AU2015101372A4 - Methods of providing service differentiation utilizing frequency-domain contention - Google Patents

Abstract

Docket No. UM1255AUOO This invention provides a weighted frequency-domain contention (WFC) scheme for providing priority differentiations among users in a random access communication system while excluding collision. Each user is classified as a high priority (HP) user or a low priority (LP) user. The users contend for channel access. The channel has plural frequency domain subcarriers, each assigned with a unique subcarrier number. In contending for the channel access, each HP (LP) user sends a request on one subcarrier selected from a first (second) subset of the frequency-domain subcarriers. In particular, different frequency domain ranges are assigned to the two subsets to achieve weighted channel-access opportunity between the HP users and the LP users. One or more winners are determined by identifying the contending user(s) having the subcarrier numbers) the smallest among all the contending users. When there are multiple winners, a signature-assisted method is employed to completely exclude collision. Abstract figure: FIG. 4. Page 23 ~- L .g~ ~ I l knI

Description

Docket No. UM1255AU00 Methods of Providing Service Differentiation Utilizing Frequency-Domain Contention Inventors: Qinglin ZHAO, Huan ZHANG, Fangxin XU, Zhijie MA, and Li FENG CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to, and the benefit of, U.S. Provisional Patent Application No. 62/211,796, filed on August 30, 2015, which is incorporated by reference herein in its entirety. LIST OF ABBREVIATIONS ACK acknowledgement AP access point BA bitwise arbitration CSMA/CA carrier sense multiple access with collision avoidance DIFS distributed inter frame space HP high priority LP low priority MAC media access control OFDM orthogonal frequency division multiplexing QoS quality of service T2F time to frequency WFC weighted frequency-domain contention WLAN wireless local area network BACKGROUND Field of the invention The present invention generally relates to a MAC protocol for a random-access communication system with multiple users contending for a channel. In particular, the present invention relates to the MAC protocol utilizing frequency-domain contention for providing QoS to different priority classes of users. List of references Page 1 Docket No. UM1255AUOO There follows a list of references that are occasionally cited in the specification. Each of the disclosures of these references is incorporated by reference herein in its entirety. [1] A. P. Jardosh, K. N. Ramachandran, K. C. Almeroth, an E. M. Belding-Royer, "Understanding Congestion in IEEE 802.1 lb Wireless Networks," Proceedings of Internet Measurement Conference, pp. 279-292, 2005. [2] S. Sen, R. R. Choudhury, and S. Nelakuditi, "Listen (on the frequency domain) before you talk," Proceedings of the 9th ACM SIGCOMM Workshop on Hot Topics in Networks, ACM, 2010. [3] X. Feng et al., "Use your frequency wisely: Explore frequency domain for channel contention and ACK," Proceedings of2012 IEEE INFOCOM, 2012. [4] S. Sen, R. R. Choudhury, and S. Nelakuditi, "No time to countdown: Migrating backoff to the frequency domain," Proceedings of the 17th annual international conference on Mobile computing and networking, A CM, 2011. [5] P. Huang, X. Yang, and L. Xiao, "WiFi-BA: Choosing arbitration over backoff in high speed multicarrier wireless networks," Proceedings of2013 IEEE INFOCOM, pp. 1375-1383, 2013. [6] H. Zhang et al., "A weighted T2F scheme for WLANs," Proceedings of the 2nd International Conference on Mobile and Wireless Technology, Springer Berlin Heidelberg, pp. 75-82, 2015. [7] L. Wang et al., "Harnessing frequency domain for cooperative sensing and multi channel contention in CRAHNs," IEEE Transactions on Wireless Communications, vol. 13, no. 1, pp. 4 4 0- 4 4 9, 201 4 . [8] S. Misra, and M. Khatua, "Semi-distributed backoff: Collision-aware migration from random to deterministic backoff," IEEE Transactions on Mobile Computing, vol. 14, no. 5, pp. 1071-1084, 2015. [9] L. Wang et al., "Attachment-learning for multi-channel allocation in distributed OFDMA-based networks," IEEE Transactions on Wireless Communications, vol. 12, no. 4, pp. 1712-1721, 2013. Description of related art With IEEE 802.11 WLANs being widely deployed worldwide, its channel efficiency has received growing concern. The IEEE 802.11 networks perform the time-domain contention. In an IEEE 802.11 network, each user must waits for a random time, before Page 2 Docket No. UM1255AUOO packet transmission. When multiple users are simultaneously backing off, the channel must remain idle, naturally leading to under-utilization. It has been shown in [1] that more than 30% reduction in throughput due to backing off Recently, [2] proposed a T2F protocol to improve the channel efficiency of WLANs. In the T2F protocol, users employ OFDM subcarriers to perform channel contention in the frequency domain, instead of the time domain. The T2F protocol arbitrates channel contention in two slots. In slot 1, each user signals on one subcarrier randomly chosen from a pool of subcarriers, and listens to this subcarrier pool at the same time by employing a second antenna. Then each user can determine the winners, who signal on the smallest-numbered subcarrier. In slot 2, the winners perform the second-round frequency-domain contention for further arbitration. Finally, the winner in the slot-2 contention transmits a packet in the next slot. By limiting the channel-contention time to two slots, the T2F protocol shortens the contention time greatly and hence improves the channel efficiency when compared to the IEEE 802.11 protocol. The T2F protocol has attracted a great deal of attention [3]-[9] since it was proposed. Among [3]-[9], references [3]-[6] are the most relevant to the present work. Ref. [3] proposed a novel MAC design called REPICK. REPICK partitions all OFDM subcarriers into two groups: identification subcarriers (each of which is assigned to a unique user) and contention subcarriers (which are used by all users for channel contention). By allowing users to simultaneously transmit ACK over identification subcarriers and contend for channel over contention subcarriers like the T2F protocol, REPICK can significantly exclude the DIFS time and significantly reduce the random backoff time, and the ACK transmission time in conventional IEEE 802.11 networks. Ref. [4] proposed Back2F protocol. The Back2F protocol extended the T2F protocol by creating backoff operations in the frequency domain. Like the T2F protocol, each user under the Back2F protocol first randomly picks a subcarrier from a specified range of subcarriers to contend for the channel, and then starts the transmission, where the subcarrier chosen by a user is regarded as the current backoff value of the user. However, unlike the T2F protocol, after the transmission is finished, all other users continue contending for the channel using the updated backoff value (rather than re choosing a new subcarrier randomly), where the updated backoff value of a user is the result that its chosen subcarrier number subtracts the minimum subcarrier number in the last contention. In the above related works, each user uniformly selects a subcarrier from the same subcarrier pool. As a result, each user has the same channel access opportunity. In reality, however, different applications have different QoS requirements. For example, a voice Page 3 Docket No. UMI255AUOO packet should have a more stringent delay requirement than a data packet, and therefore should be assigned with a higher transmission opportunity. Clearly, the protocols of T2F, B2F and REPICK cannot fulfill the QoS requirements of real-time applications. Recently, prioritized frequency-domain contention schemes [5], [6] have been proposed. In [5], it was proposed a scheme called WiFi-BA. WiFi-BA introduces a binary mapping scheme to make collision detection in the frequency domain, and a BA mechanism to contend for channel. What WiFi-BA provides is absolute priority, where HP users occupy all available bandwidth for immediate transmission, thereby starving low-LP users. In addition, WiFi-BA cannot completely exclude collision (for example, collision will occur when more-than-one users select the same random value in a binary mapping scheme) and the LP users often need several slots to contend for channel. Instead of providing absolute priority, the inventors of the present invention aim at providing relative priority (where HP and LP users coexist and share the total available bandwidth with different proportions). To this end, the inventors' previous work [6] proposed a simple scheme. This scheme retained the T2F design framework and just modified the contention policy in slot 1, where HP users use partial subcarriers and LP users use all subcarriers for contention. As a result, this scheme just provides limited priority differentiation, and does not exclude collision as in the T2F protocol. There is a need in the art for a technique for providing priority differentiation while excluding collision. SUMMARY OF THE INVENTION An aspect of the present invention is to provide a channel-access contending method for providing general and fine-grained priority differentiations among plural users in a random access communication system while excluding collision. An entirety of users comprises one or more HP users and one or more LP users. The multiple-access channel contended by the users for channel access comprises a plurality of frequency-domain subcarriers. Each of the frequency-domain subcarriers is assigned with a unique subcarrier number. The method comprises two stages. The first stage of the method is as follows. When there are one or more contending users wishing to contend for the channel access among the users, an individual contending user signals a request on one subcarrier. If the individual contending user is also one of the HP users, the one subcarrier is substantially-randomly selected from a first subset of the frequency-domain subcarriers. Similarly, if the individual contending user is also one of the Page 4 Docket No. UM1255AUOO LP users, the one subcarrier is substantially-randomly selected from a second subset of the frequency-domain subcarriers. The minimum subcarrier number among the subcarriers in the first subset is lower than the minimum subcarrier number among the subcarriers in the second subset. In addition, the maximum subcarrier number among the subcarriers in the first subset is higher than the minimum subcarrier number among the subcarriers in the second subset, but lower than the maximum subcarrier number among the subcarriers in the second subset. Afterwards, one or more winners of gaining the channel access are determined. It is done by identifying one or more of the contending users that have transmitted the requests on the selected subcarriers whose subcarrier numbers are equal to the smallest one among all the subcarrier numbers of the selected subcarriers of the one or more contending users. When it is determined that there is only one winner, this winner transmits its data over the channel. The second stage of the method is performed when it is determined that there are plural winners. Each winner first broadcasts over the channel a signature assigned to this winner. Each winner also listens to the channel so as to identify all the plural winners by detecting the signatures thereof. Afterwards, all of the winners transmit their data in a manner of one winner by another winner in a sequential order according to a pre-agreed scheme known to all the users. Other aspects of the present invention are disclosed as illustrated by the embodiments hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS The features and advantages of the present invention will be apparent from the following detailed description of the invention with reference to the accompanied drawings in which: FIG. 1 depicts a star-topology WLAN used for illustrating the T2F scheme; FIG. 2 depicts a two-round contention approach adopted in the T2F scheme; FIG. 3 depicts a star-topology WLAN for illustrating the WFC scheme as disclosed herein; FIG. 4 provides an overview of a MAC design based on the WFC scheme in accordance with one embodiment of the present invention; FIG. 5 depicts, as an example, a HP subcarrier pool and a LP subcarrier pool in the R1 stage; FIG. 6 depicts a contention resolution method used in the R2 stage and subsequent DATA transmission in accordance with an embodiment of the present invention; Page 5 Docket No. UM1255AUOO FIG. 7 illustrates a calculation of P, when n = 1 and n = 1; FIG. 8 plots the theoretical and simulation results of E( ) as In varies from 1 to 50; FIG. 9 plots the theoretical and simulation results of the system throughput versus in; and FIG. 10 plots the theoretical and simulation results of the per-user throughput when F, S, n and mi respectively change. DETAILED DESCRIPTION The present invention provides a novel OFDM-based MAC protocol, called as a WFC technique, which is based on the OFDM technology with an objective of providing QoS to different priority classes of users by utilizing frequency-domain contention. The WFC scheme as disclosed in the present invention is a significant extension of the scheme of [6]. Before detailing the WFC scheme, we provide an overview of characteristics thereof as follows. In design, the WFC scheme makes signification modifications in the two-slot contention process. In slot 1, the WFC scheme allows HP and LP users, respectively, to choose subcarriers from [1, S] and [F + 1, L], where F s S s L, and they delimit the range of the selectable subcarriers. Through suitably setting different values of S and F, the WFC scheme can provide general and fine-grained priority differentiations. For example, in a special case of S = L and F = 0 , the WFC scheme reduces to the T2F protocol; in another special case of S < L and F = 0, the WFC scheme reduces to the scheme of [6]. In slot 2, a signature-assisted method is used to completely exclude collision. In contrast, previous schemes (such as WiFi-BA, the T2F protocol, and the scheme of [6]) cannot do so. The notations used herein in the illustration of the WFC scheme are listed in Table 1 as follows. Table 1. List of symbols used. Symbol Description Page 6 Docket No. UMI255AUOO 1 U, User i 2 RI The first-round contention (for the T2F and WFC schemes) 3 R2 The second-round contention (for the T2F and WFC schemes) 4 The throughput of type- I user 5 r The throughput of type-HP user 6 FL The throughput of type-LP user 7 I' The system throughput 8 P The successful transmission probability of type- I user 9 PH The successful transmission probability of type-HP user 10 PL The successful transmission probability of type-LP user 11 F The lower boundary of LP-subcarrier range 12 S The upper boundary of HP-subcarrier range 13 L The total number of subearriers 14 Ls The length of signature sent by each user in the R2 stage 15 s, The signature of user i 16 k The variable 1 s k s Ls 17 si(k) The k th symbol of the user i's signature 18 s; (k) The complex conjugate of s, (k) 19 n The number of HP users 20 n The number of LP users 21 N The total number of all users 22 The number of winners in the Ri stage 23 E( ) The mean value of 24 LDAy7 The length of one data packet 25 TD1FS The DIFS time 26 TR I The contention time in the RI stage 27 Tm The contention time in the R2 stage 28 TDA TA One packet transmission time (including ACK time) 29 fu The probability that a type-HP user chooses an arbitrary subcarrier Page 7 Docket No. UM1255AUOO 30 fL The probability that a type-LP user chooses an arbitrary subcarrier 31 RDA7A The PHY data rate A. The T2F protocol In this section, we present the basic ideas and the drawbacks of the T2F scheme [2]. A.l. Basic ideas The T2F protocol is a scheme that provides fair channel access via frequency-domain contention for WLANs. In a T2F scheme, each user has two antennas: one for normal data transmission and another one for listening to the channel. The T2F scheme uses OFDM-based physical layer techniques. In OFDM, the whole channel is divided into a plurality of subcarriers (e.g. in IEEE 802.l la/g). Consider a star-topology WLAN illustrated in FIG. 1, where each user can hear other users and contend for the channel for data transmission. With the help of FIG. 2, the data transmission process in the T2F scheme is explained as follows. In the T2F scheme, each user first senses if the channel is idle for a duration of DIFS time. Then a two-round channel contention (i.e. R1 and R2 in FIG. 2) in two consecutive slots is performed. Finally, data transmission is performed. How the stages of R1 and R2 work is explained as follows. In the R1 stage, each user signals on one subcarrier (via the transmission antenna) randomly chosen from a pool of subcarriers, and at the same time listens to any signal on subcarriers of this subcarrier pool via the listening antenna. T2F users treat the subcarriers as integers. Then each user can determine the winners, who signal on the smallest-numbered subcarrier. As shown in the example of FIG. 2, in the RI stage, Ul and U4 select No. 5 subcarrier, and U3 and U2 select No. 8 and No. 11 subcarriers, respectively. Then U1 and U4 win because they selected the same subcarrier that has the minimum number among all the selected subcarriers. In the R2 stage, the users choosing the smallest-numbered subcarrier perform the frequency-domain contention in the same way as the R1 contention, and then transmit data in an ascending order of the chosen subcarriers. As shown in the example of FIG. 2, in the stage of R2, Ul and U4 select No. 4 and No. 8 subcarriers, respectively. After that, U1 and U4 transmit data one user after another user. Page 8 Docket No. UM1255AUOO A.2. Drawbacks In the T2F scheme, each user uniformly selects a subcarrier from the same subcarrier pool. Therefore, each user has the same channel access opportunity. However, in reality, different applications have different QoS requirements. For example, a voice packet should have a more stringent delay requirement than a data packet, and therefore should be assigned with a higher transmission opportunity. Clearly, the T2F scheme cannot fulfill the QoS requirements of real-time applications. In addition, the T2F scheme cannot exclude collisions fully. For example, if more-than-one user enter into R2 and choose the same subcarrier, it will cause collision in data transmission. B. The WFC scheme B.1. Overview of WFC The WFC scheme is an amendment of the T2F scheme. In the T2F scheme, it provides the same service for every contention user. In contrast, the WFC scheme is configured to provide weighted services for different users based on their different demands. In the present invention, we consider a one-hop star-topology WLAN where each user can hear other users as shown in FIG. 3, and assume that the WLAN system has two priority classes: a HP class and a LP class. In the WFC scheme, HP users have higher channel access probabilities than LP users. Similar to the T2F protocol, in the WFC scheme, each user has two antennas (one for transmission and another one for listening to signals), and performs the frequency-domain contention. As illustrated in FIG. 4, when a user wants to transmit data, it first senses the channel for a DIFS time, and then enters into a two-round contention process consisting of RI and R2, and chooses the winners thereafter. At last, the winners transmit data. However, unlike the T2F scheme, it is assumed that each user is assigned with a unique signature. Each signature is associated with a sequence number, and is known to all users. The WFC scheme has the following key differences in the R1 and R2 contention stages. Difference with the T2F scheme in the RI stage: In the WFC scheme, different users signal on different subcarrier pools (i.e. different subcarrier ranges). As a result, different users have different channel access opportunities. In an example, assume that U1 and U2 are HP users, whereas U3 and U4 are LP users. FIG. 4 illustrates that the HP users (i.e. Ul and Page 9 Docket No. UM1255AUOO U2) can select a random subcarrier in a first subcarrier pool [1,10], while the LP users (i.e. U3 and U4) make selections in a second subcarrier pool [4, L]. Since the subcarrier pools of HP and LP users are different, the set of HP users and the set of LP users have different probabilities of selecting the smaller-numbered subcarriers. In this example, Ul and U2 respectively choose subcarriers no. 5 and no. 8, while U4 and U3 respectively choose subcarriers no. 5 and no. 11. Since Ul and U4 choose the same subcarrier, which is the subcarrier no. 5, both Ul and U4 subsequently enter into the R2 stage for further contention resolution. Difference with the T2F scheme in the R2 stage: In the R2 stage, all winners in the RI stage send their unique signatures on the whole channel, i.e. over all subcarriers. Each winner first performs correlation among the signals transmitted on the subcarriers to detect all the transmitted signatures, and then determines the transmission order of the winners by the sequence numbers of the detected signatures. For the example shown in FIG. 4, Ul first transmits, and then U4 transmits, according to the associated sequence numbers. In the following subsections, the RI and R2 contention stages are detailed. B.2. WFC in the RI stage It is assumed that the whole channel is divided into a plurality of OFDM subcarriers. As shown in FIG. 5, in the R1 stage, consider that the HP and LP users, respectively, uniformly choose subcarriers from subcarrier pools [1, S] and [F + 1, L], where 0 s F s S s L. A smaller value of S or a larger value of F gives a higher channel access probability for the HP users. By carefully tuning the values of S and F, one can assign different channel access probabilities to the sets of HP users and of the LP users, according to their QoS requirements. In the special case of F = 0 and S = L, the RI stage of the WFC scheme is the same to that of the T2F protocol. The WFC scheme assigns different channel access probabilities to different priority classes, but it does not exclude collision - multiple users simultaneously choose the same "minimum-numbered" subcarrier (in other words, there are multiple winners). In the example shown in FIG. 4, both Ul and U4 choose the subcarrier no. 5, which is the minimum-numbered subcarrier among all chosen subcarriers. When a collision happens, multiple winners are required to enter into the R2 stage for further contention resolution. B. 3. Signature-assisted contention resolution in the R2 stage Page 10 Docket No. UMI255AUOO The WFC scheme introduces a signature set (e.g. G ={s1,S2,- .,sN S), where N is the total number of users, each unique signature s, is mapped into a two-tuple, <userID, transmissionorder>. For example, the top subfigure in FIG. 6 illustrates that each signature si, i =1,.- -,4, is mapped to the user i (i.e. U,) and the transmission order i. Before DATA transmission, the AP broadcasts the signature set and the mapping relationship. The R2 stage comprises three procedures: signature sending, signature detection and DATA transmission. Signature sending. Each RI-stage winner transmits its assigned signature on the transmission antenna, and at the same time, it keeps sensing via its listening antenna. Signature detection: Each winner uses N correlators to execute signature detection cori(L,) shown in (1), where corri(L,), i = 1, --.,N, denotes the correlation value between the received signal and the user i 's signature s,,. When con; (L,) is higher than a predefined threshold, this means user i participates in the R2 stage: coni,(LS)= s (k)y(k + A) is,(k)1 2 (1) where L, denotes the length of the signature, y(k + A), 1 s k s L,, denotes the k -th symbol of the received signal, from a shifted position A, s,(k) denotes the k -th symbol of user i's signature, and s (k) denotes the complex conjugate of s,(k). DA TA Transmission: After the signature detection, each winner knows how many users will transmit in the R2 stage, who will first transmit and who will then be transmitted by the transmission order (mapped from the corresponding signature). Taking FIG. 6 as an example, winners in the R1 stage, say U1 and U4, enter into the R2 stage and contend for the channel. According to the transmission order, Ul first transmits and U4 then transmits. C. Performance analysis In this section, we develop a theoretical model to analyze the WFC throughput and study how to achieve the proportional fairness during the saturation operation (where each user always has packets to transmit). In this model, we consider a one-hop WLAN consisting of one AP, m HP users, and n LP users, where n + in = N. We focus on the uplink traffic (that is, the traffic is only transmitted from users to the AP) and assume that the channel is perfect. C.]. Throughput Page 11 Docket No. UMI255AUOO In the following, we first express the per-user throughput and the total system throughput, and then calculate relevant parameters. Per-user throughput: In the WFC scheme, the DATA transmission time can be divided into a series of transmission periods. As shown in the bottom subfigure in FIG. 6, each transmission period consists of a DIFS interval ( TDFS), an RI contention interval (TRI), an R2 contention interval (T ), and a DATA transmission interval ( TDT x E( )), where E( ) denotes the mean of the number of the winners in the R1 stage, and T is one packet transmission time (including ACK time). Define the throughput of one type- J user, F, to be the number of bits that the type I user successfully transmits during a whole transmission period, where I = H denotes a type-HP user and I = L denotes a type-LP user. F, can be expressed as 1, = P, X LDA + (2) TDATA x E( )+ Tm + T2 + TDIFS where P, denotes the successful transmission probability of type- I user, and LDATA denotes the packet length. Total system throughput: The total system throughput, F, is the sum of each user's throughput F,. It follows that F is given by F = mFH + nFL (3) To calculate F, and F, the remaining tasks are to calculate P, and E( ). C.].1. Calculation of P, To calculate P,, we first consider a simple case that there is only one user in each class (i.e. n = 1 and in = 1), and then we extend it to the general case (i.e. n > 1 and mn > 1). The case of n = 1 and n = 1: We assume that user 1 is a HP user and user 2 is a LP user. According to the design in R1, user 1 selects a random subcarrier from the subcarrier pool [1,S], while user 2 selects from [F + 1,L], as shown in FIG. 7. In FIG. 7, only user 1 can select one subcarrier from [1, F]; only user 2 can select one subearrier form [S + 1, L]; nevertheless, both users 1 and 2 can select subcarriers from [F + 1, S]. In the WFC scheme, a user will win the contention if its selected subcarrier number is the smallest. Therefore, from FIG. 7, user 1 will win the contention absolutely if its selected subcarrier number belongs to [1,F]. User 2 will always lose the contention if its selected subcarrier number belongs to Page 12 Docket No. UM1255AUOO [S + 1, L]. Meanwhile, both users 1 and 2 have certain probabilities to win the contention if their selected subcarrier numbers belong to [F + 1, S]. In short, the successful transmission probabilities, PH and PL, when in = n = 1, are respectively given by (4) and (5): PH = P(user 1 wins absolutely in [1, F]) + P(user 1 wins in [F + 1,S]) F f(4) fH+fH (L+l-i)fL and S PL = P(user2winsin[F+1,S])= fL (S +1- i)f (5) where fH = 1/S and fL = 1/(L - F), respectively, denote the probabilities that users 1 and 2 choose arbitrary subcarriers from their subcarrier pools. In (4), 1' fH denotes the probability that user 1 wins the contention absolutely in the range of [1, F], since only user 1 can select the subcarrier from this range, while user 2 cannot; the term fH Y=F+ (L + 1 L)fl denotes the probability that user 1 wins when it selects the subcarrier i from [F + 1,S], since user 1 wins the contention in this range only when user 2 chooses the subcarrier from [i, L] with a probability of (L +1- i)fL. Likewise, in (5), the term fL =F+1 (S + - i)fH denotes the certain probability that user 2 wins when it selects the subcarrier number i from [F + 1,S], since that only user 1 chooses the subcarrier from [i,S] can ensure that user 2 wins the contention. The case of n > 1 and in > 1: We now focus on [1,F] to explain the extension ideas, as shown in Figure 7. From this figure, we know that a tagged HP user will win the contention absolutely if its selected subcarrier number i belongs to [1, F], and is the smallest compared with the selected subcarrier numbers of the rest ni - I HP users. That is, the rest in -1 HP users would choose the subcarrier from [i, S] with a probability of (S +1- i)fH . Since i varies from 1 to F, the probability that the tagged HP user wins the channel is , fH((S + 1- i)fH)"' 1 . Similarly, we can get the probabilities that any HP user and LP user win the contention when they select the subcarriers from [F + 1,S], respectively. Therefore, the probability of each HP user (P) and LP user (PL) wins the contention can be expressed as Page 13 Docket No. UM1255AUOO PH (i, n, F, S, L) P(a tagged HP user wins in [1, F]) + P(a tagged HP user wins in [F + 1, S]) fH ( j~ ((S + 1- _ )fH y)))H + IS~ ((( + - )f H )-1 x ( - i)fL ) = fi= +I((S+l-if)"+ + Hy m-1 nl (6) and PL (in, n, F, S, L) P(a tagged LP user wins in [1, F]) =f (((SF +-1)fH)' x ((L + 1- i)fL') (7) Note that when n = n = 1, (6) and (7) reduce to (4) and (5), respectively. C.1.2. Calculation ofE( ) In this section, we calculate E( ). To do so, we express the total throughput from two perspectives. On one hand, the total system throughput is the sum of each user's throughput. Let 17 and FL respectively be the throughputs of each HP and of each LP user. It is possible to having such setting on , and

T

L because the users of each class have the same subcarrier pools, and then have the same transmission probabilities and the same throughput. From (2) and (3), we can also express the system throughput as F =mInx F + n xFL = (nx PH+ n xPL) x DT TDA72 x E( )+ T, + TR 2 +TDIFS On the other hand, the total system throughput can be expressed as the ratio of the average data length to the average transmission period. Since E( ) is the average number of the winners in the RI stage, it follows that in a transmission period, the total length of data transmission is given by LDATA x E( ). Then we can also express the system throughput in terms of E(f) as F .

LDATA xE( ) TDA71 x E()+TR +TR 2 +TDIFS Combining (8) and (9), we have E()= n x PH +nxPL = mx fH(IH (sl-if H H(s+l-)fH L(L+l-i)fj) i=1 j=-2 i=F+1 j=-2 k-1 +n xfL (J(S+l-i)fHxf(L+l-i)f i=F+1 jP1 k=2 Page 14 Docket No. UM1255AUOO (10) C.2. Proportional fairness In the model under consideration, HP and LP users can, respectively, choose subcarriers from [1, S] and [F + 1, L]. In this section, we consider how to set S and F so as to achieve a predefined proportional throughput fairness ratio, y. From (6) and (7), we have F S ]H(mn,n, F, S,L) ,1(i 1-L (mn,n, F, S, L) f,"1 S + i)'" x (L +1 I)) y .=' (11)l Then, given m, n, L and y, one can find the desired S and L by numerically solving (11). D. Simulation verification In this section, we verify the accuracy of the disclosed WFC model using our C++ based simulator. The default parameter settings are shown in Table 2 below. Each simulation run lasted for 200 seconds. In all subsequent figures, the labels "ana" and "sin", respectively, denote the theoretical and simulation results. Table 2. Parameter settings in simulation. RDATA = 54Mbps TDATA ~ 223 s LDATA = 1500 bytes TSIFS TR1 = 8R TDIFS = 50 s TR 2 =3 Ls = 20 bytes L = 52 subcarriers D.1. E( ) verification In this section, we verify the accuracy of E( ), which plays an important role in modeling the throughput. FIG. 8 plots the theoretical and simulation results of E( ) as mi varies from 1 to 50, where the theoretical result of E( ) is plotted by (10), with m = n, S = Page 15 Docket No. UM1255AUOO 40, F = 10 and L = 52. From FIG. 8, it is apparent that: 1) the sin curve matches the ana curve very well, indicating that the theoretical result of E( ) is very accurate; and 2) as n increases from I to 50, the value of E( ) only changes from I to 1.8, implying that in most cases, only 1 or 2 users will enter into the 2nd-round contention. D.2. Throughput verification In this section, we verify the system throughput and the per-user throughput, where L is set to 52. FIG. 9 plots the system throughput, where the theoretical results is plotted by (3), with m = n , S = 40 and F = 10. From FIG. 9, it is apparent that: 1) the sin curve closely match the corresponding ana curve; and 2) the system throughput is a quasi-constant regardless of how in varies (for example, as m increases from 1 to 50, the system throughput varies around 0.8). The reason why the WFC scheme can achieve a high efficiency of 0.8 (i.e. the overhead being about 0.2) is that the WFC scheme just uses two slots for two rounds of frequency-domain contentions, thus limiting the contention overheads. In contrast, CSMA/CA consume more time for the time-domain contention; the results in [1] show that the overhead of CSMA/CA is more than 30%. FIG. 10 plots the per-user throughput when F, S, n and in respectively change, where the theoretical result are computed by (2) for the plots in FIGS. 10(a)-10(d). FIG. 10(a) plots the per-user throughput when F changes from 0 to 20. In FIG. 10(a), we set S = 40 and mn = n = 10. From this figure, it is apparent that: 1) the sin curve closely matches the corresponding ana curve; 2) the per-user throughput changes dramatically with F. For example, as F increases from 0 to 10, the throughput of each HP user increases from 0.046 to 0.08 whereas the throughput of each LP user decreases from 0.035 to a low level. It is because with the increase of F, the probability that each HP user wins the channel is increased in the subcarrier range of [1, F], thus increasing the throughput. In contrast, the probability that each LP user wins the channel is decreased in the subcarrier range of [F + 1, S -1], thereby lowering the throughput. FIG. 10(b) plots the per-user throughput when S varies from 30 to 50. In this figure, we set F = 10 and in = n = 10. From FIG. 10(b), it is apparent that: 1) the sin curve closely match the corresponding ana curve; 2) the per-user throughput keeps almost unchanged with S. For example, as S increases from 30 to 50, the throughput of each HP user decreases slowly from 0.08 to 0.075, while the throughput of each LP user increases Page 16 Docket No. UM1255AUOO very slowly from 0.0005 to 0.0046. It is because increasing the value of S can lead to an increase in the probability that each LP user wins the channel, but a decrease in the probability that each HP user chooses a subcarrier from [1, F]. FIG. 10(c) plots the per-user throughput when n varies from 2 to 20. In this figure, we set i = 10, F = 10, and S = 40. From this figure, it is apparent that: 1) the sin curve closely match the corresponding ana curve; 2) the per-user throughput is almost unchanged regardless of how n changes. For example, as n increases from 2 to 20, the throughput of each HP and LP user always keep unchanged as similar to FIG. 10(b). FIG. 10(d) plots the per-user throughput when in varies from 2 to 20. In this figure, we set n = 10, F = 10, and S = 40. From this figure, we can see that: 1) the sim curve closely matches the corresponding ana curve; 2) the per-user throughput decreases with an increase of m. For example, as n increases from 2 to 20, the throughput of each HP user decreases from 0.21 to 0.04, and the throughput of each LP user also decreases from 0.04 to a lower value. D.3. Proportional fairness In this section, the accuracy of the disclosed proportional-fairness model is verified. We first present the theoretical and simulation results of the proportional ratio y in terms of F and S, respectively, where y is the ratio of the throughput of HP user to the throughput of LP user, as shown in FIG. 11. Afterwards, we verify that the WFC scheme can achieve a target y . FIG. 11(a) plots the value of y when F varies from 0 to 20, where S = 40 and in = n = 10. From this figure, we can see that as F increases from 0 to 20, the value of y increases from a very low value to 1400. It is because with an increase of F, as explained for FIG. 10(a) above, the throughput of each HP user and LP user respectively increase and decrease, therefore leading to a drastic increase of the ratio y. Similarly, FIG. 11(b) plots the value of y when S varies from 30 to 50, where F = 10. From this figure, we can see that when S increases from 30 to 50, y decreases from 150 to 16. It is because as S increases, as explained for FIG. 10(b) above, the throughput of each HP user and LP user can respectively decrease and increase, thereby leading to a drastic decrease of y. Page 17 Docket No. UM1255AUOO Table 3 below compares the target value of y with its simulation result, where we set m = n = 10. As shown in this table, given the target value of y , we can obtain a pair of F and S by (11), and then use them for obtaining the simulation value of y. It is apparent that the error between the target value and the simulation value is very small. This observation manifests that the WFC scheme can well achieve the proportional fairness. Table 3. Comparison between the target and simulation values of y. y (target) F S y (simulated) 1 0 52 1.00 2 2 49 2.05 4 4 46 4.01 8 4 33 7.98 16 6 34 15.73 32 8 34 34.91 64 10 35 65.74 128 12 36 125.57 256 14 37 246.88 E. The present invention The present invention is developed based on the WFC technique disclosed above. Although the invention has been described above in embodiments predominantly based on an example application of the invention to WLANs, the present invention is not limited only to applications for WLANs. The disclosed WFC technique is applicable for a random access communication system in which plural users communicate with an AP over a multiple-access channel. The multiple-access channel is an OFDM channel where each of the users uses OFDM to communicate with the AP. An aspect of the present invention is to provide a method for contending for access to a channel by plural users in a random access communication system. An entirety of users comprises one or more HP users and one or more LP users. The channel comprises a Page 18 Docket No. UM1255AUOO plurality of frequency-domain subcarriers. Each of the frequency-domain subcarriers is assigned with a unique subcarrier number. The method is illustrated as follows. The method comprises the steps corresponding to the RI stage as follows, for providing general and fine-grained priority differentiations among the users. When there are one or more contending users wishing to contend for the channel access among the users, each contending user that is also one of the HP users signals a request on one subcarrier substantially-randomly selected from a first subset of the frequency-domain subcarriers. In addition, each contending user that is also one of the LP users signals a request on one subcarrier substantially-randomly selected from a second subset of the frequency-domain subcarriers. Each of the first and the second subsets is a subcarrier pool as introduced in Section B above. In addition, the minimum subcarrier number among the subcarriers in the first subset is less than the minimum subcarrier number among the subcarriers in the second subset. Furthermore, the maximum subcarrier number among the subcarriers in the first subset is greater than the minimum subcarrier number among the subcarriers in the second subset, but is less than the maximum subcarrier number among the subcarriers in the second subset. One or more winners of gaining the channel access are then detennined by identifying one or more of the contending users that have transmitted the requests on the selected subcarriers whose subcarrier numbers are equal to the smallest one among all the subcarrier numbers of the selected subcarriers of the one or more contending users. When it is determined that there is only one winner, the only one winner transmits data thereof over the channel. The method further comprises the steps corresponding to the R2 stage for excluding collision in the presence of more-than-one winners. When it is determined that there are plural winners, a signature-assisted contention resolution process is performed. Exemplarily, this contention resolution process is as follows. Each winner first broadcasts over the channel a signature that is assigned to said each winner. Each winner then listens to the channel so as to identify all the plural winners by detecting presence or absence of each individual user's signature. All the winners then transmit their data in a manner of one winner by another winner in a sequential order according to a pre-agreed scheme known to all the users. In one embodiment, the random access communication system is a WLAN comprising an AP and the plural users. The pre-agreed scheme is broadcast by the AP to the users. Page 19 Docket No. UM1255AUOO In one option, the subearriers in each of the first and the second subsets are consecutively arranged in a frequency domain such that each of the first and the second subsets forms a contiguous frequency subband. It is also optional that in each of the first and the second subsets, the subcarrier numbers are consecutively assigned to the subcarriers along the frequency domain. In the embodiments disclosed herein, an AP or a user (which is a computing device in general) may be implemented using general purpose or specialized computing devices, computer processors, or electronic circuitries including but not limited to digital signal processors, application specific integrated circuits, field programmable gate arrays, and other programmable logic devices configured or programmed according to the teachings of the present disclosure. Computer instructions or software codes running in the general purpose or specialized computing devices, computer processors, or programmable logic devices can readily be prepared by practitioners skilled in the software or electronic art based on the teachings of the present disclosure. The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. Page 20

Claims (5)

1. A method for contending for access to a channel by plural users in a random access communication system, an entirety of users comprising one or more high priority (HP) users and one or more low priority (LP) users, the channel comprising a plurality of frequency-domain subcarriers, each of the frequency-domain subcarriers being assigned with a unique subearrier number, the method comprising: when there are one or more contending users wishing to contend for the channel access among the users, signaling, by each contending user that is also one of the HP users, a request on one subcarrier substantially-randomly selected from a first subset of the frequency-domain subcarriers, and signaling, by each contending user that is also one of the LP users, a request on one subcarrier substantially-randomly selected from a second subset of the frequency-domain subcarriers, wherein: the minimum subcarrier number among the subcarriers in the first subset is less than the minimum subcarrier number among the subcarriers in the second subset; and the maximum subcarrier number among the subcarriers in the first subset is greater than the minimum subcarrier number among the subcarriers in the second subset, but is less than the maximum subcarrier number among the subcarriers in the second subset; and determining one or more winners of gaining the channel access by identifying one or more of the contending users that have transmitted the requests on the selected subcarriers whose subcarrier numbers are equal to the smallest one among all the subcarrier numbers of the selected subcarriers of the one or more contending users.

2. The method of claim 1, wherein the subcarriers in each of the first and the second subsets are consecutively arranged in a frequency domain such that each of the first and the second subsets forms a contiguous frequency subband. Page 21 Docket No. UM1255AUOO

3. The method of claim 2, wherein in each of the first and the second subsets, the subcarrier numbers are consecutively assigned to the subcarriers along the frequency domain.

4. The method as set forth in any of the preceding claims, further comprising: when it is determined that there is only one winner, transmitting, by the only one winner, data thereof over the channel; and when it is determined that there are plural winners, performing: broadcasting, by each winner, a signature that is assigned to said each winner over the channel; listening, by each winner, to the channel so as to identify all the plural winners by detecting the signatures thereof; and transmitting, by all of the winners, data thereof in a manner of one winner by another winner in a sequential order according to a pre-agreed scheme known to all the users.

5. The method of claim 4, wherein the random access communication system is a wireless local area network comprising an access point and the plural users, and wherein the pre-agreed scheme is broadcast by the access point to the users. Page 22