Method and apparatus estimating cell interference and noise for CDMA packet data channels

A method and apparatus for communicating data signals using a spread spectrum cellular network, the cellular network including a plurality of base stations coupled to the cellular network the mobile unit assigned to one of the plurality of base stations. The method and apparatus receives a signal of another of the plurality of base stations (target base station) and determines the interference density to the target base station from the received signal.

BACKGROUND

1. Field of the Invention

The invention relates generally to spread spectrum mobile communication networks, and more particularly, to a method and apparatus for estimating cell interference in a spread spectrum mobile cellular communication network.

2. Description of Related Art

In spread spectrum mobile cellular communication networks, the signal power level between base stations and mobile units is carefully controlled. For example in code division multiple access (“CDMA”) mobile communication system versions, a mobile unit calculates the channel quality of a signal transmitted from the base station and periodically reports this measured channel quality to the base station. The base station may then adjust the gain of future transmissions to the reporting mobile unit accordingly. In addition, the mobile unit may determine and report the channel quality of pilot signals received from other base stations. Depending on the reported channel quality of other pilot signals, communication between the mobile unit and another base station may be established.

In CDMA IS2000 standards prior to release C (1xEVDV), the channel quality for a pilot signal of a nearby base station is defined as the ratio of the pilot signal energy to the total noise and interference power as experienced by the mobile unit. In the CDMA IS2000 standard release C (1xEVDV), the channel quality (“C/I”) of a pilot signal of a nearby base station is defined as the ratio of the pilot power to the interference density (Nt), where Ntis the noise level experienced by the mobile unit when the received signal is despread using a target cell P/N sequence, excluding all same cell orthogonal signals of the target cell. A mobile unit linked to a base station operating in a CDMA IS2000 release C standard based cellular network must periodically determine the defined C/I for nearby base stations. The mobile unit may be required to determine the C/I for nearby base stations while conducting a call with its current base station and thus has limited resources to determine the defined C/I. A need thus exists for a mobile unit based system and method that may be employed to efficiently determine Ntand C/I (as defined by the CDMA IS2000 standard release C) for nearby base stations. The present invention provides such a mobile unit based system and method.

SUMMARY OF THE INVENTION

The present invention includes a system, mobile unit, method, and article of manufacture for communicating data signals using a spread spectrum cellular network. The cellular network includes a plurality of base stations coupled to the cellular network and a mobile unit is assigned to one of the plurality of base stations (active base station). The system receives a signal of another of the plurality of base stations (target base station) and determines the interference density to the target base station from the received signal. The system may synchronize an Orthogonal code sequence with the Orthogonal code sequence boundary of the target base station's pilot sequence.

In an embodiment, the system may correlate the received signal with a corresponding P/N sequence of the target base station, correlate the selected Orthogonal code sequence with the P/N correlated target pilot sequence of the target base station, and determine the energy of the Orthogonally correlated, P/N correlated, target pilot sequence. In the embodiment the Orthogonal code sequence may be a Walsh code sequence. Further, the cellular network may be a CDMA based network and each base station of the plurality of base stations represents a network cell.

In an embodiment, the system may select a code sequence that is at least quasi-orthogonal to the Orthogonal code sequences currently employed by the target base station where the selected code sequence is comprised of a repetition of a code sequence that is orthogonal to other code sequences currently employed by the target base station and the length of the selected code sequence is an integer multiple of the longest Orthogonal code sequences currently employed by the target base station. Further, the system may synchronize the selected code sequence by determining the Orthogonal code sequence boundary for the active base station's pilot signal and determining the Orthogonal code sequence boundary for the target base station's pilot signal from the determined active base station's pilot signal Orthogonal code sequence boundary.

In another embodiment, the system may correlate the received signal with a corresponding P/N sequence of the target base station and correlate a pilot Orthogonal code sequence with the target base station's P/N correlated signal. The system may further determine the power of the target base station's P/N correlated signal and determine the energy of the Orthogonally correlated, P/N correlated, received signal.

DETAILED DESCRIPTION

Throughout this description, embodiments and variations are described for the purpose of illustrating uses and implementations of the invention. The illustrative description should be understood as presenting examples of the invention, rather than as limiting the scope of the invention.

FIG. 1is a block diagram of a cellular system segment10in which the present invention may be employed. In this cellular system segment10, there are a plurality of base stations22,24,26,42, and44that are geographically separated and a mobile unit30. The mobile unit may be any wireless apparatus that includes a cellular Modulator/Demodulator (“modem”) that may communicate with a base station (“BS”)22,24,26,42, or46. For example, the mobile unit may be a cellular telephone, personal data assistant (“PDA”), or computer. In one embodiment, each base station may communicate with the mobile unit when the signal strength of the mobile unit, as received at a base station, is sufficiently strong. In one invention embodiment, the base stations support the same wireless communication protocol standard (such as the CDMA IS2000 rev. C standard). In another embodiment of the invention, the base stations may support different or multiple communication protocol standards. In addition, the mobile unit30may support a single or multiple communication protocol standards. For example, the device30may support a CDMA standard, Advanced Mobile Phone Service (“AMPS”) standard, a Time Division Multiple Access (“TDMA”) standard, and a Groupe Special Mobile (“GSM”) standard. In the example shown inFIG. 1, the mobile unit30is capable of communicating with all of the base stations22,24,26,42, or44using a CDMA standard.

InFIG. 1, the mobile unit30acquires pilot signals from several base stations22,24, and26. In this example, the pilot signal from the base stations42and44may be too weak based upon the mobile unit's current location. The mobile unit30may determine the channel quality for each base station22,24, and26, with each base station representing a cell in the cellular network segment10. The channel quality for each cell may vary due to the noise and interference experienced (i.e., “seen”) by the mobile unit. In CDMA cellular systems, non-multi-path signals transmitted from the same cell are orthogonal to each other. A mobile unit receiving a pilot signal may remove same cell signals by a despreading process. Consequently, signal interference seen using a mobile unit is commonly caused by other cell signals (between other base stations and other mobile units (not shown inFIG. 1)) and multi-path signals within the same cell. A Rake receiver may be employed to combine multi-path signals in order to generate a single, stronger coherent signal.

FIG. 2is a block diagram of an exemplary mobile unit120that may be used to practice the present invention. The exemplary device120may include a central processing unit (“CPU”)122, a random access memory (“RAM”)124, a read only memory (“ROM”)126, a display128, a user input device132, a transceiver application specific integrated circuit (“ASIC”)134, a microphone138, a speaker142, and an antenna144. The ROM126is coupled to the CPU122and stores the program instructions executed by the CPU122. The RAM124is coupled to the CPU122and stores temporary program data and overhead information. The user-input device132may comprise an input device such as a keypad, touch pad screen, track ball or other similar input device that allows the user to navigate through menus in order to place calls, in addition to performing other functions. The display128is an output device such as a CRT, LCD or other similar screen display that enables the user to read received data and to place calls.

The microphone138and speaker142may be incorporated into a handset that is coupled to the ASIC134. The microphone138and speaker142may also be separated from the handset to allow hands-free communication. In this mode, the ASIC134may include voice activation circuitry that converts voice commands into data. The data is transmitted to the CPU122via a serial bus136and may include a telephone number to be dialed.

The transceiver ASIC134includes an instruction set necessary to communicate data and voice signals over the cellular network segment10. In one embodiment, the transceiver ASIC134is a code division multiple access (“CDMA”) ASIC and the cellular network is a CDMA network that supports data and voice communication. The ASIC134is coupled to the antenna144to communicate signals within the cellular network segment10. When a data signal is received by the transceiver ASIC134, the data is transferred to the CPU122via a serial bus136. The data can include base station overhead information to be stored by the mobile unit in accordance with the methods described herein. The ASIC134may perform operations to determine channel quality. An exemplary portion of the ASIC134is shown inFIG. 4A. As shown inFIG. 4A, the exemplary ASIC134includes a radio frequency (“RF”) circuit62, a Rake receiver64, a searcher correlator66, P/N sequence generator51, demodulator processor55, and deinterleaver and Viterbi decoder53. These components,51,53,55,62,64, and66for CDMA mobile units are well known to those of skill in the art.

The searcher66may include one or more correlators. The searcher66may be employed to locate target pilot signals of nearby target base stations in an exemplary embodiment. The searcher may also be employed to find the largest multi-path peaks present in a received signal.FIG. 4Bis a simplified block diagram of a Rake receiver64that may be employed in the present invention. The Rake receiver is used to combine the identified largest multi-path peaks into a single, coherent signal. As shown in this figure, the Rake receiver64includes a plurality of fingers61, a plurality of delay units63, and an adder65.FIG. 4Cis a simplified block diagram of a finger61that may be employed in the present invention. As shown inFIG. 4C, the finger61includes a code generator69and a cross-correlator67. The code generator69provides the code for the signal to be correlated and the cross-correlator67correlates the signal to the selected/generated code, at the offset searched and determined by the searcher. A mobile unit120employing a Rake receiver64may significantly reduce noise or interference due to signal multi-path interference.

FIG. 3illustrates a block diagram of an exemplary base station100that may be used in practicing the present invention. The exemplary base station100may include a CPU102, a RAM104, a ROM106, a storage unit108, a first modem/transceiver112and a second modem/transceiver114. The first modem/transceiver112may couple, in a well-known manner, the base station100to a central cellular network control center via an Internet connection or via a wired telephone system such as the Plain Old Telephone System (“POTS”). The second modem/transceiver114couples the base station100to the cellular network segment10. The modem/transceiver114may be an Ethernet modem, telephone modem, wireless modem or other communication device that communicates with the cellular network10(FIG. 1). The CPU102directs communications between the first and second modem,112and114, respectively, for messages between the central network control center, Internet, or POTS, and one or more mobile units.

The ROM106may store program instructions to be executed by the CPU102. The RAM104may be used to store temporary program information and overhead information for other base stations in its sector (i.e., nearby base stations). The storage unit108may comprise any convenient form of data storage and may be used to store the overhead information. An exemplary portion of the modem/transceiver114is shown inFIG. 5. As shown inFIG. 5, the exemplary modem/transceiver114includes a coder52, an orthogonal signal spreader54, and an RF circuit56. In one embodiment the coder52applies a unique Pseudo-Random (“P/N”) sequence to data to be transmitted. The orthogonal signal spreader54applies an orthogonal code to the P/N coded data. In one embodiment, the orthogonal code comprises a Walsh code orthogonal sequence. The RF circuit56modulates the orthogonally spread, coded data to a predetermined radio frequency.

In accordance with the CDMA IS2000 standard release C, a mobile unit actively communicating with a base station (active base station) in a CDMA cellular network is required to determine the channel quality of nearby target base stations/cells and to report the determined channel quality to the active base station. The IS2000 standard release C states that the channel quality is equal to a ratio of the pilot energy to the interference density (Nt), where Ntis the noise level experienced by the mobile unit when the received signal is despread using a target cell P/N sequence, excluding all same-cell orthogonal signals of the target cell/BS. The active base station may report this information to a system controller (not shown) or it may evaluate the information to determine whether the mobile unit should be transferred to another target base station/cell (i.e., perform a cell switch). Using the Rake receiver64(of the ASIC134), the mobile unit30may accurately determine the interference density (Nt) for a receive path (pilot signal). Target base station pilot signals, however, are not assigned to a finger61of the Rake receiver64of the mobile unit30. The Rake receiver64is employed to combine multi-path components of CDMA signals from the mobile unit's currently assigned/active base station (cell).

In one embodiment of the present invention the searcher66is engaged to determine the pilot energy Ec and interference density Ntof target base station pilot signals. In particular, the present invention employs the searcher66to perform the processes and calculations shown inFIGS. 6A,6B, andFIG. 8.FIGS. 6A and 6Bdepict two exemplary correlation processes,70and80, that may be used in combination to determine the value of C/I.FIG. 6Adepicts an exemplary process70that may be used to determine the energy of a received target pilot signal.FIG. 6Bdepicts an exemplary process80that may be used to determine the interference density, Nt, of the received target pilot signal.

In one exemplary embodiment the processes70and80are sequentially executed by a searcher correlator66(FIG. 4A). In this embodiment, at a first time, T1, the searcher correlator66performs the first correlation process70. At a later time T2, the searcher correlator66performs the second correlation process80. In the first correlation process70, a target pilot P/N sequence is correlated with the received signal (step72). Next, the pilot Walsh code is correlated with the output signal from correlation step72(step73). The resultant signal component C is then accumulated over a period WL, where WL is equal to an integer multiple of the maximum Walsh code length used by the target cell (step74). In all current CDMA IS2000 standards, the maximum Walsh code length used in a cell is less than 256. In an exemplary embodiment, in order to increase estimation accuracy, the accumulation (step74) is performed for several Walsh code windows (step76), M times, where M is an integer. The received pilot signal power, Ec, is then estimated in step78as

At a later time T2, in an exemplary embodiment the searcher correlator66is used to perform the search second correlation process80shown inFIG. 6B. In process80, the target pilot P/N sequence is correlated with the received signal (step82). Next, an orthogonal Walsh code sequence is correlated with the resultant correlated signal (step84). The signal component C is accumulated over the period WL (step86). In an exemplary embodiment, in order to increase the estimation accuracy, the accumulation (step86) is performed for several Walsh code windows (step88), M times, where M is an integer. The received signal interference density, Nt, is estimated at step88as

1M⁢∑M⁢X2.
The orthogonal Walsh code used in step84ideally is orthogonal to all active Walsh codes in the same cell. Because the Walsh codes used in a base station may change over time, in one exemplary embodiment, a Walsh code is used that is not used for pilot signals, transmission diversity pilot signals, auxiliary pilot signals, or any channel that has a constant bit stream. This code may be determined in an IS2000 standard conforming system because such systems use specific Walsh codes for these types of signals.

In another embodiment, a Quasi Orthogonal code sequence is used as the orthogonal Walsh code in step84of process80. The Quasi Orthogonal code sequence is comprised of a repetition of a code sequence that is orthogonal to other Walsh code sequences currently employed by the target base station. Further, variations of the processes70and80may be used in other embodiments. In another process, multiple correlations, performed on different input data, may be averaged. This process may reduce the noise estimation Ntvariance. In another embodiment, the processes70and80may remove the correlation values that are too large or too small as compared with the remaining correlation values (i.e., apply a median filter to the correlation values). In any of these processes, the selected orthogonal Walsh code should be aligned with the Walsh code of the received signals in order to achieve desirable correlation performance.

FIG. 7Ais a diagram of exemplary sequence and their Walsh boundaries, including an offset, “D”, in a CDMA IS2000 standard-based system.FIG. 7Bis a flowchart, illustrating a process150, for determining the target pilot signal Walsh boundary offset D, which is the target Walsh boundary offset from the active cell, reference Walsh boundary. Since the active cell reference Walsh boundary is known to the searcher, determining D provides the target cell Walsh boundary. In step142of process150, the offset between a target P/N sequence and a reference P/N sequence is determined. In a CDMA IS2000 standard-based system, target sequences are offset by increments of 64 chips. Further, the mobile unit's base station provides the differential offset (N−K) between its reference pilot P/N sequence143and a target pilot P/N sequence141. The target sequence141and reference sequence143may have different signal propagation times (between their respective base station and the mobile unit). Step144of process150determines the chip offset, G, representing the signal propagation differential between the target pilot P/N sequence141and the reference P/N sequence143.

In one embodiment, the correlation peak of the received target sequence is used to determine the value G based on a known reference within a P/N sequence, e.g. the first logical 1 following 15 logical 0's. The Walsh boundary is obtained from a known reference boundary as follows. In process150, D is determined based on the offset “G” and “N−K”. In particular, D=((N−K)*64+G)mod Walsh_Length, where the value of Walsh_Length comprises the length of the Walsh code that is to be correlated with the target pilot signal to obtain noise level estimation (step146).

FIG. 8depicts another exemplary process90that may be used to determine the value of C/I. In process90, as shown inFIG. 8, the target pilot P/N sequence is correlated with the received pilot signal (step91). The pilot Walsh code is then correlated with the thus correlated received signal (step92). The resultant signal component C is accumulated over a period WL, where WL is equal to the maximum Walsh code length used by the pilot signal's cell (step93). In an exemplary embodiment, in order to increase estimation accuracy, the accumulation (step93) is performed for several Walsh code windows (step94), M times, where M is an integer. The total signal power is estimated at step96as

1M⁢∑M⁢x2.
At a step98, the received pilot signal power, Ec, is estimated as

1M2⁢s2.
The received pilot signal interference density, Nt, is estimated as differential between the total signal power and the received pilot signal power, Ec (at step99). Because the power estimation is summed over integer multiples of Walsh length, the correlation is ideally aligned with Walsh boundary as determined by the process150ofFIG. 7Bdescribed above in more detail.

The previous description of the preferred embodiments is provided to enable any person skilled in the wireless communications 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 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.

While this invention has been described in terms of a best mode for achieving this invention's objectives, it will be appreciated by those skilled in the wireless communications art that variations may be accomplished in view of these teachings without deviating from the spirit or scope of the present invention. For example, the present invention may be implemented using any combination of computer programming software, firmware or hardware. As a preparatory step to practicing the invention or constructing an apparatus according to the invention, the computer programming code (whether software or firmware) according to the invention will typically be stored in one or more machine readable storage mediums such as fixed (hard) drives, diskettes, optical disks, magnetic tape, semiconductor memories such as ROMs, PROMs, etc., thereby making an article of manufacture in accordance with the invention. The article of manufacture containing the computer programming code is used by either executing the code directly from the storage device, by copying the code from the storage device into another storage device such as a hard disk, RAM, etc., or by transmitting the code on a network for remote execution.