1. 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.
y 1G networks (NMT, C-Net's, AMPS, TACS) are considered to be the first
generation analog cellular systems, which started in early 1980s.
y 2G networks (GSM, cdmaOne, DAMPS) are the first digital cellular systems
launched early 1990s.
y 2.5G networks (GPRS, cdma2000 1x) are the enhanced versions of 2G networks
with data rates up to about 144kbit/s.
y 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.
y 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.
2. 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
³narrowband´ 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
3. 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:
y 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.
y 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.
y 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.
y This ³same 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.
y 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:
y 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