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12th WSEAS International Conference on COMMUNICATIONS, Heraklion, Greece, July 23-25, 2008 CONTROL SYSTEM TO REDUCE ADJACENT INTERFERENCE IN BASE STATIONS EDGAR ALEJANDRO ANDRADE GONZÁLEZ SILVIA RODRÍGUEZ CARPIO MARIO REYES AYALA JOSÉ ALFREDO TIRADO MÉNDEZ Electronic Department Metropolitan Autonomous University-Azcapotzalco Ave. St Pablo # 180, Col. Reynosa Tamaulipas, México City, México CINVESTAV-IPN MÉXICOAbstract: - This paper shows a control system to reduce adjacent interference in a cellular environment keeping thelongest distance between channels used by the handsets. This control system change dynamically the resources(forward and reverse channel) allocated to all the users of the cellular system in order to maintain less interference inthe channels (adjacent interference).Key-Words: - Cellular, Adjacent Interference, Channel, Base Station, Dynamical Allocation.1 Introduction It is important to have prototype tools to emulate the The big growing of the wireless devices in cellular communication systems for educationalradiocommunication systems and high transmission application in order to improve the knowledge processrates gives as a result the need to do an optimal of the communication system analysis.management of the spectrum and also gives as aconsequence that the electronic devices operates with 2 Design of the control systemwide bandwidth. The control system consists in three blocks: the In the cellular systems, several unwanted factors Personal Computer, the microcontroller and the logicappear like fading, co-channel interference, adjacent- for the control of the VCOs (voltage controlchannel interference, etc. The use of a limited oscillator). The VCOs has a bandwidth range from 760spectrum in the communication system makes more MHz to 870 MHz.difficult to management it maintaining the lessinterference between the users. One of the worst unwanted effects is the adjacentchannel interference, which is generated when a signalinterferes the bandwidth of another channel nearby. There are several techniques to reduce adjacentchannel interference, like filters with high order, usingthe feedforward structure in power amplifiers,modulation techniques, etc. . In this paper, we show an algorithm to reduce theadjacent channel interference in the base station Fig. 1 Control systemmaintaining the longest distance between channelsanytime. This control system allocates the channels to In the personal computer, the algorithm for thethe users maintaining the best conditions into the base allocation of the frequencies is programmed. Also, thisstation to gives a good performance reducing the software application establishes the communicationadjacent channel interference reassigned all the with the microcontroller ATMEGA8535 through thechannel of the users. With this application the students serial port.can check the management of the channels for the The information interchanging between them areusers while they are in moving into a cell. related with the code for the users (consist of three ISSN: 1790-5117 456 ISBN: 978-960-6766-84-8
12th WSEAS International Conference on COMMUNICATIONS, Heraklion, Greece, July 23-25, 2008bits), the code for the channels (consist of three bits) communication service. Then, after that the systemand the flags for the disconnection or establishment of analyzes if there are channels available or not; if therethe service for a specific user (one bit), with a specific is at least one of them, then it proceeds to allocate thestructure of the data as we can see in the frame below. channel with the longest distance between that user and the others in the system. The second task is if the user wants to finish the communication, in that case, the channel assigned to the user is released in order to be used by other user in the next establish of service. 3.1 Algorithm to establish the communication Fig. 2 Data Frame After we check that there are available channels in the system, it proceeds to assign the channel which has The microcontroller has three ports, one port for the maximum distance between the channels occupiedserial communication to receive the data to the and the new one (request).configuration of the frequencies depending of the user To achieve this goal, we have a data base of thecode and the channel code, one port to select the user channels with the maximum distance between themand the channel transmit the resulting code to the and we select the channel according to the availabilitycodecs and finally, one port for the gain control of the in that moment. The way that the channels arepower amplifiers in the RF transmission block of the allocated to the users (for example for eight users) isbase station. This last function was not showed in this showed in the table 1 and table 2.paper. The last block consists in logic for decoding the userand channel and also using a Digital to Analog Order of ChannelConverter (DAC) to choose the channel desired in the AllocationVCO for a specific user. 1 1 2 83 Design of the Algorithm 3 4 We show the algorithm explanation with eight users 4 6and eight different channels. The sequence of theprocess to allocate channels to the users is based in the 5 2following flow chart. 6 7 7 5 8 3 Table 1 Allocation showing the maximum distance between channels Where: Channels Frequencies MHz 1 760 2 775.7 3 791.4 4 807.1 Fig. 3 Algorithm general 5 822.8 6 838.5 The process to initiate the choosing of the channelsof the users, it starts when there is an event of a user, 7 854.2which asks for the resources of the system in order to 8 870establish the communication or leave the Table 2 Channels frequencies ISSN: 1790-5117 457 ISBN: 978-960-6766-84-8
12th WSEAS International Conference on COMMUNICATIONS, Heraklion, Greece, July 23-25, 20083.1 Algorithm to finish the communication This algorithm frees the channel of the user selected,but only if the user is using the last channel assigned;this means that it is not necessary to reassign all thechannels of the rest of the users that are usingresources of the system, because all of them maintainthe longest distance between themselves . In general way, when a user wants to leave thechannel assigned to him; and this user it is not the lastone (assigned), then we have to rearranged all of thechannels allocated to the users occupying the place ofthe user disconnected in order as the users enter to thesystem to ask for a channel after the user involve,giving as result, the channel allocation showing thelongest distance between themselves .4 Conclusion This system control for the management of theresources of the system (reverse channels and forwardchannels) shows a dynamical control, maintaining allthe time the users arrive asking for resources orleaving the communication, the longest distancebetween their frequencies assigned (channels)reducing the adjacent channel interference. We areworking in control of the gain in the amplifiers (lownoise amplifier and power amplifier) with the samesystem using the same frame but with the 7th bit of theframe; we use this prototype for educational purposes.Finally, this management of the channels improves theperformance of the duplexer in FDM communicationsystems.References: P. García, A. Valdovinos et all, “Método deEquilibrado basado en Algoritmos Genéticos e unEsquema de Linealización Feedforward” Spain H. Hammuda, “Cellular Mobile Radio Systems:Designing Systems for Capacity Optimization”, 1997 S. Rodríguez, “Bloque de Radiofrecuencia paraTransmisión de una Estación Base”, Thesis, México,2008 ISSN: 1790-5117 458 ISBN: 978-960-6766-84-8