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L.A. Goldobin, Yu.I. Lebedev
Effective wide application wireless interfaces technique
The results are presented of theoretical and experimental researches intended for development of modern systems of data transfer via radio-channel, with guaranteed error probability at the receiver. A theoretically proved and practically verified method of quality perfection of the data, being received in the radio link, is proposed. The method allows to provide the given parameters of noise-immunity for the minimal power of transmitters and for data transfer rate, approaching the maximal capacity of the communication channel, according to Shannon. The proposed technique of signal processing and the state of the modern microelectronics allow to develop commercial and consumer radio-systems of wide area of application with quality characteristics being principally the optimum possible.
The most evident examples of the technique applications are wireless LAN, PABX, wireless gaps in ATM channels, various multimedia systems, home systems and other types of air interfaces.
The technical solutions designed to process and transmit the high rate information flows by radio are in great demand now and will take even more attention in the nearest future. The most evident examples of the applications are wireless LAN, PABX, wireless gaps in the ATM channels, various multimedia systems, home systems.
The further movement in this direction is constrained by the absolute physical limit of the radio spectrum. That is we can transmit an information through wire or optical cable with the speed of 600 Mbps or more but to do the same using conventional radio technology it would take all the available spectrum.
Meanwhile the spectrum is strictly divided between different regions, countries and services by the Radio Regulations to prevent chaos. So in fact we have the formal law limitation even stronger than the physical one. And the problem of a system designer is to squeeze as much information as possible into the narrow patches of the spectrum determined by these limits.
The natural solution of this problem is the noiselike signals usage. This fact is widely recognized now and the noiselike signals technique more often called spread spectrum technique (SST) is the base of the most wireless LAN and the other kindred projects.
Two basic techniques are used exclusively and compete - direct sequence modulation (DS) and frequency hopping (FH). The marketed systems using these techniques are mostly located in the unlicensed ISM (industrial, scientific and medicine) and land mobile services bands.
In our opinion the approach to the spread spectrum technique mustn't be so narrow sighted and has to be aimed not only at nowadays demand but at the future generation of the wideband systems.
If so we have to recognize that:
- spread spectrum technique can and must be used not only in these relatively narrow patches of the spectrum, but in any bands where the efficacy of the spectrum usage is of prime importance. That is everywhere in fact. And the further in time the more this postulate is valid;
- conventional spread spectrum techniques such as DS and FH do not provide for the best usage of the spectrum though they are better than the old systems with concentrated spectrum.
Here below we describe very briefly the basic ideas of the technique which can do this practically nearing the Shannon limits of the information transmission rates in a given bandwidth. It was used in several developments of Russian special systems and its civil applications are not known yet.
According to Shannon using sufficiently complex system of coded signals one can transfer a discrete information with the rate
C = DF Log2(1 + PS / PN)
having any small value of the error rate. Here C - channel capacity; DF - channel effective bandwidth; PN - white noise power; PS - signal power.
A communication system having high interference immunity at the information rate nearing the channel capacity is considered to be optimal by Shannon. For its implementation the statistical structure of the signal has to be similar to the white noise, the base of the signal - M has to be sufficiently high (in limit M -> infinity).
At the initial phase of the radio systems development the binary signals with no redundancy were used. Starting from 60-s the noiselike broadband signals both FH and DS are widely used in military and data transmission systems. A noiselike signal structure effectively solves the problems of synchronization, narrowband and pulse interference rejection, Doppler and multipath effects. However one of the main disadvantages of the pseudonoise signals is their high frequency redundancy and hence very low specific information transmission rate and high demand in the frequency resources.
New classes of complex signals applications - signals of parallel structure - are actively investigated now in Russia. The trend to the parallel structures in the communication equipment leads from the bit by bit reception of an information message to reception it as a whole. That is transmission is fulfilled in a 2M alphabet instead of a binary one.
The best features are attributed to the orthogonal parallel compound signals (PCS). Transformation of an information message into parallel compound signal can be considered as its orthogonal conversion. An arbitrary ensemble of the linearly independent (orthogonal) functions can be taken as a basis ensemble. Dimensionality of the orthogonal functions and the code dimensionality of the parallel compound signal coincide, that is a complete (redundantless) code is produced.
The orthogonal function base reaches the values much more than one due to the time redundancy as distinct from the simple binary signals. The orthogonal functions base establishes the Markov interdependence between the information message bits. The PCS generation can be considered as an implementation of a pseudo-random process with Markov interdependence between the signal parameters and the information word being transmitted.
So the PCS application to a radio link enhances the quality of the information exchange and can improve the link parameters up to the potential ceiling for any non-stationary channel. Besides, an optimal signal processing algorithm application allows to adjust the communication system to a given interference environment in the channel.
Statistical structure of the multibase compound orthogonal signal is similar to the white noise since the compound signal length tends to infinity..
Investigations of the PCS with redundant coding and the signals reception as a whole, that is by the complete phrases, show that the potentially feasible information and energetics features of the signals are as follows:
- Information transmission rate nearing the channel capacity by Shannon;
- Error probability can be of any small value;
- Energy expenditure for an information bit is equal to 1.
The Shannon limit implies the signal encoding usage that allow to get any small error probability at the signal to noise ratio equal to 1. If for the wideband pseudonoise signals the Shannon limits are possible theoretically but hardly feasible practically for the parallel compound signals the technical implementation of the potential limits is practical.
Usage for the information transmission of the complex parallel compound signals whose structure can be optimized for any combination of interference gives a fundamentally new solution for the problem of providing of the specified erroneous reception probability at the receiver side of a communication system in any complex interference environment.
So an infinitely small erroneous reception probability can be reached in any interference environment - that is reaching the Shannon limits - by all the potential of the communication channel mobilization and the PCS features optimization. The fact that the potentials of a channel can be realized independently on the modulation type (wideband, narrowband or super-narrowband) is especially important.
So in conclusion of the small introduction into the PCS technique we can say referencing also the other results of their investigation the following.
- The proposed technique based on the usage of the parallel compound signals, allows to realize completely the potential of the radio channel and to optimize its parameters for the digital information transmission for different frequency bands and different data transmission rates.
- Application of the developed technique is especially effective in the systems, intended for in door operation and in other conditions for which the pronounced multipath propagation dispersion and absorption at the obstacles are characteristic as well as the fading caused by these phenomena. The latter appears due to the fact that the parallel compound signals decorrelate the beams in a multipath channel.
- The use of parallel compound signal is a radical way of noise immunity enhancing under the fading conditions.
© INTONIS, 1997
More detailed technique description can be send on request.
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