Timing advance of GSM（时间提前量）

基本概念

时间提前量TA（Timing Advance）的作用是为了补偿电波传输延迟，而根本目的则是为了提高信道编解码效率。由于GSM采用TDMA，每载频8个时隙，应严格保持时隙间的同步，没有TA就无法克服固有的无线传输劣势。

GSM的小区半径可以达到35km，从手机出来的信号需要经过一定时间才能到达基站，因此我们必须采取一定的措施（时延调整），来保证信号在恰当的时候到达基站。

简单来说，就是采用时间提前量（TA）参数调整发信时间。TA是由基站根据接收到测量报告确定的，然后发送给手机，正常通话中，当MS接近基站时，基站就会通知MS减小时间提前量；而当MS远离小区中心时，基站就会要求MS加大时间提前量。

 

解释说明

如果基站或手机不知道对端离开自己的距离（或者说对端信号的传输时延）则基站或手机的信道编码方案必须采取更多的冗余保护。而这正是手机在初始接入时的情形。

初始接入阶段：手机在随机接入信道（RACH）上发起接入请求时，基站并不知道其离开自己的距离，所以RACH的同比开销就比业务信道高很多。在基站解码手机的随机接入请求时就已经计算出了该信号的传输时延（即知道了手机离开自己的距离），基站会在随后的接入准许信道（AGCH）上以TA值的方式告诉手机其离开自己的距离，要求手机随后在业务信道（TCH）的信号发射时提前TA所代表的时间值。

呼叫进行阶段：手机发给基站的测量报告头上携带着移动台测量的时延值，而基站必须监视呼叫到达的时间，并在下行SACCH的系统消息上一每两秒一次的频率向移动台发出指令，随着移动台离开基站的距离变化，逐步指示移动台应提前发送的时间。

 

提示

1、GSM系统中为了便于手机收发射机工作，上下行是有3个时隙间隔的，因为手机不可能在同一时刻工作在上行频率和下行频率。也就是说，同样上下行占用Ts4，实际在时间上是间隔577×3um.，手机会到Ts7的时间点发射。
2、TA的取值范围是0～63，代表的时间范围是0-233us，相当于0～70公里；TA每增加1，意味着手机离开基站的距离增加约550米。这也是GSM小区的最大半径35公里的由来（我们不仅要考虑手机上行信号的传播时延，还要考虑基站下行信号的传播时延）。

3、TA 时间提前量值可以由0至233us，该值会影响到小区的无线覆盖，在给定光速下，GSM小区的无线覆盖半径最大可达到35km，这个限制值是由于GSM定时提前的编码是在0~63之间。基站最大覆盖半径算法如下：

3.7us×63×3×108m/s÷2=35km

其中，3.7us:每个比特的时长；63：时间调整的最大比特数；3×108m/s:光速。
但在某些情况下，客观需要基站能覆盖更远的地方，比如在沿海地区，如需用来覆盖较大范围的一些海域或岛屿。这种覆盖在GSM 中是能实现的，代价是须减少每载频所容纳的信道数，办法是仅使用TN为偶数的信道（因为TN0必须用做BCCH），空出奇数的TN，来获得较大的保持时间。这在北电中被称为扩展小区技术,这一技术有专门的接收处理.这样定时提前的编码将会增大一个突发脉冲的时长。即基站的最大覆盖半径为：

3.7us×(63+156.25)×3×108m/s÷2=120km

 

 

In the GSM cellular mobile phone standard, timing advance value 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 various 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 travelling 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 about 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 35km 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 receiver 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 35km.

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.