Wireless specifics

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A Wireless Communication System

Antenna

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Technologies for Cell phones to Handle Multiple Users

Unique feature for voice communications:FDMA (Frequency Division Multiple Access) in

1G cellular phone technologyTDMA (Time Division Multiple Access, e.g.,

GSM—Global Service for Mobile communications and IS-54, both use TDMA and FDMA)

CDMA (Code Division Multiple Access, e.g., IS-95, WCDMA, CDMA2000)

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Technologies for Cell Phones to Handle Up and Down Links

TDD (Time Division Duplex): forward (down link) and reverse (up link) channels use the same frequency band but alternating time slots

FDD (Frequency Division Duplex): forward and reverse channels use different carrier frequencies

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Technology used by WiFi, etc. to handle multiple users

It is a time division method.Unlike the TDMA in cell phones, in WiFi no

specific time slots are assigned to users. Instead, it is basically a first-come-first-served policy—users form a queue waiting for their turns to use the connection. It’s very much like the rule used in a bank or a computer network.

It is good for data transmission, but not that ideal for voice transmission.

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Techniques used by all

Spread Spectrum TransmissionFHSS (Frequency Hopping Spread

Spectrum)DSSS (Direct Sequence Spread

Spectrum)OFDM (Orthogonal Frequency Division

Multiplexing)

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What is Spread Spectrum Transmission

The traditional transmitted signal has a bandwidth of the same order as the information signal at the baseband. For example, the bandwidth of a voice signal is about 4 kHz (the baseband). After modulation, as it being transmitted it still occupies several thousand Hz but at a much higher frequency.

The spread spectrum signal occupies a much larger bandwidth.

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Why spread spectrum

It is robust against frequency selective fading in urban and indoor environments.

It is robust against interference emitted by machines, microwave ovens, etc.

It can be used to provide additional security, as in CDMA.

It is required by regulation to use spread spectrum in unlicensed ISM (Industrial, Scientific and Medical) bands.

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Spread spectrum methods

Frequency hopping spread spectrum (FHSS)The transmitter constantly, often randomly, shifts the

center frequency of the transmitted signal.Only the machine that knows the hopping pattern can

receive the signal.

Direct sequence spread spectrum (DSSS)Each transmitted bit is spread into N smaller pulses

(chips) before transmission.Only the machine that knows the pattern of the spread

can retrieve the signal.

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FHSS

Invented by Austrian-born movie star Hedy Lamarr to protect guided torpedoes from jamming

The shifts in frequency (hops) occur according to a random pattern that is known only to the transmitter and the receiver. (Actually it is pseudo random: It is generated by an algorithm, so it is not really random; but to a person who does not know the pattern, it looks random.)

If the center frequency moves among 100 different frequencies, the required transmission bandwidth is at least 100 times as large as the original transmission bandwidth.

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Example of FHSS

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FHSS and GSM

If the channel coincides with a deep frequency selective fading or when the cochannel interfence from another cell using the same frequency is excessive, the distortion in the received voice signal will be large. A slow frequency hopping of 217.6 hops per second can be used in GSM to tackle these problems.

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FHSS in 802.11 or WiFi

Uses 78 hopping channels each separated by 1 MHz. These frequencies are divided into three patterns of 26 hops each corresponding to channel numbers (0, 3, 6, 9, …, 75), (1, 4, 7, 10, …, 76), (2, 5, 8, 11, …, 77). These choices are available for three different systems to coexist without any hop collision.

2.5 hops per second

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FHSS in Bluetooth

Uses a fast frequency hopping (1,600 hops per second) over 79 MHz of bandwidth. That is, it hops over 79 channels each separated by 1 MHz.

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DSSS

The transmitter spreads one bit, say a one or a zero (you either have a one or a zero in the digital world), into many (N) smaller chips (they are a sequence of zeros and ones) according to a code known to the transmitter and the receiver.

The receiver, using the code and a correlator, put the spread chips together to get the original bit.

The bandwidth of the original signal will be N times wider after the spreading because the chip rate is N times faster than the bit rate. Therefore the signal will be more robust against interference and fading.

The code used for spreading and de-spreading can be secret, if only the transmitter and the receiver know it, thus providing a security measure.

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DSSS in 802.11

The code (called Baker code) used in 802.11 to spread the data bits is given by [1, 1, 1, -1, -1, -1, 1, -1, -1, 1, -1]. So one bit of data is spread to become 11 chips.

The Baker code is not a secret code, so it’s not used for security. It’s used to spread the bandwidth.

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DSSS in 802.11

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More about DSSS

The bandwidth of the transmitted DSSS signal is N times as large as that of the original signal.

CDMA uses DSSS. Each user is given a unique code that other users don’t know. Although a user can receive the signals sent to other users by the transmitter, in the de-spreading process only the signal sent to that user can be detected. The interference generated by other users is very small.

OFDM (Orthogonal Frequency Division Multiplexing)

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What is OFDM

Assume we need to send data at a speed of R symbols/sec.

We break the data sequence into N (an integer, say, 48) sub-sequences. The data rate of each sub-sequence will be R/N, much slower than the original sequence.

N carriers are used, each having a different frequency and each sending one sub-sequence.

At the receiver end, the N sub-sequences are put together to get the original data sequence.

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Advantages of OFDM

Robust against Intersymbol Interference (ISI) due to multipath propagation because:In each sub-sequence the symbols, e.g. a 1 or a 0,

are N times longer than the original symbols.The longer the symbol, the weaker the ISI (the signals

representing the same symbol but coming from multiple paths will be close enough compared with the width of the symbol so they don’t interfere with each other).

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Advantages of OFDM

Robust against frequency selective fadingTo battle the frequency selective fading (signals

at certain frequencies might be much weaker than that of other frequencies), error-control coding can be used in each subchannel.

If the signal for a particular subchannel(s) is weak, the transmitting power of that subchannel can be increased to compensate for the fading.

High spectral efficiency (high bit rate to bandwidth ratio)

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Drawbacks of OFDM

Complexity: you need to put together N signals to rebuild the original one.

Sensitive to Doppler Shift. When receiver is moving at a high speed, the received frequency will shift, too, and that can cause problem for OFDM.

High peak-to-average-power ratio (PAPR) and thus lower efficiency.

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Example: OFDM in 802.11a

64 subchannels are used, among which 48 are used for data transmission, the remaining 16 are for other purposes.

The symbol rate of each channel is 250 kilo symbols per second (ksps).

The actual data rate for the user is 48X250 ksps = 12 Msps.

The overall bandwidth is 20 MHz.

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SOFDMA (Scalable Orthogonal Frequency Division Multiple Access)

Used in 802.16e (WiMax for mobile users)The same OFDM technique will be used,

but each user may only get a part of the spectrum, depending on the application the user is running. TDMA is also used.

In 802.16-2004 (WiMax for fixed users) OFDM is used and a user get all the available spectrum. Users are separated by TDMA.

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Assigning sub-channels

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Coding techniques

Error control codingCoding for spread spectrum (CDMA)

Orthogonal codes

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Voice-oriented and data-oriented networks

Voice-oriented networks use the so-called fixed-assignment methods. Each user is assigned a slot of time, a portion of frequency band, or a specific code for the entire length of the conversation.

Data-oriented networks use random-access methods. Users share the same medium (air or wire). Since data arrive at random instances, the medium will be assigned to each user in a random fashion.

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Comparison of two methods

Fixed assignment ensures constant connection, which is needed for voice communication, but can have low utilization rate.

Random access method is more suitable for data communication, because data arrive in bursts.