标签: 跳频

除了在示波器里直接对雷达脉冲的基本参数进行测量，也可以借助功能更加强大的矢量信号分析软件。下图是用 Keysight 公司的 89601B 矢量信号分析软件结合示波器对超宽带的 Chirp 雷达信号做解调分析的例子，图中显示了被测信号的频谱、时域功率包络以及频率随时间的变化曲线。被测信号由 M8195A 超宽带任意波发生器产生， Chirp 信号的脉冲宽度为 2us ，频率变化范围从1 G~19GHz ，整个信号带宽高达 18GHz ！这里充分体现了实时示波器带宽的优势。 更严格的雷达测试不会仅仅只测脉冲和调制带宽等基本参数。比如由于器件的带宽不够或者频响特性不理想，可能会造成 Chirp 脉冲内部各种频率成分的功率变化，从而形成脉冲功率包络上的跌落（ Droop ）和波动（ Ripple ）现象。因此，严格的雷达性能指标测试还需要对脉冲的峰值功率、平均功率、峰均比、 Droop 、 Ripple 、频率变化范围、线性度等参数以及多个脉冲间的频率、相位变化进行测量，或者要分析参数随时间的变化曲线和直方图分布等。这些更复杂的测试可以借助于 89601B 软件里的 BHQ 雷达脉冲测量选件实现。这个测试软件也支持示波器的分段存储模式，可以一次捕获到多个连续脉冲后再做统计分析，下图是一个实际测试的例子。 除了雷达脉冲分析以外，借助于示波器自身的抖动分析软件或者矢量信号分析软件，还可以对超宽带的调频信号进行分析。下图是对一段在 7GHz 的带宽范围内进行调频的信号的频谱、时域以及跳频图案的分析结果。

我们如何学习通信协议，对于很多刚入行的同学来说，可能会比较迷惑。这是我在2009年1月写的一篇学习笔记，记录关于GSM跳频问题相关SPEC的学习轨迹，分享分享。     结论： 每个时隙在某一帧跳到哪个频率上，是动态地，在终端与基站建立一次通信的过程中通过专用的消息来确定的，不能由广播信道得到。 终端可以从BCCH或者DCCH中获得CA，即所有可能的跳频频点表。然后在每次建链的channel assignment消息中，终端确知MA，即从CA中选择出参与跳频的频点号，并获得HSN和MAIO，然后根据帧号计算得到当前帧应该跳频的频点号。       TS 45.002   描述： 6.2        Mapping in frequency of logical channels onto physical channels 6.2.1       General The parameters used in the function which maps TDMA frame number onto radio frequency channel are defined in subclause 6.2.2. The definition of the actual mapping function, or as it is termed, hopping sequence generation is given in subclause 6.2.3. In CTS, the specific mapping in frequency depends on the start condition defined by the parameters given in subclause 6.2.2. The hopping sequence generation for CTS is given in subclause 6.2.3. 6.2.2       Parameters The following parameters are required in the mapping from TDMA frame number to radio frequency channel for a given assigned channel. General parameters of the BTS, specific to one BTS, and broadcast in the BCCH and SCH: i)    CA: Cell allocation of radio frequency channels. ii)   FN: TDMA frame number, broadcast in the SCH, in form T1, T2, T3' (see subclause 3.3.2). For COMPACT, FN is broadcast in the CSCH, in form R1, R2 (see subclause 3.2.2). Specific parameters of the channel, defined in the channel assignment message: i)    MA: Mobile allocation of radio frequency channels, defines the set of radio frequency channels to be used in the mobiles hopping sequence. The MA contains N radio frequency channels, where 1 £ N £ 64.       For COMPACT, the reduced MA (see 3GPP TS 44.060) shall be used for a fixed amount of data blocks, see section 6.2.4. ii)   MAIO: Mobile allocation index offset.(0 to N‑1, 6 bits).       For COMPACT, MAIO_2 shall be used for the data blocks using the reduced MA. iii)  HSN: Hopping sequence (generator) number (0 to 63, 6 bits). 可见，MA，MAIO和HSN是与具体每个用户相关的参数。   具体的跳频方案，在 TS 45.002 中有描述如下： 6.2.3       Hopping sequence generation For a given set of parameters, the index to an absolute radio frequency channel number (ARFCN) within the mobile allocation (MAI from 0 to N‑1, where MAI=0 represents the lowest ARFCN in the mobile allocation, ARFCN is in the range 0 to 1023 and the frequency value can be determined according to 3GPP  TS 45.005 ), is obtained with the following algorithm: if  HSN = 0 (cyclic hopping)  then : MAI, integer (0 .. N‑1)    :        MAI = (FN + MAIO) modulo N else : M, integer (0 .. 152)         :        M = T2 + RNTABLE((HSN xor T1R) + T3) S, integer (0 .. N‑1) :        M' = M modulo (2 ^ NBIN) T' = T3 modulo (2 ^ NBIN)                                                                    if  M' N  then :                                                                                 S = M'                                                                       else :                                                                                 S = (M'+T') modulo N MAI, integer (0 .. N‑1)    :        MAI = (S + MAIO) modulo N   MA参数表，以及是MAIO和HSN参数网络通过“Messages for Radio Resources management”中某条消息传给终端的，详细内容在TS 44.018中有描述。这些消息是在某次通信过程中，网络与终端之间的三层消息，不属于某个广播信道。   44.018   中3.4.3.1描述：   Channel assignment initiation： If frequency hopping is applied, the cell allocation if present in the message is used to decode the mobile al********* If the cell allocation is not included, the mobile station uses its current cell allocation, the current CA is the last CA received on the BCCH. Afterward, the current CA may be changed by some messages sent on the main signalling link containing a CA (the possible messages are: ASSIGNMENT COMMAND, HANDOVER COMMAND and FREQUENCY REDEFINITION). Note that there are cases in which the current CA is undefined, see sub-clause 3.4.3.3.   CA 是用来解码MA的。CA是从BCCH获得，MA是从Channel assignment   消息中获得。   在 10.5.2 .1b  Cell Channel Description 对CA的格式进行详细描述： 在 10.5.2 .13        Frequency list 对CA的格式中具体频率的计算有详细的描述。通过确定的算法获得所有的16，或17，或21，或29个频点。     在 10.5.2 .21 Mobile Allocation      对MA信息字段有详细解释。 MA C i, Mobile allocation RF channel i (octet 3 etc.), i = 1, 2,..., NF The MA C i bit indicates whether or not the Mobile allocation frequency list includes the i'th frequency in the cell allocation frequency list. The cell allocation frequency list is derived from the set of frequencies defined by the reference cell channel description information element. NF denotes the number of frequencies in the cell allocation frequency list. In the cell allocation frequency list the absolute RF channel numbers are placed in increasing order of ARFCN, except that ARFCN 0, if included in the set, is put in the last position in the list, For a RF channel belonging to the mobile allocation the MA C i bit is coded with a "1"; i = 1, 2,..., NF. For a RF channel not belonging to the mobile allocation the MA C i bit is coded with a "0"; i = 1, 2,..., NF. If NF mod 8 0 then bits NF to 8n in octet 3 must be coded with a "0" in each.   10.5.2 .5 Channel Description  对MAIO和HSN参数有详细解释，并H值表示是否跳频。 ARFCN, (octet 3, bits 2 and 1, and octet 4, bits 8 to 1) The ARFCN is coded as the binary representation of the absolute RF channel number Range: 0 to 1023 H = "1": The channel selector field consists of the mobile allocation index offset, MAIO, and the hopping sequence number, HSN. MAIO, (octet 3 bit 4 to 1 high part and octet 4 bit 8 to 7 low part) The MAIO field is coded as the binary representation of the mobile allocation index offset as defined in 3GPP TS 45.002. Range: 0 to 63. HSN, (octet 4 bit 6 to 1) The HSN field is coded as the binary representation of the hopping sequence numberas defined in 3GPP TS 45.002 Range 0 to 63.