WO2007066299A2

WO2007066299A2 - Method to quasi-synchronise transmissions in ad-hoc wireless lans

Info

Publication number: WO2007066299A2
Application number: PCT/IB2006/054639
Authority: WO
Inventors: Hans-J Reumerman; Gergios Orfanos; Willi Butsch
Original assignee: Koninklijke Philips Electronics N.V.; U.S. Philips Corporation
Priority date: 2005-12-08
Filing date: 2006-12-06
Publication date: 2007-06-14
Also published as: WO2007066299A3

Abstract

In a MC-CDMA system asynchrony is minimized in order to increase the performance of the Multi-User Detector (MUD) at the receiver of a mobile station. Asynchrony is minimized by quasi-synchronizing the stations (mobile or otherwise) in the ad-hoc network to initiate transmissions only at regular intervals in time. The intervals have a duration that is equal to a multiple of the duration of a multi-carrier symbol. At present, the IEEE 802.11 standard sets a multi-carrier symbol duration to be 4 μsec.

Description

METHOD TO QUASI-SYNCHRONISE TRANSMISSIONS

IN AD-HOC WIRELESS LANS

The field of mobile radio communications aspires to support high data rate applications such as image and video. However, the ability to achieve high bit rates at low error rates over wireless channels is severely restricted by the frequency selectivity of channels used by multiple propagation paths with different time delays.

Narrow-band communications presents the advantage of being relatively immune to inter- symbol interference, but comes with the disadvantage of being susceptible to flat fading. With conventional Code Division Multiple Access (CDMA), resistance to fading is achieved by spreading the signal energy over a larger bandwidth than necessary, to contain the user signal. However, in the process of providing resistance to deep fades, the signal is affected by delay spreads to a greater extend, and experiences considerable Inter Chip Interference (ICI).

Wide-band CDMA systems have been proposed as a 3 G contender to increase data rates in wireless communication networks. However, the large frequency bandwidth of such high-speed links makes them susceptible to inter-symbol interference (ISI). Therefore, a number of Multi-Carrier CDMA (MC-CDMA) schemes have been suggested to improve performance over frequency-selective channels.

Researchers realize that there is a need to develop a technology to handle very high rate multimedia traffic, which will not only support the traffic, but also provide service to a mobile user traveling at high speed in, for example, an automobile.

As an interesting approach to combat the effect of ISI in frequency- selective fading channels, a number of communication systems based on a combination of CDMA and Multi-Carrier Modulation (MCM) have been proposed. In MCM, the entire bandwidth is subdivided into several narrow-band channels operating at lower data rates.

Orthogonal Frequency Division Multiplexing (OFDM) is on such modulation scheme that allows transmitting high data rates extremely hostile wireless channels with comparable low complexity. This transmission scheme, combined with the multiple-access scheme 'CDMA', results in what is known as a version of a MC-CDMA system.

With MC-CDMA, each data symbol is spread over multiple sub-carriers with a user specific code. Each chip of the spread symbol is transmitted on another sub-carrier. Different users, such as user 1 and user 2 share the same frequency band, i.e., the same sub- carriers, at the same time 3, as shown in FIG 1.

IEEE 802.11 or Wi-Fi denotes a set of wireless Local- Area-Networks (LAN) standards. The 802.11 standards now include a family of six over-the-air modulation techniques that all use the same protocol. One of the most popular techniques is one defined by the 'a' amendment to the original standard and that is known as the 802.1 Ia standard. The 802.11a standard uses the same core protocol as the original standard, operates in 5 GHz band, and uses 52-subcarrier orthogonal frequency-division multiplexing (OFDM) with a maximum raw data rate of 54 Mbit/s, which yields realistic net achievable throughput at the MAC layer in the mid-20 Mbit/s range.

The Medium Access Control (MAC) protocol is generally used to provide the data link layer of the Ethernet LAN system. A MAC protocol generally encapsulates payload data by adding a header (i.e. 14 byte header) containing, among other things, protocol control information (PCI) before the payload data and appending a 4-byte (32-bit) Cyclic Redundancy Check (CRC) after the payload data. The entire frame is preceded by a small idle period referred to as an inter-frame space (SIFS, PIFS, or DIFS) along with perhaps a preamble (i.e. 8 byte preamble).

The MAC protocol of MC-CDMA is an enhancement of the IEEE 802.1 Ia MAC protocol, with some modifications needed to support the Code Division Multiple Access (CDMA) Physical Layer.

Referring to FIG 2, a mobile station (MS) that is ready to transmit 10 has to select a codechannel (cch) (e.g. cchl, 2, 3 or 4). For this selection two methods are possible. The first is to select a codechannel before every packet transmission. Initially this selection is done randomly. For later transmissions, the station 10 does not select codechannels, which have already been reserved by other stations (according to the standard the considered station has set a Network Allocation Vector (NAV) for an occupied channel). The second method consists of selecting the cch with the least traffic and keeping this cch for the entire duration of the connection.

Before accessing the medium a station 10 should detect it as idle for a duration called Distributed Inter-Frame Space (DIFS), and signals the intended data transfer by transmitting a Ready To Send (RTS) packet 12.

All MSs 16, 18 that receive this control packet, and are not the intended receivers, set their NAV timer 20, interrupt their backoff down counts, and defer from the medium in order not to interfere with the transmission. If the station 14 that is the intended receiver of the RTS is idle i.e. able to receive data, it responds with a CTS packet 22, after a time called Short Inter-Frame Space (SIFS) 24. In case the station 14 that is the intended receiver is busy, the RTS transmission is repeated by the sending station 10 after a new backoff.

Mobile stations 16, 18 that receive this CTS set their NAV timer 20 as well. The sender 10 can now transmit its data packet 26 after the SIFS 24. The receiver 14 acknowledges a successful reception by an Acknowledgement (ACK) 28 at a SIFS 30 time after the end of the data frame 26. The above standard Distributed Coordination Function (DCF) procedure is followed in every cch for each data transmission.

There is a significant drawback of using the present MAC protocol with a MC-

CDMA system. In a MC-CDMA system the frequency channel is divided in many cchs by the use of different spreading sequences (codechannel, cch). In an asynchronous ad-hoc packet switched system, a transmitting MS chooses one cch for transmission. Applying Carrier Sense Multiple Access (CSMA) the MS transmits the packet of data in the selected channel with an expected probability of success after the channel (cch in MC-CDMA) is detected as idle. This method provides a simple multiple access mechanism, but parallel transmissions in different cchs, originating from different MSs are totally asynchronous to each other, with relative delays randomly distributed in [0,Ts], where Ts is the duration of one multi-carrier symbol. If this delay, which is a measure of "asynchrony" among the parallel transmissions, could be reduced, then the performance of the Minimum Mean

Square Error (MMSE) Multi-User Detector (MUD) used at the receivers of the MS-CDMA system would be enhanced.

Embodiments of the invention provide a device, system and method that reduces the asynchrony among parallel transmissions such that performance of a MUD used at the receivers of stations (mobile, stationary or otherwise) in a MS-CDMA system is enhanced. To do so, embodiments of the invention, which are generally found in a wireless network and include stations that wirelessly communicate with each other based on an IEEE 802.11 standard, communicate wirelessly by setting a predetermined amount of idle time between concurrent wireless transmissions. The predetermined amount of time is set to be a multiple of the duration of a multi-carrier symbol (It is understood that the predetermined amount of time may not be an exact multiple of the duration of a multi-carrier symbol, but is an amount of time such that the delay adds up to asynchrony to achieve synchronization.). Furthermore, embodiments of the invention require that a station may only initiate a wireless transmission at predetermined time intervals. The duration of the predetermined time interval is a multiple of duration of the multi-carrier symbol.

Other embodiments of the invention set the interframe space (between

transmissions) to be a multiple of the duration of the multi-carrier- symbol.

In yet other embodiments of the invention, a secondary IEEE 802.11

synchronization beacon is transmitted between the normally scheduled beacon interval in order to further help synchronize a plurality of stations.

In other embodiments of the invention, a plurality of stations is caused to become quasi-synchronized.

Still in other embodiments of the invention, various time intervals, including, but not limited to, backoff time, interframe space (SIFS, PISF, or DIFS), slot time, NAV, and RTS are each a multiple(s) of a predetermined amount of time. The predetermined amount of time may be equal to the time duration of a mult-carrier symbol.

The above summary of the invention is not intended to represent each embodiment or every aspect of the present invention.

A more complete understanding of the method and apparatus of the present invention may be obtained by reference to the following Detailed Description when taken in conjunction with the accompanying Drawings wherein:

FIGURE 1 is chart showing a MC-CDMA access scheme;

FIGURE 2 is a chart showing the basic functionality and timing of the MAC protocol for an MC-CDMA system;

FIGURE 3 is a chart that indicates the performance of a MUD vs. the relative delay of a users symbol;

FIGURE 4 is a timing chart showing an exemplary method of synchronizing transmissions in parallel codechannels; and

FIGURE 5 is a large scale scenario for ad-hoc WLANs in accordance with embodiments of the invention.

The MAC protocol corresponding to MC-CDMA is an enhancement of the IEEE 802.1 Ia MAC, with some modifications needed to support the MC-CDMA physical layer. In an MC-CDMA system the frequency channel is divided in many codechannels by the use of different spreading sequences. In an asynchronous ad-hoc packet switched system the codechannels for transmissions are selected randomly, the parallel transmission in different codechannels, originating from different mobile stations, are totally asynchronous to each other. Embodiments of the present invention minimize this asynchronicity in order to increase the performance of the Multi-User Detector (MUD) at the receiver. This can be accomplished if mobile stations are allowed to initiate a transmission only at or

substantially at specific points in time that are multiples of, for example, 4 μsec, the duration of a multi-carrier symbol. One of ordinary skill in the art would understand that the multiple may not be an exact multiple due to necessary time delays of electronic equipment.

The performance of the MUD is strongly bound with the relative delay between concurrent transmissions in different cchs. An example is given in FIG 3, which shows the Signal to Interference and Noise Ratio (SINR) at the MUD's output when two users are active, each transmitting the data packets encoded with a different spreading code. In other words, the graph of FIG 3 depicts the performance of a MUD at a receiver vs. the relative delay of an interferer's symbol. The symbol duration in this exemplary system is 3.2 μsec. Both the carrier strength, Pc, and the interference at the receiver, PI , comprise -68 dBm. For a small relative delay 40, the output SINR reaches a maximum of 31 dB, which corresponds to the theoretical maximum: assuming no fading and perfect interference suppression, PI=O after applying multiuser detection. The noise level for a 16.25 MHz channel and receivers with similar noise figure, as the one defined in Std.802.l la/D7 (1999), is approximately -93dBm.

SINR = Pc/ N0 + Gsp = 31

where Gsp is the spreading gain in dB:

Gsp[dB] = lOlog(SF) = 101og4 ~6dB

For larger relative delays 42, the performance of the MUD degrades and reaches a minimum for a delay of half the multi-carrier symbol duration (3.2 μsec / 2 = 1.6 μsec). As the delay increases more 44, the detector synchronizes with the next symbol, thus the output SINR increases.

Thus, in order to best enhance the performance of the system, the relative delay of concurrent transmissions should be kept low. This can be accomplished if MSs are limited to initiating a transmission only at specific times or intervals. This time or interval, as determined in embodiments of the present invention, is a multiple of the duration of a multi- carrier symbol (i.e., 4 μsec) after the reception of a beacon. In essence the interval is not always a multiple or exact multiple of the multi-carrier symbol, but is understood to be an amount of time that is a multiple of the multi-carrier symbol plus or minus an amount to compensate for electronic delays of circuitry and signals such that the asynchrony plus the amount of time adds up to achieve synchronization.

The IEEE 802.11 standard defines a procedure to synchronize the local clocks of MSs. The beacon contains the time stamp of the MS's clock sending the beacon. The beacon can be sent by any MS, and is repeated regularly, for example, every 125 msec. Upon receiving the beacon, each MS compares its own clock with the timestamp in the beacon and adopts the beacon's timestamp in the case that the time stamp in the beacon frame has a lower value than the value of the MS's own timer. Like this, MSs synchronize with the slowest running clock.

Referring to FIG 4, a timing chart is shown that depicts synchronizing transmissions in parallel codechannels. In embodiments of the invention, it was determined that synchronization of the clocks was not enough for MSs to keep a common or near common pace of 4 μsec. In embodiments of the invention, since the transmitted frames always consist of a whole multiple of multi-carrier symbols, the backoff time and interframe spaces (e.g. 50) are to be adopted to the 4 μsec multi-carrier symbol duration 52 or a multiple thereof, to allow a synchronization in physical layer basis. Therefore, embodiments of the invention will have interframe space durations (SIFS, PIFS, DIFS) equal to multiples of 4 μsec. Furthermore, embodiments of the invention have a slot duration, used for the calculation of backoff, equal to a multiple of 4 μsec also.

Still referring to FIG 4, the synchronization procedure of embodiments of the invention can be further enhanced by the medium sensing function of CSMA/CA networks. A MS 54 in idle can listen to transmissions in other cchs (cch 2 of MS2 is listening to cch 1 in FIG 4), in order to synchronize itself with the ongoing transmissions. Additionally, since the channel is divided in four smaller channels (the cchs) (see FIG 2), synchronization can be improved if MSs with low traffic transmit secondary beacons between the normally scheduled 125 msec intervals in one or a group of the cchs. Other MSs, listening to those secondary beacons would update their synchronization.

It is understood that the MSs will not be completely or perfectly synchronized to each other but instead be quasi- synchronized. First, the propagation delay 56 of both beacon and the observed transmissions will pose an error to the calculated time of the multi-carrier symbols beginning. This propagation delay will be limited in Wireless Local Area

Networks (WLAN) and Wireless Personal Area Networks (WPAN) due to the short distances covered with one link. For example, in one embodiment of the invention the longest distance to be covered between links with one hop is specified at 20m and would result in a maximum propagation delay of 68 psec.

Additionally, a MS will not be able to precisely start at or synchronize to the symbol's start time when the MS willing to transmit due to hardware constraints. For these reasons it can be assumed, in embodiments of the invention, that the starting point is met with a random deviation of, for example, about 400 psec (about 10% of the multi-carrier symbol duration or some other predetermined percentage).

Applications of this method are not only restricted to MC-CDMA systems. Even an OFDM system will improve its synchronization of the transmissions using an embodiment of the present invention that incorporates interframe and slot durations that are restricted to multiples of the multi-carrier symbol duration and greatly enhance the operation performance of a system that is a quasi- synchronous system. Furthermore, additional embodiments of the invention include stations receiving and using signals from a GPS system or a Galileo system. Such received signals from a GPS or Galileo system provide timing to a plurality of stations in an exemplary network for helping to achieve

synchronization of a station with other stations in the network.

FIG 5 is a MC-CDMA system for a large scale ad- hoc WLAN 60. Various ad-hoc sub networks are established 62, 64, 66, 68. Each network stays quasi-synchronized using methods in accordance with embodiments of the invention. The wireless networks are based on IEEE 802.11 standards, but have changes or improvements in accordance with embodiments of the invention. Each network 62, 64, 66, 68 includes a plurality of mobile stations (MS). The mobile stations are relatively stationary. Embodiments of the invention bring timing and performance advantages in large scale scenarios that consist of many sub networks.

Many variations and embodiments of the above-described invention and method are possible. Although only certain embodiments of the invention and method have been illustrated in the accompanying drawings and described in the foregoing Detailed

Description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of additional rearrangements, modifications and substitutions without departing from the invention as set forth and defined by the following claims.

Accordingly, it should be understood that the scope of the present invention encompasses all such arrangements and is solely limited by the claims as follows:

Claims

CLAIMS What is claimed is:

1. In a wireless network wherein a plurality of stations (10, 14, 16, 18) wirelessly communicate with each other based on an IEEE 802.11 standard, an improved method of communicating including: setting an idle time (50) between concurrent wireless transmissions from said plurality of stations to a predetermined amount of time, said predetermined amount of time being equal a time duration of a multi-carrier signal (52); and allowing said plurality of stations to initiate a wireless transmission only at predetermined time intervals, wherein each said predetermined time interval is equal to or a multiple of said predetermined amount of time.

2. The method of claim 1, wherein said predetermined amount of idle time between concurrent wireless transmissions is an interframe space (50).

3. The method of claim 1, further comprising providing a secondary IEEE 802.11 synchronization beacon that is transmitted by at least one of said plurality of stations between a normally scheduled (125 μsec) beacon interval.

4. The method of claim 1 , wherein at least one of said plurality of stations is a mobile station (14).

5. The method of claim 1, further comprising setting a slot duration to a multiple of said predetermined amount of time.

6. The method of claim 1, further comprising quasi- synchronizing the timing of said plurality of stations.

7. The method of claim 6, wherein said quasi- synchronizing of said timing of said plurality of stations is within a random deviation of up to a maximum percentage of the predetermined amount of time.

8. A method of quasi- synchronizing a plurality of stations (10, 14, 16, 18) that wirelessly communicate with one another using an IEEE 802.11 standard, said method comprising: setting all interframe spaces (50) that are between concurrent wireless transmissions to a time period, said time period (52) being a multiple of a duration of a multi-carrier signal; setting specified time intervals for any one of plurality of stations to initiate a wireless transmission, said specified time intervals each being the same length of time and having a duration that is a multiple of said duration of said multi-carrier signal (52).

9. The method of claim 8, further comprising transmitting, from one or more of said plurality of stations, a secondary IEEE 802.11 synchronization beacon between a normally scheduled IEEE 802.11 synchronization beacon.

10. The method of claim 9, wherein said plurality of stations are quasi- synchronized to within a predetermined percentage of the duration of said time interval.

11. The method of claim 8, wherein at least one of said stations is a mobile station (14).

12. The method of claim 8, wherein said plurality of stations are part of an ad- hoc wireless LAN system.

13. The method of claim 8, further comprises: listening, by an idle station, to an active codechannel; and quasi-synchronizing said idle station (54) using information from said active codechannel.

14. A method of quasi-synchronizing a plurality of stations (10, 14, 16, 18) that wirelessly communicate with one another using an IEEE 802.11 standard, said method comprising:

initiating, by a station (10), a wireless transmission only at times that coincide with regular intervals, each one of said regular intervals being a first multiple of a predetermined duration (52); using, by said station, an interframe space (50) between concurrent wireless transmissions that has a duration equal to a second multiple of said predetermined duration; and transmitting or receiving, by said station, a secondary IEEE 802.11 synchronizing beacon between a normally scheduled IEEE 802.11 synchronization beacon to further aid quasi-synchronization of said station with said plurality of stations, said plurality of stations being part of an network.

15. The method of claim 14, wherein said station is a mobile station.

16. The method of claim 14, further comprising: listening, by said station when it is idle, to an active codechannel; and quasi-synchronizing said station to another one (54) of said plurality of stations using information from said active codechannel.

17. The method of claim 14, wherein said predetermined duration is equal to duration of a multi-carrier symbol (1, 52).

18. The method of claim 14, further comprising using signal from one of a GPS system or a Galileo for achieving synchronization of said station to another one (54) of said plurality of stations.