wireless per nl
TRANSCRIPT
-
7/31/2019 Wireless Per Nl
1/3
GSM Transmit Frequencies
GSM-like systems have been specified for the 900 MHz (GSM), 1800 MHz (DCS-1800) and 1900
MHz (PCS-1900, USA) radio bands. In GSM, mobiles transmit in a 890-915 MHz band and receive in
a 935-960 MHz band. The duplex spacing is 45 MHz. These bands are divided into 125 bands with
200 kHz spacing. The Absolute Radio Frequency Channel Number (ARFCN) n (n = 0,1, ... 124)
identifies the uplink carrier frequency
890.0 MHz + n 0.2 MHz.
The corresponding downlink is 45 kHz higher. Extensions (E-GSM) are channel 974 to 1023, which areallocated just below the original GSM bands. DCS-1800 uses channel 512 to 885, i.e., carrier
frequencies of
1,710.0 MHz + (n-511) 0.2 MHz.
and a duplex spacing of 95 MHz. PCS-1900 operates in channels numbered from 512 to 810, with
frequencies of
1,850.0 MHz + (n-512) 0.2 MHz.
and a duplex spacing of 80 MHz.
Multiple Access
Each GSM channel uses a TDMA method to support simultaneous calls from multiple subscribers. Mobile terminals transmit in bursts of 577
microseconds. One burst fits into one time slot. Through feedback signals, the system can ensure that the signals from all users arrive exactly
in their assigned time slot, even if propagation distances to and from different users vary over time. The terminal has 28 microseconds of
guard time to power up or switch off the RF signal.
It has been noted that this pulsed transmission has may cause some interference the electronic equipment if it is not properly protected.
Particularly airbag ignition circuits have suffered interference from GSM handset transmissions.
A set of 8 TDMA slots is called a frame. In GSM, the terminal transmits and receives during different times slots. This avoids the expensive
duplex filter that otherwise would have been needed to avoid that strong transmit signals leak into the receiver.
Burst Structure
A normal burst contains
3 tail bits allowing the transmitter to power up.
57 coded data bits
1 bit stealing flag
26 bits training sequence for synchronization and training of the adaptive equalizer
1 bit stealing flag
57 coded data bits
3 tail bits
a guard time corresponding to 8.25 bits
Random access bursts carry much less data but have long guard spacings to avoid that poorly aligned burst transmissions interfere in other
slots. The frequency correction burst and synchronization bursts allow proper operation of the terminal.
Frame StructureThe GSM standard distinguishes between physical channels and logical channels. Logical channels carry user speech or data. Data or
signalling messages can travel over different physical channels. Logical channels can be
Traffic channels carrying user speech or data
The full rate channel (TCH/FS) has a net rate of 13 kbit/s.
The half rate channel (TCH/HS) has been under discussion for a long time.
TCH/F9.6, 4.8 and 2.4 provides data and fax communication at 9.6, 4.8 and 2.4 kbit/s respectively. The data occupies a full
channel, but the strength or error protection differs.
TCH/H4.8 and 2.4 carry data over a half rate channel.
Control Channels carrying network messages. Broadcast channels apply only to communication from base station to mobile.
Broadcast Control Channel (BCCH)
Frequency Correction Channel (FCCH)
Synchronization Channel (SCH)
Common Control Channels (CCCH) support the set-up of a link between mobile terminal and base station.The Random Access Channel (RACH) allows random access by the terminals to initiate a call set-up.
The base station can initiate a call using the Paging Channel (PCH)
The base station informs mobiles about which channel to use through the Access Grant Channel (AGCH)
Dedicated Control Channels (DCCH) carry control messages between network and mobile.
The Standalone Dedicated Control Channel (SDCCH) is mainly for transfer of signalling between mobile and base station.
The Slow Associated Control Channel (SDCCH) is mainly used for message to maintain a link.
converted by Web2PDFConvert.com
http://www.web2pdfconvert.com/?ref=PDFhttp://www.web2pdfconvert.com/?ref=PDF -
7/31/2019 Wireless Per Nl
2/3
The Fast Associated Control Channel (FACCH) is similar to the SDCCH but has more data capacity. This is particularly needed
whenever the mobile makes a handover from one cell to another. The FACCH "steals" transmit resources from the speech traffic
channel.
The base station can accommodate these logical channels onto radio carriers in many different ways.
Traffic Channel Frame Structure
This frame structure uses 26 frames to build a multi frame. Frames 0 to 11 and 13 to 24 carry speech or user data. Frame 12 is used as
SACCH, Frame 25 is idle, allowing the base station to measure field strengths from mobiles in other cells, when needed for handovers. 26
frames last 120 msec.
Signaling Frame Structure
The Signaling Frame Structure is a multiframe of 51 frames to accommodate control channels (FCCH, SCH, BCCH, CCCH) in the downlink
and random access (RACH) in the uplink. The 51-multiframe lasts for 235.38 msec.
Superframes
One superframe contains 1326 TDMA frames (6.12 seconds) can either carry 51 of 26-Multiframes or 26 of 51-Multiframes. One hyperframe
contains 2048 superframes which is equivalent to 2,715,648 TDMA frames.
Channel Coding
Speech or user data bits are protected using two concatenated codes. GSM has blocks of 260 bits, containing 50 class Ia bits, 132 class Ibbits and 78 Class II bits. The first step involves block coding. Three parity bits are added to allow error detection for the first 50 class Ia bits. 4
bits are added to the Class Ib bits. The 189 Class Ia and Ib bits are then convolutionally encoded at rate 1/2, i.e., the generate 378 code bits.
The coder has constraint length K = 5. Five consecutive bits are used to create the transmit bits. Every time one bit is fed into the encoder,
two bit appear at the output. The first output bit is the exor of the input bit with the third and fourth preceding bits. The second output bit is the
exor of the input bit with its immediate preceding input bit and the third and fourth preceding bits.
Figure: Convolutional encoder fortransmission in GSM. Each inputbit generates two output bits,determined by the input bit andfour previous bits. The encoder
contains four delay elements(squares in the Figure).
The class II bits remain unprotected. Together with 378 coded class I bits, each block contains 456 bits. These 456 bits are split into 4 bursts
of 114 bits each, or 8 subblocks. If the speech codec receives the information that bits are unreliable it discards the block of 260 bits and
interpolates the speech.
Interleaving
A transmission burst may easily be lost because of channel fading. Therefore portions of each block of speech data are spread over several
transmission bursts.
Data at 9.6 kbit/s
For reliable communication at a user bit rate of 9.6 kbit/s, the data terminal initially encodes the raw data, giving a bit rate of 12 kbit/s. That is,
12 kbi t/s is the bit rate over the interface between the GSM handset and the data source (PC, fax or whatever). The GSM handset then
provides for additional error protection, using convolutional encoding. For data transmission the error control and interleaving parameters
differ essentially from the case of speech transmission. The CRC pari ty bit are not used for data. The convolutional encoding still involves rate
1/2 and K=5. The 244 data bits are encoded into 488 bits. This exceed the 456 bits that can be accommodated in one block by 32 bits. So
32 bi ts redundant bits are left out. This is called "puncturing". The interleaving works over 22 frames, to ensure that bursts of errors are
effecitively spread over a longer a period of time.
Modulation
GSM uses Gaussian Minimum Shift Keying (GMSK).
converted by Web2PDFConvert.com
http://www.web2pdfconvert.com/?ref=PDFhttp://www.web2pdfconvert.com/?ref=PDFhttp://wireless.per.nl/reference/chaptr01/telephon/gsm/gsmtechm.htmhttp://wireless.per.nl/reference/contents.htm -
7/31/2019 Wireless Per Nl
3/3
www.WirelessCommunication.NL
converted by Web2PDFConvert.com
http://www.web2pdfconvert.com/?ref=PDFhttp://www.web2pdfconvert.com/?ref=PDFhttp://wireless.per.nl/reference/about.htm