AIR INTERFACE
Water Interface using the technique of Time Division Multiple Access (TDMA) to send and receive channels and signaling information between the BTS and MS. TDMA technique is used to divide each of the carriers into 8 time slots. Time slots are then marked for specific users, allowing to deal with 8 conversation simultaneously on the same carrier. Air Interface Characteristics:
FREQUENCY BAND
- "Over the water" bit rate of 270 kbps
- Distance duplex 45 MHz (GSM 900), 95 MHz (GSM 1800) or 80 MHz (1900)
- 200 kHz channel separation
- Modulation uses Gaussian Minimum Shift Keying (GMSK)
AIR INTERFACE CHANNEL
The usual path for carrying information between MS and BTS is known as Physical Channel. Differences in the Physical Channel information carriers are classified as Logical Channel. Logical Channel is divided into 2 categories, the Control Channel and Traffic Channel. Both were then divided again, there are two types of Traffic Channel and there are three categories of the Control Channel with a total of nine different types.
CONTROL CHANNEL
Control channels carry signaling information used by MS to look for RBS, synchronize itself with the RBS, and retrieval of information used for the implementation of the call set-up. There are three categories of the Control Channel, namely:
Broadcast Channels (BCH)
All BCH are transmitted point-to-multi-point downlink direction.
Frequency Correction Channel (FCCH)-Provides frequency correction information used by MS.
Synchronization Channel (SCH) - Contains the Base Station Identity Code (BSIC) and TDMA frame number is used for synchronization of MS for the structural frame of the new BTS.
Broadcast Control Channel (BCCH) - Used to broadcast general information to all MS.
Common Control Channels (CCCH)
All CCCH transmitted point to point.
Random Access Channel (Rach) - Used by the MS to request access to the system. Rach Information sent through the uplink.
Paging channel (PCH) - Used to page the MS. PCH Information sent through the downlink.
Access Grant Channel (AGCH) - Used to mark the SDCCH. AGCH information sent via the downlink.
Dedicated Control Channels (DCCH)
All DCCH sent in point to point through the uplink and downlink.
Stand-alone Dedicated Control Channel (SDCCH) - Bringing information during call setup signaling.
Slow Assosiated Control Channel (SACCH) - Send the call control data and report measurement.
Fast Assosiated Control Channel (FACCH) - Takes a critical signaling information.
TRAFFIC CHANNELS
Traffic Channel (TCH) carry voice / data. There are two types of TCH, namely: Full-Rate and Half-Rate. TCH time slots can be placed anywhere on any frequency is described in the cell, except for the first time slot (TS0) the first carrier (C0).
Full Rate - Full Rate TCH handle voice or data encoding. TCH information sent to the bit rate of 33.8 kbps.
Half Rate - With half rate channel, a MS will only use every second time slot (every other idle). As a result, two MS will be able to use the same physical channel to lead the call to a doubling path capacity.
SIGNAL PROCESSING
Before sending information through either the Air Interface Traffic Channel or Control Channel signal processing. During the change of Signal Processing, to reduce the possibility of interference during signal transmission takes place through Air-Interface
Speech coding
PCM speech code into the TRC or BSC / TRC on the A interface with the speed of 64 Kbps. Eight of these channels can provide bit rate of 512 Kbps via AirInterface, without the transmission could not. Traumatic to the TRC or BSC / TRC, providing coding segments point to talk to 20 ms. 20 ms speech segment is then entered into the speech coder. Speech coder will analyze the segment and perform the bit rate reduction. The output is a code point talking point talking to the quality acceptable, ie at 13 Kbps. 3 Kbps others in band signaling added to the signal, so the total output coming out on A-ter interfaces or A-bis interface is 16 Kbps.
Channel coding
Used to detect and fix errors that were identified during the process of signal transmission. This error signal has been measured in percent of total bits dikirim.kualitas shows the limits of signal transmission Bit Error Ratio (BER). Error bit BER is the percentage of the total bits transmitted.
Channel Coding run in TRU in the downlink and the uplink signal from MS. There are two types of Channel Coder normally used, namely block coders and convolutional Coders.
Coding channels delivering 20 ms speech pieces - pieces of the speech path that contains 260 bits of information talks. 260 bits of information are divided into:
- 50 bit is very important
- 132 bits that are important
- 78 bits that are not so important
3 bit parity is added to the 50 bits (block coding). 53 bit is shared with 132 bits are important and 4 tail bits diencoding together into 378 bits (1:2 rate). The remaining bits are not encoded.
Interleaving
Bit errors often occur because of solving the information sent through the air (air interface). This is due to the long fading affecting bits lined. The process of channel coding is only effective in detecting and correcting single errors. Interleaving solve this problem by dividing the bits of information in the form of messages and transmits these bits are not regular (this can also overcome the problem of noise). Through interleaving, ntuk single frame of information scattered through several solutions. GSM is used in two levels of interleaving, namely:
First Level
Channel coder provides 456 bits for every 20 ms speech channel
Second Level
In normal solution contained a space for every 2 bits of the block 57 bits of information. The second level of interleaving using 8 bits from 57 bits to be transmitted and placed in 8 different information bursts. Any information bursts consist of 2 blocks of 57 bits of information for each instance a different voice channels.
With the second level of interleaving we can now lose one full burst. Since the loss of one burst only affects 12.5% of all bits of each speech frame, the channel coding can to make the appropriate corrections.
Equalization
Time Dispersion occurs when the transmission signal reflected away from the RX antenna and separated from the original signal. Interference signal will be scattered by time and the symbol adjacent to each other. Recipients can not receive the original signal transmitted. Equalization is the process used to overcome the time dispersion. Equalizer to overcome the reflection that occurs in limited amounts, with the delay of 15 us who have the signal path about 4.5 miles.
Equalizer create transmission channel and calculates the possible transmitted sequences. Data is transmitted in bursts that are placed in a timeslot. Training sequences are used to create models of the canal. This model changed over time, but during a burst is considered to remain constant. The reflected signal caused by Rayleigh fading to a close reflection region. These signals have independent fading pattern compared with the direct one and can be used by the equalizer to add to his ability.
CIPHERING
Chipering is a technology used to minimize the interception of GSM. Ciphering development has a place in the form of digital signals 1 and 0 are random. Random shape is only known by the MS and BTS. The result is a signal that is unintelligible to all MS, or can only be received by stations that have the same code. Algorithm AS / 1 and AS / 2 is very possible to be used in GSM systems. Network operators have several surgical options in using the ciphering mode or other algorithms that they want.
TRAFFIC CHANNEL OFFSET
The structure of the downlink and uplink are very identical. The difference is in the setting of time or time slot; TS2 in the down link is not the same happen to TS2 in the uplink. Pengesetannya use 3 timeslot. This means that MS is not necessary, to transmit and receive at the same time.
TIMING ADVANCE
If the MS moves Base Station during the talks would be required to send a burst in time synchronization, in order to be accepted according to the timeslot in the Base Station. BS in continuing to send a value between 0 to 63, tell MS how many bits of time (3.7 us) to synchronize the time when transmitting the burst.
Frequency hopping
Frequency Hopping is where radio frequency on the physical channel to a conversation changed in the period interval. During the TDMA frame N +1, C2 is used. Call will use the same timeslot but different frequencies. There are 64 sections in the frequency hopping. One for cyclic and 63 others are random sequences.
Frequency hopping can be categorized as a tool to reduce the occurrence of interference. Two obvious types of frequency hopping implemented in the RBS 2000, namely:
Hopping Synthesizer
Hopping Synthesizer is the process of tuning the output frequency to transmission into the marked channel, send a burst of information, and retuning the output frequency to transmission to the next channel and repeat the process again.
Baseband Hopping
Baseband Hopping is the process of tuning to multiple frequency transmission to the output channel that is marked. Any information will be sent by one burst transmitter, and then burst into the transmitter that dituning held to a different output frequency, and repeat the process.
