the highest bit rate of said plurality of
source bit rates. According to one feature, the plurality of source bit
rates includes a basic bit rate R and at least one bit rate which is a
multiple M of the basic bit rate R, where M is an integer of at least 1.
The user's input selects a particular user source bit rate by identifying
a basic bit rate multiple M to a base station that is to receive the
transmission.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 shows a prior art CDMA system;
FIG. 2 shows a block diagram of a first embodiment of a transmitter unit of
a multi-code CDMA system in accordance with the present invention;
FIG. 3 shows a block diagram of a second embodiment of a transmitter unit
of a multi-code CDMA system;
FIG. 4 shows a block diagram of a third embodiment of a transmitter unit of
a multi-code CDMA system;
FIG. 5 shows a flow chart describing how a user can dynamically change the
source data bit transmission rate of the transmitter unit; and
FIG. 6 shows an illustrative base station uplink load graph.
DETAILED DESCRIPTION
With reference to FIG. 1, we describe a prior art CDMA system. The CDMA
system includes a plurality of mobile units (1 - N) which enables a
plurality of users (1 - N) to communicate with a base unit 190 at one cell
site. Illustratively, a block diagram of mobile unit 1 includes a
transmitter unit 150 and a receiver unit 160. The transmitter unit 150
includes a convolutional coder 101 which receives digital information (or
data signals) from user 1 at a first data bit rate. The output of
convolutional coder 101 is coupled to interleaver 102 and then to a Walsh
modulator 103, all of which are well known in the prior art. The output of
modulator 103 is outputted into code spreader 104, which spreads the first
data bit rate signal into a channel bit rate using a code C.sub.1 which is
unique to user 1. The output signal 104a of code spreader 104 is coupled
to coders 105 and 106. In coder 105, an in-phase code A.sub.1 further
encodes the signal 104a. Similarly, coder 106 further encodes the signal
104a using a quadrature-phase code A.sub. Q. The codes A.sub.1 and A.sub.Q
are common to mobile units 1 - N, but are unique to the cell site base
unit 190 which serves mobile units 1 - N. This ensures that mobile units 1
- N can only communicate with base station 190.
The output of coder 105 is used to modulate the carrier signal
Cos.omega..sub.c t in modulator 108. The output of coder 106 is used to
modulate the carrier signal Sin.omega..sub.c t in modulator 109. In
certain applications, an optional delay unit 107 may be utilized to
provide better spectral shaping. The output of modulators 108 and 109 are
radio frequency signals which are combined in combiner 110 and transmitted
via antenna 111 over the air to base unit 190.
The base unit 190 transmits at a different carrier frequency which is
received and decoded by mobile units 1 - N. In our illustrative example,
receiver 160 of mobile unit 1 includes a demodulator (not shown) to
demodulate the carrier frequency to obtain a channel bit rate signal which
is decoded using codes A.sub.1 and A.sub.Q and then de-spread using the
associated code sequence C.sub.1 to obtain the information data signal to
be outputted to user 1.
The base unit 190 operates in a similar manner to receiver 160 of mobile
unit 1 to receive, decode and de-spread the user 1 information data
signal. Similarly, the other mobile units, illustratively represented by
mobile unit N, operate in the same manner as mobile unit 1, except that
user N has a unique code CN to distinguish it from user 1. In mobile unit
N, the in-phase and quadrature codes A.sub.1 and A.sub.Q, respectively, as
well as the carrier frequency f.sub.c, are the same as used for mobile
unit 1.
When a higher data transmission rate is desired, one prior art arrangement
provides the user a multi-code mobile unit having a fixed number of
multiple transmitters and receiver units, each using a different spreading
code. Thus, for example, if a user required twice the bandwidth, the user
terminal would include two transmitter units 150 and two receiver units
160, which operate using different codes C.sub.1 and C.sub.2. Such an
arrangement is described in Wi-LAN Inc. Technical Bulletin No. 3 dated
October 1993 and entitled "Multicode Direct Sequence Spread Spectrum." In
such an arrangement, however, the user is allocated a predefined fixed
bandwidth and all users, when they transmit, would transmit at the same
fixed source rate.
With reference to FIG. 2, we describe our dynamic multi-code code division
multiple access (MC-CDMA) system. In FIG. 2, the units 205-211 operate in
the same manner as the previously described units 105-111 of FIG. 1 and
coder units 201-204, 221-224, and 241-244 operate the same as coder units
101-104 of FIG. 1. The units 280, 201-204, and 221-224 may each be
implemented using a Digital Signal Processor (DSP) or may be combined in
one or more DSPs. Illustratively, the DSP is shown as a separate unit 220
which controls the mobile unit 200. The combiner 254 combines the output
of code spreaders 204, 224 and 244. The serial-to-parallel unit 281
converts a user's serial digital information input, which may be up to
M.sub.max times the basic source rate R (where M.sub.max .cndot.R.ltoreq.
channel rate), into M data streams (where M is an integer
.ltoreq.M.sub.max) each of which is encoded using one of the coder units
(e.g., 201-204). The variable M is selected by a user and/or the base
station 290 depending on system status, as will be discussed in a later
paragraph. It should be noted that the Walsh modulators 203, 223 and 243
are optional, to improve the required signal-to-interference ratio, and in
accordance with an aspect of the invention may be eliminated to improve
the bandwidth multiple M.sub.max.
In an MC-CDMA system of FIG. 2, if a user 1 requests (and is allowed by the
base station 290) M times the basic source rate R, mobile unit 1 converts
the user digital stream (using serial-to-parallel unit 280) into M basic
rate streams. Each of the basic rate streams is encoded using a different
code (C.sub.1 -C.sub.M) and they are superimposed together (using combiner
254) and up-converted (using units 208, 209) for radio transmission to the
base station 290. The codes C.sub.2 -C.sub.M are derived from C.sub.1
using a subcode concatenation that is described in a later paragraph.
As shown in FIG. 2, such a system does not require modification to the
phase encoders 205, 206 or to the RF modulators 208, 209, except for using
M times the transmission power (in unit 210) to satisfy the
signal-to-interference requirements. All additional processing needed in
the MC-CDMA mobile unit 1 is done in the baseband region using digital
signal processors (DSPs). As will be described in a later paragraph, each
mobile unit 200 through 200 - N is assigned a different primary code
C.sub.1 . . . C.sub.1 ' by base station 290.
In an alternate embodiment shown in FIG. 3, variable rate convolutional
coder, interleaver and Walsh modulator (units 301-303) can be utilized in
the MC-CDMA mobile unit 300. In such an arrangement, the bandwidth of
units 301-303 is set by the input M. The serial-to-parallel unit 381 is
connected to the output of Walsh modulator 303 and converts the user's
input digital information stream into M basic data rate serial information
streams. The remaining units 304 through 311,324, 344 and 354 function in
the same manner as units 204 through 211, 224, 244 and 254 as previously
described in FIG. 2. The receiver unit 360 operates in the same manner as
unit 260 of FIG. 2.
An additional embodiment shown in FIG. 4 describes the use of a
convolutional coder, interleaver and Walsh modulator (units 401-403) which
operate at a constant chip data source rate which is C times the basic
data rate R. Because units 401-403 operate at a constant chip rate, they
are more simply realized. The user's input digital information stream (at
a data rate which is equal to M times the basic data rate R) is inputted
to a repeater 450. The repeater 450 multiplies the user digital
information stream (at a data rate of M times R) by a factor C/M such that
the resulting data bit rate is equal to C times the basic data bit rate R.
The random selector circuit 422 connects to the output of Walsh modulator
403 and randomly selects one of the C/M blocks of data. The output of
selector circuit 422 is then inputted into serial-to-parallel unit 481
which operates the same as unit 381 of FIG. 3 to generate M data streams.
Similarly, the units 404 through 411, 424 and 444 operate in the same
manner as units 304 through 311, 324 and 344 of FIG. 3. The receiver 460
operates in the same manner as unit 360 of FIG. 3.
Rate Quantization
On the transmitter side, the actual user source bit rate does not have to
be an integer multiple of the basic rate R. Each code (C.sub.1 -C.sub.M)
in MC-CDMA carries a basic rate R. M codes in parallel provide a single
user M times the basic rate R capability. If one of the codes is equipped
with sub-rate capability (i.e., variable spreading gain to provide 1/2
rate, 1/4 rate, etc.), then there is a much finer quantization in terms of
the source bit rate offering to the user. Thus, for example, the user
would be able to transmit at 3.25 times the basic rate B.
Synchronization/Acquisition
On the receiver side, the synchronization/acquisition subsystem is very
demanding even for regular CDMA systems. The MC-CDMA receiver does not
require an M-fold complexity increase in synchronization/acquisition.
Since the multipath/delay spread suffered by signals carried on the
parallel codes to/from one user would be exactly the same, one well-known
searcher circuit for acquisition will suffice for the multiple paths
receiver (RAKE) fingers of all the parallel codes.
Subcode Concatenation
To avoid the self-interference that a user employing multiple codes may
incur, the present invention provides a subcode concatenation scheme to
generate additional codes for the user. The scheme operates as follows:
Each user admitted into the system has a primary code assigned to it by
the base station. The primary codes (i.e., C.sub.1, C.sub.1 ', etc.) of
different users are PN codes, i.e., not orthogonal among different users.
The multiple codes to/from one user can and should be made orthogonal. If
C.sub.1 is the primary code of a user and the user requires a higher rate,
the additional codes, C.sub.i, will be derived from C.sub.1 by C.sub.i
=C.sub.1 .times.D.sub.i, where D.sub.i .perp.D.sub.j,i.noteq.j. Obviously,
C.sub.i .perp.C.sub.j,i.noteq.j. This orthogonality is maintained at the
receiver since the propagation variations on the parallel codes are the
same. In addition to the ability of eliminating self-interference, this
scheme helps simplify dynamic access in the sense that explicit multiple
code negotiation is not needed. The latter will be made clear in the next
section.
Dynamic Access Control
To provide the user dynamic bandwidth access control between bursts, two
different approaches may be taken: one uses a demand assignment approach;
another uses a probabilistic approach.
Taking a demand assignment approach, users (i.e., mobile units) with data
bursts to transmit or users with increased source rates must make requests
and wait for assignment by the base station. In conventional orthogonal
systems, the assignment (e.g., using well-known RAMA/TRAMA access
protocols) gives specific time slots and/or carriers to the user. In our
MC-CDMA system, only the number of codes needs to be dynamically assigned
by the base station. Each user has a unique primary code, i.e., C.sub.1,
assigned to it at call setup time. When a user is idle, a very low rate
(sub-rate) signaling channel is maintained using its primary code. Not
only does this sub-rate channel facilitate synchronization and power
control procedures, but also it is used to make multiple code requests
prior to a burst transmission. Depending on the user need and the uplink
load status, an assignment is made to the requesting user. Upon receiving
the number assignment from the base station, the user utilizes subcode
concatenation to locally generate the corresponding number of codes for
its transmission while the receiver at the base station does the same.
There is no need for explicit code negotiation. The user, then, transmits
at a power level adjusted according to the number of codes it uses as well
as the QOS it requires.
The demand assignment access described above uses transmitter-oriented
codes. By way of the low-rate maintenance channel, it assures continuous
synchronization and power control. This approach requires a dedicated
receiver and a dedicated maintenance channel for every user admitted into
the system. Alternatively, a common code or a few common codes can be
reserved for the burst access request by all users. Such access with
receiver-oriented codes will do away with the dedicated resources.
However, this approach has the disadvantage of a very significant burst
access delay due to access collision as well as the time needed to
re-acquire synchronization and power control for every burst.
Taking the probabilistic approach, adaptive access control can be employed.
One such technique is described in the co-pending, commonly assigned U.S.
patent application entitled "Controlling Power And Access Of Wireless
Devices To Base Stations Which Use Code Division Multiple Access," Ser.
No. 08/234,757, filed Apr. 28, 1994, and incorporated by reference herein.
The base station broadcasts to the mobile units the current uplink load
information. Users that have data bursts to transmit will then make
probabilistic decisions on whether to transmit. For the decision-making,
one useful criterion is that the conditional expected load, given the
current load, is optimized. Another criterion is that the conditional
probability of the system overload, given the current load, is optimized.
As described in the above-cited application, priority for users with
on-going bursts over users with new bursts may be desirable and can be
incorporated in this control mechanism. Furthermore, since MC-CDMA users
are equipped with a variable rate capability, the decision does not have
to be the probability of transmission. Instead, given the current load
information, a user can decide to transmit at a lower, yet non-zero,
source rate by using a fewer number of codes. The probabilistic approach
is attractive in that the user access can be instantaneous, and no central
controller is required to dynamically assign spectral resource among users
in the cell. The disadvantage of this approach is that the system overload
occurs with non-zero probability, which will degrade the overall spectral
efficiency.
With reference to the flow chart of FIG. 5, we describe an illustrative
sequence of system operations for both the demand assignment and
probabilistic approach. In step 501, the user inputs a request for a
connection over a common access channel used for communications with the
base station. Communications between a mobile unit and the base station
may use the previously described RAMA/TRAMA protocol or other access
protocols. In step 503, if the connection is not successful, it is
re-tried in step 505. If it is successful, then in step 507 the base
station assigns a primary code C.sub.1 to the mobile unit over the
broadcast channel. The base station prevents collision of transmissions
from each of the mobile units by selecting unique primary codes for each
mobile unit which is active. Thus, for example, one mobile unit may be
assigned a primary code C.sub.1 ; another is assigned a primary code
C.sub.1 '.
Returning to our example, following step 507, the user can then communicate
to the base station at the basic rate R using the primary code C.sub.1. If
the user desires to transmit at other than the basic rate, a request is
made as is shown in step 509. In step 509, the user requests M times the
basic rate R bandwidth for communications to the base station. Such
additional bandwidth may be required by a user that transmits in a burst
data mode. The base station, depending on the available bandwidth not
presently being utilized by other mobile units, may allow the user to
transmit at M' times the basic rate R (where M' is less than or equal to
M). In step 513 when the user receives permission to transmit at M' times
the basic rate R, the user generates the codes C.sub.2 through C.sub.M,
using the previously assigned primary code C.sub.1.
At the end of a data burst, when the user returns to an idle mode, sub-rate
signaling is maintained over the C.sub.1 channel. In step 517, it is
determined if the user has a new data burst. If the user has a new data
burst, control returns to step 509. If there is no new data burst, then in
step 519 it is determined whether or not the communications channel should
be disconnected, step 521. If the user does not wish to disconnect,
control returns to step 515.
For users operating in an isochronous mode (i.e., user is sending
continuous data transmissions), the path 522 would be substituted for
steps 515 and 517.
If the probabilistic approach is used by a user, then the following
sequence of steps is followed. Following step 507, the base station, in
step 523, broadcasts the uplink load to all users on the system. The user
monitors the uplink load and makes a probabilistic determination of being
able to transmit at a multiple M of the basic rate R. One criterion for
the decision making could be that the conditional expected load, given a
current uplink load, is optimized. Furthermore, given the current uplink
load information, a user could then decide to transmit at a lower, yet
non-zero, source data rate by using a smaller multiple M. In step 525, the
user determines at which multiple M to transmit to the base station.
Optionally, via path 526 following step 507, the base station may, in step
527, transmit to the user the probability of success for transmitting at
different multiples M of the basic rate R. Thereafter, the user uses these
probabilities to determine at which multiple M to transmit.
To illustrate how the base station handles a user request for additional
transmission bandwidth, consider the base station uplink load graph shown
in FIG. 6. Assume that a base station has a maximum bandwidth capacity of
M.sub.max (e.g., 5) times the basic rate R. At time T.sub.1 all of the
bandwidth has been assigned; hence, any request from a user for an
increased transmission bandwidth would be denied. At time T.sub.2,
however, only 40% of the bandwidth capacity is being utilized; at time
T.sub.3, 80%; at time T.sub.4, 20%; and at time T.sub.5, 0%.
At time T.sub.2 assume an active user wants to increase its bandwidth by a
multiple M=3 of its basic rate R. The base station could allow that user
to utilize a multiple 3; however, the base station would then have no
residual bandwidth capacity for newly active users or for increasing the
bandwidth of existing active users. In such a situation, the base station
would likely allow the requesting user to utilize a multiple M of 2 or 1,
thereby leaving a reserve bandwidth for other needs of the system.
Assuming the base station allows the user a multiple M=2, the new base
station loading would appear as shown in time T.sub.3 (assuming no other
changes).
In a system which enables users to use a probabilistic approach, the base
station would broadcast the uplink load shown in FIG. 6 to the users. At
time T.sub.1 a user can determine for itself what the probability of
successful transmission would be at different transmission data rates.
Certainly, a user wanting to increase its transmission data rate would
determine that it would have a higher probability of success during time
T.sub.2 than at time T.sub.1. Using the received uplink loading
information shown in FIG. 6, it is likely that a user would vary its
transmission data rate with time to optimally utilize the available
bandwidth.
As previously discussed, the base station could also broadcast to the user
the probability of a transmission success at different multiples M (not
shown in FIG. 6). The user would then decide at which multiple M to
transmit.
The above-described dynamic MC-CDMA assignment schemes provide a unique
means for providing a user with variable and dynamic bandwidth capacity
access in a wireless network. It provides access to the "peak capacity" of
a base station to a single user, without losing traditional CDMA
advantages in combating multi-path impairments.
Another feature of the proposed system enables a base station to support a
mobile unit population that is much greater than the number of base
station receivers which, in turn, is somewhat greater than the number of
simultaneous mobile transmissions supported. In such a system, we can, for
instance, classify users into two groups in order to reduce the number of
receivers required at the base station. For high activity factor or
delay-sensitive users, dedicated receivers as well as low-rate maintenance
channels could be provided at the base station. Whereas low activity
factor or non-delay-sensitive users may, instead, share receiver-oriented
codes for burst access requests to get a receiver ready, prior to a
transmission burst.
What has been described is merely illustrative of the application of the
principles of the present invention. Other arrangements and methods can be
implemented by those skilled in the art without departing from the spirit
and scope of the present invention.
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