3G: Packet Transmission in EDGE and UMTS FDD
In my last article, I highlighted some aspects of the Time Division Duplex (TDD) component of the 3GPP proposal to the ITU. We saw in particular that industry professionals believe that major companies plan to start their development with FDD systems. This month, I want to address packet transmission in evolutionary 3G systems, particularly as it pertains to EDGE and the UMTS FDD mode (aka IMT-DS).
A typical example of packet transmission is the case where several users are browsing the Web. It is expected that each user requests a relatively high transmission rate but only transmits or receives actively a fraction of the time (low duty-cycle). The rest of the time, each user simply reads or processes the data it has already received.
EDGE
One of the key features motivating the move from 2G to 3G systems is the ability to more efficiently handle the transmission of packets between the base station (BS) and the user equipment (UE). Already so-called 2.5G systems, such as GPRS (General Packet Radio Service) and its EDGE (Enhanced Data Rates for Global Evolution) enhanced version EGPRS aim at improving the transmission of packets. Two main characteristics of EGPRS are to provide higher data rates than the classical version GSM and optimize the system capacity for the transmission of packets.
While GSM uses a fixed modulation scheme (GMSK- Gaussian Minimum Shift Keying) and one slot out of 8 for transmission, EGPRS enables the use of a higher number of slots and dynamic adjustment of the transmission mechanism to adapt to the environment.
There are three possible adjustments under this scheme. First, the modulation scheme can be toggled between the more robust but less efficient GSM GMSK scheme and the classical 8-PSK scheme (with an additional rotation of 3P/8 to avoid zero crossings of the envelop). In good environments, 8-PSK provides a better use of the available spectrum. The transmitter chooses the modulation scheme and the receiver on the other side blindly determines which scheme was used.
Second, the amount of coding can be varied from a 1/1 (no coding) rate up to about 1/3. The latter is used in bad environments. Nine modulation & coding schemes are defined -- MCS-1 through MCS-9. The last adjustment option entails an elaborate retransmission scheme called Incremental Transmission (IR) or hybrid II/III ARQ. It consists in transmitting only part of the redundancy obtained through channel coding (convolutional coding) in a first step and transmitting additional (or the same) redundancy later on only if the receiver on the other side reports a decoding error. This saves system capacity at the expense of a possible decoding delay.
Based on these assertions, the bit rate available to users depends on the propagation conditions and hence on the position of the user within the cell. For example, a user close to the base station is able to use low coding and 8-PSK modulation and enjoy a high-speed connection, while users at the border of a cell experience a low-speed connection at the very same moment.
UMTS FDD
In the FDD component of UMTS, packet transmission is handled using slightly different concepts in the uplink (UL) and the downlink (DL) directions. In both directions, however, techniques have been devised that allow the BS to efficiently accommodate both high and low duty-cycle transmissions.
While a high-duty-cycle case is best accommodated on a user-dedicated connection (a user is assigned a dedicated channel or DCH), more "bursty" transmissions from multiple users are usually grouped together and mapped on a shared (or common) channel. This channel can be the FACH or DSCH on the downlink or the RACH or CPCH channels on the uplink, depending on the amount of data [1]. The idea there is to allocate as few resources as possible. It is hence possible to conceive that a cell that supports only 8 high speed channels, for example, is able to accommodate several dozens of users because each user is idle most of the time. In the same vein, each user potentially can take advantage of the maximum transmission rate supported when all other users are idle.
Another difference with EDGE for packet transmission is the use of turbo coding as an FEC scheme (EDGE mostly uses convolutional coding). Turbo coding is a technique that provides low BERs—even in noisy environments. The corresponding decoder (turbo decoder) uses an iterative process to gradually improve the quality of the received signal before making a final decision on the bits received. Turbo coding is also used in the TDD component of UMTS as well as the multi-carrier (MC) mode. As with EDGE, ARQ (Automatic Repeat Request) schemes are also used in order to ensure virtually error-free transmission of data in packet mode.
To conclude this brief discussion the handling of packet transmission, let me point out that the percentage of packet-oriented wireless traffic is expected to increase rapidly over the near years. It is hence crucial to optimize this aspect of future wireless systems such as 2.5G and 3G systems.
Reference
[1] 3GPP specifications, TS25 series (such as TS25.211 and TS25.301), http://www.3gpp.org
About the author:
Marc Barberis is a staff engineer at Synopsys in Mountain View, CA, where he is currently involved in the development of the Synopsys 3G Products. He has worked among others on the design of receivers for 2G wireless. He received an MS in electrical engineering from Ecole Nationale Superieure des Telecommunications, France. He can be reached at barberis@synopsys.com.