e-book Cellular mobile radio systems : designing systems for capacity optimization

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Assigned codes are CDMA codes that are allocated for a particular connection. When a set of sequences are generated from the same generator as described, only the seed of the 36 stage LFSR is specified to generate a family of sequences. Sequences for all the global codes, are generated using the same LFSR circuit. The signal that is upconverted to RF is generated as follows. The Logical channels are initially converted to QPSK signals, which are mapped as constellation points as is well known in the art. Similarly, two spreading codes are used to form complex spreading chip values.

The complex data are spread by being multiplied by the complex spreading code. Similarly, the received complex data is correlated with the conjugate of the complex spreading code to recover despread data. The period of the short codes is equal to the symbol duration and the start of each period is aligned with a symbol boundary. Both SU and RCS derive the real and imaginary parts of the short codes from the last eight feed-forward sections of the sequence generator producing the global codes for that cell.

The short codes that are in use in the exemplary embodiment of the invention are updated every 3 ms. Other update times that are consistent with the symbol rate may be used.

Therefore, a change-over occurs every 3 ms starting from the epoch boundary. At a change-over, the next symbol length portion of the corresponding feed-forward output becomes the short code. When the SU needs to use a particular short code, it waits until the first 3 ms boundary of the next epoch and stores the next symbol length portion output from the corresponding FF section. This shall be used as the short code until the next change-over, which occurs 3 ms later. The exact relationship between the spreading code sequences and the CDMA logical channels and pilot signals is documented in Table 5a and Table 5b.

For global codes, the seed values for the 36 bit shift register are chosen to avoid using the same code, or any cyclic shift of the same code, within the same geographical area to prevent ambiguity or harmful interference. No assigned code is equal to, or a cyclic shift of a global code. The RCS transmits a forward link pilot carrier reference as a complex pilot code sequence to provide time and phase reference for all SUs , , , and in its service area. With only one pilot signal in the forward link, the reduction in system capacity due to the pilot energy is negligible.

The SUs , , , and each transmits a pilot carrier reference as a quadrature modulated complex-valued pilot spreading code sequence to provide a time and phase reference to the RCS for the reverse link. The reverse pilot channel is subject to APC. In addition, there are pilot signals associated with access channels. Short access channel pilots SAXPTs are also associated with the access channels and used for spreading code acquisition and initial power ramp-up.

Messages are sent continuously over this channel, and each message lasts approximately 1 ms.

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The FBCH message is 16 bits long, repeated continuously, and is epoch aligned. For the FBCH, bit 0 is transmitted first. As used in Table 6, a traffic light corresponds to an Access Channel AXCH and indicates whether the particular access channel is currently in use a red or not in use a green. The values of the traffic light bits may change from octet to octet, and each 16 bit message contains distinct service indicator bits which describe the types of services that are available for the AXCHs.

One embodiment of the present invention uses service indicator bits as follows to indicate the availability of services or AXCHs. Each service type increment has an associated nominal measure of the capacity required, and the FBCH continuously broadcasts the available capacity. This is scaled to have a maximum value equivalent to the largest single service increment possible. When an SU requires a new service or an increase in the number of bearers, it compares the capacity required to that indicated by the FBCH, and then considers itself blocked if the capacity is not available.

The FBCH and the traffic channels are aligned to the epoch. Slow Broadcast Information frames contain system or other general information that is available to all SUs and Paging Information frames contain information about call requests for particular SUs.

As previously defined, the code epoch is a sequence of 29 20 chips having an epoch duration which is a function of the chip rate defined in Table 7 below. Sleep Cycle Slot 1 is always used for slow broadcast information. Slots 2 to M- 1 are used for paging groups unless extended slow broadcast information is inserted. Within each Sleep Cycle the SU powers-up the receiver and re-acquires the pilot code. It then achieves carrier lock to a sufficient precision for satisfactory demodulation and Viterbi decoding.

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The settling time to achieve carrier lock may be up to 3 Slots in duration. Having monitored its Slot the SU will have either recognized its Paging Address and initiated an access request, or failed to recognize its Paging Address in which case it reverts to the Sleep mode. Table 8 shows duty cycles for the different bandwidths, assuming a wake-up duration of 3 Slots. Three CDMA spreading code tracking methods in multipath fading environments are described which track the code phase of a received multipath spread-spectrum signal.

The first is the prior art tracking circuit which simply tracks the spreading code phase with the highest detector output signal value, the second is a tracking circuit that tracks the median value of the code phase of the group of multipath signals, and the third is the centroid tracking circuit which tracks the code-phase of an optimized, least mean squared weighted average of the multipath signal components. The following describes the algorithms by which the spreading code phase of the received CDMA signal is tracked.

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A tracking circuit has operating characteristics that reveal the relationship between the time error and the control voltage that drives a Voltage Controlled Oscillator VCO of a spreading code phase tracking circuit. When there is a positive timing error, the tracking circuit generates a negative control voltage to offset the timing error. When there is a negative timing error, the tracking circuit generates a positive control voltage to offset the timing error.

Received signal r t is applied to matched filter , which correlates r t with a local code-sequence c t generated by Code Generator The tracking circuit produces an error signal e t as an input to the code generator The code generator uses this signal e t as an input signal to adjust the code-phase it generates. Assuming that the reference user is not transmitting data so that only the spreading code modulates the carrier. The receiver passes the received signal through a matched filter, which is implemented as a correlation receiver and is described below.

This operation is done in two steps: first the signal is passed through a chip matched filter and sampled to recover the spreading code chip values, then this chip sequence is correlated with the locally generated code sequence.

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In the multipath channel described above, the sampler samples the output signal of the matched filter to produce x nT at the maximum power level points of g t. In practice, however, the waveform g t is severely distorted because of the effect of the multipath signal reception, and a perfect time alignment of the signals is not available. When the multipath distortion in the channel is negligible and a perfect estimate of the timing is available, i.

Summarizing The Analysis

When there is multipath fading, however, the received spreading code chip value waveform is distorted, and has a number of local maxima that can change from one sampling interval to another depending on the channel characteristics. For multipath fading channels with quickly changing channel characteristics, it is not practical to try to locate the maximum of the waveform f t in every chip period interval.

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Instead, a time reference may be obtained from the characteristics of f t that may not change as quickly. Three tracking methods are described based on different characteristics of f t. Prior art tracking methods include a code tracking circuit in which the receiver attempts to determine the timing of the maximum matched filter output value of the chip waveform occurs and sample the signal accordingly.

However, in multipath fading channels, the receiver despread code waveform can have a number of local maxima, especially in a mobile environment. In the following, f t represents the received signal waveform of the spreading code chip convolved with the channel impulse response. The frequency response characteristic of f t and the maximum of this characteristic can change rather quickly making it impractical to track the maximum of f t.

In a multipath fading environment, the waveform f t can have multiple local maxima, but only one median. Therefore, the Median Tracking Method of the present embodiment tracks the median of f t. The following equation determines the operating characteristic of the circuit in FIG. Tracking the median of a group of multipath signals keeps the received energy of the multipath signal components substantially equal on the early and late sides of the median point of the correct locally generated spreading code phase c n. The output of each correlator is applied to a respective first sum-and-dump bank The magnitudes of the output values of the L sum-and-dumps are calculated in the calculator and then summed in summer to give an output value proportional to the signal energy in the early multipath signals.

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The magnitudes of the L sum-and-dump output signals are calculated in calculator and then summed in summer to give a value for the late multipath signal energy. The optimal spreading code tracking circuit of one embodiment of the present invention is called the squared weighted tracking or centroid circuit. This function inside the integral has a quadratic form, which has a unique minimum.

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Based on these observations, a realization of an exemplary tracking circuit which minimizes the squared weighted error is shown in FIG. The early and late multipath signal energy on each side of the centroid point are equal. The centroid tracking circuit shown in FIG. The output signal of each correlator is applied to a respective one of L sum-and-dump circuits of the first sum and dump bank The magnitude value of each sum-and-dump circuit of the sum and dump bank is calculated by the respective calculator in the calculator bank and applied to a corresponding weighting amplifier of the first weighting bank The output signal of each weighting amplifier represents the weighted signal energy in a multipath component signal.

The weighted early multipath signal energy values are summed in sample adder to give an output value proportional to the signal energy in the group of multipath signals corresponding to positive code phases which are the early multipath signals. The magnitude value of the L sum-and-dump output signals are calculated by the respective calculator of calculator bank and then weighted in weighting bank The weighted late multipath signal energy values are summed in sample adder to give an energy value for the group of multipath signals corresponding to the negative code phases which are the late multipath signals.

The tracking circuit of FIG. The embodiment shown uses weighting values that increase as the distance from the centroid increases.