Electronics 2: Wirless Communication Internal Material

 Q. What is concept of cellular system?

Answer: Cellular system is a system for wireless communication. The total area in this ssystem is divided into smaller areas called "cells". A cell represents a coverage area of base stations. This cell has been allocated a range of frequencies.

Q. What is handoff strategies?

Answer: The process of transferring an active call or data session from one cell to another cell in a celullar network is called handoff or handover. All this is managed by MTSO.

Q. What is cell?

Answer: A small geographical coverage area of base station each with it's allocated number of RF channels is called a cell. It's size is about 20-50 km. The shape of each cell is hexagonal.

Q. Write a short note on frequency reuse.

Answer: Frequency reuse is a technique where a specified range of frequencies is used more than once in a radio system so as to increase the total capacity of the system without increasing it's allocated bandwidth. This is done using SDM (Space division multiplexing) in mobile system. Cells which are far away from each other can use same set of frequencies.

Q. Draw and explain block diagram of mobile handset.

Answer: 

Working Principle:

1. Mic: It is a sensor which converts sound (analog signal) into an electrical signal.

2. Compression & Pre-emphasis of signal is done before transmission to reduce size of data and remove noise from it.

3. FM modulator: Signal is modulated so it can travel long distances.

4. Class C RF Power Amplifier: It incrases the power of signal.

5.Duplexer: It is used to seperate transmitting and receiving singals.

6. Frequency synthesizer: It synthesizes frequency with 45 Mhz frequency seperation.

7. Microprocessor & Logic Unit : Switching of channels & power levels by base station is done through this. It also stores messages, stores called numbers, messages,etc.

8. Then the received signal is demodulated , de-emphasized and expanded back to it's original size.

9. The Speaker converts electrical signal to analog signal and gives the sound.

Q. Explain the working of GSM architecture.

Answer: It stands for Global System for Mobile. It provides services like telephony, emergency and calling& voice messaging. 

It's Uplink (mobile to base) frequency band is 890-915 Mhz & downlink (base to mobile) is 935-960 Mhz, which is 45Mhz of spacing.

The architecture of GSM is as follows:

It consists of three major subsystems:

1. Basestation Subsystem (BSS) or Raido Subsystem (RSS): It consists of MS(Mobile Station) which communicates with RSS over radio interface. It consists of a number of BTSs which are connected to a BSC, and there are also multiple BSCs. All are connected over radio interface.

2. Network & Switching Subsystem (NSS): It consists of a number of MSCs. Each MSC connected to a number of BSCs interfaced through RSS. It also consists of HLR(Home location registers) & VLR(Voice location registers).

3. Operational Support Subsystem (OSS): It helps in managing all the services and operations to ensure that they run smoothly. It consists of PSTN, ISDN and data network.

Q. What is MTSO in GSM?

Answer:  MTSO stands for Mobile telephone switching office. Currently it's known as MSC (Mobile switching Centre).It is a part of NSS (Network Switching Subsystem). It is connected to all cell sites in a region so it provides an interface between cells and the main telehone office. It performs the handoff operation.

Q. Explain the frame structure of GSM.

Answer: GSM uses TDMA for multiple access. Each channel in GSM is seperated in time slots using TDMA frame which repeat continuosly.

1. The duration of each frame is 4.615 ms. Which is agaiin divided into 8 time slots, each slot lasting about 577us.

2. The data is transmitted into small portion called as burst.

3. The burst is about 546.5 us long and remaining is allocated for guard space (about 30.5us) so as to prevent interferences with other bursts.

Q. Draw and explain working of GPRS architecture.

Answer: It stands for General Packet Radio Services. 

It consists of following components:

1. MS (Mobile Station) : It is the user's device which communicates with GPRS network.

2. Base Station Subsystem (BSS): It consists of BSCs and BTSs, where BTS transmits and receives signals from MSC and BSC controls them.

3. SGSN (Serving GPRS support Node): It maintains the location of MS and routes data packets to correct destination.

4. GGSN (Gateway GPRS Support Node): It acts as a gateway between GPRS and other networks such as internet.

5. PDN (Packet Data Network ) : It is the network that GPRS user is trying to access such as internet or private corporate network. 

Chapter 2: Short range wireless technologies & location tracking.

Q. What is Bluetooth?

Answer: It is a short range wireless communication standard which acts as a LAN but with very limited coverage area and without a need for infrastructure. It connects handheld devices and other accessories within a range.

Q. Explain the bluetooth architecture.

Answer: 

Components of architecture are: 

1. Physical Layer: 

- It includes bluetooth radio & baseband.

- Responsible for transmission and reception of data over radio interface.

- Does modulation & Demodulation of signals.

- Does power management & frequency hopping specifications.

2. Data Link Layer:

- It includes baseband, LMP (Link manager protocol) & L2CAP (Logical Link Control & Adaptation Protocol).

- LMP is responsible for establishing & managing connections between bluetooth devices.

- L2CAP provides adaptation between upper layer frame & baseband layer frame format.

- Baseband provides functions like packet segmentation & reassembly.

3. Middleware Layer:

- It includes RFcomm, At Commands, SDP & Adopted Protocols.

- Adopted protocols are ones adopted from standard models like PPP & UDP.

- RFcomm provides serial interface over bluetooth.

- SDP (Service discovery protocol) allows to discover and query services offered by bluetooth devices.

- AT commands are set of instructions sent to device to perform operations.

4. Application Layer : 

- It includes application profile.

- It allows user to interact with bluetooth applications.

Q. What is Zigbee?

Answer: It is a wireless communication protocol often designed for low power and low data rate wireless applications. Often used in smart home devices which can be controlled using phone or smart speakers.

Q. What is RFID system.

Answer: It is a radio frequency identification system, a short range wireless communication technology used generally in object tracking systems.

Q. What is GPS?

Answer: GPS stands for Global Positioning System. It is a navigation system based on satellite communication. It is based on a network of satellites, which continuosly transmit information through radio signals. The receivers receive the signals and identify locations on earth acurately.