number of ones and zeros in the spreading code.
Few or no repeated patterns In
CDMA
application,
further
of a term in the generator polynomial (explained
subsequently),
P(X),
excluding the Xn term.
Two equivalent ways of characterizing requirement of lack of correlation Two general categories of spreading sequences:
1）PN sequences 2）Orthogonal codes线性反馈移位寄存器实现（Linear Feedback
Shift
Register
Implementation）
a circuit consisting of XOR gates and a shift register implementing the PN generator for spread spectrum The LFSR contains n bits.
There are from 1 to (n-1) XOR gates.The presence or absence of a gate corresponds to the presence or absence
the PN LFSR:
A sum of XOR terms
Bn?A0B0?A1B1?A2B2???An?1Bn?1
Generator polynomial（生成多项式）
P(X)?A1?10X0?A1X?A22X???An?1Xn?Xn
Orthogonal Code: a set of sequences in which all pairwise cross correlations
are zero.
Walsh codes: the most common orthogonal codes used in CDMA applications.
A set of Walsh codes of length n consists of the n rows of an n×n Walsh matrix.
The matrix is defined recursively as:
W1??0??WnWn?W2n???WW???nn?
Error detection principles –
transmitter:
六、 编码和错误控制（Coding and Error control）
Three approaches（方法） are in common use for coping with data transmission
error:
Error detection codes（侦错码）: simply
detects the presence of an error
Error correction codes, also called forward error correction (FEC) code（前向纠错码）: not just to detect but
correct errors
Automatic repeat request (ARQ) protocols（自动重发请求协议）: receiver discards a block of data in
which an error is detected and the
transmitter retransmits that block of data
错误检测（Error Detection）
For a given frame of bits, additional
bits
that
constitute
an
error-detecting code are added
This code is calculated as a function
of the other transmitted bits
For a data block of k bits, the error
detection algorithm yields an error
detection code of n-k bits, where
(n-k) The error detection code (also check bits), is appended to the data block to produce a frame of n bits which is then transmitted Error detection principles – transmitter: For a given frame of bits, additional bits that constitute an error-detecting code are added This code is calculated as a function of the other transmitted bits For a data block of k bits, the error detection algorithm yields an error transmitted after modulation During transmission the signal is subject to noise, which may produce bit errors in the signal. detection code of n-k bits, where (n-k) The error detection code (also check bits), is appended to the data block to produce a frame of n bits which is then transmitted 区块改错码（Block Error Correction Codes） Error Correction Codes: Correct errors in an incoming transmission on the basis of the bits in that transmission Transmitter： Forward error correction (FEC) encoder maps each k-bit block into an n-bit block codeword, which is Receiver： Incoming signal is demodulated to produce a bit string which may contain errors Block passed through an FEC decoder 分组码原则（Block Code Principles） Hamming distance（汉明距离）: Hamming distance d(v1,v2) between two n-bit sequences v1 and v2 is the number of bits in which v1 and v2 disagree. If v1=011011, and v2=110001, then d(v1,v2) =3. If v1=1, and v2=1, then d(v1,v2) =2. In general, an (n, k) block code encodes k data bits into n-bit codewords With an (n,k) block code, there are 2 valid codewords out of a total of 2n possible codewords. The ratio of redundant bits to data bits, (n-k)/k is called the redundancy of the code. The ratio of data bits to total bits, k月确定了IMT-2000的网络框架标准 ，该标准明确了由ITU定义的系统接口 k/n is called the code rate. The code rate is a measure of how much additional bandwidth is required to carry data at the same data as without the code. 第九章3G的主要技术按照各组成部分的功能划分，IMT-2000系 统可以分为三大部分：移动终端即是为移动用户提供服务的设备，它与无线接入网之间的通信链路为无线链路。无线接入网实现无线传输功能可以细分为无线传输特殊功能（RTSF）无线载体通用功能（RBCF） 核心网主要作用是提供信息交换和传输 ，可以采用分组交换或者ATM网络，最终将过渡到全IP网络，并且与第二代移动通信系统核心网兼容。系统结构ITU-T的SG11/WP3工作组于1998年5 系统的无线接口是最重要的一个接口 ，当今的主流标准是 cdma2000、WCDMA以及TD-SCDMA
移动通信总结3
