Voice signals are also called voice signals. The frequency of voice signal is usually between 300 Hz and 3400 Hz. In order to turn it into a pulse signal and transmit it on the carrier, the low-frequency speech signal should be sampled and quantized first. Sampling is a common technique in analog-to-digital conversion, as shown in figure 1. The analog signal is a continuous sine or cosine wave. In order to reproduce it with a series of pulse signals without distortion, the sampling frequency should be high enough to recover the signal. According to the sampling law, the sampling frequency should be equal to twice the sampling signal frequency to avoid distortion. In the current digital mobile phone, the sampling frequency is 8kHz, which is enough to ensure the credibility of the signal. There are only two kinds of digital pulse signals: 0 and 1. The amplitude of sampling pulse wave varies greatly. To accurately describe a pulse wave, it is necessary to have a definition of its "height", which is the process of quantification. Each sample value is encoded into an 8-bit code, forming a signal of 8k× 8bit/s = 64kbit/s, which is the first step of converting speech signal into digital signal, namely analog-to-digital conversion (A/D). But the frequency band occupied by 64 kbit/s signal is too wide, which is undoubtedly a waste. Therefore, it is necessary to compress the signal. Through speech compression technology, we change the signal of 64 kbit/s into the signal of 13 kbit/s, which greatly saves the frequency band. The process from sampling, quantization to compression is called speech coding.
Figure 1 sampling frequency
After speech coding, we get a group of 13 kbit/s pulse signals that coherently reflect speech information. This group of signals will be regrouped, decomposed into important bit codes, sub-important bit codes and unimportant bit codes, and then inserted into code groups out of order, which is time division insertion. Why are you doing this? Because wireless signals will encounter all kinds of interference when transmitting in the air, and these interferences are random. If the 13 kbit/s signals sent by the speech coder are transmitted in sequence on the carrier without recombination and insertion, all the data during this period will be lost when they are disturbed for a period of time, and these signals cannot be recovered because there is no relevant data during this period. However, using interval insertion technology, even if the data is destroyed in a period of time, only a small part of a group of voice groups is destroyed, which can be completely recovered according to other pulse groups.
Of course, when coding according to related pulses, it is necessary to add some symbols to distinguish damaged pulses. This coding method can be realized by adding block convolutional codes, error correcting codes and parity codes. The above process is a process of channel coding and interleaving.
Compared with ETACS mobile phones, GSM mobile phones have a very advanced technology, that is, the confidentiality is greatly improved. There are many encryption methods for GSM mobile phones. For example, when the mobile phone is turned on and registered to access the network, it is necessary to transmit the unique access number of each user, such as the IMSI number and the minimum number. These numbers are transmitted in the air by calculating the ki value stored on the SIM card, which corresponds to the switch, so it is basically impossible to intercept and crack these information in the air. In addition, GSM system also has frequency hopping function. As the name implies, the frequency of carrier frequency is constantly changing to protect information from being stolen. This process is called encryption.
After the above-mentioned series of speech signal processing, a burst, that is, a frame signal, is formed. Binary data is converted into low-frequency modulation signal by 0.3GMSK technology, and then loaded on the carrier wave and transmitted in the form of electromagnetic wave. This is the modulation process.
The principle of SMS- short message service is simple, but the process of inputting characters on mobile phone is more troublesome. SMS has two outstanding advantages: the first is the store-and-forward mechanism. Sending data packets by SMS is done by the short message center in the mobile network instead of the end user. If the user is not in the service area, the short message is stored in the short message center and forwarded to him after the user appears, which is not available in GPRS and other services. The second is the delivery confirmation mechanism. In the circuit-switched data environment, the connection is end-to-end, so users can know whether the connection is completed and the data transmission situation. However, in the GPRS environment, the terminal connected to the GPRS bearer is always on, and it is always ready to send or receive data transparently (relative to the user). In this way, the task of checking whether the data packet is sent successfully falls on the user, and the corresponding cost will be higher for each packet data service.
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