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How to Reverse Channel works in CDMA

All MS transmit on same frequency but with different PN codes to create different logical channels. Some channels marked for Access are used for signaling and control. While Walsh Codes identifies different channels, PN codes are used to identify Cells (by 15 bit code) and MS’s (by 42 bit codes). As each MS receives a … Read more

PN Code Generation & Masking

  PN Code Generation PN Code Generation PN codes are generated from prime polynomials using modulo 2 arithmetic. The state machines generating these codes are very simple and consist of shift registers and XOR gates .   Masking will cause the generator to produce the same sequence, but offset in time PN Offset (Masking) Masking … Read more

How Walsh Code Generate in CDMA

• Definition: Orthogonal functions have zero correlation.  Two binary sequences are orthogonal if the process of “XORing” them results in an equal number of 1’s and 0’s. Example:   0000   (XOR)  0101   ——   0101 • Generation Sequence: – Seed – Repeat: right & below – Invert: diagonally   The above table of … Read more

Structure of Combined BCCH in GSM

It is used in the configuration of cells of low traffic density and small capacity. The Combined BCCH is only configured at timeslot 0. Channel combination: FCCH?SCH?BCCH?CCCH+SDCCH/4+SACCH/4 SDCCH/4: Stand-alone dedicated control channel. Each TDMA multiframe with 51 frames has 4 SDCCH; SACCH/4: Slow SDCCH/4 associated control channel; Compared with the main BCCH channel, 4 signaling … Read more

Physical Combination of Logical Channel in GSM

As shown above, CCCH=PCH+RACH+AGCH; downlink CCCH=PCH+AGCH; and uplink CCCH=RACH. In the above combinations, combination 3 and 4 must be allocated to slot 0 of the BCCH carrier configured for the cell; while combination 5 must be allocated to timeslots 2, 4 and 6 of the BCCH carrier. The FACCH works in the frame stealing mode, … Read more

Uplink Logical Channel in GSM

1. Random access channel (RACH) It is an uplink channel used for MS randomly access to network by requesting for an SDCCH. The request includes a 3bit setup reason (call request, paging response, location update request and short message request etc.) and a 5bit random reference number for MS to differentiate the access granted messages. … Read more

Definition of Logical Channel in GSM

The physical channel is the medium over which the information is carried. The logical channel consists of the information carried over the physical channels. A Physical Channel (a TS, defined by a fixed position (0-7) on a given TDMA frame) may be used to broadcast messages containing different kinds of information: traffic messages for speech … Read more

What is Frame and Channel in GSM

The major basic concept concerned with the radio path transmission of the GSM system is the burst sequence (simplified as Burst). It is a string of transmission units including more than 100 modulation bits. The burst sequence has a restricted duration and seizes a restricted radio frequency spectrum. They can be described as output from … Read more

DTX: Discontinuous Transmission

Actually, during the communication process, the mobile subscriber talks only 40% of the time and there is not much useful information transmitted during rest of the time. If all the information is transmitted to the network, it will not only be a waste of the system resources but also add more interference to the system. … Read more

What is Channel Coding

To check and correct errors during the transmission, redundancy data and the information calculated from the source data are added to the stream so as to increase the bit rate. For the voice, the length of these codes is 456 bits every 20ms. The bit rate of code stream output from the voice coder is … Read more