Data above voice systems

In the last 20 years, long-haul transmission networks were upgraded to transport digital signals due to the huge growth of the information technology market (metropolitan-area networks, interconnection of local-area networks, the Internet, digital cellular networks like GSM or CDMA, digital trunking networks like iDEN, etc).

Originally, the long-haul transmission network was designed to transport analog signals using mainly high-capacity long-distance FDM-FM analog microwave radios (Frequency Division Multiplexing/Frequency Modulation). This equipment is suited to transport voice signals from analog telephone multiplexers (it cannot transport digital signals directly). Digital microwave radios are adequate to transport digital signals, however, analog microwave radios are still necessary worldwide to transport analog television signals (NTSC or PAL), the leftover FDM-FM telephone signals and, mostly, to preserve the enormous investments made in the past.

Introduction to DAV

Hybrid data systems were developed as a cost-effective solution to transport digital signals over existing terrestrial analog microwave radio networks. In these systems, a FDM-FM telephone or television base band is transmitted with a digital base band in the same radiofrequency channel and the final result is a composite signal.

Conceptually, there are three distinct hybrid systems in terms of the occupied bandwidth:

We can define DAV as a hybrid transmission system which carries a digital data signal that occupies a portion of a microwave radio frequency spectrum above the frequencies used only for voice transmission (FDM-FM signal). DAV is transmission equipment that is used in the transition era between the analog-only telecommunications world and the digital-only telecommunications world. The importance of this type of equipment is reduced as pure digital transmission systems are deployed all over the world.

Figures 1, 2, 3, and 4 are not in scale (neither the power level on the Y-axis nor the frequency on the X-axis).

A possible fourth system — Data Above Video — is a variation of DAV. In DAVID (or DAV TV) hybrid systems the FDM-FM telephone base band is substituted by an Amplitude Modulation/Vestigial Side Band (AM-VSB) analog television signal.

Conceptually, DAVID hybrid signal spectrum is similar to the DAV spectrum. Figure 4 shows a DAVID signal plus an AM-VSB television signal. In this article, the word DAV is always used both for the real FDM-FM DAV and DAVID.

General philosophy of DAV

In a general terrestrial analog microwave radio system, the position of DAV on the transmitter side is shown on Figure 5 (i.e., DAV TX or combiner DAV) and the position of DAV on the receiver side is shown on Figure 6 (i.e., DAV RX or splitter DAV).

In the first case, DAV performs the function of combining a data stream with a telephone or television base band. In the second case, DAV performs the function of splitting the combined or hybrid signal into its original components (i.e., telephone or television plus data). Data stream is an E1 aggregate (in CEPT hierarchy) under the recommendation ITU-T G.703 or a T1 aggregate (in ANSI hierarchy) in the United States. Although, in the present time, an E1 (or T1) circuit is considered a low-capacity data circuit it is still very useful to provide digital communications to villages in the country.

DAV is always inserted before the radio switching subsystem on the transmitter side and after the radio switching subsystem on the receiver side. This rule determines that the hybrid signal will be switched when radio switching occurs. DAVID is always inserted on the transmitter side after the Sound-and-Video Combiner. On the receiver side, DAVID is always inserted before the Sound-and-Video Separator.

If DAV equipment becomes inactive it is necessary to guarantee the analog FDM-FM base band traffic continuity. This is accomplished by a manual by-pass made independently for transmitter and receiver sides. This by-pass occurs in the Intermediate Distribution Frame.

If a power failure occurs in the power subsystem of the DAV equipment the by-pass prevents the loss of the FDM-FM telephone base band.

Figure 7 shows the fundamental building blocks for DAVs.

On the transmitter side, the incoming data signal passes first through a 4-Phase Phase-Shift Keying modulator (4-PSK). By means of a combiner filter the output hybrid signal is created (the combiner filter is a low-pass filter for the analog signal and a band-pass filter for the digital signal).

On the receiver side, the combined signal is divided by the splitting filter where the digital portion is recovered by the 4-PSK demodulator.

The signal at the output of the 4-PSK modulator (= MOD 4-PSK TX) and the signal at the input of the 4-PSK demodulator (= DEM 4-PSK RX) is called data subcarrier.

Table 1 shows information on power levels, base band, pilot tones and data subcarrier in analog radio systems containing 960, 1,800, and 2,700 telephone channels as well as television. DAV equipment can also be installed on a 1,260-channel microwave analog radio system.

Data subcarrier causes interference onto the FDM-FM signal (the analog baseband). In a similar way, the FDM-FM signal causes interference onto the data subcarrier. It is a mutual interference condition and to reduce this harmful interference there are some precautions to take:

Figure 8 illustrates, conceptually, the mutual interference condition between the FDM-FM signal and the data subcarrier.

There are three different types of DAVs (according to the Data Subcarrier Frequencies — DSC):

Hybrid frequency spectrum

In figure 3 we shown a generic hybrid spectrum (FDM-FM + DAV). In most cases DAV is installed in microwave radiolinks carrying 1,800 telephone channels. Therefore, it is interesting to analyze this composite signal (= FDM-FM 1,800-channel + DAV) more deeply.

From table 1 we know that the frequency of the radio continuity pilot tone is 9,023 kHz for systems carrying 1,800 telephone channels. The analog FDM-FM baseband is from 300 to 8,248 kHz. Finally, the data subcarrier frequency is centered at 10.3 MHz. Having these values in mind, it is possible to sketch the frequency spectrum of the FDM-FM 1,800-channel + DAV composite signal (Figure 9).

Internal functions

DAV equipment performs a lot of functions and its actual block diagram is considerably more complex than that of Figure 7. In general, digital transmission systems include a series of standard building blocks independently of the modulation scheme and capacity in use.

Main functions performed at the transmitter side are:

where:

FIL = Filter Insertion Loss ( dB )

MLINPUT = Input Mismatching Loss ( dB )

MLOUTPUT = Output Mismatching Loss ( dB )

FLJOULE = Filter Internal Loss due to the Joule Effect ( dB )

In the receiver section we have the following blocks:

DAV equipments also have an alarm unit (for TX and RX) and a Bit Error Ratio Unit (the BER evaluation is based on the parity bit value).

Field deployment

DAV equipment can be installed in several types of microwave radio-relay stations, that is to say:

Testing DAV

The complete field acceptance procedure of DAV equipments (already installed in analog microwave routes) needs several tests and instruments, listed as follows.

Some previous items are not applicable to all DAV equipments available in the market (there are several differences among manufacturers).

System quality equation

In microwave radio systems equipped with DAVs the generic equation for quality is given by:

where:

CNR = Carrier-to-Noise Ratio (also known as C/N);

SNR = Signal-to-Noise Ratio (also known as S/N);

Fdsc = Data Subcarrier Frequency (MHz);

Fmea = Measurement Frequency (MHz);

Bdat = Data Signal Bandwidth (kHz);

Bvoice = Voice Channel Bandwidth (kHz);

Emph = Emphasis Improvement between Fmed and Fdsc (dB);

Lev = Data Subcarrier Relative Power Level at the Output (dB);

Log = Decimal Logarithm.

CNR is calculated for a 10E-6 BER and its value is obtained from a theoretical curve BER × CNR plus a practical additional degradation value (due to several imperfections). The obtained value depends on the modulation scheme and equipment manufacturer.

Bvoice is the effective voice band for a standard multiplex channel (from 0 to 4,000 Hz). For this type of multiplex channel Bvoice is equal to 3,100 Hz (from 300 to 3,400 Hz).

So, calculating the SNR for a DAV system with 1,800 telephone channels, for a 10E-6 BER, we have:

CNR = 16.5 decibels

Fdsc = 10.3 MHz ( table 1 )

Fmea = 7.6 MHz

Considering service bits we have the following bit rate:

The fraction 63/62 comes from the DAV’s frame format (this fraction will vary according to the way the frame is created by each equipment).

Therefore, the symbol rate (expressed in megabauds) is given by:

Bdat (expressed in MHz) is numerically equal to the symbol rate, that is to say:

Bvoice = 3,100 Hz

Emph = 2.4 decibels ( value obtained from the emphasis curve )

Lev = 10 decibels

Substituting the previous numerical values we have:

SNR = 32 decibels

This is the minimum acceptable value for the systems with 1,800 telephone channels with a 10E-6 BER. In practice, it is adopted a minimum value greater than the previous one (i.e., the limiting value of 40 decibels). SNR is measured in the test item named BER versus SNR. Figure 17 shows a plotting of BER × SNR with the theoretical SNR of 32 and the limiting SNR of 40 (note the prohibited area).

Marcello Praca Gomes Da Silva is Senior Telecommunications Engineer Transmission Coordinator with Nextel Telecomunicaçōes inRio de Janeiro, Brazil.