Spread spectrum communications have been extensively used in such applications as cellular communications and satellite communications. For cellular communications systems, the established form of spread spectrum communications is the code division multiple access (CDMA) scheme. Such communications typically use a pseudonoise (PN) code that is generated serially and cyclically. The PN code is used to modulate a carrier signal in order to spread the spectrum of the carrier signal a desired amount.
With CDMA, the signals from all users simultaneously occupy the same frequency band. The receiver discriminates the multiple signals by exploiting the properties of a PN code that is applied to the signal of each user. The receiver attempts to match in time with the PN code of the desired signal a replica of that PN code. Only then the demodulation result is meaningful; otherwise it appears noise-like. Thus, if the arriving signals have different PN codes or different PN code offsets, they can be discriminated at the receiver. Additional coding can be provided to each signal in order to ensure proper communication in case several signals arrive with the same PN code and PN code offset (e.g., orthogonal coding, use of additional PN codes, etc.).
One of the first actions that a user station must do when such a communication with another station is initiated, is to acquire the signal transmitted by the latter. The usual approach in that case is for the user station to acquire a pilot PN code signal, i.e., a signal modulated by only the PN code and carrying no data, transmitted by the other station. In order to achieve that goal, the user station employs a PN code replica of the pilot signal and tries to synchronize to the pilot signal through correlation with the incoming PN code. However, acquisition may also be based on a data modulated signal.
A PN code is a binary pseudorandom code and each of its elements is called a chip. Because of the pseudonoise code properties, the code autocorrelation results a peak only when the offset between the incoming and local PN codes is less that a chip. Acquisition techniques use that property to align the incoming and local PN codes to within less than a chip. The techniques involve the iterative utilization of the local PN code at a particular offset with respect to a specific time reference, and determination of some property related to degree of correlation. Each particular offset so utilized is termed a hypothesis.
The communication channel and the requirement for short correlation periods introduce several imperfections to the acquisition process that have as an end result the possibility of a miss of the correct chip interval even though the correct hypothesis is employed, i.e. a miss, or an acquisition indication at an incorrect hypothesis, i.e. a false alarm. One of the sources for those events is the noise in the received signal that is added by the channel. The channel noise may be the result of other interfering spread spectrum signals and/or it may be caused by the physical properties of the medium. It can cause the received signal at the correct chip interval to decrease in power, thereby leading to a miss even though the correct hypothesis is used, a miss, or it may cause a power surge at an incorrect hypothesis, thereby leading to incorrect acquisition, a false alarm. A false alarm, and to a lesser extent a miss, can add a considerable amount of time to the acquisition process.
Another source for misses and false alarms, with similar final effects as the channel noise, is the multipath signal fading that may be introduced by the channel.
Finally, due to the usual requirement for short correlation periods, usually much shorter than the PN code period, the PN properties of the pilot code are not fully exploited. The correlation period needs to be short in order to ease the implementation of the correlation process, and/or decrease the correlation time needed to test each hypothesis, and/or minimize the dB losses due to a frequency clock error. As a result, the correlation may exhibit larger peaks at incorrect hypotheses which can lead to false alarms.
The typical approach to mitigate false alarms, while ensuring an acceptably small miss probability and an acceptably small acquisition time, is to split the acquisition process into two stages. The first stage, called test stage (TS, or TS stage), is fast in order to quickly reject most of the incorrect hypotheses.
Notice that all hypotheses are incorrect except those corresponding to samples taken at the correct chip interval. Moreover, the requirements of a fast TS stage and short correlation periods together with the channel effects, can frequently increase the miss probability for the correct hypothesis. This becomes especially important for communications in environments where the signal may undergo substantial multipath fading and have its power divided among several fading paths.
To partly compensate for that outcome, the TS stage is declared successful for a particular hypothesis when the result of the corresponding correlation exceeds a threshold that is chosen to be considerably smaller than the value of the decision statistic corresponding to the exact time and to the case that all power is contained in one signal path. The threshold value is typically normalized by an estimate of the background noise, in order to mitigate variations in the signal energy caused by fading, and the normalized value is typically larger than one. Nonetheless, as a result of this use of a relatively small threshold value, many incorrect hypotheses may successfully pass the TS stage. However, most incorrect hypotheses are successfully rejected in this stage, and are rejected quickly.
The typical acquisition method then employs a second, longer stage, called verification stage (VS, or VS stage), in order to reject the incorrect hypotheses that got past the TS stage, while making certain that a correct one will be accepted with high probability. The duration of the VS stage is longer than that of the TS stage in order to more rigorously test each hypothesis and average out occasional channel imperfections. If the TS stage is not frequently passed by incorrect hypotheses, the additional time spent in the VS stage will not considerably penalize the acquisition time.
Existing VS methods comprise a single stage and can use different approaches to decide on a particular hypothesis. One such approach is to use diversity and combine the decision statistics from several individual correlations before a final lock decision is made. It is necessary to noncoherently combine decision statistics instead of having a single long correlation period in order to avoid significant dB losses that can be caused by a clock frequency error, and/or due to the limited phase stability and/or due to data modulation of the signal used for acquisition.
This approach has two important disadvantages. First, since the TS threshold has to be such that the miss probability is small even for the worst case scenario, the VS stage may be entered a large number of times during each search. Thus, if the VS stage relies on the decision statistics from many individual correlation periods, say it requires a few milliseconds, then the increase in the acquisition time can be prohibitively large. On the other hand, a shorter VS stage will easily result in several false alarms since signal fades can last up to several milliseconds for low user station speeds and carrier frequencies in the order of 1 GHz. Moreover, occasional high increases in the interfering signals levels or in the noise proportionally increase the false alarm probability while occasional deep fades of the desired pilot considerably decrease the correct detection probability. Thus, the combination of an excessive acquisition time with a large false alarm probability can make this VS method inefficient.
Another VS approach uses majority testing where several individual tests are performed on the same hypothesis and a decision is made based on the number of positive ones. Each test usually comprises several correlation periods and a subsequent comparison of a decision parameter with a threshold. Comparing to the first one, this approach has the advantage that occasional signal fades or interference/noise surges do not affect all correlation periods and may therefore be possibly isolated once a final majority decision is taken. However, many false alarms can again occur and it is not hard to see that this approach suffers from the same basic drawbacks as the first one.