GSM Transmission Process:
Stage 1: Analog to Digital (A/D) Conversion
The AID Conversion is performed by using a process called Pulse Code Modulation (PCM).PCM involves three main steps:
· Sampling
· Quantization
· Coding
The result from the process of AID conversion is 8,000 samples per seconds of 13 bits each. This is a bit rate of 104kbits/s. When it is considered that 8 subscriber use one radio channel, the overall bit rate would be 8 x 104 Kbits/s=832 Kbits/s.Recalling the general rule of 1bit per Hertz, this bit rate would not fit into the 200 KHz available for all 8 subscribers. The bit rate must be reduced somehow – this is achieved using segmentation and speech coding.
Stage 2: Segmentation and Speech Coding
Speech Coding process analyzes speech samples and outputs parameters of what the speech consists of the tone, length of tone, pitch, etc. This is then transmitted through the network to another MS, which generates the speech based on these parameters.
Segmentation: Given that the speech organs are relatively slow in adapting to changes, the filter parameters presenting the speech organs are approximately constant during 20 Ms. For this reason,when coding speech in GSM , a block of 20 ms is coded into one set of bits .
Speech coding : instead of using 13 bits per sample as in A/D convertion , GSM speech coding uses 260 bits . This calculations as 50 x 260 = 13 kbits/s . This provides a speech quality which is acceptable for mobile telephony and comparable with wireline PSTN phone .
Stage 3: Channel Coding
Channel coding in GSM uses the 260 bits from the speech coding as input to channel coding and output 456 encoded bits. The 260 bits are split according to their relative importance:
· Block 1: 50 very important bits
· Block 2: 132 important bits and
· Block 3: 78 not so important bits
The first block of 50 bits is sent through a block coder, which adds three parity bits that will result in 53 bits. These three bits are used to detect errors in a received message. The 53 bits from first block, the 132 bits from the second block and 4 tails bits (total = 189) are sent to a 1:2 convolution coder which outputs 378 bits. Bits added by the convolution coder enable the correction of errors when the message is received. The bits of the block 3 are not protected. The output from Channel coder is 456 bits representing speech samples of 20 ms.
Stage 4: Interleaving
Channel coding is most effective in detecting and correcting short error sequences. It is not suitable for handing longer sequences of bit errors. For this reason, interleaving is used to separate consecutive bits of a message to be transmitted in a non-consecutive way.
· 1st Level of Interleaving:
The channel coder provides 456 bits for every 20 ms of speech. These are interleaved, forming eight blocks of 57 bits each, as shown in the figure below.
If one burst transmission is lost, there is a 25% BER for the entier20 ms of speech (218=25%).
· 2nd Level of Interleaving:
A second level of interleave can be introduced to furt reduce the possible BER to 12.5%. Instead of sending two blocks of 57 bits from the same 20 ms of speech within one burst, a block from one 20 ms and a block from next sample of 20 ms are sent together. A delay is introduced in the system when the MS must wait for the next 20 ms of speech.
Stage 5: Ciphering Encryption
The ciphering algorithm in GSM is called the A5 algorithm. It does not add bits to the burst, meaning that the input and output to ciphering process is the same as the input: 456 bits per 20 ms.
Stage 6: Burst Formatting
Every transmission from an MSIBTS must include some extra information such as the training sequence. The process of burst formatting is to add these bits (along with some others such as tail bits) to the basic speech data being sent. This increases the overall bit-rate, but is necessary to counteract problems encountered on the radio rate path.
Stage 7: Modulation and Transmission
The bits must be sent over the air using a carrier frequency. GSM uses the GSMK modulation technique.
Air Interface:
Channels may be physical or logical.
Physical Channels:
A physical channel in GSM is defined by a frequency pair for ULIDL and a Time Slot TS of the TDMA frame. The frequency bandwidth in GSM is 200 KHz. A Time Slot TS has a duration of 0.577 ms. 8 TS from a TDMA frame; the duration of a TDMA is 4.615 ms.
Logical Channels:
Many types of logical channels exists, each designed to carry a different message to or from an MS. All information to and from an MS must be formatted correctly. Logical Channels can carry Traffic or control signals.
· Control Channels:
These are used to carry signaling or synchronization data. They are divided into three types:
· Broadcast Channels (BCH)
· Common Control Channels (CCCH)
· Dedicated Control Channels (DCCH‘S)
Broadcast Channels (BCHs)
| |||
Logical Channel
|
Direction
|
BTS
|
MS
|
Frequency Correction Channel
(FCCH)
|
Downlink,
point to
multipoint
|
Transmits a carrier frequency.
|
Identifies BCCH carrier by the carrier frequency and synchronizes with the frequency.
|
Synchronization
Channel (SCH)
|
Downlink,
point to
multipoint
|
Transmits information about the TDMA frame structure in a cell (e.g. frame number) and the BTS Identity (Base Station Identity Code (BSIC)).
|
Synchronizes with the frame structure within a particular cell, and ensure that the chosen BTS is a GSM BTS-BSIC can only be decoded by an MS if the BTS belongs to a GSM network.
|
Broadcast Control Channels (BCCH)
|
Downlink,
point to
multipoint
|
Broadcast some general cell information such as
-Location area Identity (LAI)
-maximum output power allowed in the cell and
-the identity of BCCH carries for neighboring cells.
|
Receives LAI and will signal to the network as part of the Location Updating procedure if the LAI is different to the one already stored on its SIM.MS sets its output power level based on the information received on the BCCH. The MS stores the list of BCCH carrier frequency on which RX level measurement is done for Handover decision.
|
Common Control Channels(CCCH)
| |||
Logical Channel
|
Direction
|
BTS
|
MS
|
Paging Channel (PCH)
|
Downlink,
point to
multipoint
|
Transmits a paging message to indicate an incoming cell or short message. The paging message contains the identity number of the mobile subscriber that the network wishes to contact.
|
At certain time intervals the MS listens to the PCH. If it identifies its own mobile subscriber identity number on the PCH, it will respond.
|
Random Access
CHannel (RACH)
|
Uplink,
point to
point
|
Receives access-request from MS for call setup/ Ioc. Update/SMS
|
Answer paging message on the RACH by requesting a signaling channel.
|
Access Grant Channel (AGCH)
|
Downlink,
point to
point
|
Assigns a signaling channel (SDCCH) to the MS.
|
Receives signaling channel assignment (SDCCH).
|
Dedicated Control Channels(DCCH)
| |||
Logical Channel
|
Direction
|
BTS
|
MS
|
Stand alone Dedicated Control CHannel (SDCCH)
|
Uplink and downlink,
point to
point
|
The BTS switches to the assigned SDCCH, used to call set-up signaling.TCH is assigned on SDCCH (SDCCH is also used for SMS messages to MS).
|
The MS switches to the assigned SDCCH. Call set-up
is performed.The MS receives a TCH assignment information (carrier and time slot).
|
Cell Broadcast Channel (CBCH)
|
DL link, point to multi-point, mapped on SDCCH
|
Uses this logical channel to transmit short message service cell broadcast.
|
MS receives cell broadcast message.
|
Show Associated Control Channel (SACCH)
|
Uplink and downlink, point to point
|
Instructs the MS on the allowed transmitter power and parameters for time advance.
SAACH is used for SMS during a cell.
|
Sends averaged measurements on its own BTS (signal strength and quality) and neighboring BTSs (signal strength). The MS continues to use SACCH for this purpose during a cell.
|
Fast Associated Control Channel (FACCH)
|
Uplink and downlink, point to point
|
Transmits handover information.
|
Transmits necessary handover information in access burst.
|
· Traffic Channels:
Once cell set-up procedures have been completed on the control physical channel, the MS tunes to a traffic physical channel. It uses the Traffic Channel (TCH) logical channel.
There are three types of TCH:
· Full rate (TCH): transmits full rate speech (13 Kbits/s). A full rate TCH occupies one physical channel.
· Half rate (TCHI2): transmits half rate speech (5, 6 Kbits/s).two half rate.
TCHs can shared one physical channel, this doubling the capacity of a cell.
· Enhanced Full Rate (EFR): speech coders improve the speech quality offered across one full rate TCH, but still use a full rate TCH logical channel.
Types of Frames:
· TDMA Frame:
In GSM 8 TS form (4.615ms), i.e. 8 physical channels are using the same frequency band being cyclically (Every 4.615ms) allocated to the certain user 1 application.
· Multiframe:
· Multiframes of physical channels allocated for user traffic (Traffic Channels TCH) are repetition cycles of 26 TDMA frames.
· Multiframes of physical channels allocated for signaling data are repetition cycles of 51 TDMA frames.
· Superframe:
May be:
- 51 Traffic Multiframes.
- 26 Control Multifames.
Time of Superframe =51*26*4.615 = 6.12 Sec
· Hyperframe:
2048 Superframe and has a time = 2048*6.12 = 3.5 hours.
Burst:
The content of a TS is called ''Burst'' and there are several types of burst:
· Normal Burst:
This type is used to carry the information of the traffic channels and the control channels BCCH, PCH, AGCH, SDCCH, SACCH, and FACCH.
· Frequency Correction Burst:
This is the one used by the channel (FCH) for frequency correction of the mobile.
· Synchronization Burst:
The TDMA frame number is sent on the SCH channel, which carries also the Base Station Identity Code (BSIC).
· Access Burst:
The access Burst is used by the RACH channel. The mobile sands this burst when it dose not know the distance to its serving BTS.
· Dummy Burst:
The dummy burst is sent from the BTS when there is nothing else to be sent.
0 comments:
Post a Comment