Quiz 22: Personal Communication Systems
First Generation PCS are systems where voice transmission was done using analog modulation by using frequencies around 900 Mhz. They are called as Analog phone systems. The first generation systems used FDMA (Frequency division Multiple access) technology in which available frequency was break down into 30 kHz channel and each phone was allocated a channel on full time basis. The device was supposed to transmit and receive data on this channel only. The Voice/Sound was transmitted using analog data that too on very high frequency around 150 MHz and above. There are 2 major drawback of this system: 1. One major drawback was that the channel allocated to one person cannot be used by other even if the respective person was not using the channel. 2. And another one was that the systems are insecure i.e. anyone can listen to your conversation with an all band radio receiver. Third generation PCS systems was required to create a universal infrastructure which can support both existing and future services. This means that the infrastructure should be designed such that it can use the existing services on the existing networks. There are following expectations from Third generation PCS systems: 1. Amount of information which can be transmitted in communication channel. As it is required a channel whose data rate can be varied depending on the requirement For example, there is no need of high data rate for normal text message but it is required to have high data rate for video call. 2. To improve Data communication as in today's world data communication is not confined to voice call instead it is move further to a different level of calling such as Video calls, MMS(Multimedia Message service)etc. 3. Universal worldwide standard is required so that same equipment works everywhere in the world. 4. Now a day's wireless phone is required everywhere be it vehicle, pedestrian, home or office. So it is required to have a system which is useful in all the locations. And also there is a universal personnel telecommunication terminal which can be used anywhere on variety of RF standards for fixed, mobile and satellite operations.
North American (personal communication system) PCS systems have allocated frequency in 850 MHz and 1900 MHz bands for GSM (global system for mobile communication) network. These frequency ranges are allocated as there was no space for further expansion in the 800 MHZ band as it has become very crowded. Frequencies are assigned to various service providers through auction process which is controlled by government. The government basically distributes the rights to transmit signal over specific band and in return government earn some money for itself. PCS frequency range is divided among 6 service providers in a specific region with each service provider having different uplink (mobile to base) and downlink (Base to mobile) frequency. Refer to table 22.1 in the textbook for the PCS band plan.
Interference can be explained in PCS (personal communication system) terms as when our electromagnetic signal is being disturbed or lost due to various reasons such as geography of the area, equipments (transmitters, receivers) used etcetera. As a result of interference, it is required to dial back again to the network. The following are the ways by which the interference can be prevented between base and mobile transmissions: 1. Spread spectrum technology can prevent interference between base and mobile transmissions. In this technology, a signal is generated with wider bandwidth due to which entire information (signal) is spread on that particular bandwidth so the information signal is likely to be covered by any interfering signal. But this often leads to very weak signal by making signal to noise ratio to be less than 1 but however by using proper de-spread techniques, it is possible to get the information signal decoded. 2. The digital controlled channel which uses TDMA (time-division multiple access) technology has 1 timeslot dedicated for shared channel feedback (SCF) in the forward channel (in which base transmit). This SCF field is updated by the base station and then after transmitting the same in the forward channel it is being monitored by the Mobile which helps in reducing the interference. 3. To transmit stronger signals so that signal interference noise ratio (SINR) is high and which helps to reduce the interference to a great level.
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