Systems and methods for wireless signal classification

Systems and methods for classifying a P25 Phase 1 downlink transmission as using either C4FM modulation or CQPSK modulation are provided. Some methods can include calculating a first signal-to-noise ratio (SNR) at or near a middle of a first symbol of a received wireless signal, calculating a second SNR at or near an edge of the first symbol, calculating a difference between the second SNR and the first SNR, determining whether the difference is greater than a threshold, and classifying a modulation technique of the received wireless signal as either C4FM modulation or CQPSK modulation based on whether the difference is greater than the threshold.

FIELD

The present invention relates generally to wireless communication. More particularly, the present invention relates to systems and methods for classifying a frequency modulation technique of a received wireless signal.

BACKGROUND

Project 25 (P25) is a suite of standards for digital mobile radio communications designed for use by public safety organizations in North America. Phase 1 of P25 uses a frequency-division multiple access (FDMA) protocol, and P25 radios use two different modulation techniques in outbound (i.e. downlink) transmissions: continuous 4 level frequency (C4FM) modulation and continuous quadrature phase shift keying (CQPSK) modulation. C4FM modulation is an analog modulation scheme that uses four different frequency deviations (e.g. +1800 Hz, +600 Hz, −600 Hz, and −1800 Hz) to represent two bits of communication input (i.e. a symbol). Meanwhile, CQPSK modulation is a digital modulation scheme that uses four phase changes (e.g. +135°, +45°, −45°, and −135°) to a symbol.

Both C4FM and CQPSK signals are transmitted at 4800 symbols per second and have the same frame format. Additionally, the phase changes in CQPSK modulation can be translated to C4FM frequency shifts (e.g. 135°/360°*4800 Hz=1800 Hz or 45°/360°*4800 Hz=600 Hz). Therefore, C4FM and CQPSK signals are very similar when a symbol is measured, and P25 radios have difficultly classifying whether an incoming transmission is using C4FM or CQPSK modulation.

Conventional systems and methods used to classify between C4FM modulation and CQPSK modulation use a cyclostationary feature of a modulated signal and rely on a time domain spectral autocorrelation function (SAF) to analyze the cyclostationary feature of the modulated signal and to classify modulation techniques based on a difference in locations of peaks in the SAF. However, these conventional systems and methods must generate three-dimensional graphs of the SAF for every input sample at an oversampled rated, which involves high computational complexity. Furthermore, generating these three-dimensional graphs involves FET and window smoothing, which also involves a high computational complexity.

In view of the above, there is a continuing, ongoing need for improved systems and methods.

DETAILED DESCRIPTION

Embodiments disclosed herein can include systems and methods for classifying a P25 Phase 1 downlink transmission as using either C4FM modulation or CQPSK modulation. In some embodiments, P25 user equipment or a P25 receiver can execute a method for identifying such a classification.

Embodiments disclosed herein can classify the downlink transmission by calculating a signal-to-noise ratio (SNR) for the downlink transmission at least at an edge of a symbol and at a middle of the symbol and can calculate a difference between the SNR at the edge of the symbol and the SNR at the middle of the symbol. Because C4FM modulation has a constant envelop at both the middle of the symbol and the edge of a symbol, for C4FM signals, the SNR at the middle of the symbol can be similar or the same as the SNR at the edge of a symbol. Meanwhile, CQPSK modulation does not have a constant envelop at the edge of symbol so, for CQPSK signals, the SNR at the middle of the symbol can be significantly different than the SNR at the edge of the symbol. Accordingly, embodiments disclosed herein can use the difference between the SNR at the edge of the symbol and the SNR at the middle of the symbol to classify the downlink transmission as either a C4FM signal or a CQPSK signal. More specifically, if the difference between the SNR at the edge of the symbol and the SNR at the middle of the symbol is greater than a threshold, then embodiments disclosed herein can classify the downlink transmission as a CQPSK signal, whereas if the difference between the SNR at the edge of the symbol and the SNR at the middle of the symbol is lower than the threshold, then embodiments disclosed herein can classify the downlink transmission as a C4FM signal.

FIG. 1is a diagram of a system in accordance with disclosed embodiments. As shown inFIG. 1, the system100can include a plurality of user devices102a-102d. In some embodiments, each of the plurality of user devices102a-102dcan include a respective P25 radio configured to send and receive P25 wireless communications. Each of the plurality of user devices102a-102dcan be located in a respective location within a geographic area104.

The plurality of user devices102a-102dcan receive data and voice communications from a base station106a,106bor a repeater108a,108band can transmit data to one of the base stations106a,106bor one of the repeaters108a,108b. In some embodiments, the base stations106a,106band/or the repeaters108a,108bcan relay the P25 wireless communications to one of the plurality of user devices102a-102dor an RF subsystem110. For example, a first of the plurality of user devices102acan transmit either a voice or data communication to a second of the plurality of user devices102bvia a first repeater108aor a first base station106aor both. The P25 wireless communications can originate from the RF subsystem110.

FIG. 2is a block diagram of a user device (e.g. any of the plurality of user devices102a-102d) in accordance with disclosed embodiments. As shown inFIG. 2, a user device200can include control circuitry204, which can include one or more programmable processors204aand executable control software204bas would be understood by one of ordinary skill in the art. The executable control software204bcan be stored on a transitory or non-transitory computer readable medium, including, but not limited to local computer memory, RAM, optical storage media, magnetic storage media, and the like. In some embodiments, the control circuitry204, the programmable processor204a, and the executable control software204bcan execute and control some of the methods disclosed herein.

The user device200can also include a wireless communication interface206. For example, the wireless communication interface206can include an antenna and corresponding communication hardware as would be understood by one of ordinary skill in the art for sending and receiving the P25 wireless communications.

As explained above, C4FM modulation has a constant envelope at both a middle of a symbol and an edge of the symbol, andFIG. 3is an eye diagram of a C4FM signal showing the constant envelop of C4FM signals. As shown inFIG. 3, the C4FM signal has the constant envelop at both the middle310a,310bof the symbol (e.g. where the symbol is measured) and at the edge320of the symbol (i.e. between two consecutive symbols).

In contrast, CQPSK modulation does not have the constant envelope at both the middle of the symbol and the edge of the symbol. Instead, CQPSK modulation has the constant envelop only in the middle of the symbol, andFIG. 4is an eye diagram of a CQPSK signal showing the same. As shown inFIG. 4, the CQPSK signal has the constant envelop at the middle410a,410bof the symbol, but does not have the constant envelop at the edge420of a symbol.

Embodiments disclosed herein can leverage differences between CQPSK and C4FM signals at the edge of any symbol to classify an incoming transmission as having either C4FM modulation or CQPSK modulation. For example,FIG. 5is a flow diagram of a method500in accordance with disclosed embodiments. As shown inFIG. 5, the method500can include a user device (e.g. the user device200) synchronizing a frame pattern of a received wireless signal to find frame and symbol timing as in502. For example, in some embodiments, the user device can perform frame synchronization to decode the received wireless signal. As such, the user device can synchronize the frame pattern as in502for both signal classification and signal decoding.

Then, the method500can include the user equipment calculating an SNR estimation at or near a middle of a symbol as in504, the user equipment calculating an SNR estimation at or near an edge of the symbol as in506, and the user equipment calculating a difference between the SNR estimation at the middle of the symbol and the SNR estimation at the edge of the symbol as in508. After calculating the difference, the user device can determine whether the difference is greater than a threshold as in510.

When the difference is greater than the threshold, the user device can classify the received wireless signal as a CQPSK signal using CQPSK modulation as in512. However, when the difference is lower than or equal to the threshold, the user device can classify the received wireless signal as a C4FM signal using C4FM modulation as in514.

It is to be understood that the user device can periodically or repeatedly execute the method500such that the user device can execute the method500in connection with each symbol received to classify the received wireless signal. After the user device classifies the received wireless signal, the user device can accurately decode the received wireless signal according to an appropriate modulation technique: C4FM modulation or CQPSK modulation.

FIG. 6is a plot showing results of oversampling two received P25 signals by 400 times. The x-axis of the plot inFIG. 6represents sampling points, and the y-axis represents SNR values. A first signal610can represent a C4FM signal, and a second signal620can represent a CQPSK signal. Middles of symbols can be represented by points 0, 400, and 800 on the x-axis, and edges of symbols can be represented by points 200 and 600 on the x-axis.

As shown inFIG. 6, differences between the C4FM signal610and the CQPSK signal620are most pronounced at the edges of symbols (points 200 and 600 on the x-axis), whereas SNR values are generally similar in value at the middles of symbols (points 0, 400, and 800 on the x-axis). WhileFIG. 6illustrates two signals greatly oversampled, it is to be understood that, in accordance with disclosed embodiments, SNR estimations need only be calculated twice: at the middle of a symbol (e.g. 0, 400, or 800 on the x-axis) and at the edge of the symbol (200 and 600 on the x-axis).

The embodiments described herein solve a major problem of the prior art, namely, classifying modulation in P25 communication quickly and with low computational complexity. For example, a received wireless signal can be classified as either a C4FM signal using C4FM modulation or a CQPSK signal using CQPSK modulation by only sampling twice per symbol, calculating a difference, and comparing the difference to a threshold. As such, P25 signals can be classified using minimal computational resources, thereby improving the functioning of a P25 receiver.

Although a few embodiments have been described in detail above, other modifications are possible. For example, the steps described above do not require the particular order described or sequential order to achieve desirable results. Other steps may be provided, steps may be eliminated from the described flows, and other components may be added to or removed from the described systems. Other embodiments may be within the scope of the invention.

From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific system or method described herein is intended or should be inferred. It is, of course, intended to cover all such modifications as fall within the spirit and scope of the invention.