a) Explanation

Time and frequency are limited resources in wireless systems. Multiple users share the same spectrum:

• Frequency Sharing (FDMA): Each user is assigned a separate frequency channel.
• Time Sharing (TDMA): Users transmit in separate time slots over the same frequency.

b) Diagram (Illustrative)

FDMA: Different frequency channels simultaneously. TDMA: Same frequency, different time slots.

c) Calculations

Given: Total bandwidth B_total = 40 MHz, 8 users

i) Frequency division:

ii) Time division (8 slots):

iii) Resource advantage: Same total resource used efficiently; multiplexing allows sharing of one channel among multiple users without interference.

d) Interpretation

Multiplexing improves spectrum utilization by allowing multiple users to coexist in either frequency or time, optimizing the limited wireless spectrum.

a) Definition

Duplexing allows bidirectional communication. Two types:

• FDD: Separate frequency bands for uplink and downlink, simultaneous transmission.
• TDD: Same frequency band, uplink and downlink separated in time.

b) Diagram

FDD: uplink & downlink occur simultaneously on separate frequencies. TDD: uplink & downlink alternate in time slots.

c) Calculations

Given: Total bandwidth = 30 MHz

i) FDD guard band:

ii) TDD frame (10 ms, uplink 6 ms): Downlink duration = 10 - 6 = 4 ms

iii) Uplink-to-downlink ratio:

d) Interpretation

Higher uplink traffic can use TDD effectively, while FDD is better for symmetric traffic. Guard bands in FDD ensure no interference between uplink/downlink.

a) Definition

Multiplexing combines multiple signals for transmission over a single communication channel, improving capacity and reducing infrastructure cost.

b) Block Diagram

Signals are combined using multiplexer at transmitter and separated using demultiplexer at receiver.

c) Calculations

Given: 12 channels, 64 kbps each, 10% overhead

i) Total transmitted data rate:

ii) Effective throughput:

iii) Data rate lost due to overhead:

d) Interpretation

Multiplexing allows fewer physical links, reducing cost while supporting multiple channels efficiently.

a) Explanation

Multiplexing can be applied in:

• Space: Different spatial sectors or beams.
• Frequency: Different frequency channels.
• Time: Separate time slots.
• Code: Different orthogonal codes in CDMA.

b) Diagram

Illustration of a wireless system supporting multiple users via all 4 dimensions.

c) Calculations

Given: 3 spatial sectors, 5 frequencies, 4 time slots, 8 orthogonal codes

i) Maximum simultaneous users:

ii) One sector unavailable: Users_remaining = 2 × 5 × 4 × 8 = 320 users

iii) Capacity reduction:

d) Interpretation

Multi-dimensional multiplexing significantly increases scalability and system reliability; failure in one dimension only partially reduces capacity.

a) Definition

FDM separates multiple user signals by assigning each a unique frequency band. Signals are transmitted simultaneously over different frequencies.

b) Frequency Spectrum Diagram

Each channel occupies its own frequency slot separated by guard bands to prevent interference.

c) Calculations

Given: Total bandwidth = 12 MHz, channel bandwidth = 200 kHz, guard band = 20 kHz

i) Formula for number of FDM channels:

ii) Maximum channels:

iii) Total guard band consumed:

d) Interpretation

Guard bands prevent interference but reduce spectral efficiency, making FDM bandwidth inefficient as the number of users increases.

a) Explanation

Bandwidth efficiency measures the ratio of effective channel bandwidth to total occupied bandwidth.

b) Diagram

Guard bands inserted between adjacent channels to avoid overlap.

c) Calculations (Using Question 5 data)

i) Total bandwidth lost due to guard bands: 1.06 MHz (calculated above)

ii) Bandwidth efficiency:

iii) Effective usable bandwidth: 54 × 200 = 10.8 MHz

d) Interpretation

FDM loses efficiency as guard bands increase. Proper design balances interference avoidance and bandwidth utilization.

a) Principle

FDMA allocates separate frequency channels to each user. In satellites, this allows multiple uplink/downlink channels to coexist.

b) Satellite Transponder Diagram

Each user channel occupies a distinct frequency band inside the transponder.

c) Calculations

Given: Transponder bandwidth = 36 MHz, channel = 30 kHz, guard = 10 kHz

i) Number of channels:

ii) Total guard bandwidth: (900 – 1) × 10 = 8,990 kHz ≈ 8.99 MHz

iii) Usable signal bandwidth: 900 × 30 = 27,000 kHz = 27 MHz

d) Interpretation

Guard bands consume significant bandwidth, limiting the number of users despite large transponder capacity.

a) Explanation

Increasing guard bands reduces number of users per transponder, making FDMA less efficient.

b) Diagram

Wider guard bands reduce number of active channels.

c) Calculations (Guard = 20 kHz)

New number of channels:

User capacity reduction: 900 – 720 = 180 channels

Percentage capacity loss: 180 / 900 × 100% ≈ 20%

d) Interpretation

Doubling guard bands reduces user capacity by 20%, demonstrating trade-off between interference avoidance and efficiency.

a) Definition

TDM assigns different time slots to multiple users sharing the same frequency channel sequentially.

b) Diagram

Each user transmits in their assigned time slot within a frame.

c) Calculations

Given: Frame = 20 ms, 10 users, guard time = 0.2 ms per slot

i) Time slot duration: 20 / 10 = 2 ms per slot

ii) Effective transmission time per slot: 2 – 0.2 = 1.8 ms

iii) Percentage time overhead: 0.2 / 2 × 100% = 10%

d) Interpretation

Precise synchronization is required in TDM to avoid overlapping transmissions and collisions.

a) Explanation

In TDM, each user transmits at a higher instantaneous rate (burst) during their time slot to achieve required average data rate.

b) Diagram

High peak rate during slot, zero transmission otherwise.

c) Calculations

Given: User requires 64 kbps, slot duration = 1.8 ms (from Question 9)

i) Burst data rate:

ii) Peak-to-average ratio: 711 / 64 ≈ 11.1

iii) Total frame capacity: 10 × 64 kbps = 640 kbps

d) Interpretation

High instantaneous rate allows TDM systems to multiplex multiple users efficiently but requires fast switching and buffers.

a) Explanation

TDMA/FDD combines time division multiplexing (TDMA) with frequency division duplex (FDD). Multiple users share frequency channels in time slots, and uplink/downlink use separate frequencies.

b) Diagram

Illustrates multiple users per carrier, separate frequencies for uplink and downlink.

c) Calculations

Given: Forward-link bandwidth = 25 MHz, channel = 200 kHz, 8 time slots

i) Number of RF carriers:

ii) Total simultaneous users:

iii) Users per MHz:

d) Interpretation

TDMA increases spectrum efficiency by allowing multiple users per carrier, compared to FDMA which allows only one user per frequency channel.

a) Explanation

Guard bands prevent interference between adjacent carriers in TDMA/FDD systems.

b) Diagram

Guard bands shown between adjacent frequency carriers.

c) Calculations (10 kHz guard band)

Adjusted channel bandwidth including guard = 200 + 10 = 210 kHz

i) Number of RF carriers:

ii) Total supported users: 119 × 8 = 952 users

iii) Capacity reduction: 1,000 – 952 = 48 users ≈ 4.8%

d) Interpretation

Adding guard bands improves interference control but slightly reduces user capacity.

a) Explanation

Hybrid TDM/FDM combines time and frequency multiplexing to increase channel capacity and flexibility.

b) Diagram

Multiple frequencies with time slots per frequency.

c) Calculations

Given: 6 frequency channels, 8 time slots

i) Total logical channels:

ii) One slot per frequency reserved for control: 6 × 1 = 6 control slots

Available user channels = 48 – 6 = 42 channels

iii) Percentage reserved for control:

d) Interpretation

Hybrid multiplexing enhances flexibility by combining time and frequency domains and reserving resources for control signaling.

a) Explanation

Guard time prevents overlap between consecutive time slots in TDMA due to propagation delays and synchronization errors.

b) Diagram

Illustrates insertion of guard time between slots.

c) Calculations

Given: Slot duration = 0.577 ms, guard time = 0.1 ms

i) Percentage time lost:

ii) Effective data transmission time: 0.577 – 0.1 = 0.477 ms

iii) Throughput reduction per slot: 0.577 – 0.477 = 0.1 ms lost

d) Interpretation

Guard time reduces throughput but ensures reliable, collision-free communication.

a) Explanation

CDM allows multiple users to transmit simultaneously on the same frequency by using unique orthogonal spreading codes.

b) Spectrum Diagram

Each user signal spread over the same bandwidth with unique code.

c) Calculations

Given: 64 orthogonal codes, 10% reserved for control

i) Maximum simultaneous users: 64 codes = 64 users

ii) Remaining codes: 64 × 0.9 = 57.6 ≈ 57 users for data

iii) User capacity reduction: 64 – 57 = 7 users ≈ 10%

d) Interpretation

Exceeding available codes causes interference and reduces system performance.

a) Definition

Processing gain is the ratio of spread bandwidth (chip rate) to user data rate. Higher gain improves interference resistance.

b) Diagram

Shows spreading at transmitter and despreading at receiver.

c) Calculations

Given: Chip rate = 3.84 Mcps, Data rate = 12.2 kbps

i) Formula:

ii) Calculate: 3,840 / 12.2 ≈ 314.75

iii) In dB: 10 × log10(314.75) ≈ 24.97 dB

d) Interpretation

High processing gain allows better resistance to interference and supports more users in CDMA systems.

a) Explanation

Multiple users generate multiple-access interference (MAI), which reduces effective signal quality.

b) Diagram

Shows overlapping users sharing same frequency spectrum with orthogonal codes.

c) Calculations (20 users, PG = 314.75)

i) Effective processing gain per user:

ii) Interference scaling factor: ≈ 1 / 20 = 0.05

iii) Reduction ratio: 15.74 / 314.75 ≈ 0.05 (95% reduction from single-user)

d) Interpretation

Adding users reduces effective gain, limits capacity, and increases bit error rate. CDMA planning must consider maximum simultaneous users.

a) Explanation

Comparison of multiplexing techniques:

• FDMA: Separate frequencies, simple, low interference but lower capacity.
• TDMA: Time-shared channels, higher capacity, requires synchronization.
• CDMA: Same frequency, codes differentiate users, high capacity, resilient to interference, complex.

b) Diagram

Shows user separation in frequency, time, and code domains.

c) Calculations (10 MHz bandwidth)

i) FDMA: Channel = 100 kHz, Guard = 10 kHz:

ii) TDMA: Channel = 200 kHz, 8 time slots:

iii) CDMA: Processing gain = 100: Supported users ≈ 100 users (depends on interference)

d) Interpretation

TDMA offers highest capacity, FDMA is simplest but limited, CDMA is scalable and interference-resilient but complex. System choice depends on traffic and technology.