This paper introduces the third generation (WCDMA) and fourth generation (OFDM) handset modulation schemes and their key transmission characteristics, as well as the basic concepts involved in the functions required for test amplifiers for transmission component and component development/production testing.
All examples herein select the mobile phone system downlink (base station to mobile phone) for testing.
WCDMAWCDMA (Wideband Code Multiple Access) is a defined air interface for the third generation (3G) mobile telephone network UMTS. Direct Sequence Spread Spectrum (DSSS) is used to combine the "pseudo-noise" spreading code with the user signal to transmit the user signal over the bandwidth. Different codes are assigned to different users, and multiple simultaneous transmissions are realized through the same bandwidth. Since the signal assignment code is the same, the receiver can restore (de-spread) a particular signal in the composite wideband signal. During the restoration process, all other extended signals in the broadband appear as noise.
DSSS data transmissionWith DSSS, user baseline data is modulated by one of a number of spreading codes. Such codes are also referred to as "channelization codes," each of which is a high-rate (3.84 Mbit/s), iteratively repeating pseudo-random binary sequence that "fragments" baseline data to a bandwidth of 3.84 MHz.
Figure 1(a) shows the waveforms for data transfer and data restoration, where –1 = logic 0, +1 = logic 1. The first three curves represent the transmission process. Curve 1 represents user baseline data, curve 2 represents the 8-bit spreading code assigned to each user bit, and curve 3 represents the extended signal obtained after curve 2 is "broken" at curve 1. Curve 3 represents the transmitted signal.
Figure 1 (a): User data transmitted by spreading code 1, which is restored when the receiving end generates cross-correlation with the same code (labeled as despreading code 1)
The receiving end recovers the channel data using the same spread decoding (curve 4) in combination with the transmitted signal, thus being labeled "Despreading Code 1". Curve 5 represents the recovered user data. This process is called "de-spreading", which is mathematically related to the despreading code to form a transmission spreading code. Cross-correlation is described in the "Orthogonality" section on page 3, but in summary, even if the spreading code and the despreading code increase the XOR function.
Fig. 1(b) shows the result of combining the transmitted spread signal with different spread decoding. The first three tracking curves represent the same transfer process as in Figure 1(a). The difference is that when the receiving end uses another despreading code labeled "Despreading Code 2", the data is not recovered (curves 4 and 5).
Figure 1 (b): User data is transmitted by spreading code 1, and the receiving end does not recover when cross-association is generated by despreading code 2.
OrthogonalityWCDMA uses orthogonal variable spreading factor (OVSF) codes to achieve simultaneous multi-channel transmission and to ensure channel data rate flexibility. All OVSF spreading codes are "special" and mutually orthogonal, that is, they can coexist in the 3.84 MHz transmission band without cross interference.
To achieve orthogonality, each code must have the following attributes:
â— Any two code cross-association=0
â— Autocorrelation divided by the number of chip bits per data bit = 1
â— Must have the same number of codes as -1 and 1
According to these rules, we will check the spreading codes 1 and 2 as an example.
Validate one by one according to the rules:
(1) Cross correlation=0
The cross-correlation of two sequence numbers is a measure of the similarity between the two. R(AB) is expressed as the sum of the products of the sequence bits.
Suppose A is the spreading code 1 in Fig. 1(a), and B is the spreading decoding 2 in Fig. 1(b), as follows:
A={-1, 1, 1, -1, 1, -1, -1, 1}
B={1, -1, 1, -1, 1, -1, 1, -1}
R(AB)={(-1x1)+(1x –1)+(1x1)+(-1x1)+(1x1)+(-1x–1)+(-1x1)+(1x–1)}={ 0}
As shown in the previous section, the function of cross-correlation can be easily implemented at the gate level by using XOR gates.
(2) Autocorrelation æ•°é‡ Number of chip bits per data bit = 1
Autocorrelation is essentially a cross-correlation function of sequences.
R(AA)={(-1x-1)+(1x1)+(1x1)+(-1x-1)+(1x1)+(-1x-1)+(-1x-1)+(1x1)} ={8}
R(BB)={(1x1)+(-1x-1)+(1x1)+(-1x-1)+(1x1)+(-1x-1)+(1x1)+(-1x-1)} ={8}
These two types of spreading codes have 8-bit chip bits per data bit, and the chip bits of each data bit are called a spreading factor (SF). Therefore the autocorrelation is divided by SF=1.
(3) have the same number of -1 and 1
Finally, the spreading code 1 and the spreading code 2 have the same number of -1 and 1, so the two codes satisfy the third orthogonal condition.
It should be noted that the pseudo-random code can be generated by following the rules, and its similar noise is called pseudo noise (PN).
Variable spreading factorAs shown above, both the spreading code 1 and the spreading code 2 contain an 8-bit spreading factor. The downlink spreading factor takes between 4 and 512. Under the condition of low spreading factor, when the user requires faster data transmission, the system can allocate different data transmission rates and different spreading factors. This is where the orthogonal variable spreading factor is "variable". Note that the chip rate of 3.84 Mbit/s is constant, so the data rate allocated to the user baseband is different relative to the variable SF.
The scrambling code is added after the direct sequence code is spread. The scrambling code helps the mobile phone identify the base station being contacted.
OFDM
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