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

Explore the core principles, differences, and applications of TDMA, FDMA, CDMA, and OFDMA with this medium-level quiz. Assess your grasp of how these multiple access techniques manage resources and enable efficient wireless communication in modern networks.

1. Channel Allocation in FDMA

   Which of the following best describes how Frequency Division Multiple Access (FDMA) allows multiple users to communicate simultaneously in a wireless network?

   1. Users share the same frequency but are assigned different time slots.
   2. Each user utilizes a different spreading code on the same frequency.
   3. Data is divided into subcarriers with dynamic allocation per user.
   4. Each user is assigned a unique frequency band for the entire duration of communication.

   Explanation: FDMA assigns a dedicated frequency band to each user, allowing simultaneous transmission without interference within their bands. Time slots, as mentioned in option B, are a feature of TDMA, not FDMA. Option C describes CDMA, which uses unique codes, while option D relates to OFDMA, which uses subcarriers and dynamic scheduling. Thus, only the first option correctly explains FDMA's mechanism.

2. Time Slot Assignment in TDMA

   In a TDMA system, if three users wish to send data, how does the system typically allow all users to communicate over the same frequency?

   1. Each user is given a different frequency band permanently.
   2. Each user is assigned a unique spreading code across all time slots.
   3. Users transmit sequentially in separate, recurring time slots on the same frequency band.
   4. All users randomly access the medium simultaneously without coordination.

   Explanation: TDMA works by dividing time into slots and employing a scheduling system in which users are allocated distinct time slots in a repeating sequence, all on the same frequency. Option A inaccurately describes CDMA, which relies on unique codes. Option C is the approach of FDMA, and option D describes an uncoordinated access method, which is not used in TDMA.

3. Code Usage in CDMA

   What critical feature enables Code Division Multiple Access (CDMA) systems to separate users sharing the same frequency band?

   1. Use of individual frequency bands for each user
   2. Division of data into fixed subcarriers
   3. Assignment of distinct time slots
   4. Allocation of unique orthogonal codes to each user

   Explanation: CDMA provides each user with a unique code, enabling multiple users to transmit over the same frequency band at the same time with minimal interference due to code orthogonality. Time slots are used in TDMA, and different frequency bands refer to FDMA. Division into subcarriers is a feature of OFDMA, not CDMA.

4. OFDMA Efficiency Example

   In the context of a busy cellular network, why is Orthogonal Frequency Division Multiple Access (OFDMA) especially efficient for data-heavy applications?

   1. Users are permanently locked to individual frequencies.
   2. Only one user can transmit at a time across the whole frequency band.
   3. It assigns static time slots to all users regardless of their needs.
   4. It dynamically allocates subsets of orthogonal subcarriers to users based on demand.

   Explanation: OFDMA divides the available bandwidth into many orthogonal subcarriers and assigns subsets adaptively for maximum efficiency, matching each user's needs. Option A incorrectly implies rigid time allocation. Option B doesn't reflect OFDMA's flexibility. Option D is incorrect because OFDMA supports simultaneous multi-user access by design.

5. Interference in Multiple Access Techniques

   Which technique most effectively minimizes mutual interference among users when all are operating over the same frequency spectrum?

   1. CDMA, by employing orthogonal codes for all transmissions
   2. OFDMA, by allowing random access to all subcarriers
   3. FDMA, by separating users with frequency bands
   4. TDMA, by assigning time slots but sharing frequency

   Explanation: CDMA uses mathematically orthogonal codes to allow simultaneous transmissions in the same frequency and time, significantly reducing mutual interference. TDMA and FDMA reduce interference by separating users in time or frequency, respectively, but not when all share the full spectrum simultaneously. OFDMA assigns non-overlapping subcarriers but does not inherently use orthogonal codes for user separation across the same frequency band.