GB2382277A - Constrained power adjustment adaptive modulation cellular system - Google Patents
Constrained power adjustment adaptive modulation cellular system Download PDFInfo
- Publication number
- GB2382277A GB2382277A GB0219132A GB0219132A GB2382277A GB 2382277 A GB2382277 A GB 2382277A GB 0219132 A GB0219132 A GB 0219132A GB 0219132 A GB0219132 A GB 0219132A GB 2382277 A GB2382277 A GB 2382277A
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- base station
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- sir
- modulation rate
- maximum supportable
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- 230000001413 cellular effect Effects 0.000 title claims abstract description 8
- 230000003044 adaptive effect Effects 0.000 title description 3
- 238000000034 method Methods 0.000 claims abstract description 14
- 230000011664 signaling Effects 0.000 claims abstract description 6
- 238000004364 calculation method Methods 0.000 claims description 2
- 230000005540 biological transmission Effects 0.000 abstract description 17
- 230000000694 effects Effects 0.000 description 5
- 230000003595 spectral effect Effects 0.000 description 5
- 238000013459 approach Methods 0.000 description 3
- 230000008859 change Effects 0.000 description 3
- 238000005259 measurement Methods 0.000 description 3
- 238000004891 communication Methods 0.000 description 2
- 230000006872 improvement Effects 0.000 description 2
- 230000002452 interceptive effect Effects 0.000 description 2
- 239000000654 additive Substances 0.000 description 1
- 230000000996 additive effect Effects 0.000 description 1
- 238000012937 correction Methods 0.000 description 1
- 230000006870 function Effects 0.000 description 1
- 230000000116 mitigating effect Effects 0.000 description 1
- 239000000203 mixture Substances 0.000 description 1
- 230000008450 motivation Effects 0.000 description 1
- 230000009467 reduction Effects 0.000 description 1
- 238000009877 rendering Methods 0.000 description 1
- 238000004088 simulation Methods 0.000 description 1
- 238000001228 spectrum Methods 0.000 description 1
- 230000001360 synchronised effect Effects 0.000 description 1
- 238000012546 transfer Methods 0.000 description 1
Classifications
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L1/00—Arrangements for detecting or preventing errors in the information received
- H04L1/0001—Systems modifying transmission characteristics according to link quality, e.g. power backoff
- H04L1/0015—Systems modifying transmission characteristics according to link quality, e.g. power backoff characterised by the adaptation strategy
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L1/00—Arrangements for detecting or preventing errors in the information received
- H04L1/0001—Systems modifying transmission characteristics according to link quality, e.g. power backoff
- H04L1/0002—Systems modifying transmission characteristics according to link quality, e.g. power backoff by adapting the transmission rate
- H04L1/0003—Systems modifying transmission characteristics according to link quality, e.g. power backoff by adapting the transmission rate by switching between different modulation schemes
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L1/00—Arrangements for detecting or preventing errors in the information received
- H04L1/0001—Systems modifying transmission characteristics according to link quality, e.g. power backoff
- H04L1/0023—Systems modifying transmission characteristics according to link quality, e.g. power backoff characterised by the signalling
- H04L1/0025—Transmission of mode-switching indication
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- Engineering & Computer Science (AREA)
- Quality & Reliability (AREA)
- Computer Networks & Wireless Communication (AREA)
- Signal Processing (AREA)
- Mobile Radio Communication Systems (AREA)
Abstract
A simple scheme for providing high capacity downlink for both real time and non real time services in mobile telephony applications. The invention requires only that the terminals signal at appropriate times the supportable modulation format for its measured SIR. The base station notes this and schedules transmissions to provide fair support for services. In particular, a method for allocating modulation formats to each of a plurality of terminals within a cell of a cellular radio network, comprising the steps of: in each terminal, measuring a signal to interference ratio (SIR); calculating a maximum supportable modulation rate <I>M</I> which may be supported at the measured SIR; signalling the maximum supportable modulation rate <I>M</I> to a base station of the cell; and in the base station, transmitting data to each terminal in a format according to the respective maximum supportable modulation rate <I>M</I>.
Description
<Desc/Clms Page number 1>
CONSTRAINED POWER ADJUSTMENT
ADAPTIVE MODULATION CELLULAR SYSTEM The present invention relates to modulation methods for data communication between telephones and base stations of a mobile telephone system. In particular, it relates to the adaption of the modulation method in dependence upon the interference environment of a particular mobile telephone handset.
First and second generation cellular systems primarily supported constant bit rate voice services. If data was to be sent, a channel would be allocated. The channel would have a fixed data rate, and the data would be sent in the time taken at that data rate. Third generation systems will support a mixture of fixed and variable bit rates with a packet data element. This still requires allocation of a channel, and the data will be transmitted in a time determined by the maximum data rate for that channel. The channel may have a variable data rate, but will still limit the data throughput rate, even when other channels remain unused. This means that data transmission takes longer than the theoretically required time.
However, all systems have been based on a multiple access model rather than a queuing model. In contrast, wireless LAN systems have been based primarily on a queuing model. It is likely that beyond 3G systems will merge these concepts to create a cellular system with a queuing element for non real time services and a scheduled element for real time services.
For packet services it is not efficient, in terms of throughput delay characteristics to divide up the spectrum in a multiple access fashion.
A queued approach, using a single queue in which the entire resource is available to each user in turn, provides the optimum delay performance as data may be transmitted in a very short time, but it is not possible to transmit multiple channels at the same time.
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A useful way of evaluating the performance of a particular modulation scheme is to establish the delay-throughput curve. Delay throughput performance is found to be better for the single queue approach.
The downlink, that is, the communications channel from the base station to a mobile telephone handset, is particularly addressed in the present invention. Most traffic flows in this direction, for example, when downloading files.
In a cellular system there is always a potential problem with inter-cell interference. A data signal in one cell is an interfering signal to a mobile telephone handset located in a neighbouring cell. Packet transmission suffers when multiple base stations attempt to communicate with a single handset at the same time. The fact of interference is catered for in the demodulation, but the presence of interference is unpredictable and leads to sub-optimal data transmission rates. The levels of interference will vary according to the power transmitted by the various base stations.
The dimensions available for mitigating such problems are automatic transmit power control (ATPC), adaptive modulation and inter-cell transmit scheduling.
The problem with the inter-cell effects is their dynamic nature. It may be possible to set up a packet transmission on the basis of a particular interference environment at the mobile telephone handset but this may change during the transmission of the packet, rendering the set-up of the transmission sub-optimal.
Accordingly, the present invention provides a method for allocating modulation formats to each of a plurality of terminals within a cell of a cellular radio network, comprising the steps of : in each terminal, measuring a signal to interference ratio (SIR); calculating a maximum supportable modulation rate M which may be supported at the measured SIR; signalling the maximum supportable modulation rate M to a base station
<Desc/Clms Page number 3>
of the cell ; and in the base station, transmitting data to each terminal in a format according to the respective maximum supportable modulation rate M.
A home base station, being the base station of the cell containing the terminal, and each base station in the neighbouring cells, may each transmit a sounding burst signal at their respective full power, and each terminal may receive these signals and may derive a value of SIR from the relative strength of the received signals.
Each terminal may identify the signal transmitted by the home base station and may evaluate that as the signal, whereas the signals from all other base stations may be considered to form part of the noise in the calculation of SIR.
Each terminal may compute an estimate of a maximum supportable modulation rate M in accordance with the calculated SIR and may transmit this information to the base station.
The home base station may calculate by how much, if at all, its transmit power could be reduced while still permitting the maximum supportable modulation rate M to be supported, for each terminal.
The method may further comprise the step of transmitting a second sounding burst from the home base station and each base station in the neighbouring cells at a power as determined above, and each terminal may receive these signals and may derive a second value of SIR from the relative strength of the received signals.
Each terminal may compute an estimate of a new maximum supportable modulation rate N in accordance with the second calculated SIR and may transmit this information to the base station.
The above, and further, objects, characteristics and advantages of the present invention may be more fully appreciated from consideration of the following description of
<Desc/Clms Page number 4>
certain embodiments, given by way of examples only, along with the accompanying drawings, wherein:
Figure 1 shows a graph of SIR distribution for fixed power transmissions;
Figure 2 shows a graph of capacity for fixed power transmissions;
Figure 3 shows a graph of SIR distribution with and without single stage power control; and
Figure 4 shows a graph of capacity with and without single stage power control.
Two solutions to this problem may be proposed. Firstly, one could regularly signal the changes in the environment of a mobile telephone handset by signalling to the respective base stations, but this is difficult to implement as it requires inter-cell signalling. Alternatively, one could maintain the interference environment constant during the transmission of the packets to ensure that the interference environment does not change during the transmission of the packets, and that the data transfer rate established at the beginning of the transmission remains appropriate for the duration of the transmission.
This latter option may most simply be implemented by maintaining the transmissions at constant power in all cells. According to an aspect of the present invention, the terminals (mobile telephone handsets) can measure their signal to interference ratio (SIR), determine an applicable modulation format and signal this to the base station.
This means that the available bit rate will vary as a function of the location of the terminal within the cell, since the SIR will vary according to the location of the terminal within the cell. However, this effect can be compensated by allocating more time for transmission to those terminals at the edge of the cell than to those terminals which are close to the base station, since those close to the edge of the cell are likely to be able to handle only a lesser data rate than the terminals close to the base station. Those terminals located closer to the base station will achieve a higher SIR and so will be able to use a better modulation efficiency.
<Desc/Clms Page number 5>
A simulation was performed to determine the spectral efficiency of such an approach.
The parameters were as follows :- TABLE 1
Parameter Value Range law. 3.5 Shadowing Standard deviation 8 dB No of rings of interfering clusters 2 The resulting SIR distribution is shown in Figure 1.
This leads to the distribution shown in Figure 2 of capacity based on Shannon. By operating all base stations at their maximum power, one may be sure that a signal can reach any terminal within the cell. However, this leads to difficulties, not least in that operating a base station at full power increases the noise effect of that base station's signals in the neighbouring cells.
The higher spectral efficiencies are not practical. By setting an upper limit on spectral efficiency (including forward error correction coding) of 3 bps/Hz, and clipping the spectral efficiency at this value, then the average spectral efficiency is given as about 1.27 bps/Hz. This is an extremely high figure.
It will be appreciated that if we clip the capacity at 3 bps/Hz then for any occasions in which the SIR is greater than 7, we are wasting transmitter power and generating unnecessary interference. Based on Shannon's equation and treating interference as additive white Gaussian noise, Efficiency = log2 (SIR+l), since log2 (7+1) = 3. According to Figure 2 this happens nearly 30% of the time. According to an aspect of the present invention, the overall data capacity of the mobile telephone network could be improved by reducing the transmit power under these conditions.
<Desc/Clms Page number 6>
The original motivation for fixing the transmitter power was to simplify measurements.
If an element of power control is to be introduced, in order to operate the base stations at less than full power, then this should be done in such a way as to permit practical measurements.
According to an aspect of the invention, a method of achieving this is as follows. In the following description, it is assumed that base stations are synchronised.
1. Full power measurement phase. During this phase, which can be extremely short, all base stations transmit sounding bursts at full power. A terminal can measure the total power in order to obtain the sum of interference and wanted signal and then correlate against the base station code to obtain the wanted signal power. From these the terminal can compute an estimate of the SIR for the case of all base stations transmitting at full power. For this SIR there will be an applicable maximum modulation rate, M.
2. The terminals with a reservation for downlink reception, broadcast from the base station, report the measured SIRs to their respective base stations. Each base station determines by how much, if at all, its transmit power could be reduced and still permit its current terminal to support the maximum modulation rate M. If the maximum modulation rate M cannot be supported or can only be supported at maximum transmit power then the maximum transmit power is maintained.
3. All base stations transmit a second sounding burst, similar to the first, but this time at the reduced transmit power (if applicable).
4. Terminals measure the available SIR under this new condition and compute the possible modulation rate they can support.
They then signal this new maximum modulation rate N to their base station. Because of the interference power reduction in step 2 this may be higher than the applicable
<Desc/Clms Page number 7>
modulation rate M in step I. The base station uses this new maximum modulation rate N for its subsequent transmission of data to that terminal.
The effect of incorporating this single step power control on SIR is shown in Figure 3.
The original curve is also shown for comparison. It is seen that the higher SIRs are removed and that the SIR is higher at the lower percentiles : The effect on capacity is shown in Figure 4. The average capacity is now 1.54 bps/Hz.
This represents an improvement of about 21 % over the average capacity attainable with all base stations transmitting at maximum power, illustrated in Figure 2.
Figure 3 shows a small set of cases where terminals have SIR greater than 7 (corresponding to 8.45 dB). It would be possible to introduce a further, similar stage of power control to further reduce the transmit power in these cases. However, this has been examined and found to result in a capacity improvement of only 3.5%.
Part of the attractiveness of keeping the base station's transmit power constant is that changes in the required modulation format for any given terminal are infrequent. A terminal which has not moved or whose SIR conditions have not changed need not
update its required modulation format by signalling to the base station. Thus the base y C : > station in each cell can set up schedules based on knowledge of all of the modulation formats required by its terminals. These can be changed flexibly as modulation format t requirements change but it should not be necessary to perform a complete overhaul of schedules in every frame as might be necessary if power control were added.
Accordingly, the present invention provides a simple scheme for providing high capacity downlink for both real time and non real time services in mobile telephony applications. The invention requires only that the terminals signal at appropriate times the supportable modulation format for its measured SIR. The base station notes this and schedules transmissions to provide fair support for services.
Claims (8)
1. A method for allocating modulation formats to each of a plurality of terminals within a cell of a cellular radio network, comprising the steps of: in each terminal, measuring a signal to interference ratio (SIR); calculating a maximum supportable modulation rate M which may be supported at the measured SIR; signalling the maximum supportable modulation rate M to a base station of the cell; and in the base station, transmitting data to each terminal in a format according to the respective maximum supportable modulation rate M.
2. A method according to claim 1 wherein a home base station, being the base station of the cell containing the terminal, and each base station in the neighbouring cells, each transmit a sounding burst signal at their respective full power, and wherein each terminal receives these signals and derives a value of SIR from the relative strength of the received signals.
3. A method according to claim 2 wherein each terminal identifies the signal transmitted by the home base station and evaluates that as the signal, whereas the signals from all other base stations are considered to form part of the noise in the calculation of SIR.
4. A method according to claim 2 or claim 3 wherein each terminal computes an estimate of a maximum supportable modulation rate M in accordance with the calculated SIR and transmits this information to the base station.
5. A method according to claim 4 wherein the home base station calculates by how much, if at all, its transmit power could be reduced while still permitting the maximum supportable modulation rate M to be supported, for each terminal.
<Desc/Clms Page number 9>
6. A method according to claim 5 further comprising the step of transmitting a second sounding burst from the home base station and each base station in the neighbouring cells at a power as determined in claim 5, and wherein each terminal receives these signals and derives a second value of SIR from the relative strength of the received signals.
7. A method. according to claim 6 wherein each terminal computes an estimate of a nw maximum supportable modulation rate N in accordance with the second calculated SIR and transmits this information to the base station.
8. A method substantially as described.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB0121274A GB0121274D0 (en) | 2001-09-03 | 2001-09-03 | Constrained power adjustment adaptive modulation cellular system |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB0219132D0 GB0219132D0 (en) | 2002-09-25 |
| GB2382277A true GB2382277A (en) | 2003-05-21 |
Family
ID=9921409
Family Applications (2)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB0121274A Ceased GB0121274D0 (en) | 2001-09-03 | 2001-09-03 | Constrained power adjustment adaptive modulation cellular system |
| GB0219132A Withdrawn GB2382277A (en) | 2001-09-03 | 2002-08-16 | Constrained power adjustment adaptive modulation cellular system |
Family Applications Before (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB0121274A Ceased GB0121274D0 (en) | 2001-09-03 | 2001-09-03 | Constrained power adjustment adaptive modulation cellular system |
Country Status (1)
| Country | Link |
|---|---|
| GB (2) | GB0121274D0 (en) |
Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5764699A (en) * | 1994-03-31 | 1998-06-09 | Motorola, Inc. | Method and apparatus for providing adaptive modulation in a radio communication system |
| EP1209824A1 (en) * | 2000-07-03 | 2002-05-29 | Matsushita Electric Industrial Co., Ltd. | Base station unit and method for radio communication |
| WO2002067492A1 (en) * | 2001-02-16 | 2002-08-29 | Koninklijke Philips Electronics N.V. | Network with adaptation of the modulation method |
-
2001
- 2001-09-03 GB GB0121274A patent/GB0121274D0/en not_active Ceased
-
2002
- 2002-08-16 GB GB0219132A patent/GB2382277A/en not_active Withdrawn
Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5764699A (en) * | 1994-03-31 | 1998-06-09 | Motorola, Inc. | Method and apparatus for providing adaptive modulation in a radio communication system |
| EP1209824A1 (en) * | 2000-07-03 | 2002-05-29 | Matsushita Electric Industrial Co., Ltd. | Base station unit and method for radio communication |
| WO2002067492A1 (en) * | 2001-02-16 | 2002-08-29 | Koninklijke Philips Electronics N.V. | Network with adaptation of the modulation method |
Also Published As
| Publication number | Publication date |
|---|---|
| GB0121274D0 (en) | 2001-10-24 |
| GB0219132D0 (en) | 2002-09-25 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |