CDMA: fundamentals The use of a proper code allows spreading transmitted signal and despreading received signal.

Presentazione sul tema: "CDMA: fundamentals The use of a proper code allows spreading transmitted signal and despreading received signal."— Transcript della presentazione:

1 CDMA: fundamentals The use of a proper code allows spreading transmitted signal and despreading received signal.

2 W-CDMA: general features W-CDMA is designed to allow many users to efficiently share the same RF carrier by dynamically reassigning data rates and link budget to precisely match the demand of each user in the system. As its name implies, W-CDMA is a code division multiple access (CDMA) system. As opposed to time division multiple access (TDMA), in CDMA, all users transmit at the same time. Frequency divisions are still used, but at a much larger bandwidth. In addition, multiple users share the same frequency carrier. Each user’s signal uses a unique code that appears to be noise to all except the correct receiver. Therefore, the term channel describes a combination of carrier frequency and code.

3 W-CDMA: scrambling and channelization codes The scrambling code (SC) provides a unique identity to each UE and each BS. The OVSF code allocations provide a unique identity to each channel conveyed by a UE or BS within one cell.

4 WCDMA: protocol architecture The network layer (layer 3) is based heavily on GSM standards. It is responsible for connecting services from the network to user equipment. The RLC block is responsible for the transfer of user data, error correction, flow control, protocol error detection and recovery, and ciphering. The MAC function is responsible for mapping between logical channels and transport channels. The physical layer maps the transport channels onto the physical channels and performs all of the RF functions necessary to make the system work.

5 W-CDMA: basic uplink channel without complex scrambling

6 W-CDMA: OVSF codes The spreading factor (SF) can be 4, 8, 16, 32, 64, 128, or 256, corresponding to DPDCH bit rates from 960kbps down to 15kbps. Seven sets of spreading codes are specified, one set for each spreading factor. Each code is denoted by C ch,SF,n. For example, C ch,4,2 means channelization code, SF = 4, code number 2.

7 W-CDMA: basic uplink channel with complex scrambling

8 W-CDMA: complex scrambling

9 What kind of constellation does it result if complex scrambling is applied?

10 W-CDMA: complex scrambling In the case of two channels with the same amplitude, the chip signal maps onto a QPSK constellation. The scrambling signal also maps onto a QPSK constellation.

11 W-CDMA: complex scrambling In the case of two channels with different amplitudes, the chip signal maps onto a rectangular 4-QAM constellation. The scrambling signal still corresponds to a QPSK constellation.

12 W-CDMA: HPSK modulation W-CDMA systems use Hybrid Phase Shift Keying (HPSK), also known as Orthogonal Complex Quadrature Phase Shift Keying (OCQPSK), to reduce the peak- to-average power ratio of the signal in the uplink. HPSK is a variation of basic complex scrambling that eliminates zero-crossings for every second chip point. It accomplishes this by using a specific repeating sequence as the scrambling signal and by choosing specific orthogonal codes to spread the different channels.

13 W-CDMA: HPSK modulation The repeating Walsh rotator sequence, I s = W 0 = (1,1) and Q s = W 1 = (1,- 1), is used as the scrambling signal. Only even-numbered OVSF codes are used to spread the data from the different channels. Even numbered OVSF codes consist of pairs of identical bits. For two consecutive identical chip points, the first one is rotated by +45°, and the second one by -45°, which ensures that they will be 90° apart in the final constellation and the transition between them does not go through zero.

14 W-CDMA: HPSK modulation Constellation without HPSKConstellation with HPSK

15 W-CDMA: HPSK modulation HPSK eliminates 0° phase shift transitions for every second chip point. A 0° phase transition occurs when two consecutive points are at the same place on the final constellation. This causes overshooting trajectory, which increases the peak-to-average power ratio of the signal. HPSK forces 90° transitions between pairs of consecutive points. This minimizes 0° phase transitions, which further reduces the peak-to-average power ratio of the signal.

16 W-CDMA: primary PN spreading code A primary PN spreading (scrambling) code PN (1) is applied to the final I and Q signals to allow for identification of the mobile and correlation at the receiver. The PN (1) sequence is the same for I and Q and it does not affect the number of 90° transitions. The PN (1) spreading code can instead be directly multiplied with the I and Q components of the scrambling signal before the complex scrambling.

17 W-CDMA: primary PN spreading code The PN (1) sequence does not affect the number of 90° transitions.

18 W-CDMA: secondary PN spreading code A decimated secondary PN spreading code (P) is multiplied with the Q component of the Walsh rotator, W 1 =(1,-1). P is a decimated version of the real chip rate sequence PN (2). P randomizes the direction of the phase rotation while keeping the phase difference of 90° between pairs of consecutive final points.

19 W-CDMA: secondary PN spreading code The decimated secondary PN sequence (P) randomizes the direction of the rotation.

20 W-CDMA: uplink structure

21 W-CDMA: uplink DPCH/DPCCH coding, spreading, and scrambling.

22 In Band/in Channel Measurements  Modulation domain measurements Error vector QPSK EVM Composite EVM Symbol EVM Rho Code domain power

23 Modulation Domain Measurements Error Vector The resulting constellation depends on the physical channel configuration. The constellation typically does not look like QPSK, or any other known constellation, except for some very specific channel configurations. For example, a signal with a single DPDCH (or a single DPCCH) does map onto a QPSK constellation. A signal with a DPDCH and a DPCCH at the same amplitude level maps onto a 45°–rotated QPSK constellation. You can use a regular EV measurement to evaluate the modulation quality of the transmitter for a single DPDCH, a single DPDCH, or a signal with both at the same amplitude level. More complex signals cannot be analyzed with this measurement.

24 Modulation Domain Measurements QPSK EVM A regular QPSK EVM measurement can be used to evaluate the modulation quality of the transmitter for a single DPDCH, a single DPDCH, or a signal with both at the same amplitude level. More complex signals cannot be analyzed with this measurement. QPSK EVM compares the measured chip signal at the RF with an ideal QPSK reference.

25 Modulation Domain Measurements Composite EVM The composite EVM is useful throughout the development, performance verification, and manufacturing phases of the UE life cycle. It in particular allows the: evaluation of the quality of the transmitter for a multi-channel signal. detection of spreading or scrambling errors. detection certain problems between the baseband and RF sections. analysis of errors that cause high interference in the signal.

26 By descrambling and despreading the signal the constellation and EVM for a specific code channel at the symbol level, even in the presence of multiple codes, can be analyzed. Modulation Domain Measurements Symbol EVM

27 Misurazioni in Banda/nel Canale Dominio della modulazione  Rho (  )  obiettivo: valutare la qualità di modulazioni numeriche a divisione di codice (CDMA);  definizione: rapporto tra la potenza correlata e la potenza complessiva del segnale in banda base trasmesso, caratterizzato da un solo codice (un solo canale);  procedura: la potenza correlata è calcolata rimuovendo gli offset in frequenza, in fase e nel tempo e calcolando, in banda base, la mutua correlazione fra il segnale misurato e quello di riferimento (ideale);  strumentazione: VSA, analizzatore di spettro dotato di specifica measurement personality.

28 Misurazioni in Banda/nel Canale Rho (  ) Il valore di rho è tanto più elevato quanto maggiore è il grado di “somiglianza” tra il segnale trasmesso e quello ideale. Poiché la potenza non correlata si manifesta come interferenza, bassi valori di rho compromettono la capacità di una cella nei sistemi radiomobili.

29 Misurazioni in Banda/nel Canale Dominio della modulazione  Potenza nel dominio dei codici  obiettivo: verificare se la stazione base sta trasmettendo la potenza prevista sui canali attivi (differenti codici) oppure sta trasmettendo anche sui canali inattivi, generando interferenza con altri utenti;  definizione: potenze associate alle diverse componenti (caratterizzate da diversi codici) di un segnale in banda base in modulazione CDMA;  procedura: le diverse componenti in banda base sono separate sfruttando l’ortogonalità tra i codici;  strumentazione: VSA, analizzatore di spettro dotato di specifica measurement personality.

30 Misurazioni in Banda/nel Canale Potenza nel dominio dei codici La potenza associata ai vari canali può essere valutata solo dopo la loro decodifica.

31 In Band/out of Channel Measurements ACLR is a measure of transmitter performance. It is defined as the ratio of the transmitted power to the power measured after a receiver filter in the adjacent RF channel. This is what was formerly called adjacent channel power ratio. ACS is a measure of receiver performance. It is defined as the ratio of the receiver filter attenuation on the assigned channel frequency to the receiver filter attenuation on the adjacent channel frequency. ACIR is a measure of overall system performance. It is defined as the ratio of the total power transmitted from a source (BS or UE) to the total interference power resulting from both transmitter and receiver imperfections affecting a victim receiver. The following relation holds