Lecture 9 Multiple access methods (Satellite)

Summary

This lecture explores satellite multiple access methods used to share radio spectrum among users within a satellite's footprint. Key methods discussed include Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), and Space Division Multiple Access (SDMA). The lecture explains how these methods help optimize satellite resource usage, enhance efficiency, and support various communication needs. It also covers the trade-offs and benefits of each method, including the flexibility of TDMA and CDMA and the spatial reuse efficiency of SDMA. The lecture concludes with a look at modern technologies like OFDM and OFDMA.

Takeaways

• 😀 Satellite multiple access systems allow multiple users to share the radio spectrum within a satellite's footprint.
• 😀 Frequency Division Multiple Access (FDMA) divides the satellite's bandwidth into smaller sub-bands, assigning each user a specific frequency.
• 😀 Time Division Multiple Access (TDMA) allocates the entire bandwidth to one user at a time, rotating through different users in quick succession.
• 😀 Code Division Multiple Access (CDMA) allows all users to transmit simultaneously, using unique codes to distinguish their signals.
• 😀 Space Division Multiple Access (SDMA) allows multiple users to reuse the same frequency by spatially separating their communication spots on the satellite footprint.
• 😀 The main goal of multiple access systems is to utilize satellite resources efficiently by sharing bandwidth among multiple users.
• 😀 Traffic profiles of satellite users are dynamic, meaning not all users need the full bandwidth simultaneously, leading to better resource utilization when shared.
• 😀 FDMA is easy to implement but less flexible, as guard bands between users reduce the overall system efficiency.
• 😀 TDMA provides greater flexibility than FDMA by allowing dynamic allocation of time slots to users, but it requires synchronization and overhead for control.
• 😀 CDMA is highly flexible and allows users to transmit simultaneously, but performance can degrade as the number of users increases due to increased interference.
• 😀 SDMA is particularly efficient for high-throughput satellites, enabling smaller beams to reuse the same frequencies without causing interference.
• 😀 Combining multiple access methods (like FDMA, TDMA, and CDMA) in modern satellite systems optimizes both flexibility and efficiency for handling various types of traffic.

Q & A

What is the primary goal of satellite multiple access systems?

The primary goal is to efficiently share the radio spectrum among multiple users inside a satellite's footprint, maximizing bandwidth utilization and minimizing interference.

What are the four main methods of multiple access discussed in the lecture?

The four main methods of multiple access discussed are Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), and Space Division Multiple Access (SDMA).

How does Frequency Division Multiple Access (FDMA) work?

FDMA divides the satellite's total bandwidth into smaller sub-bands, each assigned to a different user. Each user communicates on a separate frequency, with guard bands to prevent interference between sub-channels.

What are the advantages and disadvantages of FDMA?

The advantage of FDMA is its simplicity in implementation. However, it is inefficient for dynamic traffic patterns, as each user gets a fixed frequency allocation even if they are not using it, leading to potential underutilization of the spectrum.

How does Time Division Multiple Access (TDMA) improve upon FDMA?

TDMA allocates the entire bandwidth to a single user for short, rotating time slots. This method is more flexible and efficient, allowing users to share the same frequency and adjusting bandwidth allocation based on traffic demand.

What are the challenges associated with TDMA?

TDMA requires synchronization of all terminals, and there is overhead involved in managing time slots. Additionally, guard times between frames are needed to prevent interference, which adds to the system's complexity.

What is the concept behind Code Division Multiple Access (CDMA)?

In CDMA, all users transmit over the entire bandwidth at the same time but use unique spreading codes. These codes are orthogonal, meaning that the receiver can isolate each user's signal by multiplying the received signal with the corresponding code.

What analogy is used to explain the working of CDMA?

CDMA is likened to listening to a specific person's voice in a crowded room. Even though many people are speaking at once, you can still distinguish the voice you are interested in because it has a unique 'code' or 'signature' that your ear can identify.

What are the limitations of CDMA?

The main limitation of CDMA is that as the number of users increases, the bit error rate (BER) can increase, especially in non-ideal channels, as the orthogonality between codes is not perfect.

How does Space Division Multiple Access (SDMA) differ from other access methods?

SDMA uses geographic spatial separation to reuse frequencies. Different spots within the satellite's footprint can use the same frequency without causing interference, particularly in high-throughput satellites that use smaller spot beams.

How does increasing the frequency in SDMA improve efficiency?

Increasing the frequency (e.g., from Ku-band to Ka-band) allows for smaller spot beams, which increases frequency reuse efficiency by reducing the size of each beam and allowing the same frequency to be reused more often without interference.

What is the role of guard bands in FDMA and TDMA systems?

Guard bands are used in both FDMA and TDMA systems to prevent interference between users. In FDMA, they separate sub-channels, while in TDMA, they ensure that time slots do not overlap.

What is the efficiency of TDMA measured by, and how is it calculated?

The efficiency of TDMA is measured by the ratio of the useful payload (data) to the total time used for transmission, including overhead for synchronization, guard time, and control signaling. The formula takes into account the total time frame, bit rate, and overhead bits.