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Modem devices (wire and wireless)


1. Modem devices (wire and wireless)

1.1. Characteristics of modern modem equipment

Present communication systems are modem technologies implemented for specific information transmission rates.

Now the most widespread modulation types are:

a) in satellite communication – QPSK, 8PSK, 16QAM;

b) in radio relay links – BPSK, QPSK, 8PSK, 16QAM, 32 QAM, 64QAM, 128QAM, 256QAM;

c) in cable lines – QPSK, 16QAM, 64QAM, 256QAM;

d) in telephony – from 16QAM to 16384QAM.

Methods for noise-immune encoding being widely used in modern modems are encoding algorithms of Viterbi, Reed-Solomon, TCM (Trellis Code Modulation), and Turbo Coding. The latter allows to achieve utmost information transmission rates with loss no more than of 1.6 to 3 dB.

Being the most effective kind of noise-immune encoding, Turbo Coding does not still find a wide application in information transmission systems because there are certain technical problems that could not be overcome by even large western firms.
Table 1 shows calculated parameters of Turbo Coding for various encoding rates and modulation types (N = 15488) based on foreign press (see [1]):
Table 1.
Spectral efficiency
(bps/Hz)
Modulation type
Coding rate
~Eb/N0 @ 10-7
(dB)
SNR
(dB)
Shannon limit SNR
(dB)
0,5
QPSK1/40.9-2.1
-3.9
1
QPSK
1/2
1.6
1.6
0.0
216QAM
1/2
3.7
6.7
4.8
316QAM
3/4
5.9
10.7
8.5
464QAM
2/3
8.2
14.2
11.8
5256QAM
5/8
11.1
18.1
14.9
6256QAM
3/4
13.3
21.0
18.0
71024QAM
7/10
16.0
24.5
21.0
81024QAM
4/5
18.2
27.2
24.0
94096QAM
3/4
21.1
30.6
27.0
1016384QAM
5/7
24.2
34.2
30.0
1116384QAM
11/14
26.5
36.9
33.0
1216384QAM
6/7
28.8
39.6
36.0
1316384QAM
13/14
30.9
42.0
39.0

An obstacle for implementing the achieved characteristics of Turbo Coding in modern communication systems is in too high demodulation “thresholds” of existing demodulators. Thus, for the QPSK type of modulation, available demodulators begin to synchronize themselves with the carrier frequency at Signal-to-Noise Ratio (SNR) of +2.4 to +2.5 dB, and for the 16QAM type modulation – at SNR about +10 dB [2].
 
As is obvious from the Table 1, in order to implement the total possibility of Turbo Coding for the 16QAM signal at coding rate of 1/2, a demodulator should have a stable operation at 10 log PS/PN = 6.7 dB, but it upsets just at 10 dB. 
For the QPSK signal at coding rate of 1/4 a demodulator should have a stable operation at 10 log PS/PN = –2.1 dB, and at coding rate of 1/2 – at +1.6 dB, but it upsets just at +2.4 dB.

From these examples it is evident that the obstacle for effective use of Turbo Coding is a demodulator configured according to conventional technology.

1.2. Modem modernization based on new technical solutions

The unique technology developed on the basis of new technical solution allows to decrease demodulation “thresholds”, and thus to ensure effective application in Turbo Coding modems. Realization of the new modem technology provides an opportunity to obtain probability characteristics of encoding system, which correspond the values cited in the Table 1, with demodulation loss of 0.5 to 0.6 dB. In this way, the obtained system joins easily the existing radio communication infrastructure, and its technical characteristics allow to save energy and/or frequency resources in comparison with the existing systems while maintaining the same information transmission rates.

The firm “RADYNE COMSTREAM”, leader of the world modem production, has implemented, among other, the QPSK type of modulation having the coding rate of 3/4 with maximum possible efficiency in one of its satellite modems, DMD20 Universal Satellite Modem. In this case, for the error probability … of 10–7, it is necessary to ensure the SNR not lower than 5.65 dB. Let us take, for instance, the information transmission rate 2 Mbps. The occupied frequency band in this case is 2 MHz. When taking the 16QAM as the modulation type having the encoding rate of 1/2, for the same information transmission rate of 2 Mbps the necessary band is 1.5 MHz, i.e. 1.33 times less. However, in the last case, in order to provide the error probability of 10–7, the SNR is now required not worse than 7.1 dB. The difference in SNR is 1.45 dB. But the narrower band in the second case leads to reduction of the noise level by 1.24 dB.

Thus, the real difference in the SNR for signals of QPSK (3/4) and 16QAM (1/2) in the case of equal error probability is only 0.21 dB in favour of the former. This means that with the loss in the signal power by 0.21 dB the gain in the band is 1.33 times and allows to place 24 channels instead of 18 in the standard trunk having the band of 36 MHz when using 16QAM (1/2) instead of QPSK (3/4).

Realization of 16QAM (1/2) having above efficiency is possible, however, only when using the circuit with the reduced demodulation “threshold”. The adduced example demonstrates obviously the gain in the occupied frequency band.

Substitution of the 16QAM type modulation with the encoding rate of 0.375 for the QPSK type of modulation with the encoding rate of 3/4 allows to save the energy resource. Indeed, at the same information transmission rates of 2 Mbps, these both modulation types occupy the same frequency band which is equal to 2 MHz. But in the case of QPSK (3/4), in order to achieve the error probability of 10–7, the SNR must be not worse than 5.65 dB, and in the case of 16QAM (0.375) the SNR must be not worse than 3.95 dB. In the case of substituting the 16QAM (0.375) modulation for the QPSK (3/4) modulation, the energy gain is 1.7 dB, i.e., 1.48 times greater. Realization of the 16QAM (0.375) having the described efficiency is possible as well only when using the circuit with the reduced demodulation “threshold”.

It should be noted that the substitution of the modulation higher than 16QAM (i.e., 32QAM, 64QAM etc.) for the QPSK type of modulation having the encoding rate of 3/4 with corresponding reduction of encoding rates in order to maintain the same transmission rates allows to obtain still more weighty gains in energy and/or frequency resource.

The developed technology for designing modem gives a possibility for the maximum efficient realization of information transmission systems with utilization of low (1/2 and lower) encoding rates.

The data transmission in wire communication systems of subscriber access could be enhanced essentially using modem technology (“last mile” equipment), when solving the problem of increasing the information transmission rate at the “subscriber-station” sector without substitution of fiberglass telephone cables for conventional ones.

The proposed variant of Turbo Coding in combination with the modem technology permits to design systems that are especially exclusive for each course of their application (basic works [3], [4], [5], [6], as well as a series of know-how).
The Table 2 shows some characteristics of system for transmitting/receiving signals using Turbo Coding in accordance with the developed new technology:
Table  2.
Spectral efficiency
(bps/Hz)
Modulation type
Coding rate
~Eb/N0 @ 10-7
(dB)
SNR
(dB)
0.5
QPSK1/41.4-1.6
0.666
QPSK
1/3
2.1
0.24
0.75
8QAM
1/4
0.8
-0.5
1
QPSK
1/2
2.2
2.2
1
8QAM
1/3
1.9
1.9
1.5
QPSK
3/4
3.9
5.65
1.5
16QAM
0.375
2.2
3.95
1.5
32QAM
0.3
1.95
3.7
216QAM
1/2
4.1
7.1
232QAM
0.4
3.95
6.95
264QAM
1/3
3.8
6.8
316QAM
3/4
7.5
12.28
The data transmission in wireless communication systems could be implemented using the modem technology 
for unlicensed frequency bands with the PNS (pseudonoise sequence) signals having the base not less than 10.

The best parameters in the unlicensed frequency bands of 2400-2483 MHz and 5725-5850 MHz are those of modems made by firms Proxim and Adtran.
Modems made by the Proxim Corporation (10–6 BER) have the following parameters (see Table 3).
Table 3.
Model
Frequency Band (MHz)
Channel type
Sensitivity
(dBm)
Output Power (dBm)
Communication range (km)
Lynx.SCT1
31250
2400÷2483
T1
(1,544 Mbps)
-94
+27
96
Lynx.SC2T1
31650
2400÷2483
2T1
(2×1,544 Mbps)
-91
+27
88
Lynx.SCT1
31000
5725÷5850
T1
(1,544 Mbps)
-93
+20
80
Lynx.SC2T1
31600
5725÷5850
2T1
(2×1,544 Mbps)
-90
+20
77
Lynx.SC1Å1
31500
2400÷2483
Å1
(2,048 Mbps)
-93
+27
93
Lynx.SC1Å1
31400
5725÷5850
Å1
(2,048 Mbps)
-92
+20
80
Lynx.SC1Å1
31700
5725÷5850
2Å1
(2×2,048 Mbps)
-90
+20
77

Modems made by the firm Adtran (10–4 BER) and their parameters are adduced in the Table 4:
Table 4.
Model
Frequency Band (MHz)
Channel type
Sensitivity (dBm)
TRACER 4102
2400÷2483
T1
(1,544 Mbps)
-98
TRACER 4102
2400÷2483
2T1
(2×1,544 Mbps)
-96
TRACER 4202
5725÷5850
T1
(1,544 Mbps)
-95
TRACER 4202
5725÷5850
2T1
(2×1,544 Mbps)
-93

Modems made by Adtran possess approximately similar characteristics as modems made by Proxim, and the adduced parameters in sensitivity at (10–6 BER) will be the same or even worse.

According to the shown characteristics, modems of above firms are synchronized at SNR of 3 to 4 dB.

Taking into account the signal base of 10, the error probability of 10–6 is ensured at SNR of 14.5 dB.

In order to ensure the error probability 10–6 when utilizing Turbo Coding at the coding rate of 1/2, the SNR of +2.1 dB is necessary. When utilizing the developed modem technology for the unlicensed frequency bands with the use of PNS signals having the signal base not less than 10 and losses of 0.5 dB in the modem, it is possible to increase substantially the communications range and interference immunity. Modems developed according to the proposed technology, for the given SNR of –8 dB, will operate at the communications range up to 200-320 km in the line of sight (see Table 5). The sensitivity estimation data in the Tables 3, 4, and 5 are adduced for the antenna gain Ka = 3 dB, receiver gain Kr = 15 dB, and receiving circuit noise factor Kn = 3 dB.

Parameters of modem by applying the developed technology are adduced in the Table 5:
Table 5.
Model
Frequency band (MHz)
Channel type
Sensitivity (dBm)
Output Power (dBm)
Communication range (km)
DT11
2400÷2483
T1
(1,544 Mbps)
-105
+27
320
D2T12
2400÷2483
2T1
(2×1,544 Mbps)
-102
+27
270
DT13
5725÷5850
T1
(1,544 Mbps)
-104
+20
240
D2T14
5725÷5850
2T1
(2×1,544 Mbps)
-101
+20
200
DE15
2400÷2483
E1
(2,048 Mbps)
-105
+27
290
DE16
5725÷5850
E1
(2,048 Mbps)
-104
+20
240
D2E17
5725÷5850
2E1
(2×2,048 Mbps)
-101
+20
200
 
 
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