Cellular Networks and Mobile Computing COMS 6998-8, Spring 2012

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Cellular Networks and Mobile Computing COMS 6998-8, Spring 2012. Instructor: Li Erran Li ( lierranli@cs.columbia.edu ) http:// www.cs.columbia.edu /~coms6998-8 / 1/30/ 2012: Cellular Networks : UMTS and LTE. Outline. Wireless c ommunications b asics - PowerPoint PPT Presentation


Narrowing the Beam: Lowering Complexity in Cellular Networks by Scaling Up

Cellular Networks and Mobile ComputingCOMS 6998-8, Spring 2012Instructor: Li Erran Li (lierranli@cs.columbia.edu)http://www.cs.columbia.edu/~coms6998-8/1/30/2012: Cellular Networks: UMTS and LTEDoCoMo to ask for changes in Android -Nikkei

(Reuters) - NTT DoCoMo Inc (9437.T) will ask Google Inc (GOOG.O) to modify its Android operating system so that smartphones using it would put less pressure on networks, a move that could spark wider protests against the leading mobile software platform, the Nikkei reported.

The leading Japanese mobile phone service provider identified an Android application, which enables free-of-charge voice communication, as a major cause behind a service disruption that occurred on Wednesday, the business daily said.

Some Android applications send out control signals once every three to five minutes even when not in use. This translates to ten times that of a conventional mobile phone, placing additional strain on the network, the newspaper said.

A sharp rise in data consumption puts more pressure on wireless operators to speed up capacity investments, as they are struggling with clogged telecom networks to keep up with growing demand for data services on the go.

DoCoMo intends to request that Google make Android transmit control signals less often, since frequent service disruptions could hurt the popularity of Android phones, the Nikkei reported.

"Other operators have complained, some publicly, about the pressure Android apps in particular are putting on their networks," said John Jackson, analyst at British wireless consultancy CCS Insight.

The Japanese paper said that DoCoMo also hopes to team up with other mobile service providers, along with Google, to ask Android application developers to limit the frequency of control signals.

"I expect that at the very least operators worldwide will watch this dispute closely to see what remedy might be in the offing," Jackson said.

Other operators may use the dispute as an occasion to demand similar modifications, he said.

"Either way, DoCoMo's move comes at a challenging time for Google with the Android ecosystem failing to generate Apple-like (AAPL.O) revenue and OEM licensees coming under legal pressure from Microsoft Corp (MSFT.O) in particular," he added

(Reporting by Meenakshi Iyer in Bangalore and Tarmo Virki in Helsinki; Editing by Joyjeet Das, Gary Hill)1OutlineWireless communications basicsSignal propagation, fading, interference, cellular principle

Multi-access techniques and cellular network air-interfacesFDMA, TDMA, CDMA, OFDM

3G: UMTSArchitecture: entities and protocolsPhysical layerRRC state machine

4G: LTEArchitecture: entities and protocolsPhysical layerRRC state machineCellular Networks and Mobile Computing (COMS 6998-8)21/23/12Cellular Networks and Mobile Computing (COMS 6998-8)Basic Wireless CommunicationTransmitterInformation is embedded in electromagnetic radiationReceiverLossy signal and interferenceNoiseRecover information31/23/12Courtesy: Harish VishwanathCellular Networks and Mobile Computing (COMS 6998-8) Noise & InterferenceThermal NoiseGenerated due to random motion of electrons in the conductor and proportional to temperature No= KoT dBm/Hz where Ko is Boltzmanns constantReceiver Noise Figure extent to which thermal noise is enhanced by receiver front end circuitry ~ 10 dBInterference signals transmitted by other users of the wireless networkSignal transmitted by other wireless devices from different wireless networksExample: Microwave ovens near 802.11 network41/23/12Courtesy: Harish VishwanathCellular Networks and Mobile Computing (COMS 6998-8)Impact of White Gaussian Noise-10-5051015202530012345678910SNR (dB)Capacity (bits/sec/Hz)SNR = Signal PowerNoise PowerShannon CapacityC = log (1 + SNR)51/23/12Courtesy: Harish VishwanathCellular Networks and Mobile Computing (COMS 6998-8)Scattering of Signals - Multipath Fading

ReflectionDiffractionAbsorptionMultiple paths with random phases and gains combine constructively and destructively to cause significant amplitude variations

61/23/12Courtesy: Harish VishwanathCellular Networks and Mobile Computing (COMS 6998-8)Impact of Mobility

Doppler Shift =

Signal AmplitudetimeMultipath Fading71/23/12Courtesy: Harish VishwanathCellular Networks and Mobile Computing (COMS 6998-8)Flat & Frequency Selective FadingWhen the multipath delay is small compared to symbol duration of the signal, fading is flat or frequency non-selective

Happens when signal bandwidth is small

Urban macro-cell delay spread is 10 micro seconds When signal bandwidth is large different bands have different gains frequency selective fading

1-1-11Symbol81/23/12Courtesy: Harish Vishwanath8Cellular Networks and Mobile Computing (COMS 6998-8)Typical Pathloss1.010.0100.0-50-70-90-110Free space : -20 dB/decadeUrban Macro cell -40 dB/decadeShadow fadingLog-normal with std ~ 8 dBA decade : transmitter and receiver distance increase 10 times91/23/12Courtesy: Harish VishwanathCellular Networks and Mobile Computing (COMS 6998-8)Spectrum ReuseSaIbSbIaa and b can receive simultaneously on the same frequency band if SINRa and SINRb are above required thresholdThis happens if the respective transmitters are sufficiently far apartSINRa = SaIa+ NAB101/23/12Courtesy: Harish VishwanathCellular Networks and Mobile Computing (COMS 6998-8)The Cellular PrincipleBase stations transmit to and receive from mobiles at the assigned spectrumMultiple base stations use the same spectrum (spectral reuse)The service area of each base station is called a cell The wireless network consists of large number of cellsExample The network in Northern NJ is about 150 base stations for a given operatorCells can be further divided into multiple sectors using sectorized antennasEach terminal is typically served by the closest base station(s)111/23/12Courtesy: Harish VishwanathCellular Networks and Mobile Computing (COMS 6998-8)Fixed Frequency PlanningEach base is assigned a fixed frequency bandReuse of 7 nearest co-channel interferer is in the second ringReuse of 3g1g2g1g2g3g1g2g3g2g3g1g3g2g7g1g6g3g4g5g6g4g2g7g5g3g1g4g5g7g3g2g1g6g6Reuse of 1/3121/23/12Courtesy: Harish VishwanathCellular Networks and Mobile Computing (COMS 6998-8)Cellular Network EvolutionIS- 136GSMTDMAEDGETDMAUMTSCDMALTEANALOGFDMAIS-95TDMACDMACDMA 20001X-DOTDMA/CDMA

OFDM131/23/12Courtesy: Harish VishwanathCellular Networks and Mobile Computing (COMS 6998-8)The Multiple Access problemThe base station has to transmit to all the mobiles in its cell (downlink or forward link)Signal for user a is interference for user bInterference is typically as strong as signal since a and b are relatively close How to avoid interference?All mobiles in the cell transmit to the base station (uplink or reverse link)Signal from a mobile near by will swamp out the signal from a mobile farther awayHow to avoid interference?

141/23/12Courtesy: Harish VishwanathCellular Networks and Mobile Computing (COMS 6998-8)Meeting Room Analogy

Simultaneous meetings in different rooms (FDMA)Simultaneous meetings in the same room at different times (TDMA) Multiple meetings in the same room at the same time (CDMA)151/23/12Courtesy: Harish VishwanathCellular Networks and Mobile Computing (COMS 6998-8)Frequency Division Multiple AccessEach mobile is assigned a separate frequency channel for the duration of the callSufficient guard band is required to prevent adjacent channel interferenceMobiles can transmit asynchronously on the uplinkGuard Band161/23/12Courtesy: Harish VishwanathCellular Networks and Mobile Computing (COMS 6998-8)Time Division Multiple AccessTime is divided into slots and only one mobile transmits during each slotFRAME jFRAME j + 1FRAME j+2SLOT 1SLOT 2SLOT 3SLOT 4SLOT 5SLOT 6Guard time Signal transmitted by mobiles at different locations do not arrive at the base at the same time 171/23/12Courtesy: Harish VishwanathCellular Networks and Mobile Computing (COMS 6998-8)TDMA CharacteristicsDiscontinuous transmission with information to be transmitted buffered until transmission timePossible only with digital technologyTransmission delay Synchronous transmission required Mobiles derive timing from the base station signalGuard time can be reduced if mobiles pre-correct for transmission delayMore efficient than FDMA which requires significant guard band181/23/12Courtesy: Harish VishwanathCellular Networks and Mobile Computing (COMS 6998-8)Orthogonality in TDMA/FDMAEvery information signal lasts a certain duration of time and occupies a certain bandwidth and thus corresponds to a certain region in the time-frequency planeGranularity is determined by practical limitationsTime division and frequency division are invariant under transformation of the channel and retain the orthogonalityAny orthogonal signaling scheme for which orthogonality is preserved will be a useful multiple access techniquetimefrequency191/23/12Courtesy: Harish VishwanathCellular Networks and Mobile Computing (COMS 6998-8)Code Division Multiple Access Use of orthogonal codes to separate different transmissionsEach symbol or bit is transmitted as a larger number of bits using the user specific code SpreadingSpread spectrum technologyThe bandwidth occupied by the signal is much larger than the information transmission rateExample: 9.6 Kbps voice is transmitted over 1.25 MHz of bandwidth, a bandwidth expansion of ~100

201/23/12Courtesy: Harish VishwanathCellular Networks and Mobile Computing (COMS 6998-8)Spread Spectrum systemsfrequencytimecodetimeCode orthogonality is preserved under linear transformations and hence near orthogonality is preserved under signal propagation211/23/12Courtesy: Harish VishwanathCellular Network


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