Analysis and Research on GSM Mobile Phone RF System (VII)

JasonYoo

Analysis and Research on GSM Mobile Phone RF System (VII) [Copy link]

Code element simulation analysis

Because the transmission rate of GSM is 270Kb/s, the time of each code element is 3.69us. Because the transmission system with BT=0.3 is widely used now, the following analysis is made for BT=0.2 and BT=0.1.
Figure 6 below analyzes one code element in three cases, and then analyzes the influence of the phase diagram of any code element on the previous and next codes from its graph.

由观察得知，当BT乘积越小时，在一个位元区间前面位的相位函数与本位的更贴近，在一个位元区间內的信号会受到更多临近码元的干扰，显然不管是BT等于0.1，0.2还是0.3这位元的

对于后面两位的干扰要大于对前面位的干扰，因为相位是增函数。θ(t)可以写成

When the B(x)T product is smaller, each codeword will be interfered with more by neighboring codewords. If the current codeword is N(0), it will be most susceptible to N(-1), N(-2), N(+1), and N(+2). If we start with the simplest analysis, we only consider the 3-bit case, namely N(-1), N(0), and N(1). Next, we construct a phase matrix. The number of columns of this matrix is 8, namely the different combinations of the three-bit codewords, from 000 to 111. The number of rows is the number of sampling points, which is temporarily set to 4 for the convenience of analysis. Pij is the corresponding element of the matrix, where 1 ≤i≤4, 1 ≤j≤8,

Then check the corresponding phase value according to the data to be modulated. For N(i), when i ≤-2, the cumulative phase effect is basically θref. θref can be calculated using the same method of establishing a phase table. In the case of 3 bits, θi'(t) has 23 cases, that is, from 000 to 111.

θi(t)=θi’(t)+θref(t)(i-1)*T≤t≤i*T。

Of course, in actual engineering, we can consider 5 or 7 code elements at the same time for accuracy, and take 16 or 32 sampling points, and the phase moment will be amplified, but this will make the calculation more complicated. For a series of transmitted frame formats, we can use a sliding window method to process them. For example, if we use a five-bit window to analyze an 8-bit frame, we need to move four times. This article only analyzes the characteristics of a three-bit window. Therefore, when B(x)T is reduced, the inter-code interference can still be solved to a certain extent, but this requires that the DSP for receiving and processing has sufficient speed. At the same time, the value of B(x) can be reduced while taking into account some actual situations. For example, if the number of communications in the cell is small at night, the requirement for B(x) can be appropriately relaxed and maintained at 0.3, because after all, this can improve the BER. However, during the day, especially in high-density communication areas, this method will provide higher redundancy for the GSM transmission system if additional base stations are to be added in the future and the cell density is increased. Of course, there are also some new solutions to BER, mainly real-time BER detection.