Title : What Is A Gsm Network
link : What Is A Gsm Network
What Is A Gsm Network
all of us get excited when we see the symbol. but how does it work? ever wondered how millions of subscribersare served by telecom operators simultaneously? hi everyone, welcome to the world of longterm evolution (4g). today we will show how multiple users accessnetwork using multiplexing.
What Is A Gsm Network, in the last video we have shown, how two signalswere combined over a common pathway in duplexing. so, what exactly is multiplexing..??? multiplexing is the process of combining multiplesignals and transmitting them over a common channel.
then what is multiple access ?when multiplexing is used to allow multiple users to communicate over a single commonchannel, we call it multiple access. in other word, multiple access is nothingbut the application of multiplexing. so here’s a wifi hotspot sharing its internetconnection among multiple users by providing them a frequency block over which they cantransmit and receive data. the greater the size of this block, greateris the throughput. telecommunication systems, which mostly useradio waves as a communication link with end users, have employed several ways to sharethis resource. fdma which uses frequency division multiplexing,provides chunks of the frequency spectrum
to each user for data transmission. generally the data is generated at basebandand modulated at varying radio frequencies. guard bands are introduced to avoid any interference. amps or the first generation analog systemsused fdma to provide each user a duplex channel with one way bandwidth of 30 khz. tdma which uses time division multiplexing,allows multiple users to share a common frequency band by allocating different time slots. thus, in a n channel tdma, where each channelis given a time slot of t seconds, signals coming from each user will be transmittedat intervals of n*t seconds.
this technology was used in the 2g systemslike gsm, gprs and edge. each user was provided one of 8 tdma slotsin a bandwidth of 200 khz. dynamic resource allocation meant more userscould be supported but each channel could support only eight active users at a time. cdma based on code division multiplexing isa technique in which the data bits are modulated by a high frequency orthogonal sequence ofbits such as walsh codes or a pseudo random code such as gold codes to spread the signalsover a large frequency band. multiple such signals from different usersare then transmitted over the same frequency band.
in order to retrieve the signal, receivermust have the same spreading sequence, which is multiplied to this composite signal ina process called despreading. this makes cdma very secure and robust. it was used in 3gpp2 standards such as cdmaand cdma evdo (evolution data optimized) standards. wideband cdma or w-cdma spreads the signalsover a even higher bandwidth. this was used in the 3g standards of 3gppfrom umts to hspa+. where the available bandwidth was 5mhz comparedto 1.25 mhz used in cdma. there are other multiple access techniqueslike wdma(wavelength division multiple access) which is used in fibre optics and sdma(spacedivision multiple access) which refers to
spectrum reuse over non overlapping areas. as the number of users and their demand startedgrowing, communication networks evolved from 1g to 4g. radio part was significantly enhanced fromgsm to hspa+. now, focus was shifted towards better spectralefficiency by using higher modulation order like qpsk, 8psk, 16 qam, 64 qam etc. w-cdma improved upon some of the problems ofearlier generations like first, low data rates (in gsm, effectivebandwidth for a user was only 25 khz). second, congestion at peak hours.(as thefrequency chunks and time slots were very limited)
third, resource wastage (resources remained idle when users were inactive)fourth, poor spectral efficiency (due to guard bands and guard periods used for avoid interference)but exhaustion of spectrum, need for even higher data rates and spectral efficiencylead us towards ofdma. ofdma comes with several advantages over wcdmalike, bandwidth scalability,carrier aggregation, low isi(inter symbol interference). ofdma uses orthogonal subcarriers equally spaced at 15khz. users are provided a subset of these subcarriersfor data transmission.
unlike fdma or tdma, ofdma allows the usersto access variable bandwidth depending upon the resource availability. also since these subcarriers are orthogonalthere are no guard bands between them. it was first introduced in wifi and subsequentlyin wimax by ieee but as wimax lacked backward compatibility and support for mobile wirelessat its inception, operators remain stuck with w-cdma. this changed with the introduction of ltewhich brought ofdma to the forefront of today's mobile networks. so, today we showed, what were the differentmultiple access techniques that were used
in various generations of wireless communication. in our next video we will dive deep into ofdmaand sc-fdma the two pillars of 4g. hope you enjoyed the video. don't forget to subscribe to our channel tostay updated. happy learning.
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