Multiple Access Techniques: FDMA, TDMA, CDMA, OFDMA Quiz Quiz

Explore essential concepts in multiple access techniques such as FDMA, TDMA, CDMA, and OFDMA with this quiz designed for intermediate learners. Enhance your knowledge of how these methods allocate resources and maintain communication efficiency in modern wireless networks.

1. Principle of FDMA

   Which principle best describes how Frequency Division Multiple Access (FDMA) allows multiple users to communicate simultaneously?

   1. Only one user can transmit at a time within the same channel.
   2. Each user is assigned a unique frequency band for the entire communication session.
   3. All users share the same frequency and transmit simultaneously with unique codes.
   4. Each user receives the entire bandwidth in sequential time slots.

   Explanation: FDMA works by assigning a dedicated frequency band to each user for the duration of their transmission, allowing simultaneous communication. TDMA, mentioned in the second option, uses time slots instead of frequency bands. CDMA, described in the third option, relies on unique codes, not frequency separation. The last option incorrectly describes a basic access method where simultaneous communication is not possible.

2. TDMA Resource Allocation

   In a Times Division Multiple Access (TDMA) system, how are channel resources allocated among users during a phone call example?

   1. Each user is assigned repeated time slots for exclusive transmission within the same frequency band.
   2. Users are given overlapping frequency bands.
   3. Each user is given a random frequency every few seconds.
   4. All users transmit simultaneously using orthogonal codes.

   Explanation: TDMA divides each channel into time slots and assigns each user specific time slots, allowing multiple users to share the same frequency without interference. Overlapping frequency bands could result in cross-talk and inefficiency, which is characteristic of option one. CDMA is referenced in option three, which is incorrect for TDMA. Assigning random frequencies as in the last option does not reflect TDMA's structured time-based allocation.

3. CDMA Interference Management

   How does Code Division Multiple Access (CDMA) reduce interference between users who are transmitting data at the same time and on the same frequency?

   1. By using orthogonal frequency channels for each user.
   2. By scheduling transmissions in non-overlapping time slots.
   3. By assigning each user a unique spreading code to modulate their signals.
   4. By allocating narrow frequency bands to each user.

   Explanation: CDMA assigns a unique code to each user; these codes are designed to be orthogonal, which allows receivers to distinguish between simultaneous transmissions on the same frequency. Option two refers to FDMA, which uses frequency bands. Option three relates to TDMA, managing access with time slots. The fourth option incorrectly describes an OFDMA or FDMA-like approach.

4. OFDMA Distinguishing Feature

   Which feature distinguishes Orthogonal Frequency Division Multiple Access (OFDMA) from traditional FDMA using a public Wi-Fi setting?

   1. Users are assigned overlapping subcarriers, enabling simultaneous use of multiple narrowband channels.
   2. The system relies on random frequency selection for each user’s data packet.
   3. Each user is given the entire frequency spectrum, one at a time.
   4. All users share the same code but transmit at different times.

   Explanation: OFDMA splits the available spectrum into many orthogonal subcarriers and assigns them to users dynamically, allowing efficient and simultaneous use. The first option incorrectly describes TDMA with codes. The third suggests single-user access, which is not an access method at all. The random frequency selection in the fourth option does not reflect OFDMA's structured subcarrier assignment.

5. Bandwidth Efficiency Comparison

   Considering large numbers of active users in a cellular network, which multiple access technique typically offers the best bandwidth efficiency and why?

   1. OFDMA, due to its flexible allocation of orthogonal subcarriers to users based on demand.
   2. FDMA, because it reduces interference by fixed frequency assignment.
   3. CDMA, because it allows all users to share the entire frequency simultaneously and uses codes.
   4. TDMA, since it schedules many users in separate time slots.

   Explanation: OFDMA is recognized for high bandwidth efficiency, especially with many users, as it dynamically assigns resources and adapts to user demand. FDMA and TDMA are less efficient because they allocate resources in fixed partitions that can go unused. CDMA allows sharing but can suffer from interference and requires careful code management as user numbers increase. OFDMA’s fine-grained flexibility gives it an edge in modern high-density networks.