Each channel occupies one frequency band, and the spacing between adjacent frequency bands should meet the transmission signal bandwidth requirements.

Symbol time is larger than the average delay spread: less inter-symbol interference and no need for adaptive equalization

The base station is complex and large: the base station requires multiple transceiver channel equipment

A large number of bandpass filters need to be set up: to suppress adjacent channel interference

Handover is complicated: only hard handover can be used

Low frequency utilization and small capacity

The transmission rate is high, greater than the speech coding rate,

The transmission signal rate increases as the time slot N increases, requiring adaptive equalization.

No duplexer required

Base station complexity reduced, requiring only one transceiver

Strong anti-interference ability, high frequency utilization, large system capacity

Easy handover

Synchronization is difficult and expensive

Multiple users share the same frequency

Large communication capacity

When the number of users increases, the system performance gradually decreases, and vice versa.

Resistant to multipath fading

Smooth soft switching and efficient macrodiversity

High channel rate

Low signal power spectral density: strong anti-interference ability

multiple access interference

near and far effect

Space separates different users

Using wireless directional beams, the cell is divided into different subspaces to achieve orthogonal division of space.

Different wave speeds can be formed in different user directions

Can be used to combine the above three multiple access technologies

Basic idea: Divide the transmission bandwidth into a series of orthogonal and non-overlapping sub-carrier sets, and allocate different sub-carriers to different users to achieve multiple access.

Classified sub-channel OFDMA

Frequency hopping OFDMA

Main purpose: Provide addressing and media access control methods so that different devices or network nodes can communicate on multi-point networks without conflicting with each other.

Features: The principle is simple and technically easy to implement. Each workstation in the network is on an equal footing, does not require centralized control, and does not provide priority control. When the network load increases, the sending time increases and the sending efficiency drops sharply. Applied in OSI layer 2 data traffic layer

Use ACK signals to avoid conflicts

Basic unit: intra-frame interval DIFS contention window CW backoff counter

CSMA/CA comparison CSMA/CD The biggest advantages: No conflict detection procedure required Waiting time is fairly distributed On average the principle of first come first served is implemented

main difference