Multiple Access in Mobile Networks

Multiple Access in Mobile Networks

• Multiple communications need to be ensured without significant interference.
• Packet Access: The entire radio interface is allocated to a single user at a time. An example is ALOHA random access, where many terminals are connected for short periods. Repeated retransmission of corrupted data ensures successful communication sessions.
• Parallel Channel Access: Separate communications use non-overlapping or orthogonal signals.

Multiuser Packet Access

• Pure Aloha: Uncoordinated transmissions can lead to overlaps, creating collisions. This results in a low throughput.
• Slotted Aloha: Transmissions are synchronized to time slots, reducing collisions and increasing throughput compared to pure Aloha.
• Carrier-sense Multiple Access (CSMA): Transmitting stations sense the channel before transmitting to avoid collisions.

Pure ALOHA Access

• A graphical representation showing how transmissions can overlap between two nodes, creating collisions.

Slotted ALOHA Access

• A graphical representation showing how transmissions organized in time slots reduce collisions.

Throughput of ALOHA Access Schemes

• A graph comparing the throughput of pure Aloha to slotted Aloha under varying traffic loads. Slotted Aloha generally demonstrates higher efficiency at higher traffic loads.

Parallel Channel Multiple Access

• Frequency Division Multiple Access (FDMA): Active users are assigned different frequency carriers.
• Time Division Multiple Access (TDMA): Users sharing the same frequency bandwidth are allocated different time slots. Often mixed with FDMA for user groups.
• Code Division Multiple Access (CDMA): Users share the same frequency band but are assigned different codes rather than time slots or frequencies, providing flexibility.

FDMA

• Shows a graphic representation of the frequency allocation for uplink and downlink frequency bands. Guard bands prevent signal interference between adjacent channels.

Cross-Channel Interference

• The formula c(t) = a₁ cos(2nf₁t) + a2 cos(2nf2t) + a3 cos(2nf3t) represents the interference of multiple signals.
• Signals carrying information can interfere with each other when processed by non-linear amplifiers, generating new frequencies (e.g. f' = 2f₁ - f2).

TDMA

• A diagram showing how time slots are allocated to users for communication. Each user has a unique time slot, maintaining non-interference.

TDMA Frame Structure

• A graphical representation of a TDMA frame, detailing timing slots, synchronization bits, information, and guard intervals.

Dynamic TDMA and FDMA Access

• Dynamic versions of these technologies adapt to variable transmission needs more efficiently than fixed ones.

Code Division Multiple Access (CDMA)

• Describes information in a channel is encoded with a specific code sequence.
• A key concept in spread spectrum communications, where code sequences distinguish between information sources.
• Shows an important diagram of the CDMA signal characteristics.

CDMA Channel Capacity

• The formula C = W * log₂(1+S/N) relates channel capacity (maximum information transmission rate) to the bandwidth (W) occupied by the signal and the signal-to-noise ratio (S/N).

Exercise: Hiding the Signal in Noise

• A problem examining how much signal bandwidth is required to transmit data at 9 kbps given a specific signal-to-noise ratio (SNR) requirement using the channel capacity equation.

CDMA Channel Capacity with Multiple Transmitters

• How the capacity of a CDMA channel changes when multiple transmitters are sharing the same channel. The capacity increases with more transmitters with limited power while not severely reducing useful signal power.
• Describes a more complex model of multiple-transmitter CDMA, considering signal from K transmitting sources, finding an effective capacity when all sources are interfered, and assuming interference is close to Gaussian noise.
• Example on calculating the effective CDMA bandwidth requirement given a specific transmission rate and number of transmitters.

Spread Spectrum Technologies

• Direct Sequence Spread Spectrum (DSSS): The carrier is modulated with a high-rate code sequence, expanding the signal's bandwidth for better immunity to interferences.
• Frequency Hopping Spread Spectrum (FHSS): The carrier jumps among different frequencies, making it hard for interference to affect all hops.
• Time Hopping or Time-Frequency Hopping: Time slots or frequencies are determined by a code sequence for spread spectrum.

Direct Sequence Spread Spectrum (DSSS) Principles

• The principles involved in expanding the bandwidth of a signal through the modulation with a code sequence.

Example of DSSS System

• Shows the diagram of a transmitter and receiver components with the signal bandwidth expansion for interference immunity.

DSSS System Processing Gain

• The receiver multiplier's ability in compressing the signal spectrum (with synchronization) for its synchronized part while expanding the asynchronous part (i.e., noises) to achieve a better signal-to-noise ratio. An important measure of this capability is called the processing gain, denoted by variable k.
• How the receiver is able to selectively receive and effectively isolate the desired data signal amid interference and noise, illustrating the principles and importance of spread spectrum processing.