What Is a Broadband Modem?

A broadband modem is a modem that uses multiple analog voice channel bandwidths to transmit data signals.

A broadband modem is a modem that uses multiple analog voice channel bandwidths to transmit data signals.

Chinese name

Broadband Modem

Foreign name

widebandmodem

Applied discipline

Computer, communication

Introduction to Broadband Modems

A broadband modem is a modem that uses multiple analog voice bandwidths to transmit data signals. In a multi-channel carrier system, the nominal bandwidth of an analog telephone channel is 4 kHz. The 12 voice channels together form a base group with a bandwidth of 48 kHz (60 to 108 kHz). The 5 base groups are 60 voice channels combined. Together, they form a supergroup with a bandwidth of 240kHz (312 to 552kHz), and the five supergroups together form a 300-channel main group with a bandwidth of 1 232kHz (812 to 2 044kHz). According to the different frequency bands occupied by broadband modems, there are primary modems, superior modems, and primary modems.

Broadband modems are mainly used in computers and computers, where multiple low- and medium-speed data are multiplexed into high-speed data, high-speed data transmission such as digital secure phones and document fax. With the construction and development of digital networks, there are more and more high-speed data transmission using digital channels, which is both economical, convenient and reliable. Therefore, the demand for broadband modems is not very urgent, and there is no obvious progress in research and development.

Broadband Modem Basic Unit

Among broadband modems, the most commonly used are base modems, whose transmission rates are 48, 56, 64, 72, 96, 112, 128, and 144 kbit / s, using single-sideband AM, leased four-wire dedicated line circuits, full dual Synchronous transmission. The basic units of a broadband modem are:

The modulation and demodulation unit adopts the fourth type of partial response single sideband AM, and the carrier frequency is 100kHz;

Scrambler / descrambler The scrambler transforms the short-period sequence in the data signal into a long-period sequence, making the data signal a quasi-random signal; the descrambler restores the received long-period sequence to a data sequence;

The equalizer uses an adaptive equalizer to equalize the attenuation distortion and group delay distortion generated by the channel and the relevant unit in the modem;

Carrier frequency and timing system The carrier frequency system provides a carrier frequency of 100kHz for the modulation and demodulation unit. The timing system provides bit timing signals and other clock signals from both parties;

The interface conforms to CCITT V. 24. V. 10 and V. 11 suggestions.

Broadband Modem Algorithm Design

3.1 Transmission system

Wideband wireless communication mostly uses the Orthogonal Frequency Division Multiplexing (OFDM) system to improve the data transmission rate and the ability to resist multipath interference and simplify the design of equalizers. In order to improve the resistance to large-path interference, single-carrier frequency domain equalization can be considered.

3.2 System synchronization

(1) Frame synchronization

Traditional OFDM systems use the Minn algorithm to achieve frame synchronization. This algorithm is difficult to achieve reliable synchronization at low signal-to-noise ratios below 5 dB. To achieve reliable frame synchronization at low signal-to-noise ratios, spreading codes can be used in the spread spectrum system Acquisition algorithm, but how to reduce the effect of multipath interference on synchronization is a problem worthy of further research. You can use methods such as delay-related superposition and multi-segment synthesis to improve multipath resistance and synchronization performance. The above two algorithms are energy-based detection methods. Multiple correlation peaks are prone to occur when there is multipath interference. You can use FFT-based frequency-domain correlation frame synchronization algorithms to avoid multiple correlation peaks. The basic principle is that the correlation is despread. The FFT is performed on the synchronization signal. When the transmit and receive sequences are synchronized, obvious spectral peaks appear in the frequency spectrum. This method can achieve reliable frame synchronization with low signal-to-noise ratio.

(2) Bit synchronization

You can use the maximum path position synchronization algorithm based on frequency-domain correlation search to achieve reliable bit synchronization, which is based on the FFT-based frequency-domain correlation frame synchronization algorithm to search for the largest frequency-domain related spectral peak position within a symbol, and the maximum spectral peak. The position is the exact bit synchronization position. Carrier synchronization can be achieved using the SchIIlidl algorithm or M & M algorithm commonly used in OFD systems.

3.3 channel estimation

At high signal-to-noise ratios (greater than 5 dB), two repeated time-domain training sequences can be used for channel estimation. When the signal-to-noise ratio is low, the traditional channel estimation algorithm cannot effectively estimate the channel characteristics. At this time, the time-domain impulse response of the channel can be estimated by using the spreading sequence in the synchronization header, and then the channel frequency-domain characteristics can be estimated.

3.4 equilibrium

Traditional zero-forcing equalization does not generate inter-symbol interference, but in frequency-selective fading channels, especially when the channel has a frequency domain deep fading zero point, noise will be amplified. The minimum mean square error (MMSE) algorithm has better equalization performance than zero-forcing equalization, especially when the channel has deep fading zeros in the frequency domain, the channel noise will not be excessively amplified, but there will be some residual inter-symbol interference after equalization.