WO2004086668A1

WO2004086668A1 - Wireless link with adaptive modulation and different qualities of service

Info

Publication number: WO2004086668A1
Application number: PCT/CA2004/000476
Authority: WO
Inventors: Jean Belzile; François GAGNON; Naim Batani; Lévis MALENFANT
Original assignee: Socovar, S.E.C.
Priority date: 2003-03-28
Filing date: 2004-03-29
Publication date: 2004-10-07

Abstract

A method is disclosed for transmitting/receiving data in a wireless link having a variable link quality by selecting a suitable constellation using at least an indication of the link quality and encoding the data using the selected suitable constellation.

Description

WIRELESS LINK WITH ADAPTIVE MODULATION AND DIFFERENT QUALITIES OF SERVICE

TECHNICAL FIELD This invention relates to the field of wireless communications .

BACKGROUND OF THE INVENTION

Microwave link budgets are normally computed for an availability of 99.9995%. One of the important factors to consider is the fading characteristic of the channel. Many links have a fade margin of the order of 40 dB . However, such deep fades are rare occurrences.

The link is therefore over specified in order to achieve the desired availability.

Various solutions are possible for increasing the^' throughput of a point-to-point microwave link as known by someone skilled in the art. For instance, a first solution is to upgrade a transmitter with a transmitter having a larger power for instance. Unfortunately, and due to the cost necessary^' for implementing such upgrading, this change is not desirable. It will be further appreciated that such change of the transmitter requires a corresponding change of the receiver.

A second solution for increasing the throughput of a point-to-point microwave link is to increase the bandwidth for transmitting the signal. However, such increasing of the bandwidth is not desirable as it would require heavy following procedures with competent authorities .

There is a need for a method and apparatus that will overcome the above-identified drawbacks.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for increasing the throughput of a wireless link without modifying the bandwidth of the wireless link.

Yet another object of the invention is to provide a method and apparatus for increasing the throughput of a wireless link without modifying the transmitting power of the wireless link.

Yet another object of the invention is to provide a method and apparatus for adapting dynamically a throughput of a wireless link using a Quality of service (QoS) .

According to one aspect of the invention, there is provided a method for transmitting ' data in a wireless link having a variable link quality, the method comprising receiving an indication of a link quality in a wireless link, selecting one of a plurality of constellations in accordance with at least the indication of the link quality, encoding the data with the selected one of the constellations and transmitting the encoded data at a symbol rate that is substantially unchanging when the selected constellation changes. According to another aspect of the invention, there is provided a method for receiving data from a wireless link having a variable link quality, the method comprising receiving the data encoded using one of a plurality of transmitting constellations having a predetermined symbol rate, continuously detecting the constellation and decoding the received data in accordance with the detected constellation.

According to another aspect of the invention, there is provided an apparatus for providing a modulated data to be transmitted in a wireless link having a variable link quality, the apparatus comprising a plurality of constellation units, a selection unit, connected to the plurality of constellation units, for receiving primary traffic, secondary traffic, an indication of the variable link quality and for further providing the primary traffic and at least one part of the secondary traffic to a selected suitable constellation unit according to the indication of the variable link quality.

According to another aspect of the invention, there is provided an apparatus for demodulating a signal transmitted over a wireless link having a variable link quality, the apparatus comprising a correlation unit for receiving the signal and providing a signal indicative of a modulation used to transmit the signal, a plurality of constellation units, each for receiving the signal and for providing a corresponding demodulated signal, a selecting unit, connected to an output of each of the plurality of constellation units, for receiving the signal indicative of the modulation and selecting a demodulated signal according to the signal indicative of the modulation. BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:

Fig. 1 is a block diagram of a transmitter in the preferred embodiment of the invention;

Fig. 2 is a state transition diagram which shows how the decision process of the transmitter;

Fig. 3 is a block diagram which shows a Quality of Service selector in one embodiment of the invention;

Fig. 4 is a block diagram which shows a receiver in the preferred embodiment of the invention;

Fig. 5 is a block diagram which shows a correlation unit according to one embodiment of the invention.

It will be noted that throughout the appended drawings, like features are identified by like reference numerals .

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT It will be appreciate.d that the objective of the strategy is a better utilization of the available received power and channel bandwidth by adapting the transmission equipment to the characteristics of the propagation channel .

Preferably, the average transmission rate is increased by a factor of up to 4 or 5 by exploiting the fade margin present in the link budget. Specifically, the strategy consists of using larger constellations whenever the received signal power is above a desired operating point. Such situation occurs whenever little fading is present, which corresponds to over 99% of the time for typical bandwidth signals.

The additional capacity thus generated is allocated to lower quality of service sources, or variable quality of service sources. Examples of such sources are data packets transmitted over ATM or Ethernet.

Increasing the modulation level which results in an improved spectral efficiency is an efficient solution because it is transparent to the regulatory agencies and to most of the infrastructure, such as Power, Antenna, mast, link distance, RF, etc. It thus enables an easy upgrade path for existing equipment.

For example, when the channel suffers . from low to moderate fading (0 to 10 dB) , then an 8 bits/symbol/Hz constellation can be used. Whenever the fading becomes severe (30+ dB) then a 2 bits/symbol/Hz constellation is used. If the link budget is calculated for an availability of 99.9995% at 2 bits/symbol/Hz, then this is defined as the high availability traffic Supposing that the moderate to low fading conditions are prevalent over 99% of the time, then the system offers a high capacity link where most of the traffic is data based with a grade of service above 99% and the high availability traffic is always offered at a grade of service above 99.9995%.

In fact, it will be appreciated that some type of traffic may have a high requirement of reliability, such as voice traffic, some other type of traffic have a lower requirement of reliability such as data packets .

Now referring to Fig. 1, there is shown a transmitter 7 in the preferred embodiment of the invention. The transmitter 7 comprises a selection unit 9, a plurality of constellations 12, a filtering unit 20 and a RF unit 22.

The selection unit 9 comprises a quality of service (QoS) multiplexer 10, a quality of service based management unit 18 and a selecting unit 17.

Preferably, an automatic gain control (AGC) , if present, is used in order to control the peak power.

In the preferred embodiment of the invention, the quality of service (QoS) multiplexer 10 receives a primary traffic, first type of data, and a secondary traffic, second type of data. The primary traffic originates from a primary traffic source, while the secondary traffic originates from a secondary traffic source. The primary traffic has a priority level higher than the secondary traffic. Someone skilled in the art will appreciate that for instance the primary traffic may be data related to a realtime voice conversation while the secondary traffic may be data related to data packets.

The quality of service based management unit 18 provides a quality of service based controlling signal to the quality of service (QoS) multiplexer 10. In the preferred embodiment, the quality of service based management unit 18 receives link performance measures from a receiver 8, as explained below.

In ^• accordance with the quality of service based controlling signal, the quality of service (QoS) multiplexer 10 selects a suitable constellation from the plurality of constellations 12 for encoding data.

In the preferred embodiment of the invention each constellation (modulator) of the plurality of constellations 12 has the same peak power and the same bandwidth.

It will be appreciated that compatible constellations may be advantageously used in order to reduce the number of constellations of the plurality of constellations 12.

It will be appreciated that for instance in the case of an optimum situation, where the quality of service is optimum, constellation 16 may be chosen. The constellation 16 provides bn bit/symbol. In the case worst case situation, where the quality of service is poor, constellation 14 may be chosen. The bit/symbol rate of the constellation 14 is lower than the bit/symbol rate of the constellation 16.

It will be appreciated that the quality of service may take or not into account such items as link characteristics, type and quantity of traffic to transmit, desired throughput, global network performance, such as power usage, congestion, interference with and from other, radio components, etc. Still according to the quality of service based controlling signal, the quality of service (QoS) multiplexer 10 selects the primary traffic and at least one part of the secondary traffic and provides the selected primary traffic and the at least one part of the secondary traffic to the selected constellation. It will be appreciated that transmission of the first traffic respects a high level of reliability up to a predetermined maximum level of throughput while transmission of the at least one part of the secondary traffic is achieved with significant throughput at a lower level of reliability.

In the preferred embodiment and in the worst case situation, the multiplexer 10 provides the primary traffic to the selected constellation 14.

In the preferred embodiment and in other situations where the worst case scenario, the multiplexer 10 provides at least one part of the secondary traffic depending on the quality of service.

A selecting of the output of the selected constellation of the plurality of constellations 12 is performed by the selecting unit 17 in accordance with the quality of service based controlling signal provided by the quality of service based management unit 18.

The output of the constellation selected is then provided to the filtering unit 20 and then to the RF unit 22 for transmitting.

Now referring to Fig. 2, there is shown a state transition diagram which shows the decision process of the transmitter. According to a first step, an initialization of the transmitter is performed. In the preferred embodiment of the invention, the initialization of the transmitter is performed with a high availability efficiency.

According to a second step, the efficiency of the transmitter is maintained. The efficiency of the transmitter is maintained using the quality of service based management unit 18. More precisely, the quality of service based management unit 18 provides an adequate quality of service based controlling signal to the quality of service (QoS) multiplexer 10 and to the selecting unit 17 to select an adequate constellation. Such maintaining is performed dynamically.

It will be appreciated that the quality of service may vary depending on various factors such as signal distortion, interference, noise and received power level. Such factors are time-varying due to several propagation effects such as distance variations, fading, rain, movement.

Now referring to Fig. 3, there is shown an embodiment of the multiplexer 10. As explained previously, the multiplexer 10 receives the primary traffic and the secondary traffic.

As shown in Fig. 3, it will be appreciated that the multiplexer 10 enables a selection of at least one part of the secondary traffic according to a priority using the quality of service based controlling signal which is provided to a quality of service selector. A traffic mixer provides, according to the quality of service based controlling signal, the primary traffic and the selected at least one part of the secondary traffic, selected using the quality of service selector.

It will therefore be appreciated that each stream comprised in the secondary traffic are separated in a set of different priority First In First Out (FIFO) .

Preferably, bits ranging from 0 to b₀ are used for the primary traffic while bits ranging from b₀₊₁ to b_n are used for the selected at least one part of the secondary traffic. It will therefore be appreciated that b„ -bo₊i bits are available for the selected at least one part of the secondary traffic.

Now referring to Fig. 4, there is shown a corresponding receiver according to the preferred embodiment of the invention.

The receiver comprises a RF unit 50, a filtering unit 52, a correlation unit 54, a plurality of constellations 56, a quality of service-based demultiplexer 62 and a link performance receiving and collecting unit 60.

Preferably, an automatic gain control (AGC) , if present, should be used in order to control the peak power.

Someone skilled in the art will appreciate that this is usually the case in order to prevent saturation while providing good operating points.

In the preferred embodiment, the link performance receiving and collecting unit 60 collects link performance measures which are taken at various places of the receiver as shown in Fig. 4. Such collected link performance measures are provided to the transmitter using a feedback channel. Alternatively, the collected link performance measures are processed by the receiver before being transmitted to the transmitter.

The RF unit 50 receives a RF signal and provides a signal to the filtering unit 52. The filtering unit 52 filters the provided signal and provides it to the plurality of constellations 56. The filtered signal comprises a plurality of streams. The plurality of constellations 56 is preferably compatible with the plurality of constellations 12.

The correlation unit 54 monitors the incoming symbol streams ■ and identifies a correct modulation scheme used.

In the preferred embodiment, all constellations of the plurality of constellations receive simultaneously the provided signal.

The correlation unit 54 provides a constellation selection signal to the quality of service-based demultiplexer 62. The constellation selection signal is indicative of a constellation used detected to generate the identified correct modulation scheme.

It will be appreciate that the link performance measure is computed using at least one of a receiving power, a Bit Error Rate (BER) , a spectrum shape, a Mean Square Error function, etc.

The correlation unit 54 correlates the received symbol stream with the set of symbol streams. The set of - Im ¬

possible symbol stream may be generated according to various techniques such as data aided, decision directed, etc.

The set having the highest correlation result is deemed to represent the received symbol stream with the highest probability and demodulation is performed accordingly.

Now referring to Fig. 5, there is shown an embodiment of the correlation unit.

Alternatively, data provided to the quality of service (QoS) multiplexer 10 of the transmitter comprises data having an equal priority level. In such case, it will be appreciated that the amount of data provided to a constellation of the plurality of constellations 12 is varying depending on the constellation selected.

It will be appreciated that preferably a constellation is selected continuously.

The embodiments of the invention described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.

Claims

WE CLAIM :

1. A method for transmitting data in a wireless link having a variable link quality, the method comprising: receiving an indication of a link quality in a wireless link; selecting one of a plurality of constellations in accordance with at least said indication of the link quality; encoding said data with the selected one of said constellations; transmitting said encoded data at a symbol rate that is substantially unchanging when said selected constellation changes.

2. The method as claimed in claim 1, wherein said selecting is performed in accordance with said indication of link quality and a desired throughput of said data.

3. The method as claimed in one of claims 1 and 2, wherein said encoding comprises encoding a first type of said data to be transmitted having a primary priority level with any selected one of said constellations and encoding an amount of a second type of said data to be transmitted having a lower secondary priority with at least one of said selected constellations, said amount being variable in accordance with said selected constellation; wherein transmission of said first type of data respects a high level of reliability up to a predetermined maximum level of throughput and transmission of said second type of data is achieved with significant throughput at a lower level of reliability.

4. The method as claimed in claim 3, wherein said amount is zero for one of said constellations.

5. The method as claimed in claim 1, 2, 3 or 4, wherein said selecting is performed in accordance with said indication of link quality and a quality of service (QoS) for said link.

6. A method for receiving data from a -wireless link having a variable link quality, said method comprising: receiving said data encoded using one of a plurality of transmitting constellations having a predetermined symbol rate; continuously detecting said constellation; and decoding the received data in accordance with the detected constellation.

7. An apparatus for providing a modulated data to be transmitted in a wireless link having a variable link quality, comprising: a plurality of constellation units; a selection unit, connected to said plurality of constellation units, for receiving primary traffic, secondary traffic, an indication of said variable link quality and for further providing said primary traffic and at least one part of said secondary traffic to a selected suitable constellation unit according to said indication of said variable link quality.

8. The apparatus as claimed in claim 7, wherein said selection unit further receives an indication of a desired throughput, further wherein said selection unit provides said primary traffic and at least one part of said secondary traffic according to said indication of said variable link quality and further according to said indication of said desired throughput.

9. The apparatus as claimed in any one of claims 7 and 8, wherein said selection unit comprises a QOS-based management unit receiving said indication of said variable link quality and providing a selection signal; further wherein said selection unit comprises a selecting unit connected to an output of each of the constellation units, said selecting unit receiving said selection signal and providing said modulated signal using said selection signal; further wherein said selection unit comprises a multiplexer connected to each of said plurality of constellation units, said multiplexer receiving said selection signal, said primary traffic, said secondary traffic and providing said primary traffic and said at least one part of said secondary traffic to said selected suitable constellation unit.

10. The apparatus as claimed in claim 9, wherein said multiplexer comprises a priority selector receiving said secondary traffic and providing a plurality of secondary traffic streams according to a respective priority; further wherein said multiplexer comprises a plurality of First In First Out (FIFO) buffers, each receiving and buffering a corresponding secondary traffic stream; further wherein said multiplexer comprises a QOS selector receiving each buffered stream and selecting at least one buffered stream according to said selection signal; further wherein said multiplexer comprises a traffic mixer mixing said primary traffic and said selected at least one buffered stream.

11. The apparatus as claimed in any one of claims 7 to 10 further comprising an RF unit receiving said provided modulated signal and providing a corresponding RF signal for subsequent transmission.

12. An apparatus for demodulating a signal transmitted over a wireless link having a variable ■ link quality, said apparatus comprising: a correlation unit for receiving said signal and providing a signal indicative of a modulation used to transmit said signal; a plurality of constellation units, each for receiving said signal and for providing a corresponding demodulated signal; a selecting unit, connected to an output of each of said plurality of constellation units, for receiving said signal indicative of said modulation and selecting a demodulated signal according to said signal indicative of said modulation.

13. The apparatus as claimed in claim 12, further comprising an RF unit providing said signal.