3. A wireless systems composition

Wireless communication systems have specific communication channels. Exactly the term “communication channel” is used in two-way understanding [32]: in wider consideration the communication channel has message signal source, transmitting and receiving parts, information recipient, and in a narrow sense – wireless channel, in other words – open space. Further the concept of “communication channel” is using in the narrow sense.

A communication systems composition is determining by the necessity of conditions (1.1, а, б) provision. Whereby, informative signal, which enters the system with transfer rate R should be processed with some number of transformations. Such transformations are performing into separate operational modules. Let us consider basic signal transformations at the wireless system (Fig. 1.3) more detail in the parts of transmitting and receiving.

The transmitting unit of the system. Let us note, that more significant are the transformations into transmitting part of the wireless system. The receiving unit commonly makes inverse transformations to the transformations into transmitting unit. The goal is to receive original signal. The basic stages of the signal tranfromations into transmitting unit are following.

Transmitter

Reciever

Fig.1.3. Common structural scheme of the wireless communication system and its basic parameters

1. Message encoding. Original data is a sequence of symbols with duration of each of them T_b (duration of one bit). This duration determines the transfer rate of change of input information, which is R_b = 1/T_b. But no information is input as electrical signal, so could not be directly introduced into the system. So, we must perform encoding of messages that turn them into an electrical signal. In most cases, the coding is converting into an electrical signal to a rectangular shape.

2. Encoded signals filtering. It is usually formed rectangular signal as a result of encoding. A specific feature of such signals is the presence of rectangular pulses with steep fronts (front and rear). But in some cases, signals with the flat fronts may be more suitable for use in the wireless system.

3. Channel coding. There may be cases when the received signal does not satisfy the condition (1.1 a). Then one of the ways to ensure this requirement is to use the channel coding. One of the most widely used types of channel coding is the use of redundancy of transmitted messages. As a result, in addition to informational messages, additional (redundant information) is transmitted. Due to redundancy of transmitted information, some number of messages with mistakes could be fixed, that contributes to the provision of condition (1.1 a) .

4. Modulation is intended to transfer the available bandwidth for messages that are low-frequency signals (video signals) to the given frequency range for this system. But one of the many available types of modulation also makes a significant impact on the performance of wireless communication system in general. Therefore, a reasonable choice of modulation type, according to the set of technical requirements of the system, is responsible stage of wireless system design in general.

5. Antennas. In the surrounding area may be present some obstacles (buildings and other immovable and movable objects), which causes the presence of multipath wave propagation (MP), which consequently leads to worsening of the wireless system parameters. In order to reduce harmful effects of MP using MIMO technique (Multiply Input Multiply Output) with antenna that containing N_t antennas at the transmitter and N_r antennas on the receiving side.

But the design of wireless communication systems must be appropriate not only to ensure the system defined technical requirements, and in view of increasing their effectiveness. That’s why the transmitting could be made by combining multiple bits into symbols. As a result, the transfer rate of the communication channel may differ from the rate of incoming information (R_s≤ R_b).

The receiving unit of the system.

At the receiver the conversion of signals in reverse (mirror) order is implemented, unlike a transmitting unit. Therefore, major efforts in building wireless systems, primarily designed to perform basic requirements (1.1 a, b​​) for receiver.