Timing advance of GSM (time advance)

Basic Concepts

Timing advance is used to compensate for the radio wave transmission delay, while its fundamental goal is to improve the channel coding and decoding efficiency. Due to the use of TDMA in GSM, synchronization between time slots should be strictly maintained for 8 time slots per carrier frequency. Without Ta, the inherent disadvantage of wireless transmission cannot be overcome.

The radius of the GSM cell can reach 35 km, and the signal from the mobile phone can only arrive at the base station after a certain period of time. Therefore, we must take some measures (latency adjustment ), to ensure that the signal arrives at the base station when appropriate.

Simply put, the mail sending time is adjusted using the time prefix (TA) parameter. The TA is determined by the base station according to the received measurement report, and then sent to the mobile phone. During Normal calls, when the MS is close to the base station, the base station will notify the MS to reduce the time advance; when MS is far from the residential center, the base station will require ms to increase the time limit.

 

Description

If the base station or mobile phone does not know the distance from the peer end (or the transmission delay of the Peer signal), the channel encoding scheme of the base station or mobile phone must adopt more redundant protection. This is exactly what happens when the mobile phone is initially connected.

Initial access phase:When a mobile phone initiates an access request on a random access channel (Rach), the base station does not know the distance from itself. Therefore, the cost of Rach is much higher than that of the Business Channel. When the base station decodes the random access request of the mobile phone, the transmission latency of the signal has been calculated (that is, the distance from the mobile phone ), the Base Station will tell the mobile phone to leave its own distance in the TA value in the subsequent access permit channel (AGCH) and require the mobile phone to be in the business channel (TCH) later) the time value that TA represents in advance when the signal is sent.

Call phase:The measurement report sent by the mobile phone to the base station carries the delay value measured by the mobile station, and the base station must monitor the call arrival time, in addition, an instruction is sent to the mobile station every two seconds on the system message of the downstream sacch. as the distance between the mobile station and the base station changes, the instruction is gradually sent to the mobile station in advance.

 

Prompt

1. In the GSM system,The upper and lower rows have3Time Slot IntervalBecause mobile phones cannot work at the uplink and downlink frequencies at the same time. That is to say, the upper and lower lines occupy ts4, and the actual time interval is 577 × 3um. The mobile phone will launch at TS7.
2. The TA value range is 0 ~ 63, representing a time range of 0-, equivalent to 0 ~ 70 km; 1 increase in Ta means that the distance between the mobile phone and the base station is increased by about 550. This is alsoGSMMaximum radius of a residential area35Kilometers(We should not only consider the transmission delay of the upstream signal of the mobile phone, but also the transmission delay of the downstream signal of the base station ).

3. The ta time advance value can be from 0 to 233us. This value will affect the wireless coverage of the Community. Given the light speed, the wireless coverage radius of the GSM community can reach up to 35 km, this limit value is because the pre-encoding of GSM is from 0 ~ Between 63. The algorithm for maximum coverage radius of base stations is as follows:

3.7us × 63 × 3 × small m/s limit 2 = 35 km

Among them, 3.7us: the length of each bit; 63: the maximum number of BITs for time adjustment; 3x108 m/s: the speed of light.
However, in some cases, the base station needs to be able to cover farther areas, such as coastal areas, where it is necessary to cover a large range of sea areas or islands. This coverage can be achieved in GSM at the cost of reducing the number of channels supported by each carrier frequency. The solution is to only use a channel with an even tn (because tn0 must be used for BCCH ), empty the TN of the odd number to obtain a large holding time. This technology is called the extended Residential Area Technology in beidian, which has special reception processing, so that the coding in advance will increase the duration of a burst pulse. That is, the maximum coverage radius of the base station is:

3.7us × (63 + 156.25) × 3 × small m/s limit 2 = 120 km

In the GSM cellular mobile phone standard,Timing advanceValue corresponds to the length of time a signal takes to reach the base station from a mobile phone. GSM uses TDMA technology in the radio interface to share a single frequency between several users, assigning sequential timeslots to the individual users sharing a frequency. each user transmits periodically for less than one-eighth of the time within one of the eight timeslots. since the users are at varous distances from the base station and radio waves travel at the finite speed of light, the precise arrival-time within the slot can be used by the base station to determine the distance to the mobile phone. the time at which the phone is allowed to transmit a burst of traffic within a timeslot must be adjusted accordingly to prevent collisions with adjacent users. timing advance (TA) is the variable controlling this adjustment.

Technical specifications 3GPP ts 05.10 [1] and TS 45.010 [2] describe the TA Value Adjustment Procedures. the TA value is normally between 0 and 63, with each step representing an advance of one bit period (approximately 3.69 microseconds ). with radio waves traveling at about 300,000,000 metres per second (that is 300 metres per microsecond), one ta step then represents a change in round-trip distance (twice the propagation range) of AB out 1,100 metres. this means that the TA value changes for each 550-metre change in the range between a mobile and the base station. this limit of 63 × 550 metres is the maximum 35 kilometres that a device can be from a base station and is the upper bound on cell placement distance.

A continually adjusted ta value avoids interference to and from other users in adjacent timeslots, thereby minimizing data loss and Maintaining mobile QoS (call Quality-of-Service ).

Timing advance is significant for privacy and communications security, as its combination with other variables can allow GSM localization to find the device's position and tracking the mobile phone user. ta is also used to adjust transmission power inspace-division multiple access systems.

This limited the original range of a GSM cell site to 35 km as mandated by the duration of the standard timeslots defined in the GSM specification. the maximum distance is given by the maximum time that the signal from the mobile/BTS needs to reach the handler of the mobile/BTS on time to be successfully heard. at the air interface the delay between the transmission of the downlink (BTS) and the uplink (mobile) has an offset of 3 timeslots. until now the mobile station has used a timing advance to compensate for the propagation delay as the distance to the BTS changes. the timing advance values are coded by 6 bits, which gives the theoretical maximum BTS/mobile separation as 35 km.

By implementing the Extended Range feature, the BTS is able to receive the uplink signal in two adjacent timeslots instead of one. when the Mobile Station reaches its maximum timing advance, I. e. maximum range, the BTS expands its hearing window with an internal timing advance that gives the necessary time for the mobile to be heard by the BTS even from the extended distance. this extra advance is the duration of a single timeslot, a 156 bit period. this gives roughly 120 km range for a cell. [3] and is implemented in sparsely populated areas and to reach islands for example.

Timing advance of GSM (time advance)