WIRELESS COMMUNICATION END TERM EXAM 2018

 END TERM EXAM - Wireless Communication (2018)

1 (a) necessity of having large no of cell sites in a cellular system

Coverage: Having a large number of cell sites ensures that the cellular network has good coverage, allowing users to make calls and use data services in a wide geographical area. Capacity: The number of cell sites directly affects the capacity of a cellular network. With a larger number of cell sites, more users can access the network at the same time, without experiencing congestion or dropped calls. Quality of Service: A large number of cell sites also allows for better control over the quality of service, as the network can prioritize traffic, allocate resources, and adjust call quality based on network conditions.

1 (b) LMDS vs MMDs aplications of WLL technology

Feature	LMDS	MMDS
Frequency Band	High Band	High Band
Range	Short to medium	Short to medium
Capacity	High	Medium
Data Rates	High	Low to medium
Applications	Fixed Wireless Access, Metropolitan Area Networking, High-speed Internet Access, Point-to-Point Communications, Video Conferencing	Television Broadcasting, Video Conferencing, Voice and Data Services

LMDS is often used for high-speed data services and point-to-point communications, while MMDS is typically used for television broadcasting and wireless cable TV services. LMDS provides higher capacity and data rates than MMDS, but MMDS has a wider range of applications and is more cost-effective for certain use cases.

1 (c) explain different types of spread spectrum techniques

Frequency Hopping Spread Spectrum (FHSS): In this technique, the frequency of the transmitted signal is rapidly switched among several frequency channels in a pre-determined pattern. This makes the signal more difficult to intercept or interfere with, and improves resistance to jamming. Direct Sequence Spread Spectrum (DSSS): In DSSS, the data signal is spread over a wider bandwidth by modulating it with a higher-rate spreading code. This makes the signal more resistant to interference, as the spreading code effectively masks the data signal. Time Hopping Spread Spectrum (THSS): THSS is similar to FHSS, but instead of changing frequency, the transmitted signal rapidly switches among different time slots. This makes the signal more difficult to intercept or interfere with. Code Division Multiple Access (CDMA): CDMA is a multiple access technique that allows multiple users to share the same frequency band by using unique spreading codes to spread their signals. This makes it possible for multiple users to simultaneously transmit and receive data over the same channel.

1 (d) compare IS-95, cdma 2000 and WCDMA in terms of Power Control Mechanism table

Feature	IS-95	cdma2000	WCDMA
Type of Power Control	Open Loop	Closed Loop	Closed Loop
Purpose	To control the overall transmit power of the mobile device	To maintain the quality of the wireless link and to reduce interference	To maintain the quality of the wireless link and to reduce interference
Mechanism	Mobile devices transmit at a fixed power level, regardless of network conditions	Mobile devices adjust their transmit power based on network conditions, feedback from the base station, and other factors	Mobile devices adjust their transmit power based on network conditions, feedback from the base station, and other factors
Benefits	Simple and straightforward	Improves the quality of the wireless link and reduces interference	Improves the quality of the wireless link and reduces interference

Note : cdma2000 and WCDMA use more sophisticated power control mechanisms than IS-95, which only uses an open-loop power control mechanism. This makes cdma2000 and WCDMA more effective in maintaining the quality of the wireless link and reducing interference, compared to IS-95.

1 (e) Define the terms : Fade rate, Average fade duration, Depth of fading

Fade rate: The fade rate is the rate at which a wireless signal fades or decreases in strength. It is typically measured in dB per second or dB per kilometer. The fade rate is an important factor in determining the reliability of a wireless communication system, as it can affect the quality of the wireless link and the ability of the system to maintain a stable connection. Average fade duration: Average fade duration is a measure of the average length of time that a wireless signal is below a specified threshold. It is a key metric for characterizing the behavior of fading in a wireless communication system, and is used to determine the reliability and robustness of the system. Depth of fading: The depth of fading refers to the extent to which a wireless signal fades or decreases in strength. It is usually expressed in dB and is a measure of the difference between the maximum and minimum signal strengths. The depth of fading is an important factor in determining the quality of a wireless link, and can affect the ability of the system to maintain a stable connection and transmit data accurately. Together, the fade rate, average fade duration, and depth of fading are key metrics for characterizing the behavior of fading in a wireless communication system, and are used to determine the reliability, robustness, and quality of the wireless link.

2 (a) Features of cordless telephone systems

Wireless communication: Cordless telephones use radio waves to communicate with the base station, allowing you to make and receive calls from anywhere within range. Handset portability: Cordless telephones offer the convenience of a mobile phone, as you can carry the handset with you around the house or office without being tethered to a landline. Multiple handsets: Most cordless telephone systems allow you to add multiple handsets, allowing you to have multiple phones connected to the same base station and phone line. Call waiting and caller ID: Many cordless telephone systems support call waiting and caller ID, allowing you to see who is calling before you answer and to put one call on hold while you answer another. Answering machine: Some cordless telephone systems include an answering machine that allows you to record and store messages for later retrieval. Intercom and conference calling: Many cordless telephone systems allow you to make intercom calls between handsets and to conference call with multiple parties. Range: The range of cordless telephones varies, but most models offer a range of up to several hundred meters from the base station. Long battery life: Cordless telephones typically have a long battery life, allowing you to use them for several hours or even days on a single charge.

2 (b) Discuss forward and reverse Channels in IS-95 CDMA?

Forward Channel: The forward channel is the channel used for transmitting data from the base station to the mobile device. This channel is also known as the downlink or forward link. In the IS-95 CDMA standard, the forward channel uses a spreading code to spread the signal over a wide frequency band. This allows multiple users to share the same frequency band, reducing the amount of spectrum required and improving the overall capacity of the system. Reverse Channel: The reverse channel is the channel used for transmitting data from the mobile device to the base station. This channel is also known as the uplink or reverse link. In the IS-95 CDMA standard, the reverse channel also uses a spreading code to spread the signal over a wide frequency band, allowing multiple users to share the same frequency band. However, the reverse channel typically operates in a different frequency band than the forward channel, in order to reduce interference and improve signal quality.

2 (c) Consider that a geographical service area of a cellular system is 4200 km2. A total of 1001 radio channels are available for handling traffic. Suppose the area of a cell is 12 km2. How many times would the cluster of size 7 have to be replicated in order to cover the entire service area? Calculate number of channels per cell and the system capacity.

Given: Geographical service area = 4200 km2 Total number of radio channels = 1001 Area of a cell = 12 km2 Cluster size = 7 Calculations: Total number of cells required to cover the entire service area = (4200 km2) / (12 km2/cell) = 350 cells Number of times the cluster of size 7 has to be replicated = 350 cells / 7 cells/cluster = 50 clusters Number of channels per cell = 1001 channels / 350 cells = 2.87 channels/cell System capacity = Number of channels per cell * Number of cells = 2.87 channels/cell * 350 cells = 1002.95 channels So, in order to cover the entire service area, the cluster of size 7 has to be replicated 50 times. The number of channels per cell is 2.87, and the system capacity is approximately 1002.95 channels.

3 (a) PCS Architecture (told you many times)

3 (b) what is diversity in wireless communication explain different types of diversity techniques

Diversity in wireless communication refers to the use of multiple, independent channels or paths to improve the reliability and quality of the communication. The idea behind diversity is to minimize the effects of fading and interference, which can reduce the reliability of a wireless communication system.

There are several types of diversity techniques, including:

Spatial Diversity: This technique involves the use of multiple antennas at either the transmitter or the receiver, or both, to provide multiple paths for the signal to travel. Spatial diversity can be achieved through techniques such as antenna diversity, beamforming, and MIMO.

Frequency Diversity: This technique involves the use of multiple frequency channels to provide independent paths for the signal to travel. This can be achieved through techniques such as frequency-hopping spread spectrum (FHSS) and orthogonal frequency-division multiplexing (OFDM).

Time Diversity: This technique involves the use of multiple time slots to provide independent paths for the signal to travel. This can be achieved through techniques such as time-division multiplexing (TDM) and time-division duplex (TDD).

Polarization Diversity: This technique involves the use of multiple polarizations for the signal, such as vertical and horizontal, to provide independent paths for the signal to travel.

Each of these techniques provides a different way to achieve diversity, and they can be used in combination to provide even greater improvement in the reliability and quality of the wireless communication system.

4(a) Explain GSM protocol architecture in detail with the help of diagram

The Global System for Mobile Communications (GSM) is a widely used mobile communication standard. The GSM protocol architecture is divided into three main parts: the mobile station (MS), the base station subsystem (BSS), and the network and switching subsystem (NSS).

Mobile Station (MS): This is the part of the GSM system that is carried by the user. It includes the mobile phone and its associated hardware, such as the SIM card and antenna. The SIM card is a removable smart card that stores the user's subscription information, such as the telephone number, service provider information, and billing information.

Base Station Subsystem (BSS): The BSS is responsible for managing the radio link between the mobile station and the network. It includes the base transceiver station (BTS), which communicates directly with the mobile station, and the base station controller (BSC), which controls the BTS and manages the radio resource. The BSS is responsible for tasks such as call setup and tear-down, handovers between cells, and controlling the power levels of the mobile station.

Network and Switching Subsystem (NSS): The NSS is responsible for managing the core network and the call processing functions of the GSM system. It includes the mobile services switching center (MSC), which is responsible for call routing and management, and the home location register (HLR), which stores the subscription information for all mobile users in the network.

MS: Mobile Station BTS: Base Transceiver Station BSC: Base Station Controller MSC: Mobile Services Switching Center HLR: Home Location Register MSCs: Mobile Services Switching Centers So, the diagram represents the communication links between the different components of the GSM protocol architecture as: Mobile Station <---> Base Transceiver Station <---> Base Station Controller <---> Mobile Services Switching Center <---> Home Location Register <---> Other Mobile Services Switching Centers.

4(b) GSM uses a frame structure where each frame consists of 8 time slots, and each time slot contains 156.25 bits and data is transmitted over a channel at 270.833 kbps. Find time duration of a bit; time duration of a time slot; time duration of a TDMA frame; and how long must a user wait when occupying a single time slot between two successive transmissions.

The time duration of a bit in the GSM system can be calculated as follows: 1 bit / 270.833 kbps = 1 / (270.833 x 10^3 bits/second) = 3.7 x 10^-7 seconds The time duration of a time slot can be calculated as follows: 156.25 bits / 270.833 kbps = 156.25 / (270.833 x 10^3 bits/second) = 577 x 10^-6 seconds The time duration of a TDMA frame can be calculated as follows: 8 time slots x 577 x 10^-6 seconds/time slot = 4.616 x 10^-3 seconds The waiting time between two successive transmissions for a user occupying a single time slot can be calculated as follows: (7 time slots x 577 x 10^-6 seconds/time slot) + (4.616 x 10^-3 seconds/TDMA frame) = (4.039 x 10^-3 seconds) + (4.616 x 10^-3 seconds) = 8.655 x 10^-3 seconds