GSM -> Radio Links

1.Channel structure
2.Speech coding
3.Channel coding and modulation
4.Multipath equalization
5.Frequency hopping
6.Discontinuous transmission
7.Discontinuous reception
8.Power control

Since radio spectrum is a limited resource shared by all users, a method must be devised to divide up the bandwidth among as many users as possible. The method chosen by GSM is a combination of Time­ and Frequency­Division Multiple Access (TDMA/FDMA). 

The FDMA part involves the division by frequency of the total 25 MHz bandwidth into 124 carrier frequencies of 200 kHz bandwidth.  One or more carrier frequencies are then assigned to each base station.  Each of these carrier frequencies is then divided in time, using a TDMA scheme, into eight time slots.  One time slot is used for transmission by the mobile and one for reception.  They are separated in time so that the mobile unit does not receive and transmit at the same time, a fact that simplifies the electronics.

In the rest of this section, the procedure involved in digitally transmitting a voice signal in a GSM network is examined, along with some of the features, such as discontinuous transmission and reception, used to improve voice quality, reduce the mobile unit's power consumption, and increase the overall capacity of the network.

,------------------------------------------------------------------------,
|                                                                        |
| ,---,---------------,---,-----------,---,----------------,---,------,  |
| | 3 |       57      | 1 |     26    | 1 |       57       | 3 | 8.25 |  |     
| `---'---------------'---'-----------`---'----------------`---`------'  |
|  Tail   Data bits          Training         Data bits    Tail  Guard   |    
|  bits                      sequence                      bits  bits    |  
|                                                                        |
`------------------------------------------------------------------------'
				FIGURE 2

1  Channel structure

Each group of eight time slots is called a TDMA frame, which is transmitted every 4.615 ms.  TDMA frames are further grouped into multiframes to carry control signals.  There are two types of multiframe, containing 26 or 51 TDMA frames.  The 26­frame multiframe contains 24 Traffic Channels (TCH) and two Slow Associated Control Channels (SACCH) which supervise each call in progress.  The SACCH in frame 12 contains eight channels, one for each of the eight connections carried by the TCHs.  The SACCH in frame 25 is not currently used, but will carry eight additional SACCH channels when half­rate traffic is implemented.  A Fast Associated Control Channel (FACCH) works by stealing slots from a traffic channel to transmit power control and handover­signalling messages.  The channel stealing is done by setting one of the control bits in the time slot burst.

In addition to the Associated Control Channels, there are several other control channels which (except for the Stand­alone Dedicated Control Channel) are implemented in time slot 0 of specified TDMA frames in a 51­frame multiframe, implemented on a non­hopping carrier frequency in each cell.  The control channels include:

• Broadcast Control Channel (BCCH): Continually broadcasts, on the downlink, information including base station identity, frequency allocations, and frequency­hopping sequences.

   

• Stand­alone Dedicated Control Channel (SDCCH): Used for registration, authentication, call setup, and location updating.  Implemented on a time slot, together with its SACCH, selected by the system operator.

   

• Common Control Channel (CCCH): Comprised of three control channels used during call origination and call paging.

   

  • Random Access Channel (RACH): A slotted Aloha channel to request access to the network

     

  • Paging Channel (PCH): Used to alert the mobile station of incoming call.

     

  • Access Grant Channel (AGCH): Used to allocate an SDCCH to a mobile for signalling, following a request on the RACH.

2  Speech coding

GSM is a digital system, so speech signals, inherently analog, have to be digitized.  The method employed by ISDN, and by current telephone systems for multiplexing voice lines over high speed trunks and optical fiber lines, is Pulse Coded Modulation (PCM).  The output stream from PCM is 64 kbps, too high a rate to be feasible over a radio link.  The 64 kbps signal contains much redundancy, although it is simple to implement.  The GSM group studied several voice coding algorithms on the basis of subjective speech quality and complexity (which is related to cost, processing delay, and power consumption once implemented) before arriving at the choice of a Regular Pulse Excited - Linear Predictive Coder (RPE­LPC) with a Long Term Predictor loop.  Basically, information from previous samples, which does not change very quickly, is used to predict the current sample.  The coefficients of the linear combination of the previous samples, plus an encoded form of the residual, the difference between the predicted and actual sample, represent the signal.  Speech is divided into 20 millisecond samples, each of which is encoded as 260 bits, giving a total bit rate of 13 kbps.

3  Channel coding and modulation

Recall that the speech codec produces a 260 bit block for every 20 ms speech sample.  From subjective testing, it was found that some bits of this block were more important for perceived speech quality than others.  The bits are thus divided into three classes:

Class Ia  50 bits - most sensitive to bit errors
Class Ib  132 bits - moderately sensitive to bit errors
Class II  78 bits - least sensitive to bit errors

Class Ia bits have a 3 bit Cyclic Redundancy Code added for error detection.  If an error is detected, the frame is judged too damaged to be comprehensible and it is discarded.  It is replaced by a slightly attenuated version of the previous correctly received frame.  These 53 bits, together with the 132 Class Ib bits and a 4 bit tail sequence (a total of 189 bits), are input into a 1/2 rate convolutional encoder of constraint length 4.  Each input bit is encoded as two output bits, based on a combination of the previous 4 input bits.  The convolutional encoder thus outputs 378 bits, to which are added the 78 remaining Class II bits, which are unprotected.  Thus every 20 ms speech sample is encoded as 456 bits, giving a bit rate of 22.8 kbps.

To further protect against the burst errors common to the radio interface, each sample is diagonally interleaved.  The 456 bits output by the convolutional encoder are divided into 8 blocks of 57 bits, and these blocks are transmitted in eight consecutive time­slot bursts.  Since each time­slot burst can carry two 57 bit blocks, each burst carries traffic from two different speech samples.

Recall that each time­slot burst is transmitted at a gross bit rate of 270.833 kbps.  This digital signal is modulated onto the analog carrier frequency, which has a bandwidth of 200 kHz, using Gaussian­filtered Minimum Shift Keying (GMSK).  GMSK was selected over other modulation schemes as a compromise between spectral efficiency, complexity of the transmitter, and limited spurious emissions.  The complexity of the transmitter is related to power consumption, which should be minimized for the mobile station.  The spurious radio emissions, outside of the allotted bandwidth, must be strictly controlled so as to limit adjacent channel interference, and allow for the co­existence of GSM and the older analog systems (at least for the time being).

4  Multipath equalization

5  Frequency hopping

6  Discontinuous transmission

Minimizing co­channel interference is a goal of any cellular system, since it allows better service for a given cell size, or the use of smaller cells, thus increasing the overall capacity of the system.  Discontinuous transmission (DTX) is a method that takes advantage of the fact that a person speaks less that 40 percent of the time in normal conversation , by turning the transmitter off during silence periods.  An added benefit of DTX is that power is conserved at the mobile unit.

The most important component of DTX is, of course, Voice Activity Detection.  It must distinguish between voice and noise inputs, a task that is not as trivial as it appears, considering background noise.  If a voice signal is misinterpreted as noise, the transmitter is turned off and a very annoying effect called clipping is heard at the receiving end.  If, on the other hand, noise is misinterpreted as a voice signal too often, the efficiency of DTX is dramatically decreased.  Another factor to consider is that when the transmitter is turned off, there is a very silent silence heard at the receiving end, due to the digital nature of GSM.  To assure the receiver that the connection is not dead, comfort noise is created at the receiving end by trying to match the characteristics of the transmitting end's background noise.

7  Discontinuous reception

8  Power control

The mobile station measures the signal strength or signal quality (based on the Bit Error Ratio), and passes the information to the Base Station Controller, which ultimately decides if and when the power level should be changed.  Power control should be handled carefully, since there is the possibility of instability.  This arises from having mobiles in co­channel cells alternatingly increase their power in response to increased co­channel interference caused by the other mobile increasing its power.  This in unlikely to occur in practice but it is (or was as of 1991) under study.

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