What is iDEN (Integrated Digital Enhanced Network?

Integrated Digital Enhanced Network (iDEN) is a digital trunked mobile communication system developed and produced by Motorola, Inc. of the United States. It is one of the most popular trunked communication systems. Its feature is to provide users with two-way intercom function of trunked telephone and cellular phone function at the same time. The iDEN system uses time division multiple access technology in the wireless interface. The largest operator using iDEN technology is Nextel Corporation in the United States.

iDEN is a dedicated digital trunking communication system for sharing frequency, commanding and dispatching. Trunked communication system is a kind of mobile communication system for group dispatching and command communication. It is mainly used in the field of professional mobile communication. Different from ordinary mobile communication, the biggest feature of trunked communication is that voice communication uses PTT (PushToTalk) buttons to connect in a push-to-talk manner. Features such as group calling.

The predecessor of iDEN is the MIRS system, which was originally designed for cluster common network applications. Therefore, in addition to the command and dispatch business, it also has functions such as duplex telephone interconnection, data and short messages. It uses time division multiple access (TDMA) technology, VSELP (Vector Sum Excited Linear Prediction) and M-16 QAM (Quadrature Amplitude Modulation) and other technologies. It uses a duplex call structure and a special multiplexing method to make the system have low power and large capacity. Wide-area coverage.

The iDEN digital trunked communication system can provide a variety of service functions.

(1) Command and dispatch function: It can make fast and accurate command contact with terminal holders within the coverage area, so that dispatch instructions can be quickly transmitted;

(2) Duplex interconnection function: In addition to dispatching accidents, it has the same phone call function as mobile phones, and it is a supplement to the public network where there is cluster coverage but no mobile coverage;

(3) Data and short message functions: Compared with analog clusters, digital cluster communication has more reliable performance, wider coverage, and more diverse services. It is particularly advantageous for data transmission, lower costs, and greater confidentiality.

In terms of technology, iDEN has many advantages:

(1) On the basis of traditional dispatch communication, it absorbs the advantages of digital cellular communication systems and enhances the telephone interconnection function. Its wireless telephone function is on the same level as the personal mobile communication system, while adding value to digital cellular communication systems. Services such as short message service, voice mail, and circuit data based on IWF are applied to the iDEN system;

(2) iDEN adopts the traditional 800MHz spectrum to use the spectrum with high efficiency. This spectrum of spectrum is widely used in trunked communications worldwide. No adjustment is required. Discontinuous frequency points are used to allow operators to flexibly configure frequency resources. Through TDMA technology, iDEN divides a 25kHz physical channel into 6 digital communication time slots with high frequency utilization;

(3) iDEN adopts unique MI6QAM modulation technology, which makes the rate of each 25kHz physical channel (including 6 communication time slots) reach 64Kbps, and at the same time, the adjacent channel suppression reaches more than 60dB. This efficient modulation technology ensures the cluster The coexistence of digital and analog systems in the process of communication digitization. The voice coding method of iDEN adopts VSELP of 4.2Kbps, which can guarantee the quality of voice under 6: 1 compression;

(4) The cellular cell structure improves the coverage of the network. IDEN adopts a 7 × 3 cell multiplexing method to divide a base station into sector cells, expand the cell capacity, and improve the networking capabilities of large regions. At the same time, It can adopt the omnidirectional base station method and the 12 × 1 omnidirectional cell multiplexing method, adapting to local conditions and gradually developing;

(5) Cross-system scheduling communication can be realized.

The structure of the iDEN system is shown in Figure 1. Operation and Maintenance Center

As a mature cluster system, the iDEN system is supported by many technologies, of which four are more critical. They are TDMA time division technology, VSLEP speech coding technology, M-16QAM normal phase amplitude modulation technology and error control technology. With these four technologies as support, the entire system becomes more stable during operation.

TDMA Integrated Digital Enhanced Network TDMA Technology

Is to divide the time into frames at a fixed period, and then divide it into several hours

figure 2

Slot, and then the time slot allocation principle, so that the mobile station sends a signal to the base station according to the specified time slot in each frame. The base station receives the signal of the hand station in each time slot so as not to be confused. At the same time, the signals sent by the base station to multiple mobile stations will be arranged to be transmitted in predetermined time slots in order. Each mobile station only needs to receive in a specified time slot, and can receive signals on demand in the combined time slot. as shown in picture 2.

VSELP Integrated Digital Enhanced Network VSELP Voice Coding Technology

Vectored Hobbing Linear Predictive Coding (VSELP) is a vector quantization method used in the preparation of codebooks. It can maintain high speech quality at lower code rates. This coding method is a kind of CELP and has become the main development direction of CELP coding. VSELP encoders use two codebooks, one is a vector codebook and the other is a long-term prediction adaptive codebook. The linear prediction coding LPC analysis is also calculated once per frame, and its codebook is the coding sequence of the past frame. After processing a sub-frame, new bits are moved into the sequence to keep the sequence length of the entire codebook unchanged. But each sub-frame is changing, so it is called adaptive codebook. The state of the long-term filter can be used to obtain the optimal long-term prediction delay and gain, and then to find the best excitation vector from the codebook in sequence, and then to obtain a synthesized voice after a certain circuit operation.

The iDEN digital trunking system uses two optional encoders, which correspond to two time-division multiple access modes of 3 time slots and 6 time slots. One is an encoder with a source coding rate of 8 kbit / s, and uses 20 ms of speech as a coding sub-frame to obtain a 160-bit speech coding output. The other is an encoder with a source coding rate of 4.2 kbit / s. Take 30ms of speech as a coded sub-frame and get a 126-bit speech coded output. The former uses 2 timeslots out of 6 timeslots in each 25KHz channel to send these voice codes out. Compared with the latter, the coding rate is almost doubled, thereby improving the overall quality of voice connection communications. In terms of voice quality, when the scheduling system uses 6 time slots, the voice quality MOS is 3.6-3.8; when the duplex interconnect radiotelephone communication uses 3 time slots, the voice quality can exceed 4.2 or higher. From the point of view of source coding, the trunking system can guarantee lower call quality, and the voice coding rate is lower than that of the GSM system, which can undoubtedly save more bandwidth. The iDEN system uses VSELP coding technology to encode 30 milliseconds of speech into sub-frames and output 126bit, 4.2kbit / s speech coding.

Figure 3 VSELP coding block diagram

The forward error correction code signal is used to synthesize the data stream, and the output signal level is higher or lower, which can improve the audio quality and output a high-quality voice signal. VSELP technology still works well in covering the edge areas.

VSELP is excitation coding. The included speech encoder achieves high-quality speech through calculation and provides a certain error range for the channel. It is essential to apply low data rate speech encoding in remote communications.

VSELP speech encoder has the performance of reducing the complexity of calculation and improving the channel error. After using the gain quantizer, the coding efficiency is improved, and the design method of the post filter is adapted to improve the quality of the reconstructed speech. Figure 3 is a block diagram of VSELP coding.

M-16QAM Integrated Digital Enhanced Network M-16QAM Modulation Technology

The role of modulation in communication systems is critical. The so-called modulation is a process of converting a signal into a form suitable for transmission in a channel. Broadly defined modulation is divided into baseband modulation and bandpass modulation (also known as carrier modulation). Carrier modulation is the process of using a modulation signal to control the parameters of a carrier wave, even if one or several parameters of the carrier wave secretly modulate the law of the modulation signal. Modulated signals are message signals (baseband signals) from a source. These signals can be analog or digital. An unmodulated periodic oscillatory signal is called a carrier, and it can be a sine wave or a non-sine wave (such as a periodic pulse sequence). Carrier modulation is called a modulated signal, which contains all the characteristics of a modulated signal. The modulation of the baseband signal to the carrier is to achieve one or more of the following goals: First, in wireless transmission, the signal is radiated into space through the antenna in the form of electromagnetic waves. In order to obtain higher radiation efficiency, the size of the antenna must be comparable to the wavelength of the transmitted signal, while the lower-frequency components contained in the baseband signal have longer wavelengths, but the antenna is too long to achieve. However, if the spectrum of the baseband signal is moved to a higher carrier frequency through modulation, the spectrum of the modulated signal matches the bandpass characteristics of the channel. Antenna to radiate electromagnetic waves. Second, multiple baseband signals are moved to different carrier frequencies, respectively, to achieve channel multiplexing and improve channel utilization. Third, expand the signal bandwidth, improve the system's anti-interference and anti-fading capabilities, and also realize the interchange between transmission bandwidth and signal-to-noise ratio. Therefore, modulation has a great impact on the effectiveness and reliability of the communication system.

Integrated Digital Enhanced Network Error Control Technology

In the process of data transmission, especially the radio frequency channel transmission, the bit error rate is higher than that of telephone line transmission. In order to accurately transmit data, error control needs to be added. There are two general approaches.

Method 1: Use forward error correction technology to correct errors in decoding during information transmission. Forward error correction is also called forward error correction code, or Forward Error Correction (FEC) for short. It is a method to increase the reliability of data communication. In a one-way communication channel, once an error is found, its receiver will not have the right to request transmission again. FEC is a method of transmitting redundant information using data. When an error occurs during transmission, the receiver will be allowed to reconstruct the data.

Method 2: Select automatic request retransmission technology. Automatic repeat request is an error control technology that performs error detection at the receiving end in data communication and automatically requests retransmission at the sending end, referred to as ARQ. In ARQ, retransmissions continue until the codeword is successfully received. When forward error correction cannot generate data and the data is lost, this technology confirms the data that has not been received and the data that needs to be retransmitted. ^[1]