Lecture 8

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Lecture 8:Lecture 8:Introduction to UMTS Introduction to UMTS

July 6, 2008

896960896960

Introduction to Introduction to Algorithmic Wireless Algorithmic Wireless

CommunicationsCommunications

David AmzallagDavid [email protected] [email protected]

www.cs.technion.ac.il/~amzallag/awcwww.cs.technion.ac.il/~amzallag/awc

[[22]]

Multiple access revisitedMultiple access revisited

– Multiple users want to communicate in a common geographic area.

– How to allow many users as possible to communicate simultaneously?

– FDMAFDMA

– Very simple to design, narrowband, synchronization is easy, no interference among users in a cell.

– Static spectrum allocation, frequency

reuse is hard.

– AMPSAMPS (analog), the 1st generation,

use 30 KHz for each user.

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Multiple access revisited Multiple access revisited [continue][continue]

– TDMATDMA

– Can also partition time: users take turns using the channel.

– IS-54IS-54 (2G) used same 30 KHz channels, but with three users sharing them (3 slots).

– GSMGSM has 8 slots/270 KHz.

– Better suited for digital, often gets higher capacity (3 times higher here), strict synchronization and guard time needed, still susceptible to jamming, other-cell interference.

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Alternative to FDMA and TDMAAlternative to FDMA and TDMA

– What if we could allow users to share time and frequency?

– Eliminates need for tight synchronization among many different users.

– Eliminates need for expensive analog filters.

– May have favorable impact on capacity (?)

– But

– How do we separate the users?

– Won’t they interfere with each other?

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Code Division Multiple Access (CDMA)Code Division Multiple Access (CDMA)

– bits for user

– spreading code.

– “spreading factor”

– transmitted signal for user

– received signal for all users.

– channel impulse response.

– noise.

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Spreading codesSpreading codes

– The spreading code must be unique for each user.

– Ideally, they are orthogonalorthogonal to one another, i.e.

– Example: Walsh codesWalsh codes

– For a spreading factor there are 4 Walsh codes.

– In general, is a power of 2.

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How Walsh codes work?How Walsh codes work?

– Assume a noiseless channel with unity gain.

– Walsh codes allow four users to be transmitted at the at the same time and frequencysame time and frequency using four times the number of bits.

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Properties of Walsh codesProperties of Walsh codes

– Synchronization of all users is required.

– In a multipath channel, delayed copies may be received which are not orthogonal any longer!

– Only codes exist with a bandwidth expansion of , so as far as capacity, we are right back where we started with TDMA and FDMA.

– Advantages relative to TDMA and FDMA

– No guard bands or guard times are typically required.

– No equalizer is typically required, when a RAKE receiver is used.

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The near-far problemThe near-far problem

– Users may be received with very different powers:

– Users nearnear the base station are received with high power.

– Users farfar from the base station are received with low power.

– Nearby users will completely swamp far away users (inverse-square law).

– Solution: power controlpower control.

– Equalization of the received power per bit of all mobile stations at all times.

– What is special in WCDMA power control mechanism?

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Design issues in CDMADesign issues in CDMA

– So far, CDMA may be looked as stepping backwards:

– Tight synchronizationsynchronization is required to use orthogonal codes, which then break in a multipath channel anyway.

– Quasi-orthogonal codes cause self-interferenceself-interference, which dominates the performance in most CDMA systems.

– Near-far problem is a serious hindrance, requiring fast and accurate power controlpower control (that uses up bits we could otherwise send information with).

– Since there is universal frequency reuse, handoffhandoff may occurred also between mobiles in the same cell (multiple BSs can simultaneously decode the mobile’s data).

– And for all this, the required bandwidth is now times larger than it was before, so there doesn’t appear to be a capacity gain.

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A short discussion on CDMA capacityA short discussion on CDMA capacity

– It turns out there are serious advantages to CDMA in a multicell system.– Unlike FDMA and TDMA, CDMA does not rely on orthogonal frequency and

time slots that are compromised by neighboring cells.

– CDMA systems can reuse frequencies every cell! (FDMA and TDMA usually need reuse factors of 4 - 7). Capacity increased 4-7 foldCapacity increased 4-7 fold.

– In TDMA and FDMA systems:– If a user doesn’t have anything to send, the time/frequency slot allocated

to them is wasted.

– It is typically very difficult to dynamically allocate time and frequency slots.

– In CDMA systems:– If a user doesn’t have anything to send, it causes less interference to other

users of the system.

– Typically, each user needs to transmit less than half the time.

– Since interference-limited, this doubles the capacitydoubles the capacity.

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ConclusionsConclusions

– Comparing the capacity of TDMA/FDMA/CDMA is very controversial.

– In 1991, a famous QualcommQualcomm paper claimed that due to voice activity, frequency reuse, and sectorization, CDMA increased capacity by:

– Factor of 18 relative to AMPS.

– Factor of 6 relative to US TDMA (and similar for GSM).

– This turned out to be optimistic, about 1/3 of this gain actually happened (still depends who you ask).

– Still, doubling the capacity is very realistic.

– All 3G systems use CDMA for multiple access.All 3G systems use CDMA for multiple access.

K.S. Gilhousen et al, “On the capacity of a cellular CDMA system,'' IEEE Trans. on Vehicular Tech., May 1991.

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The benefits of CDMA systemsThe benefits of CDMA systems

– Increased system capacity over analog and TDMA systems.

– Good power control (solution for the near-far problem).

– Improved interference protection.

– No frequency planning required between CDMA channels.

– Fraud protection due to encryption and authentication.

– Improved handoffs with MAHO and soft handoffsImproved handoffs with MAHO and soft handoffs.

– Accommodates new wireless features, like data.

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Standards of CDMA systemsStandards of CDMA systems

– IS-95IS-95 (versions A and B)

– Version B, CDMAOneCDMAOne, enabled ISDN-like data rates to exists.

– IS-2000IS-2000 and CDMA2000CDMA2000 – A 3G system with compatibility to CDMAOne systems, allowing operators to make strategic deployment decisions in a graceful fashion.

– Changes in network only in stages.

– BSs with IP routing capabilities.

– Several types (CDMA2000-1XCDMA2000-1X) as 1XEV1XEV, 1xDO1xDO, and 1XDV1XDV.

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WCDMA main parametersWCDMA main parameters

– WCDMA is a wideband Direct-Sequence Code Division Multiple Access (DS-CDMADS-CDMA) system.

– WCDMA supports highly variable user data rates, in other words the concept of obtaining Bandwidth on DemandBandwidth on Demand (BoD) is well supported.

– WCDMA supports two basic modes of operation: Frequency Division Duplex (FDD) and Time Division Duplex (TDD).

– The WCDMA air interface advanced features: multiuser detectionmultiuser detection and smart adaptive antennassmart adaptive antennas.

– WCDMA is designed to be deployed in conjunction with GSM.

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Understanding HandoffsUnderstanding Handoffs

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Handoff strategies Handoff strategies

– When a mobile moves into a different cell while a conversation is in progress, the RNC automatically transfers the call to a new channel belonging to the new BS.

– The handoff operation not only involves identifying a new BS, but also requires that the voice and control signals be allocated to channels associated with the new BS.

– Processing handoffs is an important task in any cellular network.

– Many handoff strategies prioritizeprioritize handoff requests over call initiation requests.

– Handoff mustmust be performed successfully and as infrequently as possible and be imperceptible to the users.

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Handoff strategies Handoff strategies [2] [2]

– In order to meet these requirements, we must specify an optimum signal level at which to initiate a handoff.

– Once a particular signal level is specified as the minimum usuable signal for acceptable voice quality at the BS receiver,

a slightly stronger signal level is used as a threshold for handoff.

– The gap, given by , cannot be too large or too small.

– If is too large, unnecessary handoffs which burden the RNC may occur.

– If is too small, there my be insufficient time to complete a handoff before a call is lost due to weak signal conditions.

– Therefore, is very important.

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Handoff strategies Handoff strategies [3] [3]

– In deciding when to handoff, it is important to ensure that the drop in the measured signal level is notnot due to momentary fading and the mobile is actually moving away from the serving BS.

– Hence, the BS monitors the signal level for a certain period of time before a handoff is initiated.

– The length of time needed to decide if a a handoff is necessary depends on the speed at which the vehicle is moving.

This data can be determined from the statistics of the uplink fading signal at the BS.

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Handoffs in first generation (1G) Handoffs in first generation (1G)

– Signal strength measurements made by the BS and supervised by the RNC.

– Each BS constantly monitors the signal strength of all of its uplink voice channels to determine the relative location of each mobile user with respect to the BS tower.

– In addition to RSSI measurements, a spare receiver in each BS, locator receiverlocator receiver, is used to scan and determine signal strength of mobile users which are in the neighboring cells.

– Based on the locator receiver signal strength information from each base station, the RNC decides if a handoff is necessary or not.

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Handoffs in second generation (2G)Handoffs in second generation (2G)

– Handoff decisions are mobile assisted.

– In mobile assisted handoff (MAHO)mobile assisted handoff (MAHO), every mobile measures the received power from surrounding BSs and reports to the serving BS.

– A handoff is initiated when the power received from the BS of a neighboring cell begins to exceed the power received from the current BS by a certain level or for a certain period of time.

– Much faster than 1G method (measurements are made by mobiles and the RNC is no longer constantly monitors signal strength).

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Intersystem handoffIntersystem handoff

– During a call, if a mobile moves from one cellular system to a different system controlled by a different RNC, an intersystem intersystem handoffhandoff becomes necessary.

– A RNC starts an intersystem handoff when a mobile signal becomes weak in a given cell and the RNC cannot find another cell within its system to which it can transfer the call in progress.

– Many complicated issues are involved.

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Prioritizing handoffsPrioritizing handoffs

– One method for giving priority to handoffs is called the guard guard channel conceptchannel concept.

– A fraction of the total available channels in a cell is reserved exclusively for handoff requests from ongoing calls which may be handed off into the cell.

– Main disadvantage: reducing the total carried traffic, as fewer channels are allocated to originating calls.

– Guard channels combined with dynamic channel assignment seems a good idea.

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Practical handoff considerationsPractical handoff considerations

– There are several problems due to a wide range of mobile velocities.

– The umbrella cell umbrella cell approach:

– Different antenna heights and different power levels.

– “large” and “small” cells

– Used to provide large are coverage to high speed users while providing small are coverage to users traveling at low speed.

– Cell draggingCell dragging problem in microcell systems.

– Results from pedestrians that provide a very strong signal in uplink (e.g., in urban environment when there is a LOS between the subscriber and the BS).

– Necessary handoff may not be made.

– For good solution, handoff thresholds must be adjusted carefully.

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Practical handoff considerations Practical handoff considerations [2] [2]

– What is the typical time to make a handoff (when signal level is deemed to be below the handoff threshold)?

– Approximately,

– In first generation analog systems, this was about 10 seconds.

– In GSM systems, MAHO helps to determine the best handoff candidate, and handoffs requires only 1-2 seconds.

– Usually faster handoff supports much greater range of options for handling high speed and low speed users.

– Is handoff can be based on metrics other than signal strength?

– The co-channel and adjacent channel interference level may be measured at the BS or the mobile. This can be combined with conventional signal strength data to provide handoff if needed.

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WCDMA handoffsWCDMA handoffs

– Traditionally, handoffs are hardhard: users are either assigned to one cell or the other but not both.

– In CDMA, since all the cells share the same spectrum, soft soft handoffshandoffs are possible: multiple BSs can simultaneously decode the mobile’s data, with the RNC choosing the best reception among them.

– Soft handoffs involves an intercell handoff and is a make-before-break connection.

– Soft handoff can only occur when the old and the new BSs are operating on the same CDMA frequency channel.

– Softer handoffSofter handoff: when a mobile station is in the overlapping cell coverage area of two adjacent sectors of a base station.

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Which pilot will be used in the soft handoff process?Which pilot will be used in the soft handoff process?

– Pilot is a channel component in the downlink channel of every BS.

– The answer has a direct impact on the quality of the call and the capacity for the system.

– Setting the handoff parameters is a key element in the system.

– Which pilots are in the active, candidate, neighbor, and the other sets?

– The list of neighbor pilots is sent to the mobile when it acquires the BS or is assigned a traffic channel.

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Type of pilot groups in the handoff mechanismType of pilot groups in the handoff mechanism

– Active setActive set. The set of pilots associated with the downlink traffic channels assigned to the mobile unit. This set can contain more than one pilot since a total of 6 carriers, each with its own pilot, could be involved in a soft handoff process.

– Candidate setCandidate set. The pilots that the mobile unit has reported are of sufficient signal strength to be used.

– Neighbor setNeighbor set. A list of the pilots that are not currently on the active or candidate pilot list. The pilots in this set are identified by the BS via the neighbor list and neighbor list update messages.

– Remaining setRemaining set. The set of all possible pilots that can be possibly used by the mobile unit.

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WCDMA handoff algorithmWCDMA handoff algorithm

1. The pilot exceeds T_ADDT_ADD, and the mobile sends a PSMM (Pilot Signal Measurement Message) and transfer pilot to the candidate set.

2. The BS sends an extended handoff direction message.

3. The mobile transfers the pilot to the active set and acknowledges this with a handoff completing message.

4. The pilot strength drops below T_DROPT_DROP, and the mobile begins handoff drop time.

5. The pilot strength goes above T_DROPT_DROP prior to the handoff drop time expiring and T_DROPT_DROP sequences stopping.

6. The pilot strength drops below T_DROPT_DROP, and the mobile begins the handoff drop timer.

7. The handoff drop timer expires, and the mobile send a PSMM.

8. The BS sends an extended handoff direction message.

9. The mobile transfers the pilot from the active set to the neighbor set and acknowledge completion.

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More types of WCDMA handoffsMore types of WCDMA handoffs

– Inter-frequency hard handoversInter-frequency hard handovers that can be used, for example, to hand a mobile over from one WCDMA frequency carrier to another.

– Inter-system hard handoversInter-system hard handovers that take place between the WCDMA FDD system and another system, such WCDMA TDD or GSM.