OE-7.5 Multiple Access Techniques & Wireless Standards

What this is and why it exists

Each multiple-access scheme divides a different resource. Frequency, time, code, subcarrier, space. Once you see that, the comparison table writes itself and there is very little to memorise.

There is one standard error to avoid, and it is worth naming at the start. Duplexing is a separate question from access. Confusing the two is what makes this unit look harder than it is.

The concrete case at the end is worth studying even though newer systems have replaced it, because a real architecture makes the abstractions land.

The vocabulary

• Multiple access — how many users share one medium.
• Duplexing — how the two directions of one conversation are separated.
• Frequency division — each user gets a different frequency.
• Time division — each user gets a different time slot.
• Code division — users share frequency and time, separated by codes.
• Subcarrier division — users get different subcarriers of a multiplexed band.
• Space division — users are separated by directional beams.
• Base station subsystem — the radio side of a cellular network.

The mental model

Hold the two questions apart from the first minute.

Access asks: how do many users share the medium? Duplexing asks: how do the two directions of one conversation stay separate? They are independent choices, and a system makes both.

Duplexing has two answers. Separate the directions in frequency, so both can run at once on different frequencies. Or separate them in time, so they alternate fast enough that nobody notices. Frequency separation suits symmetric traffic like voice; time separation allows the split to be adjusted, which suits data where downloads greatly exceed uploads.

Access then has five answers, and each divides a different resource. Divide the frequency, and each user gets their own — simple, and a user who says nothing still occupies theirs. Divide the time, and users take turns on the same frequency, which suits bursty traffic much better. Divide by code, and everyone transmits at once across the same band. Codes are chosen so each receiver picks out its own signal and treats the rest as noise. Divide the subcarriers of a multiplexed band, which combines the previous topic's scheme with access and allows fine-grained allocation. Divide space, using directional beams so that two users in different directions can reuse everything else.

That is the whole comparison, and the organising question is only ever "which resource is being divided".

The concrete architecture — mobile stations, a base station subsystem, and a switching and management subsystem — is worth walking through once. It shows how the abstractions become equipment. The packet-data additions layered onto it illustrate a real network being extended for data rather than redesigned. That is what actually happens to deployed systems, and it explains a great deal about why they look the way they do.

What you should now be able to explain or do

Keep access and duplexing apart as separate questions. Compare the two duplexing choices and say which traffic each suits. Name the resource each access scheme divides. Say why time division suits bursty traffic better than frequency division. Explain how code division separates users sharing frequency and time. Walk through a concrete architecture and say how data capability was added to it.

Check yourself