• CPCH is intended to carry packet-based user data in the uplink. The reciprocal channel providing the data in the downlink direction is the FACH.  ([Holma 2007] P. 95)

crop up

• The basic reason behind a handover crops up when the air interface connection no loger fulfills the desired criteria set for it. ([Kaaranen 2005] P.113)
  

History

In 1982, the European Conference of Postal and Telecommunications Administrations (CEPT) created the Groupe Spécial Mobile (GSM) to develop a standard for a mobile telephone system that could be used across Europe.

In 1987, a memorandum of understanding was signed by 13 countries to develop a common cellular telephone system across Europe. Finally the system created by SINTEF lead by Torleiv Maseng was selected.

In 1989, GSM responsibility was transferred to the European Telecommunications Standards Institute (ETSI) and phase I of the GSM specifications were published in 1990. The first GSM network was launched in 1991 by Radiolinja in Finland with joint technical infrastructure maintenance from Ericsson.

By the end of 1993, over a million subscribers were using GSM phone networks being operated by 70 carriers across 48 countries.

Network structure

The network behind the GSM seen by the customer is large and complicated in order to provide all of the services which are required. It is divided into a number of sections and these are each covered in separate articles.

• the Base Station Subsystem (the base stations and their controllers).
• the Network and Switching Subsystem (the part of the network most similar to a fixed network). This is sometimes also just called the core network.
• the GPRS Core Network (the optional part which allows packet based Internet connections).
• all of the elements in the system combine to produce many GSM services such as voice calls and SMS.
• 
  

Packet control unit

The packet control unit (PCU) is a late addition to the GSM standard. It performs some of the processing tasks of the BSC, but for packet data. The allocation of channels between voice and data is controlled by the base station, but once a channel is allocated to the PCU, the PCU takes full control over that channel.

The PCU can be built into the base station, built into the BSC or even, in some proposed architectures, it can be at the SGSN site. In most of the cases, the PCU is a separate node communicating extensively with the BSC on the radio side and the SGSN on the Gb side.

Physical and Logical Channels

Traffic Channels (TCHs)

Full-Rate TCH

Full-Rate Speech Channel (TCH/FS) : Carries speech digitized at a raw data rate of 13 kbps, sent at 22.8 Kbps.

Full-Rate Data Channel for 9600 bps (TCH/F9.6) : Carries data sent at 9.6 Kbps. With FEC code, the data is sent at 22.8 Kbps.

Full-Rate Data Channel for 4800 bps (TCH/F4.8) : Carries data sent at 4.8 Kbps. With FEC code, the data is sent at 22.8 Kbps.

Full-Rate Data Channel for 2400 bps (TCH/F2.4) : Carries data sent at 2.4 Kbps. With FEC code, the data is sent at 22.8 Kbps.

Half-Rate TCH

Half-Rate Speech Channel (TCH/HS) : Carries speech digitized at 6.5 Kbps, sent at 11.4 Kbps.

Half-Rate Data Channel for 4800 bps (TCH/H4.8) : Carries data sent at 4.8 Kbps. With FEC code, the data is sent at 11.4 Kbps.

Full-Rate Data Channel for 2400 bps (TCH/H2.4) : Carries data sent at 2.4 Kbps. With FEC code, the data is sent at 11.4 Kbps.

(For more details about FEC channel coding, turn to [7].)

Control Channels (CCHs)

Broadcast Channels (BCHs)

Broadcast Control Channel (BCCH) - DOWNLINK -

Frequency Correction Channel (FCCH) - DOWNLINK -

Synchronization Channel (SCH) - DOWNLINK -

Common Control Channels (CCCHs)

Paging Channel (PCH) - DOWNLINK -

Random  Access Channel (RACH) - UPLINK -

Access Grant Channel (AGCH) - DOWNLINK -

Dedicated Control Channels (DCCHs)

Stand-alone Dedicated Controls (SDCCHs) - UPLINK/DOWNLINK -

Slow Associated Control Channel (SACCH) - UPLINK/DOWNLINK -

Fast Associated Control Channel (FACCHs) - UPLINK/DOWNLINK -

BCHs

• BCCH: This channel contains system parameters needed to identify the network and gain access. These paramters include the Location Area Code (LAC), the Mobile Network Code (MNC), the frequencies of neighboring cells, and access parameters.
• FCCH: This channel is used by the MS as a frequency reference. This channel contains frequency correction bursts.
  
• SCH: This channel is used by the MS to learn the Base Station Information Code (BSIC) as well as the TDMA frame number (FN). This lets the MS know what TDMA frame they are on within the hyperframe.

• PCH: This channel is used to inform the MS that it has incoming traffic. The traffic could be a voice call, SMS, or some other form of traffic.
• RACH: This channel is used by a MS to request an initial dedicated channel from the BTS. This would be the first transmission made by a MS to access the network and request radio resources. The MS sends an  Access Burst on this channel in order to request access
• AGCH: This channel is used by a BTS to notify the MS of the assignement of an initial SDCCH for initial signaling.

• SDCCH: This channel is used for signalling and call setup between the MS and the BTS.
• SACCH: This channel is a continuous stream channel that is used for control and supervisory signals associated with the traffic channels.
• FACCH: This channel is used for control requirements such as handoffs. There is no TS and frame allocation dedicated to a FAACH. The FAACH is a burst-stealing channel, it steals a Timeslot from a TCH.

Frame Structure

For a frame for traffic channe, a super frame consists of 51 multiframe that is made of 26 TDMA frames. For a frame for control channel, a super frame consists of 26 multiframe that contains 51 TDMA frames. Each TDMA frame spans 4.615 ms, consisting of 8 time slots, during each of which a user sends data called "burst". Of a normal burst, the payload (information-bearing part) occupies two 57 bit blocks.

Data Rates

The gross data rate is 32500bits/120ms = 270.83 kbit/s, resulting in 270.83/8 = 33.854 kbit/s per user. User data is actually sent at 24.7 kbit/s (57 bits * 2 / 4.615ms), excluding the overhead in the burst.

Slow Frequency Hopping

GSM employs slow frequency hopping (SFH) to mitigate the effects of multipath fading　and interference. Each burst belonging to a particular physical channel will be transmitted　on a different carrier frequency in each TDAM frame. Thus the hopping rate is equal to the　frame rate (i.e.' 217 frames/s). The only physical channels that are not allowed to hop are the broadcast and common control channels (i.e. the FCH, SCH, BCCH, PCH and AGCH).

The effect of frequency hopping on interference

In a non-frequency hopping GSM system, an MS will tend to experience interference from the same set of MSs in neighbouring co-channel cells. In a frequency hopped system, the hopping patterns (i.e. the sequence of transmission frequencies) are different in co-channel cells and the MS will experience interference from a different set of MSs on each burst. This effectively randomises the interference and each MS will experience an average level of interference.





Literature

UMTS (Universal Mobile Telecommuniations System)

<伊落丹> illidan.modeler [at] gmail.com
Northern Capital, Republic of Pandaren

System Architecture

UTRAN

Radio Resource Control

General packet radio service (GPRS) is a packet oriented mobile data service available to users of the 2G cellular communication systems global system for mobile communications (GSM), as well as in the 3G systems. In the 2G systems, GPRS provides data rates of 56-114 kbit/s.2G cellular systems combined with GPRS are often described as 2.5G. It provides moderate speed data transfer, by using unused time division multiple access (TDMA) channels.

GPRS data transfer is typically charged per megabyte of traffic transferred, while data communication via traditional circuit switching is billed per minute of connection time, independent of whether the user actually is using the capacity or is in an idle state. GPRS is a best-effort packet switched service, as opposed to circuit switching, where a certain quality of service (QoS) is guaranteed during the connection for non-mobile users.

Originally there was some thought to extend GPRS to cover other standards, but instead those networks are being converted to use the GSM standard, so that GSM is the only kind of network where GPRS is in use. GPRS is integrated into GSM Release 97 and newer releases. It was originally standardized by European Telecommunications Standards Institute (ETSI), but now by the 3rd Generation Partnership Project (3GPP).

GPRS was developed as a GSM response to the earlier CDPD and i-mode packet switched cellular technologies.

The diagram that depicts the interfaces and key components is shown below;

Interfaces in the GPRS network

Enhanced Data Rates for GSM Evolution (EDGE)

Transmission techniques

In addition to Gaussian minimum-shift keying (GMSK), EDGE uses higher-order PSK/8 phase shift keying (8PSK) for the upper five of its nine modulation and coding schemes. EDGE produces a 3-bit word for every change in carrier phase. This effectively triples the gross data rate offered by GSM. EDGE, like GPRS, uses a rate adaptation algorithm that adapts the modulation and coding scheme (MCS) according to the quality of the radio channel, and thus the bit rate and robustness of data transmission. It introduces a new technology not found in GPRS, Incremental Redundancy, which, instead of retransmitting disturbed packets, sends more redundancy information to be combined in the receiver. This increases the probability of correct decoding.

EDGE can carry data speeds up to 236.8 kbit/s (with end-to-end latency of less than 150 ms) for 4 timeslots (theoretical maximum is 473.6 kbit/s for 8 timeslots) in packet mode. This means it can handle four times as much traffic as standard GPRS. EDGE meets the International Telecommunications Union's requirement for a 3G network, and has been accepted by the ITU as part of the IMT-2000 family of 3G standards. It also enhances the circuit data mode called HSCSD, increasing the data rate of this service.

1. Web http://en.wikipedia.org/wiki/Fading
2. Web http://www.ylesstech.com/terminology.php?letter=all&id=15
   
3. [Rappaport 2001]
4. [Goldsmith 2005]
5. [Tse 2005]