Subchannel control loop

Independently controlling the transmitted power of each subchannel by a subchannel control loop is described. A transmitting station generates a channel made up of a sum of subchannels wherein each subchannel or group of subchannels is amplified with a unique gain value that is varied in accordance with subchannel power control messages from the receiving station. The receiving station generates each subchannel power control message subsequent to monitoring and calculating metrics based on that received subchannel.

BACKGROUND OF THE INVENTION 
I. Field of the Invention 
The present invention relates to communications systems. More particularly, 
the present invention relates to a novel and improved method for 
independent closed loop power control of subchannels in a spread spectrum 
communication system. 
II. Description of the Related Art 
In a code division multiple access (CDMA) spread spectrum communication 
system, a common frequency band is used for communication with all base 
stations within that system. An example of such a system is described in 
the TIA/EIA Interim Standard IS-95-A entitled "Mobile Station-Base Station 
Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular 
System", incorporated herein by reference. The generation and receipt of 
CDMA signals is disclosed in U.S. Pat. No. 4,901,307 entitled "SPREAD 
SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEMS USING SATELLITE OR 
TERRESTRIAL REPEATERS" and in U.S. Pat. No. 5,103,459 entitled "SYSTEM AND 
METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM" both 
of which are assigned to the assignee of the present invention and 
incorporated herein by reference. 
Signals occupying the common frequency band are discriminated at the 
receiving station through the spread spectrum CDMA waveform properties 
based on the use of a high rate pseudonoise (PN) code. A PN code is used 
to modulate signals transmitted from the base stations and the remote 
stations. Signals from different base stations can be separately received 
at the receiving station by discrimination of the unique time offset that 
is introduced in the PN codes assigned to each base station. The high rate 
PN modulation also allows the receiving station to receive a signal from a 
single transmission station where the signal has traveled over distinct 
propagation paths. Demodulation of multiple signals is disclosed in U.S. 
Pat. No. 5,490,165 entitled "DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM 
CAPABLE OF RECEIVING MULTIPLE SIGNALS" and in U.S. Pat. No. 5,109,390 
entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM", both of 
which are assigned to the assignee of the present invention and 
incorporated herein by reference. 
The IS-95 Over-the-Air (OTA) Interface Standard defines a set of RF signal 
modulation procedures for implementing a digital cellular telephone 
system. The IS-95 standard, and its derivatives, such as IS-95A and ANSI 
J-STD-008 (referred to collectively as the IS-95 standard), are 
promulgated by the Telecommunications Industry Association (TIA) to insure 
the operability between telecommunications equipment manufactured by 
different vendors. 
The IS-95 standard has received enthusiastic reception because it uses the 
available RF bandwidth more efficiently than previously existing cellular 
telephone technologies. This increased efficiency is provided by using 
CDMA signal processing techniques in combination with extensive transmit 
power control to increase the frequency reuse of a cellular telephone 
system. 
FIG. 1 illustrates a digital cellular telephone system configured in a 
manner consistent with the use of IS-95. During operation, telephone calls 
and other communications are conducted by exchanging data between remote 
stations 1 (generally cellular telephones) and base stations 2 using RF 
signals. Communications are further conducted from base stations 2 through 
base station controllers (BSC) 4 and mobile switching center (MSC) 6 to 
either public switch telephone network (PSTN) 8, or to another base 
station for transmission to another remote station 1. BSCs 4 and MSC 6 
typically provide mobility control, call processing, and call routing 
functionality. 
The RF signal transmitted from a base station 2 to a set of remote stations 
1 is referred to as the forward link, and the RF signal transmitted from 
remote stations 1 to a base station 2 is referred to as the reverse link. 
The IS-95 standard calls for remote stations 1 to provide 
telecommunications service by transmitting user data such as digitized 
voice data via the reverse link signal. The reverse link signal is 
comprised of a single traffic channel, and therefore is often referred to 
as a "non-coherent" signal because it does not include a pilot channel, 
and as such cannot be coherently demodulated. 
Within the reverse link signal, user data is transmitted at a maximum data 
rate of 8.6 or 13.35 kbps, depending on which rate set from a set of rate 
sets provided by IS-95 is selected. The use of a single channel, 
non-coherent, reverse link signal simplifies the implementation of an 
IS-95 cellular telephone system by eliminating the need for 
synchronization between a set of remote stations 1 communicating with a 
single base station 2. 
As mentioned above, IS-95 incorporates extensive transmit power control in 
order to more efficiently utilize the available RF bandwidth. In 
accordance with IS-95, this power control is performed by measuring the 
received signal strength and quality of the reverse link traffic channel 
when received at the base station and generating a power control command 
based on that measurement. The power control command is transmitted to the 
remote station via the forward link signal. The remote station responds to 
the power control command by increasing or decreasing the transmit power 
of the reverse link signal based on the power control command. This power 
control method is referred to as closed loop power control. The design of 
closed loop power control in a CDMA communication system is described in 
U.S. Pat. No. 5,056,109, entitled "METHOD AND APATUS FOR CONTROLLING 
TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", which is 
assigned to the assignee of the present invention and incorporated by 
reference herein. 
In IS-95 systems, the power control adjustment is performed repeatedly at 
rates on the order of 800 times per second in order to maintain the 
reverse link signal transmit power at the minimum necessary to conduct 
communications. Additionally, IS-95 also calls for transmit duty cycle of 
the reverse link signal to be adjusted in response to changes in voice 
activity by varying the transmit duty cycle in 20 millisecond increments. 
Thus, when the transmit duty cycle is lowered, the remote station 
transmits at either the set point, or the transmission is gated and the 
remote station does not transmit at all. During periods when the 
transmission is gated, the base station generates false power control 
increase commands because the reverse link signal is not detected. Since 
the remote station knows when its transmissions were gated, it can ignore 
corresponding increase commands since they are known to be false. 
To satisfy the ever increasing demand to transmit digital data created by 
networking technologies, such as the worldwide web, a more complex higher 
rate multi-channel coherent reverse link signal is provided in co-pending 
U.S. patent application Ser. No. 08/654,443 (the '443 application) 
entitled "High Data Rate CDMA Wireless Communications System" filed May 
28, 1996, assigned to the assignee of the present invention and 
incorporated herein by reference. The above referenced patent application 
describes a system wherein a set of individually gain adjusted channels 
are formed via the use of a set of orthogonal subchannel codes. Data to be 
transmitted via one of the transmit channels is modulated with one of the 
subchannel codes, gain adjusted, and summed with data modulated using the 
other subchannel codes. The resulting summed data is modulated using a 
user long code and a pseudorandom spreading code (PN code) and upconverted 
for transmission. In particular, the above referenced patent application 
describes a reverse link signal made up of Walsh sequence modulated 
subchannels including at least one traffic subchannel, a power control 
subchannel, and a pilot subchannel. 
A multi-channel reverse link increases flexibility by allowing different 
types of data to be transmitted simultaneously. Providing a pilot 
subchannel facilitates coherent processing of the reverse link signal at 
the base station which improves the performance of the link. To facilitate 
power control, time tracking and frequency tracking, it may be desirable 
to keep the average received pilot signal power to noise ratio (SNR) at a 
constant level. Note that in a CDMA based system, effective power control 
is essential to achieving a high system capacity. Usually, power control 
is divided into two parts, an open loop and a closed loop. In open loop 
power control, the mobile station measures the received forward link 
signal for a predetermined time period and adjusts its transmit power in 
response to changes in the received forward link power. The open loop 
power control as implemented in IS-95 systems is fairly slow and takes 
care of the long term channel variations (known as the corner effect). The 
closed loop power control as described previously is faster and tries to 
compensate for the effects of fading. 
In IS-95 based CDMA systems, the closed loop power control is also used to 
drive the reverse link to a desired setpoint. For example, a frame error 
rate (FER) of 1% may be desired. If the FER is too high, an increase in 
reverse link power is necessary to reduce the error rate. On the other 
hand, if the FER is lower than the desired setpoint, the reverse link 
power can be reduced. Reducing the reverse link power reduces interference 
generated and thus has a direct positive effect on the other users in the 
system. Maximum capacity is reached in a CDMA system when every user is 
transmitting at the setpoint and therefore the minimum power required to 
achieve the desired error rate. 
The operating setpoint of the system can be modified by changing the power 
control decision threshold at the base station. As a consequence, the 
total average received power of the reverse link will converge to a new 
value. This power control mechanism affects the total transmitted power. 
However, if this technique is applied to a system employing a plurality of 
subchannels as provided for in the '443 application, the relative 
strengths of each subchannel are not changed as the total transmit power 
is modified. For example, upon reaching a satisfactory power level in 
terms of received pilot subchannel power, any subsequent variation of 
transmit power to modify the received FER for a data subchannel will 
affect the pilot power, and vice versa. Since it is likely that different 
types of data which occupy separate subchannels will have different 
requirements, it is desirable to be able to control the transmitted power 
of each subchannel independently. 
SUMMARY OF THE INVENTION 
In the following description, independently controlling the transmitted 
power of each subchannel by a subchannel control loop is described. A 
transmitting station generates a channel made up of a sum of subchannels 
wherein each subchannel or group of subchannels is amplified with a unique 
gain value that is varied in accordance with subchannel power control 
messages from the receiving station. The receiving station generates each 
subchannel power control message subsequent to monitoring and calculating 
metrics based on that received subchannel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the following described exemplary embodiment of the present invention, 
the subchannel control loop controls the reverse link. Therefore, the 
transmitting station will be referred to as the remote station and the 
receiving station will be referred to as the base station. Remote stations 
may include wireless local loop stations, portable telephones, data 
terminals, and the like. It is understood that this invention could also 
be employed on the forward link alone or on both forward and reverse links 
simultaneously. 
In a channel containing N subchannels, the total transmitted power by a 
remote station, P.sub.tot, is defined as the sum of the transmitted powers 
of each subchannel: 
EQU P.sub.tot =P.sub.0 +P.sub.1 +. . . +P.sub.N (1) 
A remote station can change the setpoint of a particular subchannel, 
subchannel i, by changing the corresponding subchannel transmitted power 
P.sub.i while the operating points of the other subchannels remain 
unchanged. 
Equation (1) can be normalized by an arbitrary power P.sub.ref : 
EQU P.sub.tot =(F.sub.0 +F.sub.1 +. . . +F.sub.N)*P.sub.ref (2) 
In the exemplary embodiment, power control is accomplished by adjusting 
transmit power P.sub.ref. Each subchannel control loop operates by 
adjusting a specific one or subset of F.sub.i. 
FIG. 2 depicts an exemplary remote station. In remote station 100, a 
plurality of data signals data0-dataN enter encoders 110A-110N. The 
encoded results are interleaved in interleavers 120A-120N, then modulated 
by unique Walsh sequences W.sub.0 -W.sub.N in spreaders 130A-130N. The 
outputs of multipliers 130A-130N are amplified in gain adjust blocks 
140A-140N with unique gain values supplied from gain control processor 
180. Gain adjust blocks 140A-140N may use digital techniques or may be 
implemented using variable gain amplifiers, the design of both techniques 
is known in the art. 
In the exemplary embodiment, Walsh sequence 0 (W.sub.0) modulates a 
constant value to form a pilot signal. As such, in the exemplary 
embodiment the data input to multiplier 130A is fixed and encoder 110A and 
interleaver 120A are not needed. The gain adjusted signals are combined in 
summer 150. Summer 150 may be implemented as a digital or analog device. 
Although summer 150 is likely to be digital if gain adjust blocks 
140A-140N are digital, and analog if they are analog, it is not necessary 
to do so. The signal made up of the sum of the individually gain adjusted 
data signals is amplified in gain adjust block 160 with a gain value 
supplied by gain control processor 180. In the exemplary embodiment, gain 
adjust block 140A is not required, since pilot gain adjustments can be 
accomplished via gain adjust block 160. Alternatively, gain adjust block 
160 can be eliminated if the overall gain is factored into each of the 
subchannel gains. In either case, no loss of control is suffered since 
each subchannel gain as well as the overall signal gain can still be 
independently varied. The resultant signal from gain adjust block 160 is 
modulated and upconverted in transmitter 170 and then transmitted on 
antenna 230 through duplexer 220. As with the other gain adjust blocks, 
gain adjust 160 can be implemented using digital or analog techniques. 
Forward link data from the base station, including the power control 
message information, is downconverted and amplified in receiver 210 via 
antenna 230 through duplexer 220. The received signal is demodulated in 
demodulator 200, then deinterleaved and decoded in decoder 190. In the 
exemplary embodiment, demodulator 200 is a CDMA demodulator as described 
in the aforementioned U.S. Pat. Nos. 4,901,307 and 5,103,459. Subchannel 
power control messages from the base station are separated from the 
forward link data decoded by decoder 190 in gain control processor 180. 
These messages independently control the gain values in gain adjust blocks 
140A-140N and 160. There are a number of ways for the gain values to be 
adjusted. For example, the subchannel power control message can consist of 
N bits, wherein each of the N bits directs a corresponding subchannel to 
increase or decrease its transmitted power. In response to this message, 
each gain value is increased or decreased by a predetermined amount that 
can be used for all subchannels or can be unique for each subchannel. 
Alternatively, the subchannel power control message can contain N binary 
sequences indicating the gain values or indicating the amount of change to 
the gain values. The control messages can independently control each gain 
value or a group of gain values, and can employ a combination of 
techniques for each. 
FIG. 3. depicts an exemplary base station. In base station 300, a signal 
containing the sum of all transmitted signals from remote stations 
operating in the system enters through antenna 310 and is downconverted 
and amplified in receiver 320. PN demodulator 330 extracts the set of 
signals transmitted by a particular remote station, remote station 100 for 
example. The PN demodulated signal is directed to a plurality of Walsh 
demodulators 40A-340N. Each Walsh demodulator demodulates a corresponding 
subchannel of the signal sent by remote station 100. 
In the exemplary embodiment, the subchannels as demodulated by Walsh 
demodulators 340A-340N can be deinterleaved and decoded in decoders 
350A-350N. The data from decoders 350A-350N is delivered to comparator 
370. A useful metric for comparator 370 to calculate is that of frame 
error rate (FER). The frame error rate of each subchannel can be compared 
to a FER threshold for that subchannel as provided by threshold generator 
380. If the frame error rate of a subchannel is lower than necessary for 
the desired communication quality, the power in that subchannel can be 
reduced. Conversely, if the frame error rate of a subchannel is too high, 
that subchannel needs to have its power increased. 
In an alternative embodiment, the energy in each subchannel signal is 
summed in accumulators 360A-360N. The energy results are delivered to 
comparator 370. Receiver 320 typically contains automatic gain control 
circuitry (AGC), which normalizes the in-band energy to a predetermined 
level. Parameters associated with the AGC can be delivered to comparator 
370 to assist in normalizing energy values for comparison. Comparator 370 
compares the energy received in each subchannel with an energy threshold 
for that channel as determined by threshold generator 380. The energy 
thresholds are calculated to ensure a certain quality of service on the 
respective subchannel. Each subchannel's power can be adjusted based on 
the comparison. The power can be reduced if the threshold is exceeded and 
increased if the threshold is not exceeded. Moreover, the two embodiments 
can work in conjunction with one another by permitting the energy 
thresholds to be varied in response to the frame error rate or other 
signal quality metric. 
Many other alternatives are envisioned for comparison in comparator 370. 
When decoders 350A-350N use the Viterbi algorithm, Viterbi decoder metrics 
can be provided for comparison. Further examples include comparison of 
symbol error rate instead of frame error rate and computation of a cyclic 
redundancy check (CRC). The thresholds can be signaled to threshold 
generator 380 by base station controller 4 as shown in FIG. 1, or they can 
be calculated in threshold generator 380 itself. 
In the exemplary embodiment, comparator 370 makes a determination based on 
the received subchannels whether or not to increase or decrease the power 
level of each of the subchannels. Based on that determination, message 
generator 390 creates a power control message to be sent to the remote 
station to modify any subchannels, if necessary. The power control 
messages can be transmitted as signaling data or punctured into the data 
stream as described in IS-95, or any other method of signaling capable of 
relaying the message to the mobile station. As discussed previously, the 
message can be a simple up or down command per subchannel or as 
complicated as sending the exact gain values for each. Furthermore, each 
subchannel can be controlled independently, or alternatively, subchannel 
power control messages can control groups of subchannels. The power 
control message is modulated in modulator 400, upconverted and amplified 
in transmitter 410 and transmitted to remote station 100 via antenna 420. 
Remote station 100 modifies the gain value associated with each subchannel 
as described previously, and thus the subchannel control loop is closed. 
In an alternative embodiment, the gain values for gain adjust blocks 
140A-140N can be calculated in an open loop manner. A predetermined gain 
calculation algorithm in gain control processor 180 can be used to 
calculate the individual gain adjust values based on the received energy 
of the forward link signal. For example, different subchannels are likely 
to have different coding for error correction and thus the error rates 
will vary for a given drop in received power due to a fade. Empirical 
studies can be used to develop the predetermined gain calculation 
algorithm. 
In another alternative embodiment, if both forward and reverse links employ 
this invention, open loop calculations of received energy of a forward 
link subchannel can be used to adjust the gain of a corresponding reverse 
link subchannel, and vice versa. In situations where there is symmetry or 
partial symmetry with respect to the forward and reverse links, the 
received energy in a subchannel can be used in calculations for 
determining the power level of the corresponding transmit subchannel. A 
combination of open and closed loop techniques can also be employed. 
The previous description of the preferred embodiments is provided to enable 
any person skilled in the art to make or use the present invention. The 
various modifications to these embodiments will be readily apparent to 
those skilled in the art, and the generic principles defined herein may be 
applied to other embodiments without the use of the inventive faculty. 
Thus, the present invention is not intended to be limited to the 
embodiments shown herein but is to be accorded the widest scope consistent 
with the principles and novel features disclosed herein.