FAQ_ S2LP FEC 规则

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FAQ_ S2LP FEC 规则 [复制链接]

本文作者：ST工程师Joshua Zhu

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关键字 ：S2-LP, sub 1GHZ 

 

 

问题: 客户使用S2-lp来开发他们的产品。现在它们可以使能FEC在两个基于S2-lp的设备之间进行通信。但他们计划使能FEC的条件能兼容其它sub 1Ghz的产品。如: 

1). 设备基于S2-lp， 使能 FEC作为TX，其它sub 1Ghz的产品为RX(可采用软方式实现FEC)； 

2). 设备基于S2-lp， 使能 FEC作为RX，其它sub 1Ghz的产品为TX(可采用软方式实现FEC)； 

 

ST工程师解答：

1). FEC 解释: 

• In convolutional codes, each block of k bits is mapped into a block of n bits 

BUT these n bits are not only determined by the present k information bits 

but also by the previous information bits. 

• S2-LP implements a (n,k,L)=(2,1,2) encoder. The output of each 

convolution depends on one input and on 3 values memorized in the shift 

register. 

• Rate 1/2 (k = 1, n = 2) 

• Constraint length L = M + 1 = 4 (M bit memory) 

• When FEC is enabled the number of transmitted bits is roughly doubled 

hence the on-air packet duration in time is roughly double as well. 

2). Viterbi: 

• S2-LP implements a Viterbi algorithm that works on 6-bit soft-decision. 

• The Viterbi algorithm compares the received coded sequence with the 

encoded sequences resulting from the encoding of all possible input 

message sequences. 

• The metrics are calculated as difference between the frequency deviation 

measured and all the possible frequency deviation expected. 

3). Interleaving: 

• In order to improve the effectiveness of convolutional encoding, matrix 

interleaving is applied to the encoded data at the output of the convolutional 

encoder. 

• The symbols from the output of the encoder are written raw-wise into a 4x4 

matrix buffer starting from the upper-left cell and read column-wise starting 

from the lower right cell. 

• Each pair of encoded symbols corresponding to one single encoded bit is 

packet into a single matrix cell. For each encoded symbols pair (so(n), s1(n)) 

the symbol so(n) is transmitted first on air. 

• Note that interleaving is always enabled together with FEC for the Basic and 

STAck packet formats.