Patent Document

CROSS REFERENCE 
   This continuation application claims priority from U.S. application Ser. No. 09/359,855, filed Jul. 22, 1999 now abandoned entitled “Forward Link Power Control in a Cellular System Using N T /I 0  Values” and currently assigned to the assignee of the present application. 

   BACKGROUND OF THE INVENTION 
   I. Field of the Invention 
   The present invention relates to communication systems in general, and to power control in a communication system in particular. 
   II. Description of the Related Art 
   There are many in prior art communications systems that require a measurement of the strength of a signal received by a mobile station. For example, during handoff of a mobile station from one base station to another a determination of the strength of the signals received by the mobile station is desirable for determining when to perform the handoff. One such handoff technique is disclosed in U.S. Pat. No. 5,267,261, entitled “MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM,” assigned to the assignee of the present invention. 
   In the improved technique of U.S. Pat. No. 5,267,261 the mobile station monitors the signal strength of pilot signals transmitted by neighboring base stations within the system. The mobile station sends a signal strength message to a system controller via the base station through which the mobile station is communicating. Command messages from the system controller to a new base station and to the mobile station in response to the signal strength are thus used to establish communication through the new and current base stations. The mobile station detects when the signal strength of a pilot signal corresponding to at least one of the base stations through which the mobile unit is currently communicating has fallen below a predetermined level. The mobile station reports the measured signal strength indicative of the corresponding base station to the system controller via the base stations through which it is communicating. Command messages from the system controller to the identified base station and mobile station terminate communication through the corresponding base station while communications through the other base station or stations continue. 
   It is known for the power control information transmitted from the mobile station to be inserted into a dedicated control channel separate from the traffic channel. However, it is desirable to decrease the need for separate control channels. Additionally, while it is preferable for the power of the energy of the signal sent on the traffic channel to be used to determine the power control parameters, it is known, for the control information to be based upon the error rate rather than the signal-to-noise ratio because the signal-to-noise ratio of the traffic channel is difficult to measure. For example, in some current systems, the time between errors is used to indicate the error rate. The error rate is then used to determine the quality of the traffic channel. Furthermore, it is difficult to obtain power control information and utilize it in time to respond to the conditions indicated in the power control information. 
   SUMMARY OF THE INVENTION 
   A system and method is taught for estimating the relative amount of power that is provided on the traffic channel using a calculated amount of noise that is present on a pilot channel. This estimate may then be used for several purposes, including controlling the power level of transmissions within a communication system having a base station, a mobile station and a plurality of channels including a communication channel and a pilot channel. The mobile station measures the ratio of the amount of energy received per symbol to the amount of interference received. The amount of energy received over the pilot channel is used to determine the amount of noise received in the pilot channel. The signal-to-noise ratio of the communication signal is determined according to the determined signal strength value and the pilot channel noise value. Accordingly, in one embodiment of the disclosed method and apparatus, the power level of a transmission is controlled according to the calculated signal-to-noise ratio. 
   A system and method is also taught for estimating the noise level in a communication channel within the communication system. The pilot signal includes a pilot energy component and a pilot noise component. The pilot energy component is removed from the pilot channel signal to provide a remaining pilot signal. As noted above, the amount of noise in the channel is estimated based upon the amount of noise in the pilot channel. 
   As noted above, in accordance with systems, the power of transmitted signals is controlled based upon an indication of the amount of power received by the intended receiving device. In such systems, the power levels of transmissions are controlled by determining difference between the signal-to-noise ratio of a received signal and the desired signal-to-noise ratio. A transmitter transmits the difference signal between the base station and the mobile station. 
   The pilot channel is divided in time into frames and the power control signal is inserted into each frame. Thus, information representative of the strength of the communication signal is transmitted to the base station by way of the pilot channel within each frame. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein: 
       FIG. 1  shows an exemplary illustration of a cellular communication system; 
       FIG. 2  shows a power control subchannel within the cellular communication system of  FIG. 1 ; 
       FIG. 3  is a flow chart illustrating the steps performed to determine signal-to-noise ratio of a received traffic signal; 
       FIG. 4  is a detailed flow chart illustrating certain steps of  FIG. 3 ; and 
       FIG. 5  is a block diagram of the disclosed apparatus. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An exemplary illustration of a cellular communication system is provided in  FIG. 1 . The system illustrated in  FIG. 1  can use various multiple access modulation techniques for facilitating communications between a large number of system mobile stations (or mobile communication devices), and the base stations. These techniques include CDMA spread spectrum modulation. 
   In a typical CDMA system, the base stations transmit a unique pilot signal, including a pilot carrier, upon a corresponding pilot channel. For example, in accordance with one embodiment of the disclosed method and apparatus, the pilot signal is an unmodulated, direct sequence, spread spectrum signal transmitted at all times by each base station using a common pseudorandom noise (PN) spreading code. The pilot signal allows the mobile stations to obtain initial system synchronization in addition to providing a phase reference for coherent demodulation and a reference for signal strength measurements. Furthermore, the received pilot signal can be used to estimate the arrival time, phase, and amplitude of the received traffic signal. In accordance with one embodiment of the disclosed method and apparatus, the pilot signal transmitted by each base station is modulated with the same PN spreading code with different code phase offsets. 
   A system controller  10 , also referred to as a mobile switching center (MSC)  10 , typically includes interface and processing circuitry for providing system control to the base stations. The controller  10  also controls the routing of communication device calls from the networks (such as the public switched telephone network (PSTN)) to the appropriate base station for transmission to the appropriate mobile station. The routing of calls from mobile stations through base stations to the PSTN is also controlled by the controller  10 . 
   The controller  10  can be coupled to the base stations  12 ,  14 ,  16  by various means such as dedicated phone lines, optical fiber links or by microwave communication links. In  FIG. 1 , three base stations  12 ,  14 ,  16  and a communication device (such as a mobile station)  18  are illustrated. The mobile station  18  consists of at least a receiver, a transmitter, and a processor. The base stations  12 ,  14 ,  16  typically include processing circuitry for controlling the functions of the base stations  12 ,  14 ,  16 , and interface circuitry for communicating with both the mobile station  18  and the system controller  10 . 
   The arrows  20 A– 20 B shown in  FIG. 1  represent the possible communication link between the base station  12  and the mobile station  18 . The arrows  22 A– 22 B shown in  FIG. 1  represent the possible communication link between the base station  14  and the mobile station  18 . Similarly, the arrows  24 A– 24 B shown in  FIG. 1  represent the possible communication link between the base station  16  and the mobile station  18 . 
   After a mobile station  18  processes a received signal, the resulting signal is a composite of a desired signal and a noise signal. The signal-to-noise ratio averaged over some period of time is a good measure of the strength of the received signal. For example, in a CDMA system the signal-to-noise ratio of the received signal can be averaged over a block. The mobile station  18  can therefore estimate the signal-to-noise ratio and compare the estimate with the value the mobile station  18  actually received. In accordance with one embodiment of the disclosed method and apparatus, the mobile station  18  sends to the base stations  12 ,  14 ,  16  the resulting difference between the measured and expected values of the signal-to-noise ratio as a parameter (FWD_SNR_DELTA) represented in units of decibels. The parameter (FWD_SNR_DELTA) is preferably transmitted on a reverse link power control subchannel. 
   In determining the expected signal-to-noise ratio, the mobile station  18  calculates a signal-to-noise ratio that will result in an average forward link fundamental block erasure rate equal to the forward link fundamental block erasure rate configured by the base stations  12 ,  14 ,  16 . In calculating the expected signal-to-noise ratio, the mobile station  18  assumes that successively lower rate blocks are transmitted with three decibels less power per PN chip. In accordance with one embodiment of the disclosed apparatus, the mobile station  18  performs maximal ratio combining of the receive paths. 
   In addition to calculating the expected signal-to-noise ratio, the mobile station  18  must determine the signal-to-noise ratio of the received traffic signal. The flowchart of  FIG. 3  is a high level flowchart that illustrates the steps that are performed in order to determine the signal-to-noise ratio of the received traffic signal. Initially, the traffic and pilot signals are received together with any noise on the channel (STEP  300 ). While filters remove noise that is out of the frequency band over which the traffic and pilot signals are transmitted, noise that is in-band, is passed. The received traffic signal is decovered by the particular Walsh code used to channelize the traffic channel. Likewise, the pilot channel is decovered by the particular Walsh code used to channelize the pilot channel (STEP  302 ). Once the pilot and traffic channels have been decovered, the per symbol signal to interference Ε S /I o  is measured (STEP  304 ). Next, the noise to interference, N t /I o  is measured (STEP  306 ). Once these values are measured, the per symbol signal-to-interference, Ε S /N t  is divided by the N t /I o  to yield the per symbol signal-to-noise ratio, Ε S /N t  (STEP  308 ). This value is then provided to devices that use the per symbol signal-to-noise ratio Ε S /N t  to control the system (STEP  310 ), such as by performing power control of the forward link transmit signal. The details as to how STEPs  304  and  306  are performed are provided in the flowchart in  FIG. 4 . 
   It should be understood by those skilled in the art that prior to decovering to separate the orthogonal channels, each of the traffic channels is included in the noise on the pilot channel. Likewise, the pilot signal and each of the traffic channels, except the traffic channel of interest, are included in the noise of the traffic channel of interest. Once decovered, the noise in the traffic channel includes only energy associated with non-orthogonal signals. It should further be understood by those skilled in the art that an automatic gain control device is typically used to ensure that the total received signal is received at an essentially constant value. Accordingly, all of the signal values are referenced to the total received signal strength, I o . Nonetheless, this is not noted in the equations that follow. Accordingly, the total received traffic signal can be represented as:
 
 r   T   =s   T   +n   T   EQN. (1)
 
where s T  represents the desired traffic signal and n T  represents the noise in the received traffic signal. It will be understood that:
 
 s   T   =Σd   k Ε s,k   1/2   EQN. (2)
 
where d k  is the k th  symbol within the symbol stream or data stream of the traffic channel; and Ε S,k  is the total received energy of the traffic channel over the k th  symbol. The sum is taken for all k from 1 to n, where n is the total number of symbols in a frame. It should be noted that in an alternative embodiment of the disclosed method and apparatus, the number of symbols, n may differ from the number of symbols in a frame.
 
   In many cases, a “rake” receiver is used to combine signals received from different sources or signals from the same source that have traversed different paths (and thus are delayed with respect to one another). In such cases, the total received traffic signal is attained by multiplying the traffic signals received on each independent path by the associated pilot signals. This multiplication results in each received traffic signal being weighted by the relative strength of the associated pilot signal. These products are then summed to form the total received traffic signal r T . The following equation represents this sum:
 
 r   T   =Σr   T,i   &lt;r   p,i &gt;  EQN. (3)
 
where the sum is taken over the subscript i from 1 to m, r T,     i    is the received traffic signal for the i th  path, m is the total number of paths, and the brackets which enclose the term r p,i  indicate the fact that the pilot signal may be filtered by a low pass filter to reduce any fluctuations in the amplitude of the pilot over short periods in time.
 
   The total received pilot signal for a particular path can be represented as:
 
 r   P,i   =s   P,i   +n   P,i   EQN. (4)
 
where s P  represents the received pilot signal and n P  represents the pilot noise.
 
   In addition, the pilot signal value s p,i  is equal to the data times the square root of the energy per symbol, E s  and a scaling factor. This relationship can be represented as follows:
 
 s   P,i =αΣ( d   k Ε s,k   1/2 )  EQN. (5)
 
where: α is a scaling factor which takes into account the relative transmission gains of the traffic and pilot channels and the integration lengths for each channel; the sum is taken over the subscript k from 1 to n; n is the total number of symbols; d k  is the k th  symbol of the symbol stream or data stream of the pilot channel; and Ε S,k  is the total received energy of the pilot channel over the k th  symbol. The symbol stream d is essentially either a positive one or a negative one representing the state of the information modulated on the pilot channel. In the case of the pilot signal, it is typical for the data to have a constant value of one. Therefore, the data, d can be dropped from the equation. In multiplying a traffic signal with a pilot signal, Eqn. (2) can be substituted into Eqn. (1), and Eqn. (5) can be substituted into Eqn. (4). The resulting product is then:
 
 r   T =[(Σ d   k Ε S,k   1/2 ) i   +n   T,i ]·[αΣ( d   k Ε S,k   1/2 ) i   +n   p,i   ]=dαΕ   S +noise  EQN. (6)
 
   However, if the noise n P  of the pilot signal r P  and the noise n T  of the traffic signal r T  are uncorrelated, then the product r T  is essentially a scaled unbiased estimator of the traffic data multiplied by the traffic signal energy. This is due to the fact that the uncorrelated noise will not sum up. However, the correlated data does sum up. Accordingly, an assumption can be made that the noise is negligible (i.e., insignificant and can be ignored). It can reasonably be assumed that the noise n P  of the pilot signal r P  and the noise n T  of the traffic signal r T  are uncorrelated, because the pilot signal r P  and the traffic signal r T  are transmitted on orthogonal channels. 
   Since d is essentially random and unknown, it is desirable to eliminate d from Eqn. (6). In accordance with the disclosed method and apparatus, in order to eliminate d from Eqn. (6), a dot product is performed. The dot product is taken between the estimator dαΕ T  and the symbol stream d after decoding and re-encoding of the received traffic signal (STEP  400 ). By decoding the traffic information, the information is essentially extracted from the received signal. Re-encoding the information returns the information to the state in which it existed before the decoding. Since the data sequence is relatively well known after the decoding operation, performing this dot product allows the data sequence to be taken into account when determining the energy of the received signal. That is, the dot product projects the data onto the received signal. Accordingly, the energy in the information symbols is summed and the energy in the noise is not since the noise is uncorrelated. Naturally, the more symbols that are summed, the greater the ratio of symbol energy to noise. The result of the dot product operation is:
 
αΕ T ·αΕ T =(αΕ T ) 2   EQN. (7)
 
   In order to estimate the traffic channel signal energy, the scaling factor α is removed from Eqn. (7). Scaling factor α can be represented as:
 
 α=G   P   /G   T   ·L   P   /L   T   EQN. (8)
 
where G P  is the pilot signal transmission gain, G T  is the traffic transmit signal transmission gain, L P  is the integration period of the pilot signal, and L T  is the integration period of the traffic signal. While the pilot integration period L P  and the traffic integration period L T  are known, the relationship between the pilot signal gain G P  and the traffic transmit signal gain G T  is typically not known in cases in which power control factors change the gain of the traffic channel.
 
   Therefore, in order to eliminate the scaling factor α, the mobile station  18  determines the pilot energy by computing the dot product of the pilot signal with itself. This produces a biased estimate of the pilot energy Ε P  which is a scaled biased estimate of the traffic channel signal energy, Ε P =α 2 Ε T . Therefore, a biased estimate of the traffic channel signal energy Ε T  can be determined by squaring the unbiased estimator, αΕ T  of the traffic signal energy and dividing it by the biased estimator of the pilot signal energy Ε P :
 
Ε T =(αΕ T ) 2 /(α 2 Ε T )  EQN. (9)
 
   As noted above in Eqn. (7), the energy per symbol, Ε S  can then be attained by normalizing the value Ε T  with respect to a symbol (i.e., by dividing by the number of symbols over which Ε T  was determined, such as the number of symbols per frame) (STEP  402 ). Accordingly:
 
Ε T   /n=Ε   S   EQN. (10)
 
where n is the number of symbols over which Ε T  was determined.
 
   If the fundamental block rate of the received traffic signal is known (STEP  404 ), then the normalized dot product associated with the block rate is selected (STEP  406 ). However, if the fundamental block rate is not known (STEP  404 ), then the dot product that has the maximum value is selected (STEP  408 ). 
   The disclosed method and apparatus takes advantage of the fact that the pilot signal has a known constant data sequence. Since the data sequence is known, the pilot channel signal can be easily differentiated to isolate the noise content (STEP  410 ). In accordance with one embodiment of the disclosed method and apparatus, this is done by inverting the pilot channel, shifting the inverted pilot channel signal one symbol in time with respect to the unshifted pilot channel signal, and summing the shifted inverted pilot channel signal with the unshifted pilot channel signal. This can also be done by decovering the pilot channel with Walsh code W 64   128  and integrating over the frame. This particular Walsh code is a pattern of alternating positive ones and negative ones. Thus, the sum of the energy in the pilot channel over a discrete number of symbols is zero, thereby isolating the remaining N T  term. This permits a determination of the normalized noise of the pilot channel. 
   The desired value, which is the signal-to-noise ratio is Ε S /N T,  can be attained by simply dividing the value Ε S  by the value N T . 
     FIG. 5  is a simplified block diagram of the disclosed apparatus. A radio frequency (RF) receiver  501  receives the incoming signal and does RF processing in known fashion. The received signal is then coupled to a processor  503 . A Walsh decovering circuit  511  within the processor  503  decovers each of the traffic channels and the pilot channel. It will be understood by those skilled in the art that the Walsh decovering circuit  511  may be implemented either as software run on the processor  511 , as a circuit which is implemented using discrete components, an application specific integrated circuit (ASIC) which is distinct from the processor  511 , or in any other manner that would allow the decovering procedure to be accomplished, as is well known in the art. Once decovered, the traffic channel signal is coupled to a decoder  507 . Similar to the decovering circuit, the decoder  507  may be implemented using discrete components, an application specific integrated circuit (ASIC) which is distinct from the processor  511 , or in any other manner that would allow the decoding procedure to be accomplished, as is well known in the art. The decoding process results in the information that was originally encoded by the transmitter that transmitted the received signal. This information is then coupled to a re-encoder  509 . The re-encoder  509  may be implemented using discrete components, an application specific integrated circuit (ASIC) which is distinct from the processor  511 , or in any other manner that would allow the re-encoding procedure to be accomplished, as is well known in the art. Once the re-encoding function has been performed, the processor  503  performs the functions described above to determine the Ε S /N T  value. This value is then coupled to a communication network controller  505 , such as the processor within a base station that is responsible for controlling the forward link power control, or the processor within a mobile cellular telephone that is responsible for communicating the amount the forward link power control should be adjusted in order to maintain a desired transmission power. It should be noted that the particular use to be made of the Ε S /N T  value is not intended to be limited by the particular embodiments that are disclosed herein, but should be understood to include all possible applications of this quality value. 
   The mobile station  18  can also calculate the measured normalized signal-to-noise ratio, Ε f /N t , on a per frame basis. The normalized per frame signal-to-interference ratio Ε f /R is measured, where R is the total signal received and E f  is the energy of the desired signal during a single frame. The per frame noise-to-interference ratio, N t /R, is then measured. Ε f /I 0  is then divided by N t /R in order to calculate Ε f /N t . 
   The normalized per frame signal-to-noise ratio, Ε f /N 0  used to calculate the normalized per frame signal can be calculated as follows: the dot product of the re-encoded symbols d and the soft decisions dαΕ T /I 0  can be computed for each rate of the fundamental block. The result can be squared and divided by the estimated pilot energy as shown:
 
Ε P   /I   0 =α 2 Ε T   /I   0   EQN. (11)
 
   The dot products for each rate of the fundamental block can be normalized using the ratio of the number of symbols in a full rate block to the number of symbols in the block. If the fundamental block rate is not known the maximum normalized dot product can be selected. The per frame noise-to-interference ratio, N t /I 0  can then be measured by accumulating the energy in a forward code channel over the frame. 
   Signals representative of the signal-to-noise ratio can be used for the control of power transmission levels in the system and method of the present invention. In the preferred embodiment of the invention for example, the base stations  12 ,  14 ,  16  can use the FWD_SNR_DELTA value sent to it by the mobile station  18 . The FWD_SNR_DELTA value is sent to the base stations by the mobile station  18  on the power control subchannel of a reverse frame n to adjust the forward gain (FWD_GAIN) it applies to a forward frame n+1. 
   In order to calculate FWD_SNR_DELTA, the mobile station  18  can use an expected signal-to-noise value along with the calculated signal-to-noise value. The per frame expected signal-to-noise ratio Ε f /N t  can be calculated as follows. The mobile station  18  can set the initial expected value equal to the signal-to-noise ratio of the first fundamental block that it successfully decodes. If the fundamental block is erased the mobile station  18  increases the expected value of Ε f /N t . Otherwise, the mobile station  18  decreases the expected value of Ε f /N t . 
   The increase step size P i  and the decrease step size P d  are determined by the desired forward link fundamental block erasure rate R e  and the maximum rate of increase of Ε f /N t . This maximum rate of increase can be defined as P m . Then, P d =(R e P m )/(R e −1) and P i =(P d /R e ). P m  can have a value of one-half. 
   If the power control subchannel FWD_SNR_DELTA is not erased by the base stations  12 ,  14 ,  16 , the forward per symbol signal-to-noise ratio delta flag (FWD_SNR_VALID) is set to 1. Otherwise, the base stations  12 ,  14 ,  16  set both the FWD_SNR_DELTA and FWD_SNR_VALID values to 0. The forward gain applied by the base station transmitter to forward transmit frame n+1 is then calculated as follows:
 
FWD_GAIN[ n+ 1]=|FWD_GAIN_MIN, where FWD_GAIN adj &lt;FWD_GAIN_MIN |FWD_GAIN_MAX, where FWD_GAIN adj &gt;FWD_GAIN_MAX |FWD_GAIN adj , otherwise  EQN. (12)
 
where FWD_GAIN adj =FWD_GAIN[N]*10 −X , and superscript X is determined according to FWD_SNR_DELTA and FWD_SNR_VALID. It will be understood, however, that any method of calculating FWD_GAIN can be used in accordance with the system and method of the present invention.
 
   Referring now to  FIG. 2 , there is shown a portion of power control subchannel  30 . Power control subchannel  30  is suitable for use in the communication system of  FIG. 1 . For example, power control subchannel  30  can be used to transmit FWD_SNR_DELTA from the mobile station  18  to the base stations  12 ,  14 ,  16  in order to control the power level of transmissions to the mobile station  18 . 
   Power control subchannel  30  can be located within a pilot channel carrying a plurality of power control groups  34 . For example, sixteen power control groups  34  can form each of a plurality of frames  38  within the pilot channel. Each power control group  34  can contain a plurality of pseudorandom noise words  38 . In practicing the method of the present invention one or more pseudorandom noise words  38  can be removed and replaced with power control information  40 . 
   The removed pseudorandom noise words  38  can be any noise words  34  within the length of power control group  34 . However, in a preferred embodiment, noise words  38  located towards the center of power control group  34  are used. It is preferred that power control information  40  instruct a transmitter to increase or decrease the transmit power level a specified amount or to leave the transmit power level unchanged, as shown in Eqn. (12). Furthermore, it is also preferred that the transmission of frame  38  containing power control information  40  in this manner be repeated several times in order to increase reliability. 
   It will be understood that any power control information can be transmitted by puncturing the power control information into selected positions within a power control group  34 . In addition, it will be understood that this method of puncturing power control information into the pilot channel may be advantageously applied to any of the methods for determining power control information set forth herein. 
   The foregoing description of the preferred embodiments of this invention is provided to enable a person of ordinary skill in the art to make and use the invention claimed herein. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the principles described can be applied to other embodiments without the use of any inventive faculty. Therefore, the present invention is not to be limited to the specific embodiments disclosed but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Technology Category: 5