Introduction to mobile radio communications.

Spread spectrum.

Modulation techniques.

Introduction.

Mobile communication has seen an unprecedented increase in the number of users over the past years. Its success in part is due to the various technologies that have been developed. It is not unlike other stationary communication systems in many respects however it does differ in others, one obvious one being the environment the signal and data propagate over. The generic mobile communication system.

Basically this is a transmitter and a receiver see figure Generic mobile communication. There are many varied and complex techniques associated with such a system the figure serves as a basic introduction. As the signal is communicating over spatial (unguided media) it may encounter obstacles.

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Figure. Generic mobile communication.

Spread spectrum techniques.

A number of techniques are used in mobile communications that are described as spread spectrum techniques, named because they increase the bandwidth required. This effectively increases the bit rate of the signal and gives the signal a unique characteristic.

• Example the data to be transmitted say a '1' is purposely increased in the number of bits transmitted. • A four times increase could change a '1' into say '1010' • The pattern used depends on the "spreading code" used • Obviously here the bit rate has increased by four but the data is still only '1

Direct sequence spread spectrum. (DS-SS).

The basic technique is shown in the figure basic spread spectrum where two bits of a data signal m(t) are spread by the so called spreading code s(t) here multiplication of the two signals is shown see figure Basic DS-SS using multiplication.

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Figure. Basic DS-SS using multiplication.

Another way to implement DSSS is to XOR the data stream with the spreading, often the spreading code is a Pseudo noise generator, here a simple periodic one is shown.

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Despreading.

Despreading can be implemented by multiplication or XOR of the spread data with the original spreading code.

Mathematically generated spreading codes.

Code division multiple access (CDMA) is used in mobile telephony and one method uses specially generated spreading codes. A number of mathematical techniques exist that are used to generate the spreading code one such is Walsh sequences, which are formed from Hadamard matrices.

The 2nd order Hadamard is:

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higher orders increase as a power of 2 i.e. H2, H4, H8,H16 ….

Subsequent matrices are formed from the Kronecker product, which multiplies by H2 as follows:

Hd = Hd/2 ( H2 in that each value of the present matrix is multiplied by H2 to give the next order,

The actual Walsh sequence re-orders the Hadamard matrix into order by the number of zero crossings in the rows of the matrix. I.e. when reading the code a zero crossing is when the code changes from a 1 to a -1 and a -1- to a 1. The matrix is re-ordered such that the row are re-ordred in increasing number of zero crossings. The matrix formed now gives a number of rows each row contains a spreading code.

Despreading.

Despreading of Hadamard generated codes uses correlation.

Frequency hopping spread spectrum (FH-SS).

This is another method of spread spectrum, in this method the spreading sequence is based on a frequency selection. This is best understood with reference to the figure frequency pattern diagram. At discrete intervals in time the frequency is assigned according to the graph.

Here the bit has been assigned four different frequencies.

| | |

|A |A0rect(t/T) |

|B |A1rect(t/T) |

|C |A2rect(t/T) |

|D |A3rect(t/T) |

Each symbol could represent a number of binary digits (if binary symbols were being represented) i.e:...