Τεχνική Spread spectrum (SS)Τεχνική Spread spectrum (SS) Δέκτης RAKEΔέκτης RAKE Χωρητικότητα WCDMAΧωρητικότητα WCDMA Έλεγχος Ισχύος (Power Control)Έλεγχος Ισχύος (Power Control) Διαφορική Μετάδοση (Uplink και downlink)Διαφορική Μετάδοση (Uplink και downlink) Μεταπομπές στο WCDMAΜεταπομπές στο WCDMA Δομή δικτύου UMTSΔομή δικτύου UMTS Spread Spectrum systems

Τεχνική Spread Spectrum The spread spectrum techniques: Direct sequence, DS (WCDMA, IS-95) Frequency Hopping (GSM) In direct sequence user bits are coded by a unique binary sequence (=Code).The bits of code are called chips. The chip rate (W) is typically much higher than the bit-rate (R). The signal spreading is achieved by modulating the data modulated signal a second time by wideband spreading signal The used code in WCDMA is pseudorandom sequence which is constructed by combining two codes: Orthogonal Variable Spreading Factor, OSVD-code Cell specific scrambling code

If Tx and Rx are using the same code which are synchronized, the received narrowband user data is amplified with the factor of W/R = Processing Gain. Other power component (=interference) coming to the receiver (other users, background noise) won't have PG Processing gain includes spreading gain and channel coding gain The processing gain is different for different services over 3G mobile network (voice, www-browsing, videophone) due to different R This means that the coverage area and capacity is different for different services Τεχνική Spread Spectrum

Τεχνική Spread spectrum

Τεχνική Spread Spectrum

Διαλείψεις σε μια διαδρομή multipath Φαινόμενο multipath. Ανάλογα με τις φάσεις των συνιστωσών η συνισταμένη είτε ενισχύεται είτε εξασθενεί. Αυτό προκαλεί την εικόνα των διαλείψεων στην περιβάλλουσα του λαμβανομένου σήματος. Στον δέκτη RAKE κάθε διαδρομή περιλαμβάνει πολλές συνιστώσες σήματος. Κάθε διαδρομή θεωρείται ότι υφίσταται διαλείψεις ανεξάρτητες από τις λοιπές διαδρομές(typically Rayleigh fading).

Καθυστέρηση Διαδρομής Το φαινόμενο multipath προκαλεί πολλές κορυφές στην έξοδο του προσαρμοσμένου φίλτρου (MF) του δέκτη Αντιστοιχεί μια διαδρομή σε κάθε κορυφή

Συγχρονισμός Προσαρμοσμένο φίλτρο Δέκτη Για να συμπτυχθεί το λαμβανόμενο σήμα με τον κατάλληλο κώδικα πρέπει να υπάρχει συγχρονισμός. Αυτός ανιχνεύεται με το κατάλληλο Προσαρμοσμένο φίλτρο

Δέκτης RAKE

Multipath Channel

Χωρητικότητα WCDMA

Ισχύς Λήψης (ενός χρήστη στον BS) συναρτήσει του #χρηστών στη κυψέλη Χωρητικότητα WCDMA N=-103 dBm ρ=5 dB R=12.2 kbps W=3.84 Mcps

Μετάδοση WCDMA

Έλεγχοι Ισχύος Το πρόβλημα “Near-far”

Βρόγχοι ρύθμισης Ισχύος Ο έλεγχος ισχύος (PC) διασφαλίζει πως κάθε MS και BS ενώ εκπέμπουν αρκετή ισχύ για την μετάδοση της πληροφορίας, προκαλούν την ελάχιστη δυνατή παρεμβολή στους λοιπούς χρήστες. Υπάρχουν 3 διαφορετικοί έλεγχοι: 1) Έλεγχος Ανοικτού βρόγχου για αρχική ρύθμιση ισχύος του MS 2) Έλεγχος Κλειστού βρόγχου (TPC) ( με συχνότητα of 1.5 KHz στο UMTS ) Αντισταθμίζει τις διαλείψεις (fast and slow fading) Χρησιμοποιείται τόσο στο UL όσο και στο DL Θέτει μια (σταθερή) τιμή στόχου ποιότητας (SIR) target στα MS / BS 3) Έλεγχος εξωτερικού βρόγχου Θέτει το (SIR) target του PC ανοικτού βρόγχου βάσει του FER που ανιχνεύεται στον Ελεγκτή του Δικτύου (RNC) Αντισταθμίζει μεταβολές στις συνθήκες μετάδοσης Ρυθμίζει την τιμή του (SIR) target για την επίτευξη τιμής στόχου για τα FER/BER/BLER ανάλογα με: – την ταχύτητα του MS και – την διαθέσιμη πολυδιαδρομική διαφορησιμότητα Χρησιμοποιείται τόσο στο UL όσο και στο DL

Έλεγχος Κλειστού Βρόγχου Fast closed loop PC (TPC)

Έλεγχος Κλειστού Βρόγχου Fast closed loop PC (TPC)

Ο έλεγχος κλειστού βρόγχου προσπαθεί να διατηρήσει σταθερό SIR. Αυτό όμως δεν διασφαλίζει, το απαιτούμενο FER (που μπορεί να θεωρηθεί σαν το κριτήριο ποιότητας της ζεύξης). Επομένως η πληροφορία για το FER μεταδίδεται στον έλεγχο εξωτερικού βρόγχου που ρυθμίζει το SIR target ή SIR setpoint σύμφωνα με αυτήν. Έλεγχος Εξωτερικού Βρόγχου ( Uplink )

Intra-system handovers Intra-frequency handovers softer (MS is connected to two sectors of the same BS simultaneously) soft (MS is connected to two sectors of different BSs simultaneously) hard (MS is connected to only one sector at time. Causes short dealys) Inter-frequency handovers hard (MS is connected to only one sector at time working at different frequencies. Causes short dealys) between cell layers (from large macrocells to small microcells, for example) Inter-system handovers Handover WCDMA  GSM900/1800 Handover WCDMA  some other system as well WCDMA Handover types

In Soft handover the mobile is connected to two or more base stations (or sectors in the case of softer HO) at the same time. This means that the same information flows through many BSs. The receiver combines these signals. The MS enters to soft handover state if the difference between measured pilot signals (or meas. pilot Ec/I0 to be exact) from several BSs are within the threshold value See figure: when the mobile is close to the cell border between BS1 and BS2 and transmits with a very high power it might interfere the neighboring cell (BS2). However, with the soft handover this mobile is connected also to BS 2 which allocates RAKE fingers for that mobile and collects all its signal energy. The mobile is also power controlled by both of BS. Soft/softer handover

+ Decreases interference from one sector/BS to another  Increases capacity Without soft/softer handover the interference power would be very high because the same frequency is used in adjacent cells + Coverage gain due to diversity The signal to other BS can be temporarily very weak. Because of the soft/sofer handover branch the resulting signal can be relatively good Seamless handover No delays in the handover – Requires additional resouces at the BS + additional signalling Why use soft / softer handovers ?

Softer Handover softer handover is handled by BS internally softer handover probability about % no extra transmissions between BSs needed basically same RAKE MRC processing as for multipath/antenna diversity (BS / S). More RAKE fingers needed. provides additional diversity gain softer handover does create additional interference and needs BS LPA resources

Softer handover probability about % Extra transmissions needed UL / DL soft HO diversity processing very different MS: MRC RAKE combining RNC/BS: frame selection Soft handover Except for the TPC symbol exactly the same information (symbols) is sent via air. Differential delay in order of fraction of symbol duration The combination point (UL) depends on the network architecture (RNC or BS)

Seamless coverage extension for CDMA with existing GSM network Capacity extension for GSM with load sharing between WCDMA and GSM Service control - different services to different networks Intersystem Handover Load reason handover Coverage reason handover Service reason handover

UL receiver diversity Two antennas receive multipaths with different phases sum field (receive envelope) of the antennas are uncorrelated With diversity combining (maximum ratio combiningm MRC, in WCDMA) the total received SIR is maximised UL diversity decreses the received Eb/N0 and transmitted Eb/N0 better radio performance Additional antenna is just like additional multipath, so it needs additional RAKE finger

Space diversity is not applicable in MS because the separation between two MS antenna would be too small  branches would be correlated The DL diversity has been implemented as Tx diversity Two methods: Open loop diveristy: two BS antennas have been coded differently and the decoder can combine the signals optimally closed loop diversity: the mobile asks BS to change the phase difference between Tx antennas to obtain optimum performance Gain against fast fading can be achieved, almost as much as in UL diversity DL receiver diversity

In CDMA coverage and capacity are tight together: When the number of users increases, the interference levels increases and therefore the needed powers in order to keep constant quality. Due to infinite power resources this means that the coverage decreases. This leads to Cell Breathing: the coverage area changes as the load of the cell changes Therefore, the coverage and the capacity has to plan simultaneously Effective control of cell breathing by radio resource management (RRM) is needed in WCDMA Coverage and capacity planning