Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2001-08-29
2004-01-13
Tran, Khai (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C370S342000
Reexamination Certificate
active
06678314
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to spreading factor detection, and more particularly, but not explicitly, spreading factor detection or determination in a communication system that is based on direct sequence spread spectrum technique or any analogous technique.
BACKGROUND OF THE INVENTION
In a communication system data is transmitted as a plurality of data symbols in subsequent data or radio frames. The signals carrying the data may be transmitted with variable data symbol transmission rates (data speeds), wherein the transmission rate may be different in subsequent frames of the transmission. For example, in a cellular CDMA (Code Division Multiple Access) system data is encoded for transmission by processing data symbols to be transmitted by a spreading code for each transmission channel. The effect of a spreading code is to spread the frequency band of a transmission to a chip rate which is larger than the actual data or information symbol rate. This results that more symbols is transmitted than what the actual number of information symbols is since at least some of the symbols are repeated. For example, if the used spreading factor is eight, eighth symbols (referred to as “chips”) are transmitted for every information symbol.
According to one possibility the length of the spreading code (i.e. the number of “chips” per a data symbol) is defined by a spreading factor. It is, however, noted that the length of the spreading code may also remain constant, i.e. the length thereof may not necessarily depend on the data rate. The spreading factor is sometimes expressed by a definition ‘chip rate’: ‘data symbol rate’ or by ‘data symbol duration’: ‘chip duration’, wherein the ‘data symbol duration’ equals 1: ‘data symbol rate’ and the ‘chip duration’ equals 1: ‘chip rate’. The term spreading factor will be used in the following, although also other terms, such as spreading ratio or processing gain, may sometimes be used in this context.
It is possible to set up such a variable-rate (multi-rate) connection where the data symbol rate of the information symbols or bits, and thus the spreading code used in the spreading modulation of these symbols, may vary from frame to frame (e.g. every 10 ms). The data rates used in such a connection are not arbitrary, but for each frame duration, one of the plurality of predefined data rates is used. Moreover, although not necessarily so, every higher data symbol rate may be dividable by a lower data symbol rate, the division factor being for example 2
k
for k≧0. This specification presents an example employing variable-rate connection spreading factors 4, 8, 16, 32, 64, 128 and 256 and corresponding data rates. However, it is evident that also other spreading factors and data rates may be used without departing from the basic concept of the variable data rate (or multi-rate) transmission. To give an example of a possible relation between the spreading factors (SF) and the data symbol rates, in a CDMA with a chip rate of 4.096 MHz the relation may be such that spreading factors 4, 8, 16, 32, 64, 128 and 256 correspond data rates 1024, 512, 256, 128, 64, 32 and 16 ksps (kilosymbols per second), respectively. However, the relation between the spreading factors and data rates may be different. For example, factors such as the number of code channels and the employed channel coding method and possible use of puncturing may influence the relation between the end user bit rate and the spreading factor.
When a signal is transmitted between a base station and a mobile station (either on the uplink or the downlink) in a communication system that is based on direct sequence spread spectrum technique, the receiving station may need to establish from the received signal some information about the communication path along which the signal has travelled. This procedure is referred to herein as “channel estimation”. The channel estimation is typically carried out in a channel estimation entity. Various techniques are known for channel estimation. A channel impulse response generated by the estimation entity is required in order to properly decode and process incoming data.
In case a dedicated data channel is used for the channel estimation, it is possible to use the spreading factor and/or the data transmission rate of the received signal in the channel estimation. It is also possible, and in some cases even necessary, to utilise spreading factor information when estimating signal-to-interference ratio (SIR) in a data channel. However, if the spreading factor and/or data rate of a received radio frame is not directly available at the receiving station, the correct or most probable value thereof has to be determined by some means, e.g. by trying several possible data spreading factors for the transmission. For example, in the CDMA the data symbols in said radio frame are usually obtained by means of detecting the complex data symbols assuming that the highest-possible data symbol rate is used for transmission, and using the lowest-possible spreading factor in the receiving station. For example, when the length of the spreading code depends on the spreading factor, the shortest possible spreading code (e.g. with spreading factor SF=4) has to be known so that all the other spreading codes can be constructed from the shortest spreading code by multiplying the shortest spreading code by an integer. In practice, if the turnover time of the spreading code, i.e. the spreading factor, is not known, the receiver unit must perform the first despreading based on an assumption that the shortest possible code (minimal SF) was used for the transmission. For example, assuming that the data rate of the received radio frame was 1024 ksps in the first place, this corresponds to the spreading factor of 4 (at 4.096 Mcps) used in the detection of the data symbols. The result of this detection should be a data vector of all detected complex information symbols corresponding to the received radio frame.
The components of a signal that has propagated over a multipath channel have to be combined or summed at the receiver end in order to receive a signal that corresponds the original signal prior spreading and transmission. An example of the combination techniques is so called maximal ratio combination (MRC). The MRC is based on computing a weighted average for the signal components such that more weight is given for the signal components that have travelled over the strongest signal paths.
However, after combining the spread signal a correct spreading factor has to be used, otherwise the received data cannot be properly represented or used otherwise and/or will be lost. Therefore the above assumption of the data rate may not be enough, but a more accurate determination of the actual data rate of the signal at the transmitting station during the transmission may be required. For example, if the basic assumption is incorrect, this stage may then give a decision that instead of the assumed and initially used 1024 ksps (with spreading factor 4 and frequency 4.096 MHz) the actual data rate in the received radio frame was 256 ksps (corresponding to the spreading factor of 16). After the determination, as many adjacent samples or chips as is indicated by the spreading factor are summed together to get one actual data symbol. In the above example, this would lead to summing up every four adjacent samples to get one actual data symbol (4×4=16). Therefore it would be advantageous to be able to have information of the actual data symbol rate and/or spreading factor of the transmission already at the time of the detection of the signal at the receiver circuitry.
The transmission between the stations may or may not include explicit information of the spreading factors and/or data rates of the transmitted signal. If the variable-rate connection includes explicit information on the used spreading factors and/or data rates for each received radio frame, this information is usually given in a separate channel, for example in an DPCCH (Dedicated Physical Control
Juntti Markku
Pajukoski Kari
Squire Sanders & Dempsey L.L.P.
Tran Khai
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