A brief discussion on the specific application of real-time signal processing in communications

In the current DSP market, communication equipment is its largest user, the following are a few examples:

(1) Digital Hive System

Digital cellular systems use general-purpose DSPs to implement functions such as speech synthesis, error-correction coding, baseband modems, and system control.

a. Speech synthesis, speech compression and coding are the earliest and most widely used applications of DSP. In wired and wireless communications, vector encoders are used to compress speech signals into channels with limited bandwidth.

b. Error correction coding. Forward Error Correction (FEC) schemes are widely used in telecommunication applications to reduce the bit error rate in noisy channels. As more and more data is passed through channels with limited bandwidth, the need for improved FEC technology becomes more urgent. Cyclic redundancy check (CRC) and bit parity check are still used for simple error detection. However, more complex forward error correction schemes such as Verbi convolutional coding and Reed-Solomon (RS) coding are often used for multi-bit error detection and correction. In general, linked coding schemes are better than simple schemes in reducing bit errors. For example, the IS-54 voice channel specification for digital cellular mobile phones in the United States is jointly guaranteed by CRC and convolutional codes.

c. Baseband modulation and demodulation. DSP can be used to implement baseband modulation and demodulation functions. These functions include timing recovery, automatic gain and frequency control, symbol detection, pulse shaping, and matching filters. Many functions were previously implemented in hardware. With the development of high-performance DSPs and the growing need for multi-purpose hardware design, for example, the IS-54 standard requires that each terminal be able to handle three modulation schemes: FM, FSK, and DQPSK, which are now often implemented in DSP through its software.

(2) Software Radio

Software radio, which has received full attention and developed rapidly in recent years, uses the same hardware equipment and different software modules to adapt to communications in different frequency bands and different modulation methods. In software radio, the most important and challenging part is the high-performance A/D, D/A converter and the real-time signal processor with DSP as the core. The concept of software radio was proposed and initially applied in military communications; but in recent years, it has also shown broad application prospects in various civilian communication systems. The high real-time requirements of software radio for DSP are strongly promoting the development of DSP, including the performance of single-chip processors, the ability of multiple processors to work together, the environment for DSP software development, and the DSP real-time operating system.

(3) Equalization and channel estimation

Another computationally intensive task for the DSP is channel simulation to estimate echo, noise, or intersymbol interference. Line echo cancellation is an application suitable for DSP implementation in public wired telephones. Echo and noise cancellation is equally important for wired and wireless communications.

Equalization is another channel estimation technique used to eliminate inter-symbol interference caused by channel delay spread. [page]

(4) Speech and character recognition algorithms

In addition to completing core applications, DSP is often used to complete user interface tasks. This is because DSP-based products have strong design flexibility, allowing system designers to add additional tasks to their DSPs to make fuller use of their processing capabilities. In mobile phones, its voice dialing function can be realized by DSP. When using personal digital assistant (PDA) technology, combining computer and communication applications, human-computer interface design becomes more important.

(5) Modem

The complex mathematical operations in the modem are mainly completed by the DSP or ASIC composed of DSP cores. In portable computers, when the size, weight and power consumption of the modem are particularly concerned, the requirements for DSP are higher. The speed of the modem continues to increase with the increase of DSP speed, and by 1997 it had reached 56kbps.

(6) Integrated Services Digital Network (ISDN)

ISDN significantly reduces costs while greatly increasing data transmission rates, thereby enabling the development of new services that require high-speed data transmission rates and real-time information processing, such as interactive publishing, video conferencing, telemedicine, and distance learning.

Take video conferencing as an example. Because a wide bandwidth is required to transmit video images and sounds, traditional video conferencing requires the rental of expensive broadband channels. However, the H.320 encoding implemented by the latest DSP-based video conferencing system is compatible with international video conferencing and ISDN video telephony. In other words, video conferencing can be run on the 6 basic rate interfaces of ISDN (i.e. 6×64kbps).

The dual-mode modem implemented by DSP can work in analog mode on analog channels and in digital format on ISDN channels because the ISDN system is digital.

(7) Multimedia

Multimedia is driven by converting traditional analog video and sound signals into digital signals. Movies and TV can already be loaded into VCDs and DVDs for home entertainment, electronic games, commercial exhibitions, teaching, etc. Digital video data will soon be transmitted through channels such as cable television, telephone lines, cellular phones and satellites. The large amount of data generated by the digitization of video and sound signals must rely on high-performance DSPs to reduce the requirements for storage space and transmission bandwidth. The tasks that need to be completed by DSP include encoding/decoding of video and audio signals, color space conversion, echo cancellation, filtering, error correction, multiplexing, bit stream protocol processing, etc.