WO2007113730A1

WO2007113730A1 - Mobile station for a multi-channel based communications network

Info

Publication number: WO2007113730A1
Application number: PCT/IB2007/051061
Authority: WO
Inventors: Hans-Juergen Reumerman; Georgios Orfanos; Bernard Walke
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2006-03-31
Filing date: 2007-03-27
Publication date: 2007-10-11

Abstract

A mobile station for a multi-channel based communications network comprises a receiving and sending unit (2) and a control unit (4) for controlling the receiving and sending unit (2). Thereby, after transmission of information to a succeeding mobile station, a guard interval (8) is included, during which no further information is sent to the succeeding mobile station to enable transmission of the information from the succeeding mobile station to further mobile stations. Therewith, the utilization of the communications network is optimized and the overall throughput is increased.

Description

Mobile station for a multi-channel based communications network

The present invention relates to a mobile station for a multi-channel based communications network and to a method for transmitting information over such a network. More particularly, the present invention relates to a mobile station for a multi-carrier code division multiple access wireless local area network and to a method for transmission of information over such a network. Further, the communications network may also be a IEEE 802.1 la,e based communications network extended for operation in multiple channels.

State of the art document US 2004/0264475 Al describes an architecture for multi-channel carrier-sense multiple access systems using an 802.11 protocol. The architecture known from US 2004/0264475 Al comprises: in a MAC for a station, plural transmit queues, a queue selection mechanism, and a holding queue; a physical layer having multiple channels therein; and in a receiver for a station, a re-ordering buffer for ordering packets in a proper sequence prior to the packets leaving the receiver. Thereby, the 802.11 wireless local area network protocol uses carrier sense multiple access/collision avoidance for its access mechanism. A feature of this mechanism is that it senses the channel selected by the transmitting station prior to transmission, and if the channel is found to be busy, the station defers transmission for a pseudo-randomly chosen period of time. In addition, collisions are avoided by having each station maintain a network allocation vector based on the duration values of frames to be transmitted, thereby providing an increase in throughput over what might be achieved in a given single channel system.

The architecture of US 2004/0264475 Al has the disadvantage that although the throughput between two involved stations is high, the overall performance of the entire network is limited.

It is an object of the invention to provide a mobile station and a method for a multi-channel based communications network with an improved overall throughput. This object is solved by a mobile station as defined in claim 1 and by a method as defined in claim 9. Advantageous developments of the invention are mentioned in the dependent claims.

The present invention has the further advantage that it allows an efficient sharing of system resources among the mobile stations and prioritizes the forwarding mobile station. Additionally, a forwarding mobile station can transmit more than one packet in parallel, which increases overall performance of a multihop connection and reduces the delay.

The measure as defined in claim 2 has the advantage that the duration of the guard interval in which the mobile station is pausing, is optimized with respect to a necessary duration for data transmission determined by a transmission window duration. Such a transmission window duration may be a part of the information, i.e. the data packet to be transmitted, or may be determined by a measurement of the duration that was necessary to transmit a certain data packet transmitted. The measure as defined in claim 3 has the advantage that further information may be sent from the mobile station to a succeeding mobile station without interfering with the first information forwarded by further mobile stations over a channel used for forwarding the first information.

The measure as defined in claim 4 has the advantage that a direct acknowledgement, for example a acknowledgement or a negative acknowledgement, can be omitted to reduce the traffic over the network. Due to the use of the upper transmission power for sending at least a part of the first information, the preceding mobile station is enabled to successfully receive a part of the first information already sent from the preceding mobile station to the mobile station. Hence, the preceding mobile station is enabled to determine that the mobile station continues transmission of the first information, and therefore the preceding mobile station is enabled to determine that the first information has been successfully transmitted to the mobile station. Hence, reception of the first information by the preceding mobile station is a reliable indirect acknowledgement.

The measures as defined in claims 5 and 6 have the advantage that, for example from an unique identification number stored in the header information of the first information, the preceding mobile station determines whether the first information has been successfully transmitted or not at an early time instant so that it must listen only to a part of the first information sent from the mobile station. Further, the other part of the first information may be sent with a lower transmission power than the upper transmission power to reduce a channel interference.

The measure as defined in claim 7 has the advantage that the mobile station is not transmitting to an already busy mobile station so that collisions are reduced. Thereby, the not available timer may be a network allocation vector.

Further, the measure as defined in claim 8 has the advantage that collisions in multihop scenarios are reduced, while smart back-off deployment is enabled.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment described hereinafter.

The present invention will become readily understood from the following description of preferred embodiments thereof made with reference to the accompanying drawings, in which like parts are designated by like reference signs and in which: Fig. 1 shows a mobile station according to a preferred embodiment of the present invention;

Fig. 2 illustrates a data transmission method according to a first embodiment of the present invention;

Fig. 3 illustrates the utilization of channels by the method according to the first embodiment of the present invention;

Fig. 4 illustrates a data transmission method according to a second embodiment of the present invention;

Fig. 4A illustrates an information sent by a data transmission method according to the second embodiment of the present invention; Fig. 5 illustrates a spreading method according to an embodiment of the present invention;

Fig. 6 illustrates an utilization of channels by a method according to an embodiment of the present invention;

Fig. 7 illustrates an utilization of channels by a method comprising smart back-off according to an embodiment of the present invention;

Fig. 8 shows a star scenario arrangement of mobile stations to illustrate an embodiment of the present invention;

Fig. 9 shows a diagram showing a simulated scenario for multihop transmissions; Fig. 10 shows a diagram showing a carried end-to-end traffic per multihop connection vs. offered load;

Fig. 11 shows a diagram showing a mean end-to-end queuing delay per multihop connection vs. offered load; Fig. 12 shows a diagram showing a mean end-to-end service time per multihop connection vs. offered load;

Fig. 13 shows a diagram showing a cumulative distribution function (CDF) of queuing delay per hop for 0.75 Mbit/sec offered load per connection;

Fig. 14 shows a diagram showing a CDF of queuing delay per hop for 1.25 Mbit/sec offered load per connection;

Fig. 15 shows a diagram showing a CDF of end-to-end delay per multihop for 0.75 Mbit/sec offered load per connection; and

Fig. 16 shows a diagram showing a CDF of end-to-end delay per multihop connection for 1.25 Mbit/sec offered load per connection.

Fig. 1 shows a schematic structure of a mobile station 1 according to a preferred embodiment of the invention. The mobile station 1 can be used in multi-carrier code division multiple access wireless local area networks. The mobile station 1 and the method described below are applicable but not limited to a multihop medium access control protocol on the basis of IEEE 802.1 la,e. The mobile station 1 and the method for such a mobile station are, generally, applicable for wireless communication systems with a multichannel structure.

The mobile station 1 comprises a receiving and sending unit 2 connected with an antenna 3. Further, the mobile station 1 comprises a control unit 4 for controlling the receiving and sending unit 2. Thereby, the receiving and sending unit 2 and the control unit 4 are connected over a connection 5. The mobile station 1 comprises a timer unit 6 that is connected via a connection 7 with the control unit 4. The timer unit 6 comprises one or more not available timers, wherein the control unit 4 can set one of the available timers to a not available time interval for a channel CCHl , CCH2 (Fig. 3).

Fig. 2 illustrates a method according to the first embodiment of the present invention, and Fig. 3 illustrates a utilization of channels CCHl, CCH2 by such a method. The method and the utilization of channels CCHl, CCH2 is illustrated with reference to mobile stations MSl, MS2, MS3, MS4, each of which is arranged according to the mobile station 1, as shown in Fig. 1.

At timing tl a ready-to-send (RTS) packet is sent from mobile station MSl. This ready-to-send packet is received from mobile station MS2 between timing tl to timing t2. After a short interframe space from timing t2 to timing t3, the mobile station MS2 answers the ready-to-send packet with a transmission of a corresponding clear-to-send (CTS) packet sent during the time interval from timing t3 to timing t4. Thereby, the ready-to-send packet and the clear-to-send packet are both transmitted over channel CCHl. The clear-to-send packet sent from mobile station MS2 is received by mobile station MS 1 , and after a short interframe space interval from timing t4 to timing t5, the mobile station MSl begins data transmission of data Dl from timing t5 to timing t6. This data Dl is received by the mobile station MS2. After reception of the data Dl sent from mobile station MSl and a short interframe space interval from timing t6 to timing t7, the mobile station MS2 sends an acknowledgement between timing t7 and timing t8 to the mobile station MS 1 , when the data Dl has been successfully received.

Then, the control unit 4 of the mobile station MSl determines a successful transmission of the data Dl and controls the receiving and sending unit 2 of the mobile station MS 1 so that a transmission to the mobile station MS2 is halted during a guard interval 8. Hence, the receiving and sending unit 2 of the mobile station MSl is controlled so as to pause transmission to the succeeding mobile station MS2 during the guard interval 8. In this case, the guard interval 8 begins at timing t8 and ends at timing tl6. During the guard interval 8 the mobile station 2 forwards the information comprising the data Dl over the channel CCHl to a further mobile station MS3, as described in the following.

After a back-off BO beginning at timing t8, the mobile station MS2 sends a ready-to-send packet to the mobile station MS3 over the channel CCHl from timing t9 to timing tlO. After a short interframe space interval from timing tlO to timing tl 1, the mobile station MS3 answers the ready-to-send packet from mobile station MS2 with a clear-to-send packet, wherein the clear-to-send packet is sent from timing tl 1 to timing tl2. The mobile station 2 receives the clear-to-send packet and, after a short interframe space interval from timing tl2 to timing tl3, the data Dl is forwarded to the mobile station MS3 during the time interval from timing tl3 to timing tl4. The mobile station MS3 receives the data Dl and, after a short interframe space interval from timing tl4 to timing tl5, answers with an acknowledgement between timing tl5 and timing tl6, because the data Dl has been successfully received. At timing tl6, the guard interval 8 of mobile station 1 expires so that the receiving and sending unit 2 of the mobile station 1 continues with transmission of further information to the mobile station MS2. This means that after a back-off BO from timing tl6 to timing tl7 a ready-to-send packet is sent to the mobile station MS2 during the time interval from timing tl7 to timing 118. It should be noted that the mobile station MSl may transmit the further information to mobile station MS2 over another channel, for example channel CCH2, to avoid collision with data sent from mobile station MS3 when the mobile station MS3 is connected with mobile station MS4 over channel CCHl.

As shown in Fig. 2, from timing tl8 to timing tl9, data Dl is transmitted from mobile station MS3 to mobile station MS4 parallel to the transmission of data D2 from mobile station MSl to mobile station MS2, wherein different channels CCHl, CCH2 are used to increase overall performance of the multihop connection and to reduce a possible delay in information transmission.

It should be noted that channel CCH2 is not used by any one of mobile stations MSl, MS2 or MS3 during the time interval from timing t2 to timing tl6, but may be used by other mobile stations, for example mobile station MS4, for data transmission.

During the time interval from timing t2 to timing t8, a not available timer is set for channel CCHl in the timer unit 6 of mobile station MS3, because channel CCHl is used by the adjacent mobile station MS2 and the neighboring mobile station MSl. Hence, by means of the not available timer set for mobile station MS3 with respect to channel CCHl from timing t2 to timing t8, a interference with a transmission between mobile station MSl and mobile station MS2 is avoided.

Further, the not available timer set in the timer unit 6 of the mobile station MS3 from timing t2 to timing t8 is not only set to the channel CCHl, but also with respect to the mobile stations MSl and MS2. The control unit 4 of the mobile station MS3 determines the other mobile stations MSl and MS2 using the channel CCHl from this information set in the timer unit 6, and controls the receiving and sending unit 2 of the mobile station MS3 so as to pause transmission to mobile station MS2 over the free channel CCH2 during the not available time interval from timing t3 to timing t8. This applies also to a succeeding mobile station, for example to the mobile station MS4 succeeding mobile station MS3.

Fig. 4 illustrates a data transmission method according to a second embodiment of the present invention. From timing tl to timing t6 information is transmitted from the mobile station MSl to the mobile station MS2, as described in detail with reference to Fig. 2 and 3. But, at timing t20 that corresponds to timing t7 in Figs. 2 and 3, the mobile station MS2 continues directly with transmission of data Dl. Thereby, as shown in Fig. 4A, a part 10 of the data Dl comprising a header information 11 is sent with an upper transmission power, and another part 12 of the data Dl is sent with a normal transmission power. Thereby, the control unit 4 of the mobile station MS2 determines the upper transmission power for sending the part 10 of the data Dl as a maximum of a transmission power required to transmit information to the mobile station MS 1 and a transmission power required to transmit information to the mobile station MS3. Hence, the receiving and sending unit 2 of the mobile station MS2 can successfully send the part 10 of the data Dl both to the mobile station MSl and the mobile station MS3. The mobile station MSl identifies the data Dl between timing t20 and t21 as the data Dl already send between timing t5 and t6 from the header information 11. Hence, the control unit 4 of the mobile station MSl determines reception of the part 10 of the data Dl comprising the header information 11 as an indirect acknowledgement for a successful transmission of data Dl to the mobile station MS2. The other part 12 of the data Dl may be transmitted with a normal transmission power, wherein the normal transmission power is determined as the transmission power necessary to transmit information from the mobile station MS2 to the mobile station MS3.

It should be noted that the receiving and sending unit 2 of the mobile station 1 may stop reception of the data Dl after reception of the header information 11 at timing t24, wherein the timing t24 is between the timing t20 and the timing t21. The mobile station MS3 may answer the received data Dl with a clear-to-send packet after a short interframe space interval between timing t21 and timing t22. The transmission of the clear-to-send packet is then between timing t22 and timing t23. It should be noted that the clear-to-send packet between timing t22 and timing t23 may be sent after reception of the part 10 of the data Dl so that the other part 12 of the data Dl is transmitted from the mobile station MS2 to the mobile station MS3 after reception of the clear-to-send packet. Therewith, utilization of the communications network may be further optimized in case of a huge or large amount of data Dl.

The header information 11 may be the serial number or a unique identification number of the data packets to be transmitted or acknowledged. Further, it should be noted that delaying the reception of a packet in a channel CCHl, CCH2 for a certain time, for example the short interframe space interval SIFS, enables parallel reception in two channels CCHl and CCH2. This applies for mobile stations 1 equipped with more than one receiver.

Besides reducing interference, multihop transmissions are essential for coverage extension and interconnection of different subnetworks. In rich scattering environments, especially in home and office environments, the coverage of Wireless Local Area Networks (WLAN)s is considerably decreased. One of the ways to provide continuous coverage is deploying multihop networks. A multihop functionality extension for the Multi- Carrier Code Division Multiple Access (MC-CDMA) based Distributed Coordination Function (DCF) is presented. The described Medium Access Control (MAC) protocol deals efficiently with the problem of packet forwarding without raising the system's complexity, while exploiting the multi channel structure of the MC-CDMA based system. It must be noted that the proposed solutions are not limited to MC-CDMA networks but can be easily adapted by other systems with multi channel structure, where multiple channel separation is not necessarily done in code domain.

A multihop connection consists of consecutive links, which enable the data transfer between two Mobile Stations (MS)s that cannot establish a direct radio link, and its realization requires the support of the network. A relay function is required, providing the functionality of forwarding MSs, for relaying of data packets to the next node of a multihop connection. Such relay functions can be implemented either in the first, second or third layer of the ISO /OSI reference model.

For the IEEE 802.11 MAC protocol, several relay functions for the MAC sublayer are possible. Based on the legacy MAC rules, a data-driven cut-through MAC is possible, for efficient forwarding. A receiver initiated protocol, namely RObust Ack-Driven Media Access Protocol (ROADMAP), can be used which avoids the problem of traffic prediction and reduces the overhead for multihop transmissions. Further reduction of overhead, is achieved with the Multiple Access with ReduCed Handshake (MARCH) protocol, where implicit Acknowledgements (ACK)s are used.

The main focus of the above protocols is multihop support for single channel networks, operating on basis of the IEEE 802.11 Wireless Local Area Network (WLAN). The focus here is the extension of the Medium Access Control (MAC) layer functionality to support packet relaying, in modified Distributed Coordination Function (DCF) for Multi- Carrier Code Division Multiple Access (MC-CDMA) based WLANs. For this reason, a simple relaying operation is taken into consideration and further optimized for the support of the MC-CDMA Physical layer (PHY layer). Therefore, multihop connections of up to 3 hops are considered, where the same Physical Layer mode (PHY mode) is used for all data transmissions within a multihop transmission. It is assumed that necessary information from layer three (routing) is known and provided to the MAC sublayer from network layer, and that all MSs are equipped with one transmitter only. The following gives a description of the MC-CDMA technique and the MC- CDMA based WLAN protocol. Thereafter, a detailed presentation of the multihop MAC protocol is given. Then, an extended presentation and discussion on simulation results is made. First, an overview of the main protocol for MC-CDMA based DCF is given.

Fig. 5 shows the MC-CDMA spreading mechanism with spreading factor 4, as an example.

In MC-CDMA, each symbol of the output data stream of a user is multiplied by each element of the user's spreading code. The MC-CDMA chips are formed in this way and placed via Inverse FFT (IFFT) in several narrow band subcarriers. Multiple chips are transmitted in parallel on different subcarriers. This method is called "frequency spreading".

In conventional Direct- Sequence CDMA (DS-CDMA), each user symbol is transmitted in the form of many sequential chips, each of which is of short duration and has a wide bandwidth. In contrast to this, due to the Fast Fourier Transform (FFT) associated with Orthogonal Frequency Division Multiplexing (OFDM), MC-CDMA chips are long in time duration, but narrow in bandwidth. Consequently interchip interference is reduced, and synchronization is easier compared to other spread spectrum techniques.

If a Spreading Factor (SF) of 4 is chosen, the symbol of one user is divided into 4 fractions and each of them is transmitted in parallel in 4 different subcarriers. Since the channel utilizes 48 data subcarriers, a total of 48/4=12 symbols of the same user can be transmitted in parallel to use the complete channel bandwidth.

From the systems point of view, one subcarrier carries a fraction of the users symbol, and can therefore carry additional load, coming from symbols of other users. At the end the symbol that is transmitted in one subcarrier consists of the sum of 4 fractions of 4 symbols that belong to 4 different users (see Fig. 5 for a MC-CDMA system with SF=4). Next, a PHY layer of MC-CDMA based DCF is described. In the MC-CDMA based DCF, orthogonal Walsh Hadamard spreading codes of length 4 are used, obtained from the rows of the 4^th order Hadamard matrix:

+1 +1 +1 +1

+1 -1 +1 -1

H₄ = +1 +1 -1 -1

+1 -1 -1 +1 Unlike Universal Mobile Telecommunications System (UMTS), where each transmitter uses a unique spreading sequence, in the proposed system with four available sequences, this is not feasible. Therefore, the concept of a Codechannel (CCH or cch) is used. A CCH is a spreading sequence, which is not explicitly assigned to a MS, but shared among a number of MSs. Each MS considers each spreading sequence as a subchannel of the frequency channel. Consequently, the frequency channel is divided (logically) by the four spreading sequences in four subchannels, the CCHs.

For channel coding, the K=7 convolutional encoder is used and at receiver's side besides the convolutional decoder the Minimum Mean Square Error (MMSE) Multiuser Detector (MUD) is applied. The rest of the PHY parameters have similar values to the ones used in conventional IEEE 802.1 Ia.

Next, MC-CDMA based DCF is described.

The MC-CDMA based DCF is a development of the IEEE 802.1 Ia WLAN MAC protocol, with modifications needed to support the MC-CDMA PHY layer. In this case, the frequency channel is divided into 4 parallel codechannels (SF = 4). Each of them can be accessed by the MSs applying the DCF.

A MS ready to transmit has to select a cch. Initially this selection is done randomly. For later transmissions, the MS does not select CCHs which have already been reserved by other MSs (according to the standard the considered MS has set a Network Allocation Vector (NAV) for an occupied channel).

According to the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) procedure, after detecting the medium idle for time DCF Interframe Space (DIFS), a MS defers for a certain time, called backoff, before transmitting its packet in order to avoid collisions. The duration of the backoff time is defined by:

Backoff Time = Random x aSlotTime

where Random is a uniformly distributed random integer number in interval [0, CW], and aSlotTime equals, for example, 9μs. The Contention Window (CW) has a starting value of 7, is doubled after a collision and reduced after a collision resolution.

Fig. 6 shows the multichannel approach for the IEEE 802.11 MAC.

If the countdown of the MS's backoff timer, carried out in steps of aSlotTime (9μs), is not interrupted by another transmission, the MS can initiate data transfer by transmitting a Ready To Send (RTS) packet in the selected codechannel, as depicted in Fig. 6.

All MSs, which receive the RTS frame, and are not the intended receivers, interrupt their backoff down counts and set their NAV. The NAV denotes the time a MS must defer from the medium in order not to interfere with an ongoing transmission. Compared with the standard IEEE 802.11 WLAN, in the MC-CDMA system each MS utilizes separate NAV states for each CCH.

The intended receiver, if idle i.e. able to receive data, responds to the RTS frame with a Clear To Send (CTS) frame, after a time Short InterFrame Space (SIFS). The SIFS time is mainly the transceiver turnaround time, as each MS is assumed to be equipped with one transceiver. We assume that MSs are equipped with four correlators and thus can monitor all four cchs in idle state. Similar to the RTS frame, MSs which receive this CTS set their NAV timer as well.

The sender can now transmit its data packet after SIFS. The packet is acknowledged in case of successful reception by an ACK frame, sent from the receiver with a delay SIFS after reception's end. Should two or more MSs access the same cch, on the same frequency channel, at the same time, a collision occurs. A retransmission attempt is started with a new RTS frame after backoff. The above procedure is followed in every codechannel for each data transmission.

In CDMA networks, the number of simultaneous transmissions can be increased until the Signal to Interference and Noise Ratio (SINR) at the receiver decreases to a limit that sets them unable to correctly receive and detect the incoming packets. Therefore, power control plays a major role for the systems capacity. In the proposed system power control is applied by means of an efficient calculation using the RTS and CTS frames.

Next, Smart Backoff is described in further detail with reference to Fig. 7. Instead of selecting a CCH randomly, a MS can apply Smart Backoff for prioritized medium access.

The Smart-Backoff procedure, shown in Fig. 7, allows a MS to iterate between CCHs during backoff thus directly reduces the delay of a data transfer. For this purpose, MSs applying Smart Backoff monitor all CCHs during backoff and mark the moment a CCH gets idle. If the backoff down count is interrupted one of the three cases shown in Fig. 7 occurs:

Another CCH seems idle. The MS has to monitor the CCH for at least a DIFS interval to determine whether it is really idle and then it can continue the down count of backoff timer in this CCH. Another CCH is determined idle and the MS can directly continue its backoff timer down count in this CCH.

No CCH is idle. The MS must wait.

Should two or more CCHs be idle when the backoff down count is finished, the MS will choose one of them, preferably the one that is idle for the longer period, for its transmission. Alternatively, the MS can transmit two or more packets in parallel, if after Smart Backoff procedure the correspondent amount of CCHs is idle.

Next, the multihop MAC protocol is described in further detail.

The progress of a multihop connection spanning over 3 hops is shown in Fig. 2. MSl is the initiating node, transmitting data packets over MS2 and MS3 to the final destination MS4. In this case, every forwarding MS is responsible for the correct transmission in the next hop, as with its own data. Signalization of the route is done, using the four address fields in MAC overhead as follows:

Address 1 : Contains the source address of the multihop connection (MSl). Address 2: Contains the next hop (MS2). Address 3: Contains the address of the final receiver. (MS4). Address 4: Contains the address of the second forwarding mobile station (MS3).

MS2 signals the correct reception of a data packet, with an ACK, and prepares the transmission to MS3 starting a new backoff. A new backoff is started in MSl too, and a new the transmission between MSl and MS2 would delay the transmission between MS2 and MS3. In order to prioritize packet relaying at MS2, a multihop guard interval 8 is introduced for MSl (Fig. 3). MSl, being the initiator of the multihop connection MSl to MS4, has to provide time for forwarding station MS2 to forward the data packet to MS3. For this reason MSl abstains for an interval, equal to the transmission window duration of the certain data packet. After the above guard interval 8 expires, MSl can initiate a transmission according to the carrier sensing rules. Depending on the scenario topology, MSl can transmit in parallel to MS3 (shown on the bottom left of Fig. 2), using another CCH, which increases overall performance of the multihop connection and reduces the delay.

In order to improve performance, Smart Backoff can be used at transmitting MSs, which enables parallel transmissions. Especially in the case of forwarding MSs serving two multihop connections, like the star topology in Fig. 8. Parallel transmissions at MS3 are essential for achieving lower delays. In such topologies though, Smart or Parallel Backoff might increase the number of collisions.

In Fig. 8 two multihop transmission take place over the common forwarding MS2: one connection from MSl to MS4 and another from MS3 to MS5. MS3 sets, according to the modified DCF, its NAV timer for the corresponding CCHl upon receiving the RTS frame from MSl (Fig. 3), or the corresponding CTS from MS 2. Smart Backoff would lead MS 3 to another idle CCH (CCH2 in Fig. 3), where it can proceed with backoff count down. A transmission from MS3 would interfere in this case with the ongoing data transfer from MSl to MS2. To overcome this problem, an extend NAV is proposed, the NAV per CCH and MS. According to the new NAV, each MS receiving a RTS and/or CTS, sets its NAV timer for the denoted duration of transmission, on the channel in which the control frame was received, and marks additionally the involved MS(s) as occupied. This precaution prohibits collisions in multihop scenarios, while it enables Smart Backoff deployment in multichannel networks. Next, experimental simulation results are described.

A representative multihop scenario is shown in Fig. 9. Besides a bottleneck station MS7, the scenario comprises four multihop connections of 1 to 3 hops. All transmitting MSs are capable of Smart Backoff and parallel transmissions in two CCHs are allowed for MS7, facing highest traffic. The applied values for further simulation parameters are given in Table I.

End-to-end connections are named as follows: connection between MS2 and MS4 is referred to as con. 1, between MS5 to MS9 as con. 2, between MS6 and MSlO as con. 3 and between MSl 1 and MS12 as con. 4.

Table I: Simulation Parameters

Fig. 10 presents the carried traffic per connection with the offered load. The one hop con. 1 , reaches in saturation the maximum cch MAC level capacity for the applied PHY mode, namely 2.4 Mbit/sec. Similarly, the 2 hop con. 4 reaches an end-to-end carried traffic of 1.2 Mbit/sec, that corresponds to half the cch capacity. The achieved maximum carried traffic for con. 2 and con. 3 averages to 0.8 Mbit/sec/ for each connection and is limited from the common forwarding station (MS 7), that competes for channel access, prior to every transmission, with one of the two transmitters MS5 and MS6.

In Fig. 11, mean end-to-end queuing delay per connection is shown. The end- to-end queuing delay comprises the queuing delay at all queues for a specific data packet. Results comply with the above throughput analysis: Multihop connections with more hops carry lower traffic and face high end-to-end queuing delay. The direct con. 4 achieves the lowest end-to-end queuing delay (as a direct link), while con. 3 and con. 2 suffer from high end-to-end queuing delay, rapidly raising with offered load. The mean end-to-end queuing delay at saturation load (which is different for every connection), is for all multihop connections approximately the same.

Important for the analysis of system's behavior, is the mean end-to-end service time per multihop connection, presented in Fig. 12. In these measurements, the service time over all hops is considered. For the direct link (con. 4), 3.1msec are needed in average for a complete transmission. Accordingly, the two hop con. 1 requires double service time, since two complete transfers are performed. In both cases, mean end-to-end service time is constant for different offered load, which reveals the collision free operation of those connections. For, con. 2 and con. 3, mean service time has the expected value of 9.3msec and 6.2msec, respectively, for low offered load only. For higher load, RTS collisions and RTS timeouts (no CTS response) for transmission attempts from MS5 and MS6 to the common forwarding station MS7 raise the required service time.

In Figs. 13 and 14, the CDFs of queuing delay per hop are presented, for 0.75 and 1.25 Mbit/sec offered load per connection, respectively. In the first case (0.75 Mbit/sec/connection offered load), the offered load is chosen at the saturation point of con. 2 and con. 3, which achieve the lowest carried traffic. The highest queuing delay comprises 200msec for transmissions of MS5. Its queuing delay distribution is similar to the one of MS6, since both MSs are the sources of two multihop connections sharing the same first forwarding station MS7. Furthermore, the multihop guard interval prohibits after a successful data packet transfer the immediate transmission of next data packet, raising the delay of next packets in the queue. Additionally, some collisions occur among MS5 and MS6, which increase further the delay. The second highest queuing delay is achieved by transmissions of MS2, owing to the multihop guard interval for prioritization of forwarding station MS3. The direct link between MSl 1 and MS12 follows, with better queuing delay performance. In this case, queuing delay is affected by the ability of Smart Backoff to detect a free cch.

Queuing delay distributions of the hop between MS7 and MS8, and the hop between MS7 and MSlO, are almost equal. The 3.1msec stepwise raise of queuing delay at 63% and 73% respectively, is the evidence of contention between MS7 and MS5 or MS6. In case MS5 (or MS6) transmits a data packet to MS7, MS5 (or MS6) defers for a duration equal to the multihop guard interval 8. MS7 competes then with MS6 (or MS5) on channel access and in 37% (27%) of the cases, MS6 (MS5) gains control of a cch, blocking MS7 with its RTS. The outcome is an increased queuing delay at MS7, equal to a transmission cycle (3.1msec). After its transmission, MS6 (MS5) defers according to the multihop guard interval duration, and MS7 competes for medium access with MS5 (MS6). Should MS5 (MS6) win the competition, MS7 sets its NAV and delays its transmission further, for another 3.1 msec. The steps on queuing delay diagrams for MS7 (MS7 to MS8 and MS7 to MSlO) give evidence to this situation, which might repeat, up to 3 times.

Best performance on queuing delay is achieved at MS3 and MS8, which are neither using the multihop guard intervals 8, as the last forwarding MSs of con. 1 and con. 2 respectively, nor are they participating in a second multihop connection. Particularly, MS8 can transmit concurrently with MS5 in another cch.

In Fig. 14, the Cumulative Distribution Function (CDF)s of queuing delay per hop are presented, for the case of 1.25 Mbit/sec offered load per connection. Network operation shows the same performance characteristics as in previous case, with the difference of higher queuing delay for MS5 and MS6, which are now in overload. Additionally, data packets at MS2 experience increased queuing delay, with distribution similar to the one of MS6 in Fig. 13, as now the offered load is chosen at the saturation point of MS2 (and not at the saturation point of MS6, as it was the case in Fig. 13). The CDF of end-to-end delay, measured as the delay between the arrival of a data packet in the network and the reception of the ACK at the last hop, is depicted in Fig. 15, for 0.75 Mbit/sec offered load per connection. For con. 2 and con. 3, the end-to-end delay has a similar distribution, due to the common forwarding station MS7. The reason for the small difference between the two CDFs is the one more hop at con. 2. Similar results are shown in Fig. 16, presenting the end-to-end delay for 1.25 Mbit/sec offered load per connection. The network operates in saturation and the high offered load introduces high end- to-end delay for data transfer.

As a result, an efficient forwarding method for the MC-CDMA based DCF is described. Performance evaluation results show the ability of the multihop network to achieve an overall good performance. Using Smart Backoff, forwarding MSs, which participate in more than one multihop connection can improve their performance, and achieve higher throughput. Technical solutions proposed in this chapter for the realization of relays, such as the NAV timer per MS and cch, can be adopted from other wireless systems with multi channel structure, which don't necessarily use MC-CDMA. Table of some Abbreviations:

ACK Acknowledgement cch Codechannel

CDF Cumulative Distribution Function

CDMA Code Division Multiple Access

CSMA/CA Carrier Sense Multiple Access/ Collision Avoidance

CTS ClearToSend

CW Contention Window

DCF Distributed Coordination Function

DIFS DCF InterFrame Space

DS-CDMA Direct Sequence-CDMA

IEEE Institute of Electrical and Electronics Engineers

IFFT Inverse Fast Fourier Transformation

FFT Fast Fourier Transformation

MAC Medium Access Control

MARCH Multiple Access with ReduCed Handshake

Mbit/sec Megabits per second.

MC-CDMA Multi-Carrier Code Division Multiple Access

MMSE Minimum Mean Square Error

MS Mobile Station

MUD Multiuser Detector

NAV Network Allocation Vector

OFDM Orthogonal Frequency Division Multiplexing

PHY layer Physical layer

PHY mode Physical Layer mode

QoS Quality o f S ervice

ROADMAP RObust Ack-Driven Media Access Protocol

RTS RequestToSend

SIFS Short InterFrame Space

SF Spreading Factor

UMTS Universal Mobile Telecommunications System

WLAN Wireless Local Area Network Although an exemplary embodiment of the invention has been disclosed, it will be apparent to those skilled in the art that various changes and modifications can be made which will achieve some of the advantages of the invention without departing from the spirit and scope of the invention. Such modifications to the inventive concept are intended to be covered by the appended claims in which the reference signs shall not be construed as limiting the scope of the invention. Further, in the description and the appended claims the meaning of "comprising" is not to be understood as excluding other elements or steps. Further, "a" or "an" does not exclude a plurality, and a single processor or other unit may fulfill the functions of several means recited in the claims.

Claims

CLAIMS:

1. Mobile station (1) for a multi-channel based communications network, especially for a multi-carrier code division multiple access wireless local area network, which mobile station comprises: a receiving and sending unit (2) for receiving and/or sending information, a control unit (4) for controlling said receiving and sending unit (2), wherein said receiving and sending unit (2) is adapted to send a first information to a succeeding mobile station, wherein said control unit (4) determines a successful transmission of said first information and is adapted to control said receiving and sending unit (2) so as to pause transmission to said succeeding mobile station during a guard interval (8), wherein said receiving and sending unit (2) is adapted to send a second information to said succeeding mobile station after said guard interval (8) expired, and wherein said guard interval (8) is determined to enable said succeeding mobile station to forward said first information before reception of said second information.

2. Mobile station according to claim 1, characterized in that said control unit (4) is adapted to determine said guard interval (8) at least nearly equal to a transmission window duration of said first information.

3. Mobile station according to claim 1 or 2, characterized in that said receiving and sending unit (2) is adapted to send said second information to said succeeding mobile station over a channel that is different from a channel used by said succeeding mobile station to forward said first information.

4. Mobile station according to claim 1, characterized in that said receiving and sending unit (2) is adapted to receive said first information from a preceding mobile station, that said control unit (4) determines an upper transmission power for sending at least a part of said first information as a maximum of a transmission power required to transmit information to said preceding mobile station and a transmission power required to transmit information to said succeeding mobile station.

5. Mobile station according to claim 4, characterized in that said receiving and sending unit is adapted to send a part of said first information comprising a header information with said upper transmission power and to send another part of said first information with a transmission power required to transmit information to said succeeding mobile station.

6. Mobile station according to claim 5, characterized in that said part of said first information comprising said header information is sent to said preceding mobile station as an indirect acknowledgement, wherein said header information is an extended header information comprising an identification number for said information to be transmitted or acknowledged.

7. Mobile station according to claim 1, characterized by a timer unit comprising at least a not available timer (6), wherein said control unit (4) is adapted to set said not available timer to a not available time interval for a channel used by other mobile stations, wherein said control unit (4) determines said other mobile stations using said channel and wherein said control unit (4) is adapted to control said receiving and sending unit (2) so as to pause transmission to said succeeding mobile station over any possible channel during said not available time interval when said succeeding mobile station is one of said other mobile stations using said channel.

8. Mobile station according to claim 7, characterized in that said not available timer (6) is an extended not available timer (6) set for said channel and said other mobile stations involved by using said channel.

9. Method for transmission of information in a multi-channel based communications network, especially in a multi-carrier code division multiple access wireless local area network, which method comprises the steps of: sending a first information from a first mobile station to a second mobile station; sending said first information from said second mobile station to a third mobile station; pausing transmission from said first mobile station to said second mobile station during a guard interval (8) to enable said second mobile station to finish said sending of said first information to said third mobile station before a second information is sent from said first mobile station to said second mobile station.

10. Method according to claim 9, characterized in determining an upper transmission power for sending at least a part of said first information as a maximum of a transmission power required to transmit information from said second mobile station to said first mobile station and a transmission power required to transmit information from said second mobile station to said third mobile station; sending a part of said first information comprising a header information with said upper transmission power to both said first mobile station and said third mobile station, wherein a reception of said header information at said first mobile station is regarded as an indirect acknowledgement for said first information transmitted from said first mobile station to said second mobile station.

11. Method according to claim 9 or 10, characterized in determining a not available time interval for an at least locally used channel; determining mobile stations using said channel; and pausing transmission form another mobile station to any one of said mobile stations using said channel over any possible channel during said not available time interval.