3G Mobile System

What is 3G Mobile System?

•Broadband Service
•Packet Based Transmission
•Transmission of text, digitized voice, video at data rate higher than 2 Mbps
•Consistent set of services to mobile computer and phone users wherever they are located in the world

3G Spectrum Allocation

3G Terms

•IMT 2000
–Third generation mobile systems as defined by ITU
–Global recommendation
•3GPP
–3rdGeneration Partnership Project (Forum for a WCDMA standardization)
–Involved: ETSI (Europe), ARIB (Japan), TTA (Korea), T1P1 (USA), TTC (Japan) and CWTS (China)
•UMTS
–Third generation telecommunication system that is subject to specifications produced by 3GPP
•WCDMA
–Air Interface technology adapted for UMTS Terrestrial Radio Access (UTRA)
•UTRA-FDD
–WCDMA in 3GPP, FDD mode
•UTRA-TDD
–WCDMA in 3GPP, TDD mode
•CDMA2000
–Air Interface technology proposal from TR45.5 (USA) on evolution of IS-95 (CDMA)

UMTS System Characteristics

•W-CDMA : 5 MHz
•Carrier Spacing : multiples of 200 kHz
•W-CDMA spreading rate = 3.84 Mchip/s
•Chip Rate = 3.84 MHz
•Raised cosine filtering with roll-off 0.22
•Information bit rate: between 8 kbit/s and 2 Mbit/s(currently up to 384 Kbit/s)
•Spreading Factor (SF): 4 -256
•Multiple Access Scheme : Wideband DS-CDMA
•Duplex Scheme: FDD and TDD modes
•Carrier Spacing: 4.4 –5.4 MHz
•10 ms frame with 15 time slots
•Node synchronization: asynchronous
•Highly variable data rates, data rate constant within 10 ms frame
•Bandwidth on demand, efficient resource usage
•Multiple services with different variable data rates over one physical channel

Characteristic to WCDMA

•RAKE receiver takes advantage of multipath propagation
•Fast power control keeps system stable by using minimum power necessary for links
•Soft handover ensures smooth handovers

Multiservice Environment

•Data speed–In RAN1 bit rate varies from 8 kbps up to 384 kbps–Variable bit rate also available–Bit rate gradually grows up to 2 Mbps
•Service delivery type–Real-time (RT) & non real-time (NRT)
•Quality classes for user to choose–Different error rates and delays
•Traffic asymmetric in uplink & downlink
•Common channel data traffic (FACH)
•Inter-system handovers

Air Interface
•Capacity and coverage coupled -“cell breathing”
•Neighbor cells coupled via interference
•Soft handover
•Fast power control
•Interference limited system (e.g. GSM frequency limited)

Key features of WCDMA

•Soft handoff: user equipment (UE) and base stations use special rake receivers that allow each UE to simultaneously communicate with multiple base stations. The diversity gain associated with soft handoff is known as the "soft handoff gain factor".

•Multipath reception: the rake receivers also allow the UE to decode multiple signals that have traveled over different physical paths from the base station. For example, one signal may travel directly from the base station to the UE, and another may reflect off a large building and then travel to the UE. This phenomenon, "multipath propagation", also provides a diversity gain. The same effect occurs on the uplink from the UE to the base station.

•Power control: transmissions by the UE must be carefully controlled so that all transmissions are received with roughly the same power at the base station. If power control is not used, a “near-far” problem, where mobiles close to the base station over-power signals from mobiles farther away, occurs. The base station uses a fast power control system to direct the mobile to power up or power down as its received signal level varies due to changes in the propagation environment. Likewise, on the downlink, transmissions from the base stations are power-controlled to minimize the overall interference throughout the system and to ensure a good received signal by the UE.

Frequency reuse of 1: every base station in the CDMA system operates on the same frequency for a given carrier, so no frequency planning is required. As every site causes interference to every other site, careful attention must be paid to each site's radio propagation.

Soft capacity: capacity and coverage are intertwined in CDMA, depending on the number of users in the system and the amount of interference allowed before access is blocked
for new users. By setting the allowed interference threshold lower, coverage will improve at the expense of capacity. By setting the threshold higher, capacity will increase at the expense of coverage. Because of the fundamental link between coverage and capacity, cells with light traffic loads inherently share some of their latent capacity with more highly loaded surrounding cells.

WCDMA Compared to GSM and IS-95 CDMA

WCDMA vs. GSM
WCDMA has some similarities with GSM technology, however, it is a fundamentally different technique for allowing multiple users to share the same spectrum and as a result it has many differences.


WCDMA vs. IS-95 CDMA