- •49 Cordless telecommunications
- •49. 1 Introduction
- •49. 1. 1 Analogue technology
- •49. 1. 2 H. F. /V. H. F. Analogue ct
- •49. 1.3 European 900 mHz analogue standard
- •49. 2 Digital technology
- •49. 2. 1 Areas of application
- •49. 2. 2 Main service principles
- •49. 2. 3 Traffic capacity
- •49. 2. 4 Digital cordless standards
- •49. 3 Ct2/cai digital specification
- •49. 3. 1 Radio aspects
- •49. 3. 2 Frame multiplex structure
- •49. 3. 3 Protocol structure
- •49.3. 4 Transmission plan and digital codec
- •49. 3. 5 Implementation
- •49. 4 Dect digital specification
- •49. 4. 1 Dect protocol architecture
- •49. 4. 2 Intel-working Units (iwu)
- •49. 4. 3 Spectrum resource
- •49. 4. 4 Detail radio aspects
- •49. 4. 5 Radio operational features
- •49. 4. 6 Frame structure
- •49. 4. 7 Transmission plan and digital codec
- •49. 4. 8 Implementation
49. 4. 7 Transmission plan and digital codec
DECT is subject to the same constraints as applied to CT2/CAI in terms of transmission performance. It is notable that both use the same codec standard and hence offer the same basic speech quality level. However, since DECT employs TDMA working the inevitable consequence is that DECT loop delay amounts to about 25ms. Such a delay cannot he tolerated in the local public network in the presence of speech echoes from the DECT handset. At present two approaches are being taken towards the amelioration of this effect:
By designing handset with virtually zero echo return. This calls for high orders of acoustic coupling loss between handset ear and mouth pieces approaching 46dB.
A combination of good handset design combined with the use of acoustic echo cancellation or control.
The latter seems to be preferred. Unfortunately the problem of echo cancellation and control in terminal equipment is not an independent design exercise as care needs to be taken to ensure that there are no unfavourable reactions with similar devices already in place on long distance connections, e. g. over a satellite link.
49. 4. 8 Implementation
To achieve success the basic production cost of DECT must be low. Thus the same objectives that were seen to he important to CT2/CAI's cost basis are equally applicable to DECT, i.e. the adoption of common components and large scale production to achieve the full cost benefits of scale.
Furthermore, equipment installation practices must be developed that can guarantee standards of communication quality that the customer has come to expect from the more conventional wired terminal equipment.
Exercise 1 Learn the following words and word combinations
1 |
to concatenate |
последовательно соединять |
2 |
throughput |
пропускная способность (сети) |
3 |
timeslot |
временной интервал |
4 |
digital codec |
цифровой кодер-декодер |
5 |
to release |
деблокировать, освобождать |
6 |
acoustic loss |
акустические потери |
7 |
a delay |
задержка |
8 |
bit delay |
задержка на символ |
9 |
frame delay |
задержка на кадр |
10 |
gate delay |
время задержки сигнала на логический элемент |
11 |
loop delay |
задержка в контуре |
12 |
real array delay |
задержка сигнала в матрице при считывании |
13 |
select array delay |
задержка сигнала в матрице при выборе |
14 |
to assess |
оценивать |
15 |
assessment |
оценка |
16 |
loss |
затухание, потеря; ослабление |
17 |
accumulated loss |
остаточное затухание |
18 |
attenuation loss |
потери на затухание |
19 |
echo return |
эхо-сигнал |
20 |
to handover |
переключатель |
21 |
disturbance |
помехи, повреждения, неисправность |
22 |
beacon |
радиомаяк, передатчик маяка, передающая станция маяка |
23 |
cyclic redundancy check (CRC) |
контроль циклическим избыточным кодом |
24 |
acoustic coupling |
акустическая связь |
25 |
echo cancellation |
компенсация эхо-сигналов |
Exercise 2 Read the text
Exercise 3 Give the Russian equivalents for the following English ones:
1 |
to cater for the traffic requirements |
2 |
the maximum throughput |
3 |
to be able to operate on all timeslots |
4 |
cluttered, multi-path environments |
5 |
channel bit error ratios |
6 |
to inform the controller |
7 |
the broadcast beacon transmission |
8 |
a cyclic redundancy code (CRC) |
9 |
to protect the data |
10 |
the channel set up phase |
11 |
the problem of echo cancellation |
12 |
to achieve the full cost benefits of scale |
13 |
acoustic coupling loss |
14 |
the more conventional wired terminal equipment |
Exercise 4 Answer the following questions:
1 |
What requirements influenced the basic design of the radio system? |
2 |
Was an FDMA/TDMA/TDD system produced as the result of putting all these requirements into practice? |
3 |
Is it a general requirement that all base and portable terminals shall be able to operate on all timeslots? |
4 |
Have you read the information about the main radio operation features (49.4.5) and about the frame structures (49.4.6)? |
5 |
Can you retell the main content of these parts in Russian? |
6 |
Do DECT and CT2/CAI use the same codec standard and offer the same basic speech quality level? |
7 |
Why does DECT loop delay amount to about 25ms? |
8 |
How can the problem of echo cancellation and control in terminal equipment be solved? |
9 |
Do you know how to achieve great success in implementation of DECT? |