ECE 5221 Personal Communication Systems

1. ECE 5221 Personal Communication Systems Prepared by: Dr. Ivica Kostanic Lecture 23 – Basics of 3G - UMTS Spring 2011

2. Property Value Access scheme DS-CDMA with FDD and asynchronous operation Bandwidth 5MHz Chip rate 3.84 Mc/second Spreading Variable: UL 1-512 (power of 2), DL(1-256) Single code user rates UL (after coding) 15,30,60,120,240,480, 960 kb/sec Up to 6 code aggregation Single code user rates DL(after coding) 15,30,60,120,240,480,960,1920 kb/sec Up to 3 code aggregations Frame Length 10ms Power control Open loop and closed loop with 1500 commands/sec Detection Coherent on both UL and DL Diversity support DL TX Diversity, DL and UL RAKE receiver reception, UL space diversity W-CDMA (UMTS-FDD) • 3G and 2G completely different air interfaces • Advanced radio resource management – required by diverse 3G applications • Multi-rate spreading • W-CDMA is interference limited • Provides soft capacity and • Coverage, capacity, qualitytradeoffs Summary of W-CDMA properties UL – uplink; DL - downlink

3. DS SS Systems - basic principles Processing of the signal for a single CDMA user RF modem part is independent of CDMA • Three basic stages • Spreading • “RF Modem” • De-spreading

4. DS CDMA - multiple users • After spreading signals from multiple users are summed • Signals from multiple users co-exist in time and frequency • The spreading codes have to be orthogonal

5. Example of DS CDMA - two users same PG Processing gain (PG) is the ratio of chip and bit rates

6. UMTS Voice Example At RF (before de-spreading) Processing gain At the base-band (after de-spreading) • Required S/N ratio for voice after de-spreading is around 5dB • Signal can have S/(N+I) of -20dB and still be received successfully • DS CDMA allows demodulation of signals that are below interference and/or noise floor Note: processing gain is derived through reshaping of the power spectrum density in the frequency domain Vocoder rate 12.2kbps Chip rate 3.84Mbps

7. Orthogonal Variable Spreading Factor codes (OVSF) OVSF code generation • UMTS-FDD uses OVSF codes • OVSF codes preserve orthogonality even for different code lengths • Codes are designated with 2 numbers • first number is the length • second number is the position in the code tree • OVSF codes are orthogonal if they are not on the same path from the root of the code tree

8. OVSF code - orthogonality Example: Illustration of the orthogonality OVSF codes are orthogonal if they are not on the same path Consequence: assignment of a given code eliminates all codes down the path Note: in CDMA unique code is the channel. In 3G one may have many low rate or few high rate “channels”

9. W-CDMA variable spreading - equal powers User 1 and user 2 have different data rates User 1 and user 2 use codes of different length User 2 has a smaller processing gain Decision making process is easier for user 1

10. W-CDMA variable spreading - equal Eb/Nt User 2 adjusts its power to compensate for smaller processing gain With power adjustments, both users have same symbol energy after de-spreading

11. Multipath and Rake RX (1) • Terrestrial environment – multipath propagation of RF signal • Multipath propagation results: • Signal dispersion: at the RX energy is dispersed among multiple components • Signal fading: each component is subject to fading • Signal dispersion – energy reaches received through resolvable multipath components • Components are resolvable if their relative delay is larger than a chip interval • In W-CDMA one chip time interval corresponds to 78m of path difference • Resolvable multipath components are combined using maximum ratio combining (MRC) Power delay profile example for 5MHz channel

12. Multipath and Rake RX (2) • Signal fading – each delay position usually consists of several components • The components have random phases – causes fading • Fading occurs at the scale comparable to ½ of a wavelength (~ 7cm) • Fading may be as much as 30dB deep • Fading is mitigated through interleaving and coding Example signal variations due to fast fading

13. Operation of Rake RX • Rake receiver consist of multiple receiving “fingers” and searchers • Rake RX algorithm • Identify the time delay positions with significant energy and assign them to fingers • Demodulate resolvable multipath receptions at each of the fingers • Combine demodulated and phase adjusted symbols across all active fingers and present them to the demodulator • Typically phone’s rake receiver has minimum of • Three fingers • One searcher • Delay resolution for a searcher is typically 14/-1/2 chip interval Simplified diagram of a rake receiver Note: fingers may be used for multi-paths or for different cells in soft handover

14. Power control • WCDMA implements power control on both uplink and downlink • On the UL - two loops for power control • Inner (fast loop) • Outer (slow loop) • Inner loop (uplink) • Fast adjustments of MS TX power so that target SIR at the base station is met • Rate: up to 1500 power adjustments/sec • Outer loop (uplink) • Adjustment of the SIR target at the base station • On the DL • Provide increase of power for edge of the cell mobiles • Compensate for some fast fading effects Note: SIR target usually changes as a function of propagation environment and mobile’s speed

15. Soft/softer handovers • Softer – mobile is in communication with sectors of the same cell • Soft – mobile is in communication with sectors of different cells • Essential interference mitigation tool • Ensures that the mobile is power controlled by all cells that cover certain area • Prevents interference by reducing “near-far” problem • Form mobile standpoint – soft and softer are essentially the same • From the system perspective soft and softer differ in number of allocated resources Softer handover Note: soft handover requires additional resources between Node B and RNC Soft handover