Patent Application
20080013494
I. Field of the Invention
The present invention relates to communication systems in general and, in particular, to improving the transmission of information signals in a communications system.
II. Description of the Related Art
CDMA communication systems are very sensitive to peak transmit power and are generally limited by interference related to transmit power levels. One interference related limitation is the so called “Near-Far Problem”. In this problem as transmit power increases during a transmission it causes more interference in other channels. To deal with this additional interference the other channels must increase their own transmit power. The increase in transmit power by the other channels in turn generates more interference for all the channels. This avalanche effect occurs until the system is stabilized and all the channels are satisfied. Therefore, in order to maximize the capacity of such a system it is desirable that each user transmit only the minimum power necessary to achieve a required quality of service. Another problem that can degrade the performance of other links in a transmission system is a waveform that contains a discontinuous power pattern. This problem compounds the Near-Far Problem.
Transmit power amplifiers provide another area where interference can limit the capacity of CDMA communication systems. The maximum output power of transmit power amplifiers is determined by a number of design parameters including power dissipation and unwanted emissions. Unwanted emissions are those that are outside the bandwidth of the input signal. Most of the unwanted emissions occur due to intermodulation within the power amplifier. Intermodulation is caused by high transmit power levels that drive the amplifier into a nonlinear region.
Unwanted emissions are often limited by regulatory bodies, such as the FCC. Industry standards may also set limits on unwanted emissions in order to avoid interference with the same or another system. To maintain unwanted emissions within the desired limits, the output power of the transmit power amplifier is selected so that the probability of exceeding the emission limits is very small. When a waveform having a nonlinear envelope is amplified, the maximum output is determined by the portion of the waveform that has the highest power level. Additionally, if the requested output power exceeds the maximum permitted output power, a transmitter can limit the output power to the maximum permitted level in order to keep the unwanted emissions within the prescribed limits.
Referring now to
For example,
Total power waveform 30 represents the total power of transmission waveforms 20a-n summed together. Because pilot portions 22 and data portions 22 are at high levels within transmission waveforms 20a-n, the corresponding portions 32, 36 of total power waveform 30 are high. Because voice portions 26 vary and are usually low, portion 34 of total power waveform 30 can vary from close to zero to an intermediate level 34.
The invention is a method for limiting the peak transmit power in a CDMA communication system. At least one of first and second high transmit power regions are separated into a plurality of high transmit power subregions. The high transmit power subregions of the plurality of high subregions are shifted by time offsets of differing durations to provide a plurality of time offset subregions. First and second low transmit power regions are also provided. At least one of the first and second low transmit power regions is also separated into a plurality of transmit power subregions and the low transmit power subregions are shifted by time offsets of differing time durations. The subregions can be time offset by a predetermined time duration or by a random time duration.
The features, objects, and advantages of the present invention will become more apparent form the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify corresponding elements throughout and wherein:
Referring now to
However, in a preferred embodiment of the present invention transmit waveforms 50 are time offset with respect to each other in such a way that the high power levels A do not line up with each other. In this way the high levels and the low levels of the various transmit waveforms 50 are averaged out. This results, most importantly, in a lower peak transmit power in the communication band. As previously described, a lower peak transmit power reduces unwanted emissions and interference.
Referring now to
In one preferred embodiment of the invention, the averaging of the high transmit levels A of transmit waveforms 74a-n is accomplished by providing each successive waveform 74a-n with the same fixed offset when a new waveform 74a-n is added to the communication band. Thus, for illustrative purposes, transmit waveforms 74a-n are identical to each other except that they are time offset from each other by differing multiples of the fixed time offset t0.
For example, if transmit waveform 74a is the first signal to be transmitted by a communication band, it can be transmitted with zero offset. If transmit waveform 74b is the next signal to be transmitted within the communication band it can receive time offset t0 with respect to transmit waveform 74a. If transmit waveform 74c is the next signal to be transmitted it can be time offset by t0 with respect to transmit waveform 74b. This is equivalent to a time offset of 2t0 from waveform 74a. Each subsequent transmit waveform 74a-n to be transmitted by way of the communication band can then receive an additional offset t0 in the same manner. It will be understood however that it is not always possible to shift every waveform by any time offset that may be required by this method.
Referring now to
In another embodiment of the present invention, the various portions within time slots 72 of transmit waveforms 74a-n can be separated from each other and transmitted in any of the possible sequences. For example, within time slot 72 data portion 90 can be separated from the remainder of transmit waveform 74a and transmitted first. Pilot portion 78 can be separated and transmitted next after data portion 90. The remaining portions within time slot 72 can also be transmitted in any sequence. Applying this technique to the waveform of graphical representation 50, portions A, B, and C can be transmitted as ABC, ACB, or in any other order. Furthermore, the sequences can be varied from one transmit waveform 74a-n to the next
Improved results can be obtained in the method of separating and reordering the portions of transmit waveforms 74a-n by randomly changing the sequence of the transmissions of the waveform portions. This results in further averaging and smoothing of the contributions to the total transmit power made by the various waveforms. New transmission sequences can be continuously produced by a random number generator. In this case both the transmitter and the receiver must have knowledge of the parameters of the random number generator in order to permit decoding by the receiver.
In addition to using a fixed time offset t0 for each new waveform, it is possible to select an individual offset for each new waveform according to an algorithm. For example, the new time offset can be selected by determining which of the possible offsets is being used by the lowest number of existing calls. Additionally, the individual offsets can be determined by a peak power algorithm adapted to provide a minimum increase in the peak transmit power according to the shape or expected shape of the new transmission signals. The algorithm can be a heuristic one. In order to perform this function the peak power minimization algorithm must be able to predict the transmit power waveform over a period of time, for example over a transmit frame.
Referring now to
For example, the total transmit power of some known systems can be calculated as:
where:
(1−α)<1
is the forgetting factor,
When a new channel set up is required in order to add a new transmission waveform, the base station can compute the transmit power waveform W resulting from the addition of the new channel. The base station can then compute the resulting power vectors corresponding to each of the possible time offsets as follows:
(
where cyclk( ) is an operator that produces a cyclic shift of the vector W by k elements. The new channel can then be set up with the time offset that corresponds to the (
It will be understood that when a waveform such as transmission waveform 50 is separated into sections having power levels A, B and C, the transmission sequence of the sections can be selected in a similar heuristic manner. For example, the resulting peak transmit power can be determined for each possible transmission sequence and the transmission sequence resulting in the lowest peak transmit power can be selected.
Referring now to
The order of the transmission of the interleaved subregions can be a predetermined order, a random order, or any other order understood by those skilled in the art. Separation and interleaving of transmission waveforms in this manner provides excellent averaging of transmission waveforms and minimizing of peak transmit power. When regions within a transmit power waveform are interleaved in this manner the receiver must wait for the end of a slot before it can begin decoding.
The previous description of the preferred embodiments is provided to enable a person skilled in the art to make or use the present invention. The various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive faculty. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed. It will be understood that all of the methods disclosed herein can be used at the time of call set up or at any time during a transmission after set up.
Additionally, it will be understood that the various methods can be combined with each other in any manner. In particular, all of the separable waveform methods can be used independently or in conjunction with the previously described time shifting based methods, with or without the random or heuristic methods. Furthermore, the various methods disclosed herein can be performed either at the time of call setup or at any time during transmission of the transmission waveforms.
This continuation application claims priority from co-pending application Ser. No. 09/867,844 filed May 29, 2001, entitled “Signal Splitting Method for Limiting Peak Power in a CDMA System,” which is a divisional of Ser. No. 09/144,408, filed Aug. 31, 1998, entitled “Signal Splitting Method for Limiting Peak Power in a CDMA System, now issued as U.S. Pat. No. 6,396,817 on May 28, 2002, all of which are assigned to the assignee of the present invention.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 09867844 | May 2001 | US |
| Child | 11768863 | Jun 2007 | US |
