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The transmission medium is a resource that can be subdivided into individual

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The transmission medium is a resource that can be subdivided into individual channels according to different criteria that depend on the Multiple Access technology used.

There are several solutions to implement Multiple Access for wireless systems:

• FDMA: Frequency Division Multiple Access

Frequency spectrum contains several carriers, signals of different users are separated by placing them in separate carriers.

• TDMA: Time Division Multiple Access

Time is divided into slots, signals of different users are separated by placing the signals into separate time slots.

• FDMA/TDMA: Frequency and Time Division Multiple Access

The time-frequency plane is divided into small separate areas defined by the association of one Time Slot and one carrier.

• CDMA: Code Division Multiple Access

Users occupy the same frequency at the same time, thus frequency and time are not used as discriminators and signals of different users are separated by modulating them with different sets of codes.

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Frequencies are identified with their ARFCN which unambiguously corresponds to a

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Frequencies are identified with their ARFCN which unambiguously corresponds to a couple of frequencies (one Uplink, one Downlink) associated to each established link.

For the GSM 900:

F uplink = 890 MHz + 0.2n for 1 < n < 124

F downlink = F uplink + 45 MHz

For the E-GSM 900:

F uplink = 890 MHz + 0.2n for 0 < n < 124

F uplink = 890 MHz + 0.2 (n-1024) for 975 < n < 1023

F downlink = F uplink + 45 MHz

For the GSM 1800:

F uplink = 1710.2 MHz + 0.2 (n-512) for 512 < n < 885

F downlink = F uplink + 95 MHz

For the GSM 1900:

F uplink = 1850.2 + 0.2 (n-512) for 512 < n < 810

F downlink = F uplink + 80 MHz

The GSM spec. 5.05 specifies the modulation mask of a GSM modulated signal as

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The GSM spec. 5.05 specifies the modulation mask of a GSM modulated signal as it is depicted above.

The spectrum lies around the carrier frequency with a 3 dB bandwidth of 271 kHz.

The spacing between two consecutive carriers is 200 kHz.

200 kHz apart from the carrier, the spectrum of the GSM signal is 30 dB lower than its maximum, and 60 dB lower at 400 kHz from the carrier.

GSM specifications state that system and equipment must operate with specific

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GSM specifications state that system and equipment must operate with specificratios of carrier to interference signals:

• C/Ic or useful signal over interfering signal at the same frequency may be aslow as 9 dB,

• C/Ia1 or useful signal over interfering signal at ± 200 kHz may be as low as-9 dB,

• C/Ia2 or useful signal over interfering signal at ± 400 kHz may be as low as-41 dB,

• C/Ia3 or useful signal over interfering signal at ± 600 kHz may be as low as-49 dB.

A TDMA frame, 8 successive Time-Slots (TS), has a duration of 60/13 ms or

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A TDMA frame, 8 successive Time-Slots (TS), has a duration of 60/13 ms or4.615385 ms.

A TS, has a duration of 15/26 ms or 0.576923 ms.

A physical channel is made of the recurrence of the same TS taken from successiveframes of the same channel.

A basic unit of measure in transmission on a radio path is a burst, a series of 114

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A basic unit of measure in transmission on a radio path is a burst, a series of 114modulated bits of information. Bursts have a finite duration and occupy a finite part of theradio spectrum. Bursts are sent in time and frequency windows called slots.The normal burst shown in this slide is made of:

• Tail bits: three "0" bits at the beginning and end to help avoid loss of synchronization.• Information: speech, data, and signaling.• A training sequence: a list of bits known by the receiver allowing it to demodulate the

burst.• Stealing flags (S): indicate if information is either user's data (includes speech) or

signaling data whenever the TCH has been ‘stolen’.• A guard band: bits where nothing is transmitted to allow for overlap due to the variable

distance from the mobile telephone to the Base Transceiver Station. This is necessaryif the timing advance is not exactly right.

• Normal Burst bears traffic channels, its associated channel (slow and fast), StandAlone, and the broadcast Control Channels (BCCHs).

Other burst are defined with regard to their time-amplitude profile:• Access burst: used in the uplink direction during the initial phase of transmission when

propagation delay (timing advance) between the mobile telephone is not yet known.The training sequence and tail are longer than those of a normal burst to increase theprobability of successful demodulation.

• Frequency correction burst: to enable the mobile telephone to find and demodulate asynchronization burst to the same cell.

• Synchronization burst: time synchronization of the mobile station, the first burst amobile telephone needs to be able to demodulate successfully.

• Dummy burst: dummy sequence to replace data if there is nothing to transmit, forexample, Broadcast Control Channel filling.

The Base Transceiver Station BTS (GSM) can be split into three kinds of modules:

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The Base Transceiver Station BTS (GSM) can be split into three kinds of modules:

• one Base Common Functions BCF, performing all common functions of the site,

• one or several transceivers TRX, one per TDMA frame,

• an antenna coupling system, one per cell.

Control channels are intended to carry signaling or synchronization data. Three are defined:

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Control channels are intended to carry signaling or synchronization data. Three are defined:Broadcast Channels (BCHs), Common Control Channels (CCCHs), Dedicated Control Channels(DCCHs).

Broadcast channels are point-to-multipoint unidirectional (downlink) control channels from the fixedsubsystem to the mobile station.

• First, BCHs include a Frequency Correction Channel (FCCH) that allows an MS to accuratelytune to a Base Transceiver Station (BTS).

• Then BCHs contain the Synchronization Channel (SCH), which provide TDMA frame-orientedsynchronization data to an MS.

• Last, BCHs include the Broadcast Control Channel (BCCH) intended to broadcast a variety ofinformation to MSs, including cues necessary for the MS to register in the network.

Common Control Channels (CCCHs) are point-to-multipoint channels that are primarily intended tocarry signaling information for access handling functions. The CCCHs include:

• Paging Channel (PCH), which is a downlink channel used to page MSs.• Access Grant Channel (AGCH), which is a downlink channel used to assign an MS to a

specific Dedicated Control Channel (DCCH).• Cell Broadcast Channel (CBCH), which is downlink channel used to broadcast miscellaneous

short messages to the MSs.• Random Access Control Channel (RACH) is an uplink channel which allows an MS to initiate

a call.

Dedicated Control Channels are point-to-point, bi-directional control channels. Two types ofDCCHs are used:

• Stand-alone Dedicated Control Channels (SDCCH) whose allocation is not linked to theassignment of a traffic channel (TCH). They bear information about authentication, locationupdates, and assignment to traffic channels (TCHs).

• Associated Control Channels are linked to the existence of a traffic channel (TCH). The FastAssociated Control Channel (FACCH) or burst-stealing is a control channel obtained bypreemptive dynamic multiplexing on a TCH. The Slow Associated Control Channel (SACCH),also known as a continuous data stream, is allocated together with a TCH or an SDCCH.

Channels are reused at regular distance intervals. The mechanism that governs this

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Channels are reused at regular distance intervals. The mechanism that governs this process is called frequency planning.

The slide shows an example of an N = 12 frequency plan where the available frequencies of a GSM network are placed.

This set of 12 cells is called a frequency reuse pattern and is generally used for a BCCH frequency plan.

Fractional Reuse is a method used to maximize spectral efficiency as it can

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Fractional Reuse is a method used to maximize spectral efficiency as it canincrease the capacity of the network by more than 100% while ensuring perfect end-user quality and without additional spectrum allocation or new sites introduction.

Fractional Reuse consists in defining a TCH frequency plan which differs from theoriginal BCCH frequency plan. Instead of following a conventional pattern, TCHtransmitters are hopping on various frequency groups:

− 3 groups for the 1x3,

− 1 group for the 1x1.

The counterpart of this is of course the reuse distance reduction that allows a betterspectral efficiency. Basically, the quality is maintained with the frequency loadcontrol:

− 50% for the 1/3,

− 20% for the 1/1.

A practical example of a 4*3 reuse frequency pattern is displayed here. Each color

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A practical example of a 4*3 reuse frequency pattern is displayed here. Each colorrepresents a frequency group.

As fast fading effects are strongly linked with the wavelength and hence with the

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As fast fading effects are strongly linked with the wavelength and hence with the frequency, the statistical behavior of a signal emitted at two different frequencies is not the same.

Frequency hopping is one of the features introduced in the GSM standard to

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Frequency hopping is one of the features introduced in the GSM standard toimprove both performance and system capacity. The purpose of frequency hoppingis to decrease the degradation effect of fast (Rayleigh) fading, by averaging thesignal and the interference in the network.

The principle of frequency hopping is to allow the GSM transmissions to hop on atime slot basis between the frequencies allocated to a sector. By changing thefrequency of a call every time slot, the number of burst impacted by the interferenceis limited and the channel content can be rebuilt thanks to GSM coding techniques(error correction, interleaving).

As illustrated on the above slide, the Frequency Hopping law can be cyclic orpseudo-random.

What is called frequency load is simply the ratio between the number of hoppingtransmitters in a sector and the number of hopping frequencies. This parameter iscrucial in a network with frequency hopping, since it represents the time fraction fora given frequency being used in the network.

The parameters used to set the Frequency Hopping sequence are the following

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The parameters used to set the Frequency Hopping sequence are the followingones:

• FN : Frame Number (GSM time),

• HSN : Hopping Sequence Number [0, 63],

• Nf : Number of hopping frequencies,

• MAI : Mobile Allocation Index,

• MAIO : Mobile Allocation Index Offset between 0 and (Nf - 1),

• MA : Mobile Allocation.

The hopping sequence generation algorithm uses the Hopping Sequence Numberto distinguish between the 64 possible pseudo-random sequences and produces foreach frame a Mobile Allocation Index. This parameter is therefore GSM time andHSN dependent. The number of frequencies in the Mobile Allocation is also used asan input in order to produce a MAI value within the range of the frequencyallocation. This value (MAI) is increased by the MAIO assigned to each time slot.

HSN and MAIO properties :

• Sequences bearing different HSN will statistically collide 1/Nf of time,

• Sequences bearing the same HSN but different MAIO are orthogonal,

• HSN = 0 correspond to cyclic frequency hopping,

Thus, on a same site an identical value of HSN for each sector provides the sameMAI. A different MAIO for each transmitter ensure the hopping sequence laws areorthogonal.

There are two types of frequency hopping techniques: baseband FH and

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There are two types of frequency hopping techniques: baseband FH andsynthesized FH.

Baseband frequency hopping.

Using baseband frequency hopping, each transmitter is dedicated to one frequencyand is connected to all the TDMA processing boards via the FH bus. Baseband FHis used with cavity coupling system. It requires exactly the same number offrequencies as the number of transmitters.

In this case, all frequencies to hop on have to be physically present on the site.Automatic reconfiguration of the hopping sequence is provided as part of thebaseband frequency hopping package in the event of a failure of a transmitter in thehopping sequence.

There are two types of frequency hopping techniques: baseband FH and

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There are two types of frequency hopping techniques: baseband FH andsynthesized FH.

Synthesized frequency hopping.

Using synthesized frequency hopping, each transmitter is associated to one TDMAand can transmit on all the frequencies. Synthesized FH is used with hybridcoupling systems and it can use more frequencies than the number of transmitters.

The main issue is to ensure that the BCCH frequency is transmitted all the time (onevery TS of the TDMA) at a constant power even if there is no call to transmit (novoice or data burst). This is achieved by a specific configuration which consists indedicating a transmitter to the BCCH frequency (so the TDMA in charge of theBCCH does not hop).

The number of frequencies is usually greater than the number of transmitters inorder to have the smallest fading margin in the link budget.

Synthesized FH not only brings the same benefits as baseband Frequency Hopping(high voice quality and high resistance to interference and Rayleigh fading), but it isalso a key element for the fractional frequency reuse patterns, providing significantcapacity increase by allowing fractional technique with less transmitters thanfrequencies to hop on.

The start of an uplink TDMA frame is delayed with respect to the downlink by a fixed

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The start of an uplink TDMA frame is delayed with respect to the downlink by a fixedperiod of three Time Slots. Why?

Staggering TDMA frames allows the same TS number (TN) to be used in both thedown and uplink while avoiding the requirement for an MS to transmit and receivesimultaneously.

On the radio path, propagation delays cannot be ignored. Indeed, 1 km corresponds

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On the radio path, propagation delays cannot be ignored. Indeed, 1 km correspondsto a propagation delay of 3.33 µs (compared with a bit period of 48/13 = 3.7 µs).

But the BTS receives continuously, and has its own scheduling. The mobile stationmust itself balance the propagation delay, in order to avoid overlapping in the framereceived by the BTS.

This is why the system takes into account these timing delays and orders the mobilestation to transmit with an anticipation called the Timing Advance.

As specified in the GSM recommendations, there is a constant frame delay of 3

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As specified in the GSM recommendations, there is a constant frame delay of 3 timeslots between the reception and the transmission.

In order to avoid such overlapping due to propagation delay, a timing advance δδδδ is computed by the BTS and sent to the mobile.

This value of timing advance is then used by the mobile to emit its bursts earlier.

Handover is initiated by the network based on radio subsystem criteria (RF level,

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Handover is initiated by the network based on radio subsystem criteria (RF level, quality, distance) as well as network-directed criteria (current traffic loading per cell, maintenance requests, etc.).

In order to determine if a handover is required due to RF criteria, the MS makes radio measurements on neighboring cells. These measurements are reported to the serving cell on a regular basis.

Neighboring cell list is provided by the system to the MS. This list details BCCH and BSIC allocation in the neighboring cells.

BSIC = Base Station Identity Code = Base Color Code + Network Color Code.

Network Color Code: first three bits of BSIC. Each PLMN is assigned a NCC.

Base station Color Code: last three bits of BSIC. BCC is used to identify one of the cells sharing the same BCCH frequency. Neighboring cells may, or may not, have different BCC.

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An example of a traditional system is GSM: it is a narrowband (NB) system.

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An example of a traditional system is GSM: it is a narrowband (NB) system.

GSM means Global System for Mobile Communications. It is one of the leading digital cellular systems.

Description of the spectrum

To get some chance to be properly received, the transmitted signal must override interference and noise. That is why the signal must be transmitted at a higher power level than any other interference, especially extra-noise coming from other systems, resulting in adjacent channel interference and co-channel interference.

For one given channel, at the carrier frequency Fc, most of the wanted signal power is contained in the Main Lobe.

The adjacent channels, which transmit other signals, are the channels located at the carrier frequencies Fc-1 just before and Fc+1 just after the frequency Fc. They are shifted from +/- 200 kHz.

The co-channel signal is a signal that is transmitted at the same Fc frequency but in another distant cell, and possibly interferes with the wanted signal.

An example of a spread spectrum system is UMTS: it is a wideband (WB) system.

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An example of a spread spectrum system is UMTS: it is a wideband (WB) system.

UMTS means Universal Mobile Telecommunications System. It is a ThirdGeneration (3G) mobile technology that delivers voice, data, audio and video towireless devices anywhere in the world through fixed, wireless and satellitesystems.

Description of the spectrum

A wideband signal is used to transmit the wanted signal, but with low energy inorder to create as little interference as possible.

Signals behave like Additive White Gaussian Noises, each signal contributionsuperimposes on the previous one. And for one given channel, the wanted signal,the co-channel signals and the noise are all transmitted in the same frequencyband.

Direct Sequence CDMA defines two types of codes:

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Direct Sequence CDMA defines two types of codes:

Channelization Codes

Users data is modulated by a channelization code. The orthogonality properties of OVSF enable the UE to recover its bits without being interfered by other users. This is true only if the system is synchronous, which is the case in downlink, but not in uplink. Thus, the OVSF codes are not used to separate users in uplink and therefore different users can use the same code. But they can be used to distinguish the different physical channels of one user.

Scrambling Codes

The scrambling operation is used for base station and mobile station identification. In downlink, the same scrambling code can be used on different channels in a cell, but different scrambling codes are used in different cells. In uplink, scrambling codes are used to differentiate users.

Scrambling codes reduce the interference between neighboring cells in downlink since same channelization codes are used.

It is important to maintain good cross-correlation properties between the different scrambling codes in order not to decode an interferer.

Once allocated to the mobile users, these codes do not change in the midst of communication, unless the base station can be notified of the change.

Similar to the reuse of frequency in GSM, scrambling codes are reused.

All W-CDMA users occupy the same frequency at the same time, thus frequency

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All W-CDMA users occupy the same frequency at the same time, thus frequency and time are not used as discriminators.

W-CDMA operates by using CODES to discriminate between users. The receiver will ‘hear’ all the transmitter signals mixed together, but by using the correct code sequence, it can decipher the required transmission channel and the rest is background noise.

Spreading sequences are actually unique streams of 1 and -1 which compose the code associated with a user. Therefore, users are discriminated thanks to spreading codes.

Many code channels are individually “spread” with their associated “code” and then added together to create a “composite signal”.

In the receiver, the composite signal is correlated with a replica of the code used to spread the data to be recovered. Thus, low cross-correlation between the desired users and the interfering users is important to suppress multiple access interference.

Good auto-correlation properties are required for initial synchronization and to reliably separate multi-path components. The correlation between two bit strings of the same length is defined as the “degree of similarity” between them:

• When the correlation is determined between two copies of the same string, it is called auto-correlation.

• When the correlation is determined between any two same length strings, it is called cross-correlation.

The possibility to operate in either FDD or TDD mode is allowed for efficient

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The possibility to operate in either FDD or TDD mode is allowed for efficient utilization of available spectrum according to frequency allocation in different regions. FDD and TDD are defined as follows:

FDD

Duplex method whereby UL and DL transmissions use 2 separate frequency bands:

• Uplink 1920 MHz - 1980 MHz; Downlink 2110 MHz - 2170 MHz.

• Bandwidth: each carrier is located on the center of a 5 MHz wide band.

• Channel separation: nominal value of 5 MHz that can be adjusted.

• Channel raster: 200 kHz (center frequency must be a multiple of 200 kHz).

• Tx-Rx frequency separation: nominal value of 190 MHz. This value can be either fixed or variable (minimum of 134.8 and maximum of 245.2 MHz).

• Channel number: the carrier frequency is designated by the UTRA Absolute Radio Frequency Channel Number (UARFCN). This number is sent by the network (for the uplink and downlink) on the BCCH logical channel and is defined by Nu= 5 * (Fuplink MHz) and ND= 5 * (Fdownlink MHz).

TDD

Duplex method by which UL and DL transmissions are carried over the same frequency band using synchronized time intervals. The carrier uses a 5 MHz band, although there is a low chip rate solution (1.28 Mcps). The available frequency bands for TDD is 1900-1920 MHz and 2010-2025 MHz. Only hard handovers are possible.

A time slot is a unit of time which carries chips. Each time slot lasts 0.667 ms and

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A time slot is a unit of time which carries chips. Each time slot lasts 0.667 ms and is made of 2560 chips.

A radio frame is a processing unit which consists of 15 time slots. Its duration is 10ms. It is always made of 38,400 chips, leading to a fixed chip rate of 3.84 Mcps. The radio frame corresponds to the scrambling code period.

The cell System Frame Number (SFN) is broadcasted on the BCCH, and goes from 0 to 4095. It is used for paging and system information scheduling.

Other synchronization counters have been defined, like BFN (NodeB Frame Number, from 0 to 4095), RFN (RNC Frame Number, from 0 to 4095) and CFN (Connection Frame Number, from 0 to 255 or 0 to 4095 for PCH).

The Connection Frame Number (CFN) is the frame counter used for the L2/transport channel synchronization between UE and UTRAN. It is sent on the Iub interface, between L1 and MAC.

The RNC Frame Number (RFN) and the NodeB Frame Number (BFN) are used for RNC-Node B synchronization.

The different physical channels are the following:

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The different physical channels are the following: • SCH (Synchronization CHannel): used for the cell search procedure. There

are primary and secondary SCHs. Downlink.• CCPCH (Common Control Physical Channel): used to carry common

control information such as cell information, … (a primary exists (P-CCPCH), plus additional secondary ones (S-CCPCH)). Downlink.

• CPICH (Common Pilot Control CHannel): used to help the UE to get correct signals from the network. It is the phase reference. Downlink.

• DPDCH (Dedicated Physical Data CHannel): used to carry dedicated data generated at layer 2 and above (coming from DCH). Uplink and Downlink.

• DPCCH (Dedicated Physical Control CHannel): used to carry dedicated control information generated at layer 1 (such as pilot, TPC, TFCI bits). Uplink and Downlink.

• PDSCH (Physical Downlink Shared CHannel): used to carry data information coming from several users. Downlink.

• PICH (Page Indication CHannel): carries an indication to inform the UE that paging information are available on the S-CCPCH. Downlink.

• PRACH (Physical Random Access CHannel): used to carry random access information when a UE wants to communicate with the network. Uplink.

• PCPCH (Physical Common Packet Channel): used to carry data coming from several users. Uplink.

• AICH (Acquisition Indicator CHannel): this is used to inform a UE that the network has well received its access. Downlink.

Depending on the classification seen on the previous page, the main FDD

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Depending on the classification seen on the previous page, the main FDD measurements taken by the UE and transmitted to the higher layers are:

• Intra-frequency measurements: CPICH_RSCP and Ec/No of the cells using the same frequency as those of the Active Set.

• Inter-frequency measurements: CPICH_RSCP and Ec/No of the cells that are using different frequencies than the Active Set.

Measurements are not only used for handover procedures, but can also be used for power control, cell reselection, or UE positioning.

RSSI: Received Signal Strength Indicator, the received wide band power, including thermal noise and noise generated in the receiver, within the bandwidth defined by the receiver pulse shaping filter.

RSCP: Received Signal Code Power, the received power on one code measured on a given physical channel (P-CPICH or DPCCH for example).

ISCP: Interference Signal Code Power, the interference on the received signal.

SIR: Signal to Interference Ratio, is defined as: SF x (RSCP / ISCP).

Ec/No: The received energy per chip divided by the power density in the band. The CPICH Ec/No is identical to P-CPICH_RSCP / RSSI.

In conventional radio technologies (AMPS, TDMA and GSM), the desired signal

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In conventional radio technologies (AMPS, TDMA and GSM), the desired signal must be strong enough to override interferences. The figure of quality is the C/I (carrier to interference ratio), which corresponds to a Ec/No in CDMA.In W-CDMA all users occupy the same frequency band at the same time! The figure of quality is the Eb/No (Energy per bit to interference spectral density ratio).At the receiver, as the codes are orthogonal and known, only the power of the intended user is despread.After despreading (decoding), correct data recovery requires a given Eb to No ratio that corresponds to a binary error rate (BER), or BLER. Under this Eb/No, the noise will generate too many errors. The noise is mainly generated by the other users transmitting at the same time and at the same frequency, but using a different set of codes. The target Eb/No depends on the service, the bit rate, the environment, the speed of the mobile, the receiver antenna and decoding algorithms.Therefore, in order not to cross this maximal noise level, all the users have to share the power and optimize its usage. The despreading process results in a processing gain. The larger the Spreading Factor, the larger the gain. This means that using a larger Spreading Factor, we can reduce the power of transmission (and therefore the background noise). Thanks to this property, spread signals can operate at negative signal to noise ratios (dB scale), given that the processing gain is high enough.Uplink W-CDMA interferences come mainly from nearby users. The transmission power of all users must be tightly controlled so that their signals reach the base station at the lowest possible level. This way, interferences are controlled and the famous near-far problem is alleviated.

From the UE perspective, there are 3 different ways to classify the cells that may

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From the UE perspective, there are 3 different ways to classify the cells that may be involved in the handover procedures:

• Cells belonging to the Active Set are the cells involved in the soft handover and that are communicating with the same UE. This one has to simultaneously demodulate and combine the signals coming from these cells.

• Cells belonging to the Monitored Set, that do not belong to the active set, but that are monitored by the UE depending on a list transmitted by the UTRAN.

• Cells belonging to the Detected Set, which are detected by the UE, but that are neither in the Active Set nor in the Monitored Set.

B = Bandwidth.

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B = Bandwidth.

Tb = Tbit = duration of a digital sample of the original data stream.

Tc = Tchip = duration of a chip = duration of a digital sample in the spreadingsequence.

SF = Spreading Factor = number of chips per bit in the spreading sequence. A SFcomposed of n chips causes a spread of the signal by a factor n.

For UMTS, the spread rate (chip rate) is a constant fixed to 3.84 Mchips/s.

N = Noise level.

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N0 = Noise level.

I0 = Interference level.

Ec = Energy chip = power of the spread (wanted) signal.

Eb = Energy bit = power of the de-spread (wanted) signal.

Ec / I0 = SIR of the wanted signal.

Eb / N0 = SNR required to ensure correct data recovery after de-spreading.

PG = Processing Gain = gain obtained through the de-spreading process andproportional to the bandwidth of the spread signal.

Reminder

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Reminder

The required Eb/No is the minimal value under which the system generates toomany errors (due to noise) to recover a signal.

Case 1

The signal to transmit has a large bandwidth, and a high transmission rate. But theavailable signal Eb/No is lower than the required Eb/No.

The transmission power required is high; the covered area has a radius R1.

Case 2

The signal to transmit has a smaller bandwidth, and a lower transmission rate. Itsavailable signal Eb/No is higher than the required Eb/No.

With the same transmission power as in the first case, the covered area has aradius R2 larger than R1.

CDMA systems show a certain relation between capacity and coverage, so the

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CDMA systems show a certain relation between capacity and coverage, so thenetwork planning process itself depends not only on propagation but also on cellload. Thus, the results of network planning are sensitive to the capacityrequirements, which makes the process less straightforward. UMTS forces radionetwork planners to abandon the coverage first, capacity later approach.

Furthermore, for a given design load, due to the large difference in services bit ratesand QoS requirements, UMTS networks exhibit several cell ranges possibilities. Themain parameter which are then linked with a typical cell range are:

• RF environment properties,

• Service provided,

• Design load.

Interference Margin

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Interference Margin

This is the degradation in the power budget due to traffic loading (also known asinterference degradation margin or noise rise above thermal noise).

This factor is calculated based on the cell load versus the maximal asymptoticcapacity through the relationship:

Interference Margin = 10.log{1/(1-L)}

where L represents the Load factor expressed as a percentage of the maximal load.

As a result:

• A 50% loading shrinks the cell coverage by 3 dB,

• A 60% loading by 4 dB,

• A 70% loading by 5.2 dB and so on.

Note that the interference margin, as given in the above equation, includes bothintra-cellular interference and inter-cellular interference.

There are three main types of handover:

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There are three main types of handover:

• Inter RNC Handover: the Mobile hands over from one cell to another,belonging to a different RNC.

• Intra Node B Handover: the Mobile hands over from one cell to another,belonging to the same Node B.

• Hard Handovers:

- Inter frequency: used to change the radio frequency band of the connectionbetween the Mobile and the UTRAN.

- Inter mode: used to change the mode between FDD and TDD.

- Inter system: used for handover from/to a non-UTRAN system to/fromUTRAN.

To cover all the needs to guarantee quality, four different Classes of Service have

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To cover all the needs to guarantee quality, four different Classes of Service have been defined for UMTS, depending on their respective constraints.

Real time content is categorized as ‘Conversational’ or ‘Streaming’, whereas non real time content (data transmission) is categorized as ‘Interactive’ or Background in UMTS:

• Real-time or near real-time applications, preserve time relation (variation) between information entities of the stream:

— Conversational class: symmetrical, delay untolerant (real-time)

— Streaming class: asymmetrical, delay tolerant, jitter tolerant.

• Data transmissions with very low error rate:

— Interactive class: low error rate, delay untolerant

— Background class: low error rate, delay tolerant.

Main attributes are:

• service availability,

• delay tolerance (latency),

• delay variation (jitter),

• throughput,

• packet loss rate.

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The WCDMA system normally carries user data over dedicated transport channels,

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The WCDMA system normally carries user data over dedicated transport channels, or DCHs, which brings maximum system performance with continuous user data. The DCHs are code multiplexed onto one RF carrier. In the future, user applications are likely to involve the transport of large volumes of data that will be bursty in nature and require high bit rates.

HSDPA introduces a new transport channel type, High Speed Downlink Shared Channel (HS-DSCH) that makes efficient use of valuable radio frequency resources and takes into account packet data services burstiness.

This new transport channel shares multiple access codes, transmission power and use of infrastructure hardware between several users. The radio network resources can be used efficiently to serve a large number of users who are accessing bursty data. To illustrate this, when one user has sent a data packet over the network, another user gets access to the resources and so forth. In other words, several users can be time multiplexed so that during silent periods, the resources are available to other users.

There is no more fast Power Control with HSDPA and the High Speed Downlink

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There is no more fast Power Control with HSDPA and the High Speed Downlink Shared Channel is transmitted at a constant power while the modulation, the coding and the number of codes are changed to adapt to the variations of radio conditions.

Where R4 dedicated downlink PS data channels offer a constant data rate using power adaptability, HSDPA shared channel opposes PS data rate variability.

HSDPA operation requires three new physical channels to be introduced:

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HSDPA operation requires three new physical channels to be introduced:

• The HS-PDSCH (DL) on which is mapped the HS-DSCH. It carries only the data payload. The SF is equal to 16 and up to 15 codes can be reserved to HS-PDSCH per cell. One HS-DSCH can be mapped onto one or several HS-PDSCH (the maximum number of codes is given by UE capabilities).

• The HS-SCCH (DL) that carries the HS-PDSCH associated signaling. The SF is fixed to 128. It indicates to which UE data is intended to, on which codes and with which parameters. There are as many HS-SCCH transmitted during a TTI as scheduled user number.

• The HS-DPCCH (UL) that carries feedback information (ACK/NACK/CQI) to handle retransmissions (SF=256).

HSDPA operation also requires to use already existing R4 physical channels:

• The DPCH (UL/DL) is needed to carry the higher layers signaling. Dedicated physical channel is also required in UL to carry the corresponding user data (PS I/B) as HS-PDSCH is DL only.

The configuration of the OVSF code tree can provide up to 15 SF16 codes allocated

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The configuration of the OVSF code tree can provide up to 15 SF16 codes allocated to HS-PDSCH and up to 4 SF128 codes for HS-SCCH.

All the R4 common channels (CPICH, P-CCPCH, S-CCPCH) are allocated in the top of the tree and take the equivalent of a SF32 code. All remaining OVSF codes can be used for non-HSDPA services (speech, multi-RABs...)

In the above example, the HS-PDSCH SF16 codes are allocated and reserved by the RNC at the bottom of the tree. Immediately above, the HS-SCCH SF128 codes are allocated. These codes are allocated at cell setup and cannot be used or preempted for other services.

All the remaining codes are therefore contiguous and left for further DCH allocations. This includes associated DCH as well as any other calls mapped on DCH (e.g. speech calls, streaming, etc).

Note that the maximum configuration (15 HS-PDSCH codes and 4 HS-SCCH codes) leaves no room in the OVSF tree for DCH (due to common channels occupancy) so it is not even possible to allocate associated SRB for HSDPA calls.

The scheduler is a key element of HSDPA that determines the overall behavior of

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The scheduler is a key element of HSDPA that determines the overall behavior of the system and, to a certain extent, its performance. For each TTI, it determines which terminal (or terminals) the HS-DSCH should be transmitted to and, in conjunction with the AMC, at which data rate.

Several algorithms can be used for the scheduler.

The Fair scheme schedule users trying to serve them all with evenly shared data rate.

The Round Robin scheme schedules users according to a FIFO approach. It provides a high degree of fairness between users, but at the expense of the overall system throughput (and therefore spectral efficiency), since some users can be served even when they are experiencing destructive fading (weak signal).

The CQI scheme schedules users with the highest C/I for the current TTI. This naturally leads to the highest system throughput since the served users are the ones with the best channel. However, this scheme makes no effort to maintain any kind of fairness among users. In fact, users at the cell edge will be largely penalized by experiencing excessive service delays and significant outage.

The Proportional Fair scheme offers a good trade-off between RR and CQI schemes. This results in all users having equal probability of being served even though they may experience very different average channel quality. This scheme provides a good balance between the system throughput and fairness.

Adaptive Modulation and Coding (AMC) is a fundamental feature of HSDPA. It

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Adaptive Modulation and Coding (AMC) is a fundamental feature of HSDPA. It consists in continuously optimizing the user data throughput based on the channel quality reported by the UE (CQI feedback). This optimization is performed using adaptive modification of the coding rate, the modulation scheme, the number of OVSF codes employed and the transmit power per code.

Different combinations of modulation and channel coding rate (based on the Transport Format and Resource Combinations or TFRC) can be used to provide different peak data rates. Essentially, when targeting a given level of reliability, users experiencing more favorable channel conditions (e.g. closer to the NodeB) will be allocated higher data rates.

The above figure shows an illustration of the user throughput evolution for one single OVSF code in function of the channel quality as a result of AMC.

Twelve categories have been specified by Release 5 for HSDPA UEs according to

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Twelve categories have been specified by Release 5 for HSDPA UEs according to the value of several parameters among which are the following:

• Maximum number of HS-DSCH codes that the UE can simultaneously receive (5, 10 or 15).

• Minimum inter-TTI interval, which defines the minimum time between the beginning of two consecutive transmissions to this UE. If the inter-TTI interval is one, this means that the UE can receive HS-DSCH packets during consecutive TTIs, i.e. every 2 ms. If the inter-TTI interval is two, the scheduler needs to skip one TTI between consecutive transmissions to this UE.

• Supported modulations (QPSK only or both QPSK and 16QAM),

• Maximum peak data rates at the physical layer (number of HS-DSCH codes x number of bits per HS-DSCH / Inter-TTI interval).

These twelve categories provide a much more coherent set of capabilities as compared to R4 which gives UE manufacturers freedom to use completely atypical combinations.

The maximum achievable data rate depends on the UE category but also on the

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The maximum achievable data rate depends on the UE category but also on the instantaneous radio conditions it is exposed to. Each UE category has therefore a reference table specifying the supported combinations between the reported CQI values, the number of codes and the radio modulation (QPSK or 16-QAM).

Instantaneous radio channel conditions are known at the UTRAN level thanks to the periodical decoding of the Channel Quality Indicator sent by the UE to the NodeB onto the HS-DPCCH. The UE first estimates the Carrier over Interference ratio (C/I). From this estimate the UE then determines a CQI (with a maximum HS-DSCH BLER target of 10%) and then it sends this indication back to the NodeB. The NodeB takes this input into consideration in order to adapt the throughput to the UE.

Note: a UE reporting a CQI value of 0 is not scheduled by the NodeB.

The UE receives some physical layer parameters via RRC:

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The UE receives some physical layer parameters via RRC:

• HS-SCCH set to monitor consisting of up to 4 HS-SCCH codes per UE,

• Repetition factor of ACK/NACK.

The UE monitors all the HS-SCCHs from the set it has been allocated. It despreads and decodes the first slot of the corresponding HS-SCCHs.

If the UE detects data intended to it by recognizing its UEId on the first slot, it decodes the rest of the subframe to get all the relevant information needed to receive HS-DSCH (transport block size, HARQ process number, redundancy version, New Data Indicator).

In parallel the UE may start to despread the HS-PDSCH codes it has been allocated (the first slot of HS-SCCH contains the channelization code set and the modulation scheme information).

After HS-PDSCH decoding, the UE send the ACK/NACK and possibly repeats it over consecutive HS-DPCCH subframes.

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Several area types have been defined in UMTS to handle user mobility:

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Several area types have been defined in UMTS to handle user mobility:• Location Area (LA): this notion is strictly equivalent to the GSM one:

—an LA contains a group of cells, each cell belonging to one LA,—LAs are used by the Core Network (CS domain) to get information on the

user location when in the idle mode,—one LA consists of a number of cells belonging to the RNCs that are

connected to the same CN node, i.e. one 3G_MSC/VLR,—the mapping between one LA and the RNCs is handled within the

MSC/VLR owning this LA.• Routing Area (RA): this notion is strictly equivalent to the GPRS one:

—an RA contains a group of cells, each cell belonging to one RA,—RAs are used by the Core Network (PS domain) to get information on the

user location when in the idle mode,—one RA consists of a number of cells belonging to RNCs that are

connected to the same CN node, i.e. one 3G_SGSN,—the mapping between one RA and RNCs is handled within the SGSN

owning this RA.• UMTS Registration Area (URA): when the UE is in connected mode and is

using common and/or shared channels, the network can locate it on the cell basis or on the URA basis, depending on its level of activity and mobility.

• Service Area (SA), may consist of several PLMNs: one SA may consist of one country, be a part of a country or include several countries.

A GSM system is basically designed as a combination of two major subsystems:

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A GSM system is basically designed as a combination of two major subsystems: the radio subsystem called the BSS (Base Station Subsystem) and the NSS (Network and switching SubSystem).

In order to ensure that network operators have several sources of cellular infrastructure equipment, GSM decided to specify not only the radio interface, but also the main interfaces that identify different parts. There are three different dominant interfaces, namely:

• the Um interface between the MS (Mobile Station) and BTS (Base Transceiver Station);

• the Abis interface between BTS and BSC (Base Station Controller);

• the A interface between the BSS and NSS.

The BSS includes the equipment and functions related to the management of connections on the radio path, including handover treatment. It mainly consists of a BTS, a BSC, and its associated component part: the TRAU (Transcoder/Rate Adaptation Unit).

The NSS includes the equipment and functions related to end-to-end calls, management of subscribers, mobility, and interfaces with the fixed network, called the PSTN (Public Switched Telephone Network).

In particular, the NSS consists of MSC (Mobile services Switching Centers), VLR(Visitor Location Registers), HLR (Home Location Registers), and AuC (Authentication Center).

The support of GPRS (General Packet Radio Service) does not represent a major

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The support of GPRS (General Packet Radio Service) does not represent a major upgrade to the existing GSM infrastructure.

The largest impact is the addition of two new network entities, the SGSN (Serving GPRS Support Node) and the GGSN (Gateway GPRS Support Node).There is no hardware impact on the BTSs and overall the GPRS represents a software upgrade of the BSS, except for the introduction of PCUs to support the packet-oriented nature of the Gb interface, logically between the PCU and the SGSN.

The architecture of the GPRS is designed so that signaling and high-level data protocols are system independent. Only the low-level protocols in the radio interface must be changed to be capable of operating the same service.

The main functions of the SGSN are:

• to detect and record GPRS MS in its service area

• to send/receive data packets to/from the MS

The main function of GGSN is to forward data packets between an external packet network and the GPRS network.

In addition to routing and data transfer functions, the SGSNs and GGSNs collect charging statistics that are commonly used as a basis for billing.

3GPP Release 4 is a further enhancement of 3GPP Release 1999.

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3GPP Release 4 is a further enhancement of 3GPP Release 1999.

The Core Network becomes a Bearer Independent Circuit switching Core network or BICC.

The Call Server architecture has been accomplished by splitting MSC and GMSCs into two entities:

a MSC server (VLR) or a GMSC Server,

a Media GateWay for Circuit (CS-MGW).

This functional split has resulted in three new interfaces:

The Nc interface requirements are served by BICC which is an evolution of ISUP specified by the ITU-T.

The Mc interface was created as a result of the split of MSC and GMSC application functionality, into server and Media Gateway functions.

The Nb interface provides User plane transport over a packet switched network. The Nb User plane protocol is largely identical to the ATM Iu UP protocol. The BICC protocol on the Nc interface now contains sufficient information to establish bearer connections between Media GateWays.

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