GSM multiple access scheme

GSM Multiple Access Scheme

The Multiple Access Scheme defines how the GSM radio frequency can be shared by different simultaneous communication communication between different mobile stations located in different cells. GSM uses a mix of Frequency Division Multiple Access (FDMA) and Time Tim e Division Multiple Access (TDMA) (TDMA) combined with frequency hopping for its Multiple Access Scheme. Each user is given a pair  of frequencies (one for uplink and one for downlink) and a time slot during a time frame. The time frame provides the basic unit of  logical channels. GSM FREQUENCY SPECTRUM Frequency Allocation There are two frequency bands of 25 MHz each that have been allocated for the use of GSM. The band 890 - 915 MHz is used for  the uplink direction (from the mobile station to the base station). The band 935 - 960 MHz is used for the downlink direction (from the base station to the mobile station) [1 [1].

Figure 1: GSM Frequency Bands [2 [ 2] FDMA AND TDMA FDMA divides the frequency spectrum into small slices, which are assigned to the user. Since the radio spectrum is limited and users do not free their assigned frequency until they are completely finished with it, the number of users in the system can be quickly limited [1 [ 1]. As the number of users increases, the required frequency spectrum also increases. TDMA allows many users to share a common channel. The unit of time in TDMA is called a burst. Each user is assigned its own burst within a collection of  bursts called a frame. Carrier Frequencies GSM uses TDMA within a FDMA structure. As a result, different users can transmit using the same frequency, but they can't transmit at the same time. A 25MHz frequency band is divided using an FDMA scheme into 124 one-way carrier frequencies. Each base station is assigned one or more carriers to use in its cell. A 200kHz frequency band separates the carrier frequencies from each other. Normally, Normally, a 25MHz band should be divisible into 125 carrier frequencies but in GSM the 1st carrier frequency is used as a guard band between GSM and other services that might be working on lower frequencies.

Figure 2: Frequency Division in the Uplink Spectrum

Bursts Each carrier frequency is then d ivided according to time using a TDMA scheme. Each of the carrier frequencies is divided into a 120ms multiframe. A multiframe is made up of 26 frames. Two of these frames are used f or control purposes, while the remaining 24 frames are used for traffic.

Figure 3: Structure of a Multiframe Each frame can in turn be divided into 8 bursts, and each of the 8 bursts is assigned to ta single user. In a TDMA system, a burst is the unit of time, and each burst lasts for approximately 0.577 ms.

Figure 4: Structure of a Frame Burst Structure In GSM, there are 4 different types of bursts. A normal burst is used to carry speech and data information. The structure of the normal burst is shown below. Each burst consists of 3 tail bits at each end, 2 data sequences of 57-bits, a 26-bit training sequence for equalization, and 8.25 guard bits. There are 2 stealing bits (1 for each data sequence) that are used by Fast Access Control Channels. The frequency correction burst and synchronous burst have the same length as normal burst. They have different internal structures to differentiate them from normal bursts. The frequency correction burst is u sed in Frequency Correction Channels (FCCH) and the synchronous burst is used in Synchronization Channels (SCH). The random access burst is shorter than a normal burst, and is only used on Random Access Channels (RACH).

Figure 5: Burst Structure [1]

CHANNELS  A channel relates to the recurrence of one burst in every frame. The channel is characterized by both its frequency and its position

within the TDMA frame. This characterization is cyclical, and the channel p attern repeats every 3 hours. There are two major  categories of channels in GSM: traffic channels, and control channels. Channels can also be classified as being dedicated or  common. Dedicated channels are assigned to a mobile station, while common channels are used by idle mobile stations.

Traffic Channels Traffic channels transport speech and data information. A traffic channel using a group of 26 TDMA frames called 26-Multiframe. In this standard, traffic channels for uplinks and downlinks are separated by 3 bursts. Because of this, the mobile station does not need to transmit and receive at the same time. A full rate traffic channel uses 1 time slot in each of the traffic frames in a multiframe.

Control Channels Control channels deal with network management messages and channel maintenance tasks. These channels can be used by either idle or dedicated mobile stations. Some of the common channel types are: • • • • •

•

Broadcast Control Channels Frequency Correction Channels Synchronization Channels Random Access Channels Paging Channels  Access Grant Channels

Broadcast channels are used by the base station to provide the mobile station with network synchronization information. There are 3 functions that a broadcast channel can have. The broadcast control channel (BCCH) provides that mobile station with the parameters it needs to identify and access the network. A synchronous channel (SCH) gives the mobile station the training sequence needed to demodulate the information transmitted by the base station. The Frequency Correction Channel (FCCH) supplies the mobile station with the frequency of the system to synchronize with the network. Every GSM cell broadcasts exactly one FCCH and one SCH, which are defined to be on time slot 0 in the TDMA frame [ Paging Channels are used to alert the mobile station of incoming calls. Random Access channels are used by the mobile station to request access to the network. The base station uses an Access Grant Channel to inform the mobile station about which channel it should use.

Figure 11: The GSM demodulation obtained using direct down-conversion

Figure 12: Byte reconstruction using cosine and sinus multiplication