Mobile Communications

Course: Communication Systems

Dr. Muhammad Saleem Awan

Mobile communication is burdened with particular propagation complications, making reliable wireless communication more difficult than fixed communication between and carefully positioned antennas.

• The antenna height at a mobile terminal is usually very small, typically less than a few meters. Hence, the antenna is expected to have very little 'clearance', so obstacles and reflecting surfaces in the vicinity of the antenna have a substantial influence on the characteristics of the propagation path.

• The propagation characteristics change from place to place and, if the terminal moves, from time to time.

What is OFDM?

• Modulation technique – Requires channel coding– Solves multipath problems

OFDM modulationSource

coding

I/Q-mod., up-converter

I/Q RFChannel coding / interleaving

Transmitter:

InfoSource

OFDM de-modulation

Source decoding

Down-converter, I/Q-demod.

I/Q RF

Decoding / deinter-leaving

Receiver:Radio-channel

InfoSink

e.g. Audio 0110 01101101

fPSD

* fPSD

fc-fc

OFDM – Orthogonal Frequency Division Multiplexing

Multipath Radio Channel

Multipath Propagation• Reflections from walls, etc. • Time dispersive channel

– Impulse response:

• Problem with high rate data transmission:– inter-symbol-interference

[ns]

p () (PDP)

Multipath Radio Channel

Inter-Symbol-InterferenceTransmitted signal:

Received Signals:Line-of-sight:

Reflected:

The symbols add up on the channel

Distortion!Delays

OFDM Technology

Concept of parallel transmission (1)

Channel impulse response

1 Channel (serial)

2 Channels

8 Channels

Time

In practice: 50 … 8000 Channels (sub-carriers)

Channels are transmitted at different frequencies (sub-carriers)

Multipath Radio Channel

The Frequency-Selective Radio Channel

• Interference of reflected (and LOS) radio waves– Frequency-dependent fading

-10

-5

0

5

10

15

20

Frequency

Power response [dB]

OFDM Technology

Concept of parallel transmission (2)

Channel impulse response

1 Channel (serial)

Channeltransfer function

Channels are “narrowband”

2 ChannelsFrequency

Frequency

8 ChannelsFrequency

FrequencyTime

Signal is “broadband”

Implementation and System Model

Concept of an OFDM signal

Ch.1

Ch.2 Ch.3 Ch.4 Ch.5 Ch.6 Ch.7 Ch.8 Ch.9 Ch.10

Saving of bandwidth

Ch.3 Ch.5 Ch.7 Ch.9Ch.2 Ch.4 Ch.6 Ch.8 Ch.10

Ch.1

Conventional multicarrier techniques

Orthogonal multicarrier techniques

50% bandwidth saving

frequency

frequency

Spread SpectrumAn increasingly popular form of encoding for wireless communications is known as spread spectrum.

• The use of spread spectrum makes jamming & interception more difficult and improves reception. • The basic idea of spread spectrum is to modulate the signal so as to increase significantly the bandwidth (spread the spectrum) of the signal to be transmitted.

Spread Spectrum

A comment about pseudorandom numberspseudorandom numbers:

These numbers are generated by an algorithm using some initial value called the seed. The algorithm is deterministic and therefore produces sequences of numbers that are not statistically random.

• The important point is that unless you know the algorithm and the seed, it is impractical to predict the sequence. Hence, only a receiver that shares this information with a transmitter will be able to successfully decode the signal.

Frequency Hopping Spread Spectrum

The signal is broadcast over a seemingly random series of radio frequencies, hopping from frequency to frequency at split-second intervals.

A receiver, hopping between frequencies in synchronization with the transmitter, picks up the message.

Frequency Hopping Spread Spectrum

Frequency Hopping Spread Spectrum

Direct Sequence Spread Spectrum

Each bit in the original signal is represented by multiple bits in the transmitted signal using a spreading code known as chipping code.

The chipping code spreads the signal across a wider frequency band in direct proportion to the number of bits used.

Therefore, a 10-bit chipping code spreads the signal across a frequency band that is 10 times greater than a 1-bit chipping code.

Direct Sequence Spread Spectrum

Note that an information bit of 1 inverts the pseudorandom bits in the combination, while an information bit of 0 causes the pseudorandom bits to be transmitted without inversion.

Direct Sequence Spread Spectrum

The amount of spreading that is achieved is a direct result of the data rate of the pseudorandom stream; the greater the data rate of the pseudorandom input, the greater the amount of spreading.

Direct Sequence Spread Spectrum

In this case, the information stream and the pseudorandom stream are both converted to analog signals and then combined, rather than performing the exclusive-or of the two streams and then modulating.