TDMA and FDMA

-

Multiple access scheme

The multiple access scheme defines how different simultaneous communications, between different mobile stations situated in different cells, share the GSM radio spectrum. A mix of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA), combined with frequency hopping, has been adopted as the multiple access scheme for GSM.

FDMA and TDMA
Using FDMA, a frequency is assigned to a user. So the larger the number of users in a FDMA system, the larger the number of available frequencies must be. The limited available radio spectrum and the fact that a user will not free its assigned frequency until he does not need it anymore, explain why the number of users in a FDMA system can be "quickly" limited.
On the other hand, TDMA allows several users to share the same channel. Each of the users, sharing the common channel, are assigned their own burst within a group of bursts called a frame. Usually TDMA is used with a FDMA structure.
In GSM, a 25 Mhz frequency band is divided, using a FDMA scheme, into 124 carrier frequencies spaced one from each other by a 200 khz frequency band. Normally a 25 Mhz frequency band can provide 125 carrier frequencies but the first carrier frequency is used as a guard band between GSM and other services working on lower frequencies. Each carrier frequency is then divided in time using a TDMA scheme. This scheme splits the radio channel, with a width of 200 khz, into 8 bursts. A burst is the unit of time in a TDMA system, and it lasts approximately 0.577 ms. A TDMA frame is formed with 8 bursts and lasts, consequently, 4.615 ms. Each of the eight bursts, that form a TDMA frame, are then assigned to a single user.

In GSM 900, 124 Channels, each 200 kHz wide are used for FDMA. All the channels are additionally separated in time via a GSM TDMA frame, i.e. each 200 kHz carrier is subdivided into frames that are repeated continuously. The duration of a frame is 4.615 ms.

Source : Wikipedia

Comments

Post a Comment

Popular posts from this blog

SCFT - Single Cell Function test

-
Single Cell Function test SCFT is for verifying whether individual BTS works well or not, by making a single cell function test of BTS hardware and software. Essential items to be tested are as follows: 1) BTS Transmitter Output Measurement Test 2) Initial parameter establishment (Pilot PN/System ID/Site ID/Frequency, Neighbor List) 3) Call Origination and Termination Test 4) Softer Handoff Test/Antenna installation check (Direction, Tilt, Transmission Line) 5) Single Cell Coverage Test 6) Noise Floor Test 7) Parameter check.
Read more

Troubleshooting for GSM KPIs (SD Block & SD Drop)

-
KPIs to be monitored: ·             SD Blocking ·             SD Drop SDCCH Channel: ·    SDCCH channel is a dedicated channel which is using for LAC updation, Call Setup, SMS in Ideal mode. It works in UL & DL SD Blocking: ·    SD Blocking means that you are not getting SD resource for the call origination. When MS connects with Network then RACH and AGCH are provided. After AGCH, SDCCH is provided but if SDCCH is not provided at this time due to some problem or due to unavailable of SD by BSC. KPI Formula in Ericsson: ·             SDCCH CONGESTION = (CCONGS / CCALLS) * 100 ·             CCONGS - Congestion counter. ·  ...
Read more

PS core network

-
Image
The PS core network consists of two parts, the traditional General Packet Radio Service (GPRS) core network and the new Evolved Packet System (EPS) core network. The PS core network connects mobile subscribers to Packet Data Networks (PDNs) such as the Internet, corporate networks or operator services networks.  a functional overview of the PS core network. Logical entities and interfaces are shown in black and form a 2G/3G network, while the blue boxes and lines in Figure  show the LTE/EPC related parts. The LTE/EPC network can be regarded as an extension and evolution of existing 2G/3G networks. From a node implementation perspective, there is no strict border between the two networks. The corresponding logical entities of GPRS and EPS networks can be deployed concurrently in Ericsson PS core nodes, such as the SGSN-MME and EPG; these are marked in red. In addition, Ericsson GPRS nodes can be upgraded with EPC functionality by software only. Network evolution i...
Read more