GENERAL 3G RELATED QUESTIONS

By Zahid Ghadialy (zahidtg@yahoo.com)

Last Updated: 31/08/2005

What does the abbreviation 3G (or UMTS or XXX) stand for?

Please see Vocabulary Section for the abbreviation you are looking for.

I am looking for a particular definition related to 3G/UMTS, where can i find it?

Please see Terms and Definitions section for the definitions you are looking

I am looking for UMTS/3G related job, what are good places to look for them?

Please see Jobs Section for the job you are looking. If you know of any other good links, please let us know so we can add it.

What are these 1G, 2G, 2.5G, 3G and 4G

All these terms specified are the Generations of the mobile networks.

  • 1G networks (NMT, C-Net's, AMPS, TACS) are considered to be the first generation analog cellular systems, which started in early 1980s.
  • 2G networks (GSM, cdmaOne, DAMPS) are the first digital cellular systems launched early 1990s.
  • 2.5G networks (GPRS, cdma2000 1x) are the enhanced versions of 2G networks with data rates up to about 144kbit/s.
  • 3G networks (UMTS FDD and TDD, cdma2000 1x EVDO, cdma2000 3x, TD-SCDMA, Arib WCDMA, EDGE, IMT-2000 DECT) are the latest cellular networks that have data rates 384kbit/s and more.
  • 4G is mainly a marketing buzzword at the moment. Some basic 4G research is being done, but no frequencies have been allocated. The Forth Generation networks are expected to be ready for by around 2012.

What is TDD and FDD?

TDD stands for Time Division Duplex and FDD stands for Frequency Division Duplex. They are different modes of CDMA. In FDD mode of transmission both the Transmitter and the Receiver transmit simultaneously. This simultaneous transmission is possible because they are both on different frequencies. In TDD mode of operation either Transmitter or Receiver can transmit at one time. This is because they use the same frequency for the transmission.

Which mode is more common, TDD or FDD?

At present all the major 3G Networks are using FDD mode of operation. As far as i am aware there are no commercial TDD networks at the moment. Recently T-Mobile announced that they wil install TDD Network in Czech Republic. See News Section for more details.

Can you expand on the FDD mode of operation?

In the FDD mode of operation, the uplink and downlink use separate frequency bands. These carriers have a bandwidth of 5 MHz. Each carrier is divided into 10-ms radio frames, and each frame further into 15 time slots. The frequency allocation consists of one frequency band at 1920-1980 MHz and one at 2110-2170 MHz. These frequency bands are used in FDD mode both by the UE and the Network. The lower frequency band is used for the Uplink (UL) transmission and the upper frequency band is used for the Downlink (DL) transmission. The frequency separation is specified with 190 MHz for the fixed frequency duplex mode and with 134.8MHz to 245.8MHz for the variable frequency duplex mode.

Can you expand on the TDD mode of operation?

The TDD mode differs from the FDD mode in that both the uplink and the downlink use the same frequency carrier. There are 15 time slots in a radio frame that can be dynamically allocated between uplink and downlink directions. Thus the channel capacity of these links can be different which is very advantageous especially when people are downloading stuff on their mobiles. The chip rate of the normal TDD mode is also 3.84 Mcps, but there exists also a “narrowband” version of TDD known as TD-SCDMA. The carrier bandwidth of TD-SCDMA is 1.6 MHz and the chip rate 1.28 Mcps. TD-SCDMA has been proposed by China and potentially has a large market share in China if implemented.

What is TDD HCR and TDD LCR?

HCR stands for "High Chip Rate" and is same as 3.84Mcps TDD described above. LCR stands for "Low Chip Rate" ans is the same as TD-SCDMA described above.

Can you expand on the unequal bandwidth concept in TDD?

The HCR TDD uses 10ms radio frame that is divided into 15 time slots each being able to carry a chip sequence of 2560 complex valued chips. At least one slot has to be reserved for Downlink (DL) transmission to allow for broadcast information and one for Uplink (UL) transmission in order to realize customer’s access to the system. The remaining slots can be arbitrarily distributed to either direction in order to adapt to the asymmetry of requested services. The LCR option, a 10 ms radio frame is divided into two sub-frames of 5 ms duration. Each of the sub-frames contains seven time slots. Transmission bursts fitting into a single slot contain 864 complex valued chips. The first time slot is always used for DL transmission, the latter six can be divided into UL and DL transmission adaptively, starting with the time slots used for UL. Unlike transmission in the HCR mode, the time slots used for LCR transmission in a certain direction have to be grouped together. Between the first two slots in each sub-frame special synchronization and pilot signals are included.

If FDD is so popular why would people use TDD mode of operation?

Juha Korhonen in his book Introduction to 3G Mobile Communications has summarised the reasons for TDD mode being used. They are:

  • The main reason for TDD use is spectrum allocation. The spectrum allocated for IMT-2000 is asymmetric, which means that an FDD system cannot use the whole spectrum, as it currently requires symmetric bands. Thus the most obvious solution was to give the symmetric part of the spectrum to FDD systems, and the asymmetric part to TDD systems. The proposed spectrum allocations for UTRAN TDD are 1,900–1,920 MHz and 2,010–2,025 MHz.
  • Many services provided by the 3G networks will require asymmetric data transfer capacity for the uplink and downlink, where the downlink will demand more bandwidth than the uplink. A typical example of this is a Web-surfing session. Only control commands are sent in the uplink, whereas the downlink may have to transfer hundreds of kilobits of user data per second toward the subscriber. As the TDD capacity is not fixed in the uplink and downlink, it is a more attractive technology for highly asymmetric services. The base station can allocate the time slots dynamically for the uplink or downlink according to current needs.
  • Another reason for TDD is easier power control. In the TDD mode both the uplink and downlink transmissions use the same frequency; thus, the fast fading characteristics are similar in both directions. The TDD transmitter can predict the fast fading conditions of the assigned frequency channel based on received signals. This means that closed-loop power control is no longer needed, but only open loop will be sufficient. However, openloop control is based on signal levels, and if the interference level must be known, then this must be reported using signaling.
  • This “same channel” feature can also be used to simplify antenna diversity. Based on uplink reception quality and level, the network can choose which base station can best handle the downlink transmissions for the MS in question. This means less overall interference.
  • Since the UE only has to be active (receiving or transmitting) during some of the time slots. There are always some idle slots during a frame and those can be used for measuring other base stations, and systems.

All the advantages above make TDD look better option than FDD. Why not use only TDD mode? Are there any problems?

The following are the problems that make TDD unpopular:

  • The main problem is interference from TDD power pulsing. The higher the mobile speed, the shorter the TDD frame so that fast open-loop power control can be used. This short transmission time results in audible interference from pulsed transmissions, both internally in the terminal and with other electronic equipment. Also, the timing requirements for many components are tighter. Both problems can be solved, but the solutions probably require more costly components.
  • A TDD system is prone to intracell and intercell interference between the uplink and downlink. The basic problem is that in adjacent cells, the same time slot can be allocated for different directions. It may happen that one UE tries to receive on a slot while another UE nearby transmits on the same slot. The transmission can easily block the reception attempt of the first UE. This problem can be prevented if all base stations are synchronized, and they all use the same asymmetry in their transmissions. However, this is costly (time-synchronous base stations), and also limits the usability of the system (fixed asymmetry).

How is UMTS subscriber differentiated from a GSM subscriber?

UMTS subscriber differentiated from a GSM subscriber based on SIM card. For UMTS and GSM subscriiber the SIM is different. UMTS subscriber uses USIM while GSM one uses SIM.

Can 2G SIM be used to access 3G Services?

Section 13.1 of 22.101 says: For access to services, provided by PS or CS CN domains, a valid USIM shall be required. Optionally, SIM according to GSM phase 2, GSM phase 2+, 3GPP release 99, 3GPP release 4 specifications may be supported. I guess its upto the network to allow or disallow 2G SIM accessing 3G services.





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