Wireless Communication - Study Mode
[#66] What is Mobile communication?
Correct Answer
(A) Allows to communicate from different locations without the use of physical medium
Explanation
Solution: Mobile communication is a wireless communication system that allows users to communicate over a distance using portable devices. The key characteristic is the mobility of the communication devices. This means the devices can move around while maintaining a connection. Let's analyze the options: Option A: Allows to communicate from different locations without the use of physical medium This is the correct answer . Mobile communication inherently relies on wireless transmission, meaning no physical wires or cables connect the communicating devices. The signal travels through the air (or other mediums like satellite) to reach its destination. Option B: Allows to communicate from different locations with the use of physical medium This is incorrect. If a physical medium is used, it's not considered mobile communication in the true sense. Think of a wired telephone network – it allows communication between different locations but requires physical cables. Option C: Allows to communicate from same locations without the use of physical medium While this might describe some wireless local area networks (WLANs), it doesn't capture the essence of *mobile* communication. The emphasis is on the ability to communicate while *moving* between locations. Option D: Allows to communicate from same locations with the use of physical medium This describes a wired local communication scenario, not mobile communication.
[#67] Various air interface formats used by EDGE are also known as . . . . . . . .
Correct Answer
(A) Modulation and coding schemes
Explanation
Solution: Explanation: First, let's define some key terms: Air Interface: The air interface refers to the radio frequency (RF) link between a mobile device (like a cell phone) and a base station (cell tower) in a wireless communication system. It defines how data is transmitted and received over the airwaves. Modulation: This is the process of varying one or more properties of a periodic waveform, called the carrier signal, with a message signal such that the message signal is contained within the variations of the carrier. It's how information is encoded onto the radio waves. Coding Scheme: This refers to the techniques used to add redundancy to the data for error correction and detection during transmission. It helps improve the reliability of the communication even in the presence of noise or interference. Correct Answer: A: Modulation and coding schemes EDGE (Enhanced Data rates for GSM Evolution) uses various combinations of modulation and coding schemes to achieve different data rates and robustness levels. The air interface is defined entirely by these schemes. The choice of modulation and coding scheme directly impacts the spectral efficiency (how much data can be transmitted in a given bandwidth) and the robustness of the communication (its ability to withstand interference). Different schemes offer different trade-offs between speed and reliability. Why other options are incorrect: B: Coding schemes: While coding schemes are a crucial part of the EDGE air interface, they are only one aspect. The modulation method is equally important and forms a complete definition of the air interface with coding. C: Modulating air interface: This is not a standard term. The air interface is characterized by a combination of modulation and coding, not just modulation alone. D: Air interface coding schemes: Similar to option B, this is too narrow. The complete definition requires both modulation and coding schemes.
[#68] Cordless telephone system will not work under which of the following criteria?
Correct Answer
(D) Within a city
Explanation
Solution: A cordless telephone system uses radio waves for communication between the handset and the base station. The range of these radio waves is limited. Cordless telephones use short-range radio frequencies to communicate. This means there's a limited distance the handset can be from the base station before the signal becomes too weak to work effectively. Option A: Within a home . Cordless phones are designed to work within a typical home. The range is usually sufficient to cover a house. Option B: Within a building . Similar to a home, a cordless phone *could* work within a building, depending on the building size and construction materials which may interfere with the signal. Larger buildings or those with significant interference could cause problems. Option C: Within campus . A campus is a significantly larger area than a home or building. The range of a cordless phone is not designed for such distances, and significant signal degradation and interference are highly likely. Option D: Within a city . A city is even larger than a campus. The distance and interference from various sources (other wireless devices, buildings, etc.) would make a cordless phone completely impractical and unusable. Therefore, the correct answer is Option D: Within a city , because the range of a cordless phone is far too limited for such a large and interference-prone environment.
[#69] 2G CDMA standard, IS-95, was proposed by which company?
Correct Answer
(B) Qualcomm
Explanation
Solution: Question: 2G CDMA standard, IS-95, was proposed by which company? Definitions: 2G (Second Generation) Wireless Technology: Represents the second generation of cellular network technology. It marked a significant improvement over 1G (analog) systems by introducing digital transmission, leading to improved voice quality, increased capacity, and enhanced security. CDMA (Code Division Multiple Access): A channel access method used by radio communication systems. Unlike other multiple access schemes like TDMA (Time Division Multiple Access) or FDMA (Frequency Division Multiple Access), CDMA allows multiple users to share the same frequency band simultaneously by using different codes to distinguish between them. This is achieved by spreading the signal across a wider bandwidth using a unique code for each user. IS-95: A specific standard for CDMA-based 2G cellular networks. It defines the air interface specifications (how the mobile phone communicates with the base station) for the system. Correct Answer: B: Qualcomm Qualcomm played a pivotal role in the development and standardization of IS-95, the prominent CDMA standard for 2G networks. They were instrumental in developing the underlying CDMA technology and were heavily involved in its commercialization. Why other options are incorrect: A: Nippon Telephone and Telegraph (NTT): While NTT is a major telecommunications company, it wasn't the primary proponent of the IS-95 standard. Their contributions to 2G and beyond were significant in their own right, but not specifically focused on IS-95 development. C: Bellcore and Motorola: Both Bellcore (now Telcordia Technologies) and Motorola contributed to the advancement of cellular technology, but they weren't the principal developers behind the IS-95 CDMA standard. They had their own involvement in other cellular technologies and standards. D: AT&T Bell Laboratories: AT&T Bell Labs also played a significant role in the evolution of telecommunications, including contributions to early cellular systems. However, the core development and proposal of the IS-95 standard are primarily attributed to Qualcomm.
[#70] Which of the most widely used model for signal prediction in urban areas?
Correct Answer
(A) Okumura model
Explanation
Solution: First, let's define what a signal prediction model is in the context of wireless communication. A signal prediction model is a mathematical formula or algorithm used to estimate the received signal strength at a specific location in a wireless communication system. These models are crucial for planning and designing wireless networks, ensuring adequate signal coverage and minimizing interference. The correct answer is A: Okumura model . The Okumura-Hata model (often shortened to just Okumura model) is an empirical model that is widely used for predicting path loss in urban areas. It's based on extensive measurement data collected in Tokyo, Japan. Its popularity stems from its relative simplicity and reasonable accuracy for many urban environments. It considers various factors affecting signal propagation, such as frequency, distance from the transmitter, and the type of environment (urban, suburban, rural). While not perfect, its ease of use and relatively good accuracy for a wide range of scenarios make it a preferred choice for initial estimations in urban planning. Now let's address why other options are less suitable: Option B: Attenuation factor model: This is a more general term and doesn't represent a specific, widely used model like Okumura. Attenuation factors are components within various models, but they don't stand alone as a complete predictive model. Option C: Ericsson Multiple Breakpoint Model: While a valid model, it's not as widely used or established as the Okumura model. It might be preferred in certain specific scenarios or by particular vendors, but it lacks the general acceptance and broad application of the Okumura model. Option D: Log distance path loss model: This is a very basic model, suitable for simple estimations but often lacking the accuracy and detail needed for realistic urban environments. It primarily accounts for distance-based signal attenuation, neglecting crucial factors like terrain, building density, and other environmental effects significant in urban areas. The Okumura model, on the other hand, incorporates these factors, leading to more precise predictions. In summary, while several models exist for signal prediction, the Okumura model stands out due to its widespread use, relative simplicity, and acceptable accuracy for urban environments, making it the most appropriate answer.