CN1418445A - Use of silence detection for improved received voice quality during search of neighbor set - Google Patents

Abstract

A method of a mobile terminal searching signal strength of neighboring base stations within a cellular communications network. The method includes receiving a signal, transmitted at a first frequency, at a mobile terminal from a first base station. The received signal is processed at a signal processing unit within the mobile terminal. The received signal is analyzed and a period of silence within the signal is determined. Thereafter, the mobile teminal ceases signal processing and searches alternate frequencies of neighboring base stations and determines a signal strength measurement for each of the neighboring base stations. After performing the neighboring base station searching, the mobile terminal restores signal processing of the signal through the first base station at the first frequency.

Description

Improving Received Signal Quality Using Silence Detection in Adjacent Group Search

field of invention

The present invention relates to a method for searching adjacent base stations to determine the switching of a call in conversation, in particular to using the silent period of the received voice in a call in conversation to complete the search for alternative frequencies of adjacent base stations, reducing or eliminating A method of misinterpreting speech due to word or word breaks.

Background of the invention

According to the TIA/EIA-95 standard code division multiple access (CDMA) mobile terminal should have the function of completing mobility management or "equipment maintenance". To this end, the mobile terminal continuously searches a set of neighboring base stations, or "pilots", a list of which is provided to the mobile terminal by the base station, to determine the most suitable one.

When an adjacent pilot frequency is found to be on a different CDMA frequency, the ongoing voice signal processing on the current operating frequency will be temporarily stopped due to the intervention of the new frequency and the measurement of the strength of the pilot frequency. The time it takes before the previous CDMA frequency is recovered and speech processing is allowed to continue creates a "hole" in the received speech. In a preferred embodiment of the invention, each frequency search takes about 20 ms before speech processing resumes. If there are multiple frequencies to be searched, the above-mentioned "hole" will become very large, and the voice quality will drop sharply. For example, when the hole happens to be in the middle of a syllable, it may cause problems in understanding words. Variety.

At present, most CDMA networks use a single frequency to transmit signals, so the mobile terminal does not need to search for other alternative frequencies, and therefore does not forcibly interrupt the voice processing. But with the expansion of the network and the growth of the number of users, CDMA service providers have to use multiple frequencies.

The same dilemma exists when searching for adjacent base stations in a Time Division Multiple Access (TDMA) network. The hole reduction technology currently used in the TDMA system is to stagger the search interval, which can reduce the overlap of multiple holes, to a certain extent It can reduce the trouble of the problem, but it can't completely solve the problem.

Therefore, it is necessary to find a method of searching for alternative frequencies emitted by adjacent base stations without distorting the speech of the ongoing call.

Summary of the invention

The invention utilizes the naturally occurring interruption in the conversation to complete the strength measurement of the candidate frequency signal sent by the adjacent base station. For the user, the intensity measurement at this time is carried out during the interruption of the conversation, which can only cause little or no distortion of the received speech.

The invention comprises the steps of: receiving a signal by a receiver, and processing the signal by a digital signal processor. The speech part of the signal is sent to the vocoder, and then sent to the speaker to be played to the user. The user's speech is first sent to the digital signal processor through the vocoder, and then sent to the base station through the transmitter. During the processing of the transmitted and received signals, the digital signal processor detects periods of silence. During the silent period, the processing of the signal is suspended, and the strength of the candidate frequency signal sent by the adjacent base station is tested to determine whether there is a candidate frequency for handover. Once the signal strength measurement is completed, the digital signal processor will resume sending and receiving. signal processing.

The method can provide the user with a perceptually coherent speech signal. During the alternate frequency search, a previously stored speech signal can be fed to the vocoder to prevent possible interruptions or other disturbances that could interfere with the user's listening. A variable search period can further be used to prevent noticeable distortion.

Brief introduction to the drawings

Figure 1 is a schematic diagram of a mobile network according to the present invention;

Fig. 2 is each constituent unit in the mobile terminal used among the present invention;

Fig. 3 is a flow chart of gradually completing the search for candidate frequencies of neighboring base stations in a wireless communication network.

Detailed description of the invention

In the following description, the same reference symbols denote the same or corresponding parts in several drawings. Referring now to the illustrations, the method of the present invention for searching candidate frequencies of adjacent base stations will be described. This method is very useful in the mobile communication network shown in FIG. 1 . The mobile communication network represented by numeral 10 comprises a plurality of base stations 12 which can be connected to other terrestrial communication networks such as the public switched telephone network (PSTN) 18 via a mobile services switching center (MSC) 14 . Each base station 12 is located in an area called a serving cell and provides services thereto. In the mobile communication network 10 in the figure, one base station 12 is generally configured for each serving cell.

In each cell there may be multiple mobile terminals 16 communicating with base station 12 via radio links. Base stations 12 allow users of mobile terminals 16 to communicate with each other or with users connected to the PSTN 18 . The MSC 14 is responsible for routing calls to and from the mobile terminal 16 via the corresponding base station 12.

Handover occurs when an ongoing call being handled by a first base station 12 is transferred to another base station 12 for handling. A handover is required when the quality of the received signal drops below an acceptable level while another base station is available that can provide a high quality signal. Network management to increase system capacity may also force an ongoing call to be handed over from a first base station 12 to a second base station 12 with more capacity available.

The mobile terminal 16 completes signal strength measurements of neighboring base stations 12 in order to determine when a handover is required in accordance with TIA/EIA-95. The current base station 12 will send a set of neighboring base station lists, usually referred to as neighboring cell lists, at each call initiation or handover completion. The neighbor cell list identifies the frequencies of the neighbor base stations 12 that are potential handover targets. The mobile terminal continuously reports the results of these measurements to the current base station 12 . The mobile terminal 16 may perform the signal strength measurement on a fixed or variable cycle.

The measurement report provided by the mobile terminal 16 gives the current base station 12 a list of signal strengths from neighboring cells measured by the mobile terminal at the current location. The mobile communication network 10 knows which adjacent cells still have unoccupied wireless channels that can be allocated and used during the handover. From these available channel lists, the mobile communication network 10 selects the serving cell most suitable for handling the current call from a quality of service and overall interference point of view. An appropriate traffic channel assigned to the target base station 12 is selected and the mobile terminal 16 is caused to tune to the selected traffic channel in the target cell. At the same time, the MSC 14 also transfers the call from the current base station 12 serving the mobile terminal to the base station 12 of the target cell.

Figure 2 shows a mobile terminal 16 used in a CDMA mobile network, comprising: a receiver 102 for receiving and transmitting signals; a digital signal processor 104 for extracting voice data from received signals and providing information to a vocoder 110 Vocoder 110 outputs voice information through loudspeaker 114; In order to send outgoing message, receive voice message from microphone 112, convert into digital code by vocoder 110, utilize spread spectrum technology to expand the whole CDMA channel through digital signal processor 104 again Audio stream on the 1.25MHz frequency band; the output signal of the vocoder 110 is sent to the transmitter 116 for transmission; the central processing unit (CPU) 106 receives control/data messages for controlling the functions of the mobile terminal 16; the memory 107 can be searched During this period, the received speech is stored and sent to the vocoder 110, which will be described below.

The CDMA signal received by receiver 102 is quantized and filtered by a Walsh code correlator to discriminate it from other signals in the frequency band. In addition, the signal needs to be processed by a pseudo-noise sequence correlator to eliminate the added pseudo-random PN code, and further extract the desired signal from other transmitted signals or the same frequency.

The recovered signal includes voice/voice messages and control/data messages. Voice/speech messages are processed by a Viterbi decoder located within DSP 104 to detect and correct errors in signal transmission. Preferably, DSP 104 decodes the signal after receiving a predetermined number of bits. Arriving signals are not stored in the DSP 104 until a predetermined number of bits is reached or no additional signals are received over time. In a preferred embodiment of the present invention, DSP 104 receives voice/speech data for about 20 ms before starting to decode. Once the voice/voice message has been decoded, it is sent to a vocoder 110 to convert the received digital signal into an analog audio signal for output through a speaker 114 .

Outgoing messages are processed in a reverse process. The speech signal received by the microphone 112 is sent to the vocoder 110 for compression and conversion into digital codes. Then the message is sent to DSP 104 to be encoded by a convolutional encoder, then orthogonally encoded with Walsh codes and PN sequences, and then sent to the base station 12 that handles the call through the transmitter 116.

In the voice/voice messages sent and received, there are natural interruptions between syllables, words or sentences during the conversation, or silent periods. At this time, there is no voice/voice data to be processed by DSP104, and these silent periods can be used for searching adjacent frequencies. Almost all of the periods of silence that occur during conversations are longer than the time required to search for neighboring base stations.

For received calls, no speech/audio signal is sent to vocoder 110 during DSP 104 search. The received signal is stored in the DSP 104 and processed until the search job is completed. An interruption in the signal to the vocoder 110 may result in a hum or pop being heard in the speaker 114 . In addition, the user may also perceive a change in the background noise of the call. In order to ensure that the above search will not affect the user's listening, the DSP 104 can send the voice/speech signal previously stored in the memory 107 to the vocoder 110 as background noise. It is processed by vocoder 110 to output to speaker 114 with minimal hum or pop.

In order to further ensure that the search for the candidate frequency of the adjacent base station does not affect the user, the DSP 104 can change the repetition period of the search. For example, the search for the frequency of the first adjacent base station needs to detect two frequencies, and a total search time of about 40ms is required. The DSP 104 first processes incoming signals for a period of time until another period of silence is detected and then continues to search, thus continuing to search for the second, third, fourth and fifth adjacent frequencies. This variable period reduces the possibility of being noticed by the user when the search time is longer than the silent period. Variable repetition rate systems are currently applied to Time Division Multiple Access (TDMA) mobile terminals and are well known in the art.

Figure 3 describes the various steps of the present invention. During a call, mobile terminal 16 receives signals from base station 12 that include voice/voice messages and control/data messages. These messages are received by receiver 102 and sent to DSP 104 for processing (block 300). After receiving the search request, DSP 104 analyzes the voice/voice message sent and received (block 310); if the message does not include the period of silence (block 320), then continue to analyze the next voice/voice frame to determine whether to include the period of silence. If the DSP 104 detects a period of silence, it will temporarily stop the processing of the voice/voice message and search for adjacent frequencies. During the suspension period of this message processing, the background noise stored in the memory 107 can be sent to the vocoder 110 processing to prevent aural inconsistencies output to the speakers (block 330).

The DSP 104 changes the operating frequency currently used to transmit and receive signals to complete the search, and the receiver 102 receives the signals on the adjacent frequency list and channel quality measurements, such as strength, BER and WER, etc. (block 340). After adjacent frequency detection is complete, the receiver switches back to the previous CDMA frequency and transmit and receive signal processing resumes (block 350). The channel quality measurements are sent to the base station 12 to determine when to hand off the current ongoing call to a neighboring base station (block 360).

The present invention is described in terms of CDMA networks, but is equally applicable to other networks, including TDMA and Frequency Division Multiple Access (FDMA) networks. These networks all contain signal processing units for processing the sending and receiving signals. These signal processing units have the function of judging the silent periods in the sending and receiving signals, so as to use these silent periods to search for adjacent base stations. The completion of these searches will not cause damage to the Impact of sending and receiving voice/voice messages.

Various modifications and changes may occur to those skilled in the art who have read and referred to the above description of the invention without departing from the scope defined in the following claims.

Claims (14)

1. A method for a mobile terminal to search for adjacent base station signal strengths in a CDMA cellular mobile network, comprising: receiving, at the mobile terminal, a signal transmitted at the first frequency from the first base station; signal processing in the signal processing unit; Identify periods of silence in the signal; After that, stop signal processing, and search for at least one candidate frequency signal of at least one adjacent base station; After at least one candidate frequency is found, the processing of the first frequency signal passing through the first base station is resumed. 2. The method of claim 2, wherein the speech signal output to the user is a continuous signal without interruption. 3. The method of claim 2, further comprising sending the stored speech to the vocoder while searching for the candidate frequency. 4. The method of claim 1, further comprising receiving a request to search for an alternate frequency prior to determining the quiet period. 5. The method of claim 1, wherein the silent period includes sending and receiving messages. 6. The method of claim 1, wherein the neighbor base station search uses a variable repetition rate. 7. The method of claim 1, wherein periods of silence are formed by natural breaks in conversation. 8. a method for carrying out adjacent base station alternative frequency searches in a CDMA wireless communication network, comprising the following steps: receiving a signal of the first frequency at the mobile terminal; Signal processing by a digital signal processor; Send the processed message to the vocoder; determine periods of silence in the received signal, and stop sending processed messages to the vocoder; Searching for at least one candidate base station operating on the second frequency to complete signal strength measurement; After completing the search for candidate base stations, send the processed message to the vocoder. 9. The method of claim 8, sending the processed message to the vocoder after completing the search of the candidate frequency during the silent period to generate uninterrupted speech output. 10. The method of claim 8, further comprising sending the previous voice message to the vocoder while searching for at least one candidate base station. 11. The method of claim 8, wherein the step of determining a silent period includes determining a silent period for transceiving signals. 12. A method for testing candidate frequencies in a CDMA wireless communication network, comprising the following steps: Receive an incoming signal at the receiver; Incoming signal processing in the digital signal processor; sending the processed incoming call signal from the digital signal processor to the vocoder; processing of outgoing messages in a digital signal processor; Determining the silent period in the incoming signal and the outgoing signal in the digital signal processor; conduct alternate frequency tests; and After the candidate frequency test is completed, the incoming signal is sent from the DSP to the vocoder and the outgoing message is processed at the DSP. 13. The method of claim 12, wherein the silent period in the incoming signal and the outgoing signal is determined after receiving a request to test the candidate frequency. 14. The method of claim 12, wherein during the search for the candidate frequency, the transmission of the processed incoming signal from the digital signal processor to the vocoder is stopped and the processing of the outgoing message is stopped.