Lecture 19: Multiple access communicationDANIEL WELLER

TUESDAY, APRIL 2, 2019

AgendaMultiuser communications

Orthogonal coding and multiplexing

Multiple access communication schemes

How can so many people use their phones to communicate at the same time? We’ll begin to explore this and related questions in today’s lecture on multiple access communications.

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Multiuser communicationsEarly modes of multiuser communications went to great lengths to provide dedicated communications links for each pair of users. For instance, cities were blanketed with massive webs of wires:

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The sky was once filled with wires. Let’s be glad we’ve moved to a mostly wireless future. Credit (left): Tekniska

museet/flickr; right: IEEE Global History Network

Multiuser communicationsTo regulate wireless communications, the Federal Communications Commission created licenses that particular users (e.g., radio or TV stations) could buy to provide dedicated bandwidth that others could not use:

However, providing every single userwith their own dedicated spectrumis very wasteful – why?

In today’s lecture, we will begin to understand other ways of sharing communications channels among many users.

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Cellular networks: leading the wayBefore modern cellular networks, mobile wireless radio communications generally operated in an uncoordinated way.

◦ As users moved around, their communications could clash with other users relying on the same channel or bandwidth.

Cellular networking changed this by dividing a region into a multitude of cells, each containing its own base station responsible for communicating with the users within its cell.

◦ Depending on the expected density of users, the size of cells can be quite large or very small.

◦ The base stations are responsible for handing off users (e.g., phones) from one cell to another as they move among them.

◦ This way, frequency ranges could be shared among users and recycled as they relocate.

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In the beginning: 1G cellular networksThe analog Advanced Mobile Phone System (AMPS) was deployed in the US in 1983. It assigned different frequency ranges to users, via a scheme called Frequency Division Multiple Access (FDMA).

◦ We’ll learn more about FDMA later.

Each user in a cell had a dedicated frequency band; base stations had to keep track of which bands were in use as users moved between cells and reallocate bands to prevent multiple users’ transmissions from clashing with each other.

◦ FCC only allocated about 25 MHz for downlink and 25 MHz for uplink frequencies, allowing up to 832 downlink/uplink channels of about 30 kHz each.

◦ This approach is not scalable – more channels would require more bandwidth.

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From analog to digital: 2G was bornAnalog transmissions were bandwidth hungry, mostly limited to speech/audio, and had limited privacy or security (they were unencrypted).

Two other multi-access approaches were adopted for digital transmissions: Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA).

◦ TDMA allocated users with brief time slots for transmitting part of their digital signal.

◦ CDMA employed “codes” to allow users to label their messages, enabling simultaneous transmission.

2G networks enabled digital media such as SMS (text messages), ringtones, and even data to be shared over a cellular network. Thus the smartphone became possible!

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CDMA

Need for speed: 3G mobile broadbandBuilding on the digital multiple access technologies of 2G, 3G gained speed through more efficient use of bandwidth. Whereas 2G used circuit-switching, 3G introduced packet-switching:

◦ Circuit-switching: All information in a signal travels together on the same path, at the same speed.

◦ Packet-switching: Information is divided into packets, which are transmitted separately, possibly using a multitude of different paths.

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Need for speed: 3G mobile broadbandThis move to higher speeds is a major evolutionary step in cellular communications:

◦ In 3G, the minimum speed was boosted to 2 million bits/second for stationary/walking users and 384,000 bits/second for moving vehicles – much faster than 2G networks could achieve!

◦ Thus, wireless broadband and data applications became more common, and even more demanding applications like video streaming became possible in limited cases.

However, packet-switching was used solely for data in these networks, to ensure compatibility with the older 2G networks for making phone calls. It’s not until 4G that packet switching became the norm for all communications.

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Cellular like the internet: 4G LTEBy moving to a full internet-like communications architecture with packet-switching for all communications, 4G networks could make use of bandwidth more efficiently than ever before.

◦ Voice/audio is sent via packets

◦ Users are identified via internet-style addressing; the cellular network takes on modern routing capabilities

◦ Data rates improved by a factor of 10; streaming video and other data-intensive applications become widespread

◦ The Long Term Evolution (LTE) standard guided 4G development in a coordinated way over the last 15 years through the 3rd Generation Partnership Project (3GPP).

◦ Future efforts to expand the capabilities and applications of cellular networks fall under the heading of “5G”◦ A major piece of many next-generation technologies including smart cities, self-driving cars, Internet of Things, AI everywhere

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TDMA

Time Division Multiple AccessLet’s see how these technologies work. First, let’s start with Time Division Multiple Access (TDMA):

◦ Idea: enable multiple users to share a single channel by assigning each user a “slice” of time during which only that user can communicate.

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Time Division Multiple AccessHow are time slices assigned?

◦ Each base station tracks user requests for transmit/receive time

◦ Requests are prioritized by type (e.g., voice calls, then data)

◦ Balancing assures users do not have to wait too long due to other users’ heavy usage (this is why many networks will throttle excessive usage)

◦ However, users are responsible for honoring time slice allocations; base station does not explicitly control user transmission times.

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It’s my turn!

No, it’s my turn!

Alexa, whose turn is it?

Frequency Division Multiple AccessInstead of assigning a time slice to the entire channel, a base station can assign users different ranges of non-overlapping frequencies they are allowed to use:

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FDMA

(1G Cellular Networks)

Frequency Division Multiple AccessHowever, as users relocate, and their needs change, maintain a fixed assignment of frequencies becomes wasteful. Hence, modern FDMA is actually a time-frequency assignment hybrid:

This is the norm for 2G TDMA/FDMA networks like Global system for mobile (GSM) networks

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F/TDMA

(2G Cellular Networks)

TDMA, FDMA, and orthogonalityBy ensuring frequencies are not shared at a given slice or moment in time, TDMA and FDMA both implement a form of orthogonality, since only a single user is transmitting a nonzero sinusoid at any given frequency at any given time.

◦ Different time slices are orthogonal

◦ Different frequency ranges are orthogonal

◦ Different time/frequency slice ranges are orthogonal

Is this the only way to ensure orthogonality? No.◦ In fact, data coding provides another

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Code Division Multiple AccessCode Division Multiple Access uses orthogonal data coding sequences to separate user signals without enforcing either time slicing or frequency range constraints:

CDMA: Each user has access to all frequencies, all the time!

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CDMA

All frequencies, all the time...

Code Division Multiple AccessUnlike TDMA/FDMA, each user can transmit continuously and has access to the full channel.

◦ Robust in challenging environments with varying channel characteristics (e.g., during bad weather)

◦ Does not require waiting to communicate, reducing latency and avoiding excessive delays

CDMA: Each user users a different code to transmit.◦ If the receiver knows the transmitter’s code, the receiver can extract the data from just the transmitting

user, irrespective of other transmissions on the same channel.

◦ This requires the transmissions’ codes to be orthogonal to each other. This way, multiplying a received signal by one user’s code will automatically zero out the contributions from every other user.

◦ Start with a low-bandwidth signal, and multiple by a high-bandwidth “spreading code”. The result will be high bandwidth, but allow simultaneous reuse of that bandwidth by other users.

◦ Number of concurrent users not limited by bandwidth available, just by number of orthogonal codes!

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ExampleLet’s see an example of this for a digital signal:

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CDMA example

Low-Bandwidth Signal:

High-Bandwidth Spreading Code:

Mix is a simple multiply

… and transmit.

Call one period of this spreading code, cA

ExampleWhat if we multiply the mixed high-bandwidth signal by the spreading code again?

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CDMA example

To Decode / Receive, take the signal:

Multiply by the same Spreading Code:

… to get ... How does this compare to the original message?

ExampleWhat if we take a different spreading code, orthogonal to this code? <cA, cB> = 0

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Take the same signal:

Multiply by the wrong Spreading Code:

… you get ...

… which clearly hasn't recovered the original signal. Using wrong code is like being off-frequency.

What if we use the wrong code?

Call one period of this code, cB

Not consistent

CDMA orthogonalityIf we decode with the right spreading code,

◦ The eight decoded bits all match (all +1 or all -1)

◦ Averaging the eight decoded bits yields the original low-bandwidth signal

If we decode with an orthogonal code,◦ The eight decoded bits will be sort of “random” (not all the same)

◦ Averaging the eight decoded bits equals zero.

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Take the same signal:

Multiply by the wrong Spreading Code:

… you get ...

… which clearly hasn't recovered the original signal. Using wrong code is like being off-frequency.

What if we use the wrong code?

CDMA example

To Decode / Receive, take the signal:

Multiply by the same Spreading Code:

… to get ...

CDMA orthogonalitySuppose code length is N bits

User A has code cA, user B has code cB.

Then, choose cA and cB to be orthogonal: <cA, cB> = 0

When user A sends “+1”, it sends +cA; when sending “-1”, it sends -cA.

User B sends “+1” as +cB and “-1” as -cB.

At the receiver end, we get ±cA ± cB. ◦ If we multiply by cA, what do we get?

◦ What if we multiply by cB?

◦ This is why orthogonality is important.

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CDMA and noiseNow suppose a real CDMA-coded transmission has noise.

◦ How does noise affect a digital (binary) signal?

Key idea: noise is random, so inner product between noise and cA or cB will be small.

Thus, what is result of decoding user A’s signal? User B’s signal?

How much noise can CDMA tolerate for a length N code, and still decode the right value?

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CDMA and noiseIn fact, CDMA can decode signals well below the “noise floor”:

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CDMA

… below the noise floor! … and still be heard!

Trust me. We'll prove it later.

Noise floor

CDMA data

ActivityNow, we’ll divide the room roughly in half and simulate a “CDMA” communication with noise.

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cB = -1 1 -1 1cA = -1 -1 1 1

ActivityLet’s each try to decode each other’s messages:

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cA = -1 -1 1 1cB = -1 1 -1 1

Activity result (divide noise by 2)L+R+N/2 = 0 1 -1 4 | 3 1 -1 -4 | 2 1 1 -1 | -3 -1 -3 1 | 2 0 -2 3 | -1 -4 2 1 | 0 1 -1 1

cA = -1 -1 +1 +1 cB = -1 +1 -1 +1

Left decodes by multiplying by cB, averaging:

Right decodes by multiplying by cA, averaging:

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Activity result (divide noise by 2)L+R+N/2 = 0 1 -1 4 | 3 1 -1 -4 | 2 1 1 -1 | -3 -1 -3 1 | 2 0 -2 3 | -1 -4 2 1 | 0 1 -1 1

cA = -1 -1 +1 +1 cB = -1 +1 -1 +1

Left decodes by multiplying by cB, averaging:

0 1 1 4 | -3 1 1 -4 | -2 1 -1 -1 | 3 -1 3 1 | -2 0 2 3 | 1 -4 -2 1 | 0 1 1 1

1.5 -1.25 -0.75 1.5 0.75 -1 0.75

Right decodes by multiplying by cA, averaging:

0 -1 -1 4 | -3 -1 -1 -4 | -2 -1 1 -1 | 3 1 -3 1 | -2 0 -2 3 | 1 4 2 1 | 0 -1 -1 1

0.5 -2.25 -0.75 0.5 -0.25 2 -0.25

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AnnouncementsNext class: Amplitude modulation

ECE 2066: No lab today (Lab 6 is due next Tuesday)

Homework 7 out (due April 11).

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