Novel Time-Frequency Reservation Aloha Scheme for OFDMA Systems

IEEE WCNC 2011 - MAC

Elias Yaacoub1, Mohamad Adnan Al-Alaoui2, and Zaher Dawy2 1QU Wireless Innovations Center (QUWIC), Qatar Science and Technology Park, Doha, Qatar 2Department of Electrical and Computer Engineering, American University of Beirut, Beirut, Lebanon Email: [email protected]; {adnan, zaher.dawy}@aub.edu.lb

Abstract— A novel reservation Aloha scheme is presented subcarriers are grouped into a set of subchannels, where for OFDMA systems. The proposed scheme is based on two each subchannel consists of a fixed number of consecutive dimensional reservation in time and frequency. The proposed subcarriers [3], [13]. approach is compared to other OFDMA extensions of reservation Aloha. The proposed scheme is shown to be superior in Slotted Aloha over OFDM was first proposed in [3]. terms of increasing sum-rate, reducing the number of users in In [8], a backoff scheme for multichannel slotted Aloha is outage, and reducing the collision probability in the reservation presented, where backoff is done on different subchannels phase. rather than on a different time slot on a single subchannel. Index Terms— OFDMA, reservation Aloha, opportunistic In [9], a class of multichannel MAC schemes based on the transmission, probabilistic scheduling RTS/CTS (Ready-To-Send/Clear-To-Send) dialogue and on Aloha contention resolution are studied. The studied class uses one subchannel, the control subchannel, for transmission I. INTRODUCTION of the RTS/CTS dialogues, while the other subchannels are Aloha is one of the first algorithms for random access [1], assigned to data transmissions. The RTS packets contend on [2]. Aloha is commonly studied under a single channel the right to use one of the data subchannels and the winner assumption, in which user contention happens over only in a contention is reserved the right to use one of the data one channel. In single channel slotted Aloha, users transmit subchannels to transmit its data packet in a collision free packets in fixed length time slots. When more than one manner. It was shown in [9] that for a fixed total bandwidth user transmit in the same time slot, collision occurs [3]. scenario, although the collisions are reduced, the sum-rate To reduce the impact of collisions, reservation Aloha was of the multichannel MAC schemes is inferior to that of the proposed [1]. In reservation Aloha, the fixed length time corresponding single channel MAC scheme, which sends slots (as in slotted Aloha) are preceded by small reservation the RTS/CTS packets and data packets on a single shared request slots. Requests are transmitted in the minislots using channel. To mitigate this drawback, channel sensing was the slotted Aloha random access technique. Hence, collisions adopted in [14], where carrier sense multiple access with occur in the reservation phase, but not in the transmission collision avoidance (CSMA/CA) over OFDMA is presented. phase. No slotted reservation is available in [14], but a novel backoff Reservation Aloha is used in satellite networks, e.g., [4], mechanism over transmission slots is studied. and in wireless local area networks (WLANs), e.g., [5]. In [11], a reservation Aloha scheme for OFDMA is pre- Priority reservation Aloha over a single channel for appli- sented. In the contention period, users compete for subcar- cations in vehicle-to-vehicle communication (e.g., audio and riers, and a user reserves the subcarrier during the whole video streaming are given higher priorities) is investigated transmission time until the next contention period. Users in [6]. Classical Aloha (without reservation) is recently being access the channel and are aware of their channel state investigated in the context of underwater acoustic sensor information (CSI). No capture effects are considered. With networks [7]. capture effects, even if collisions occur in the contention In state-of-the-art and next generation wireless commu- period, the base station (BS) may be able to detect one of nications systems, orthogonal frequency division multiple the contending users and allow it to use the corresponding access (OFDMA) is adopted as the accessing scheme, e.g., in transmission slot. In [15], users are not aware of their CSI. the UMTS long term evolution (LTE) and WiMAX. Several Capture effect is used to resolve collisions: users with better extensions of Aloha, slotted Aloha, and reservation Aloha channels get to access the channel and hence enhancements over OFDMA have been presented in the literature [3], in reservation Aloha are obtained due to exploiting multiuser [8], [9], [10], [11], [12]. In OFDMA, a set of orthogonal diversity. In [10], a reservation scheme for reservation Aloha with CSI is presented. Each user reserves the subchannel for the whole transmission time of a frame, and collision This work was made possible by a NPRP grant from the Qatar National Research Fund (a member of The Qatar Foundation). The statements made resolution (capture effect) is assumed. In [12], a Markov herein are solely the responsibility of the authors. model of the wireless channel is adopted: users transmit on

978-1-61284-254-7/11/$26.00 ©2011 IEEE 7 a subchannel in good state using slotted Aloha, and do not transmit on a subchannel in the bad state. In this paper, a novel approach for reservation Aloha over OFDMA is presented. Reservation is performed on T T a frequency-time grid, as opposed to classical reservation (NTTI -2)T Aloha where time slots are reserved over the whole band- (a) TDMA/OFDMA Aloha width [1], [6], and as opposed to the extensions of reservation Aloha to OFDMA [9], [10], [11], [16], where a subchannel is reserved over the whole transmission period of the frame. We consider an uplink scenario where a group of users are T T communicating with a single receiver, such as an access point (NTTI -2)T (AP) in a WLAN or a base station (BS) in a cellular network. (b) FDMA/OFDMA Aloha Hence, the terms BS and AP are used interchangeably in this paper. The proposed approach subdivides the transmission frame into several time-frequency slots, and users contend for a particular transmission slot over a certain subchannel. T T (N -2)T Hence, in a given Aloha frame, several users might transmit TTI on the same subchannel but at different time slots, and several users might transmit at the same time over different subchannels. The proposed scheme is compared to other schemes in relatively high load scenarios and is shown to lead to significant improvements. Pilot Measurement and Slot Reservation Ack Transmission The paper is organized as follows. The proposed approach Channel Estimation and the system model are presented in Section II. The (c) Proposed OFDMA Aloha simulation results are presented and discussed in Section III, where the proposed method is compared to other schemes in the literature. Finally, in Section IV, conclusions are drawn Fig. 1. The different types of OFDMA reservation Aloha. and extensions for future research are described. sequentially through its subchannels, sorted in decreas- II. PROPOSED APPROACH ing order of CSI. It decides to transmit over a subchannel In classical reservation Aloha, the reservation is over with a probability 𝑝𝑇 . If it decides to transmit, it transmission slots in the time domain. All the bandwidth randomly selects one of the 𝑁TTI − 2 small reservation is used for transmission, e.g., [1] and [17]. We will refer slots over that subchannel and transmits a reservation to this scheme, when applied in an OFDMA system, by signal in that slot. TDMA/OFDMA Aloha. It is shown in Fig. 1 (a). With ∙ The user estimates its achieved rate on the selected slot. OFDMA being widely used in state-of-the-art wireless com- If it is not sufficient to achieve its target rate, it moves munications systems, reservations are made over subchan- to the next subchannel and repeats the same operation. nels, and a single user is allowed to transmit over a reserved When it goes through all subcarriers without achieving subchannel until the next frame where a new reservation is its target rate, it moves back to the first subcarrier and performed, e.g., [10], [11], [12], and [16] . We will refer repeats the process, until it achieves its target rate or to this scheme by FDMA/OFDMA Aloha. It is shown in until a maximum number of allowed slots is reserved. Fig. 1 (b). The proposed scheme is a novel method to ∙ At the end of the reservation phase, the BS transmits an perform reservation slotted Aloha over OFDMA. It consists Ack message containing 𝑁sub × (𝑁TTI − 2) bits, repre- of allowing users to compete over transmission slots, or senting the reservation slots over all subchannels, with transmission time intervals (TTIs), over all the available 𝑁sub the number of subchannels. When a reservation subchannels. It is shown in Fig. 1 (c). Each frame of duration was successfully made on a given TTI over a certain 𝑁TTI is subdivided into three phases: a pilot transmission subchannel, the corresponding bit is set to 1. When a and channel estimation phase of duration 1 TTI, a reservation collision has occurred during the reservation phase, or phase of duration 1 TTI, and a transmission phase of duration when no reservation was made, the bit will be set to 0. 𝑁TTI − 2, with each TTI having a duration 𝑇 . The approach Hence, if a user has attempted to reserve a slot and can be described as follows: found a 1 in the corresponding bit in the Ack message, ∙ The BS transmits a pilot signal over the available sub- it knows that the slot was successfully reserved. If, on channels. Each user measures the received pilot power the other hand, it finds a 0, it knows that a collision has and estimates its CSI over each subchannel. occurred and hence it refrains from transmission on that ∙ Each user sorts its subchannels in decreasing order of slot. CSI. With the proposed approach, collisions occur only in the ∙ In the reservation phase, there are 𝑁TTI − 2 small reservation phase, but not in the transmission phase, which reservation slots over each subchannel. Each user goes leads to avoiding unnecessary transmissions. It should be

8 𝑑 noted that the proposed approach can be easily modified to where 𝑅𝑘,𝑖 is the discrete rate of user 𝑘 over subcarrier 𝑖. accommodate collision resolution and capture effect. In fact, Conversely to continuous rates, which can take any non- the BS can transmit, in the Ack message, the ID of the user negative real value according to the Shannon capacity for- who successfully reserved each slot, and a zero otherwise. mula log2(1+𝛾𝑘,𝑖), discrete rates represent the quantized bit Hence, if more than one user competed over the same slot, rates achievable in a practical system as follows: they will know which one won the competition. However, ⎧ 𝑟 ,𝜂≤ 𝛾 <𝜂 without capture effect, the Ack message can be significantly  0 0 𝑘,𝑖 1  shortened since only one bit is needed for each slot. The pilot ⎨𝑟1,𝜂1 ≤ 𝛾𝑘,𝑖 <𝜂2 𝑑 signal transmitted by the BS at the beginning of each frame 𝑅 (𝛾 )= 𝑟2,𝜂2 ≤ 𝛾𝑘,𝑖 <𝜂3 𝑘,𝑖 𝑘,𝑖  (4) allows the users to keep their synchronization with the BS. . . . . ⎩ 𝑟𝐿−1,𝜂𝐿−1 ≤ 𝛾𝑘,𝑖 <𝜂𝐿 A. System Model where 𝜂𝑙 represents the SNR target in order to achieve the We consider a scenario with a single BS or AP and users rate 𝑟𝑙 with a predefined BER. Note that in the limit, we have competing for resources to communicate with that BS by 𝑟0 =0, 𝜂0 =0, and 𝜂𝐿 = ∞. Consequently, the sum-rate of using the proposed Aloha scheme. Users are assumed to the system is given by: always have data to transmit as in [12]. Each user needs to ∑𝐾 ∑𝑁 satisfy a target average data rate 𝑅𝑇 . If the average data rate 𝑅 = 𝑅𝑑 (𝛾 ) is not achieved by a user after a certain number of frames tot 𝑘,𝑖 𝑘,𝑖 (5) 𝑘=1 𝑖=1 𝑁frames, the user is assumed to be in outage. Users regulate their transmissions in order to achieve 𝑅𝑇 after 𝑁frames.This is performed as follows: The number of bits that should III. RESULTS AND DISCUSSION be transmitted in order to achieve 𝑅𝑇 after 𝑁frames is This section presents the simulation results obtained by given by 𝑁𝑏,𝑇 = 𝑅𝑇 ⋅ 𝑁frames ⋅ 𝑁TTI ⋅ 𝑇 . Denoting comparing the proposed approach to other schemes in the by 𝑛𝐹 the number of the current frame in a window of literature, in terms of sum-rate, percentage of users in outage, 𝑁 𝑁 length frames, and by 𝑏,𝑛𝑓 the number of bits transmitted and collision probability. 𝑛 in frame 𝑓 , the number∑ of previously transmitted bits is 𝑁 (𝑝) = 𝑛𝐹 −1 𝑁 expressed as 𝑏,𝑛 𝑛 =1 𝑏,𝑛𝑓 . Consequently, a user 𝐹 𝑓 A. Simulation Model makes enough reservations in frame 𝑛𝐹 in order to transmit (𝑝) 𝑁𝑏,𝑇 − 𝑁 The simulation model consists of a single BS or AP with 𝑁 = 𝑏,𝑛𝐹 𝑏,𝑛𝐹 bits. Hence, a user attempts 𝑁frames − (𝑛𝐹 − 1) users uniformly distributed within its coverage area. Each (𝑝) frame consists of 𝑁TTI =10TTIs, i.e., eight TTIs are used to subdivide the remaining (𝑁𝑏,𝑇 − 𝑁𝑏,𝑛 ) bits equally over 𝐹 for transmission. Each user attempts to achieve its target the remaining 𝑁frames − (𝑛𝐹 − 1) frames. rate, and it is considered in outage if it fails to achieve that rate after 𝑁frames = 100 frames. The duration of a TTI is B. Throughput Calculations considered to be 1 msec, sufficient to transmit 12 symbols over each subcarrier [18]. The results are averaged over 𝐾 We consider a single cell uplink OFDMA system with 50 iterations of 100 frames each. We use 𝑝𝑇 =0.5, unless 𝑁 𝑘 users and subcarriers to be allocated. For each user and otherwise specified. 𝑖 subcarrier , the transmit power, channel gain, and total noise The total bandwidth considered is 𝐵 =5MHz, subdivided 𝑃 𝐻 𝜎2 power are respectively denoted by 𝑘,𝑖, 𝑘,𝑖, and 𝑘,𝑖.The into 25 subchannels of 12 subcarriers each [13]. The max- signal-to-noise ratio (SNR) is given by imum mobile transmit power is considered to be 125 mW. 𝑃 ⋅ 𝐻 All mobiles are assumed to transmit at the maximum power, 𝛾 = 𝑘,𝑖 𝑘,𝑖 𝑘,𝑖 𝜎2 k=1,...K;i=1,..., N (1) and the power is subdivided equally among all subcarriers 𝑘,𝑖 allocated to the mobile. The channel gain over subcarrier 𝑖 The peak power constraint of user 𝑘 is given by: corresponding to user 𝑘 is given by:

∑𝑁 𝐻𝑘,𝑖,dB =(−𝜅 − 𝜆 log10 𝑑𝑘) − 𝜉𝑘,𝑖 +10log10 𝐹𝑘,𝑖 (6) 𝑃𝑘,𝑖 ≤ 𝑃𝑘,max k=1,..., K (2) 𝜅 𝑖=1 In (6), the ﬁrst factor captures propagation loss, with a constant chosen to be 128.1 dB, 𝑑𝑘 the distance in km This means that the power spent by the user over all its from mobile 𝑘 to the BS, and 𝜆 the path loss exponent, allocated subcarriers should be lower than its maximum which is set to a value of 3.76. The second factor, 𝜉𝑘,𝑖, transmission power 𝑃𝑘,max. captures log-normal shadowing with zero-mean and an 8 dB Total rate of user 𝑘 is deﬁned as follows: standard deviation, whereas the last factor, 𝐹𝑘,𝑖, corresponds 𝑎 ∑𝑁 to Rayleigh fading with a Rayleigh parameter such that 𝑑 𝐸[𝑎2]=1. The SNR thresholds of the various modulation 𝑅𝑘 = 𝑅𝑘,𝑖(𝛾𝑘,𝑖) (3) 𝑖=1 and coding schemes obtained from [19] are shown in Table I.

9 5 Proposed − 64k FDMA/OFDMA Aloha − 64k 4.5 TDMA/OFDMA Aloha − 64k Proposed − 128k 4 FDMA/OFDMA Aloha − 128k TDMA/OFDMA Aloha − 128k Proposed − 256k 3.5 FDMA/OFDMA Aloha − 256k TDMA/OFDMA Aloha − 256k 3

2.5

2 Sum−Rate (Mbps)

1.5

0.5

0 0 5 10 15 20 25 30 Number of Users

Fig. 2. Sum-rate of the different schemes for a cell radius of 500 m and different target rates.

100

90 Proposed − 64k FDMA/OFDMA Aloha − 64k 80 TDMA/OFDMA Aloha − 64k Proposed − 128k 70 FDMA/OFDMA Aloha − 128k TDMA/OFDMA Aloha − 128k 60 Proposed − 256k FDMA/OFDMA Aloha − 256k TDMA/OFDMA Aloha − 256k 50

30 Percentage of Users in Outage

0 0 5 10 15 20 25 30 Number of Users

Fig. 3. Percentage of users in outage for a cell radius of 500 m and different target rates.

B. Results with Different Target Rates reservation Aloha were made in [17]. Attempts to enhance the stability of multichannel Aloha were presented in [3]. In this section, we present simulation results for a cell radius of 500 meters and three different target rates: 64 kbps, 128 kbps, and 256 kbps. Fig. 2 shows the sum-rate results, Using the proposed scheme with 𝑅𝑇 =64kbps, all the Fig. 3 shows the percentage of users in outage, and Fig. 4 users are served even when the number of users reaches 30. displays the collision probability results. For a small number With 𝑅𝑇 = 128 kbps, degradation occurs when the number of users (up to four users), all schemes perform comparably of users exceeds 25, and with 𝑅𝑇 = 256 kbps, stability is and all users achieve their target data rate. When the number lost when the number of users exceeds 20. In fact, when of users increases, the TDMA/OFDMA and FDMA/OFDMA the number of users and/or the target data rate increases, reservation Aloha schemes degrade signiﬁcantly and lose more packets need to be transmitted at the same time in their stability. This is not the case for the proposed scheme order to achieve the target data rate for all users. This leads which shows better stability when the number of users to an increase in collision probability during the reservation increases. In fact, it is widely known in the literature that phase, as shown in Fig. 4. Consequently, less transmissions Aloha is an unstable algorithm. In [3], it was shown that occur during the transmission phase which leads to a sum- this unstability property also applies to OFDMA Aloha. rate degradation, as shown in Fig. 2, and hence the number Attempts to enhance the stability of single channel slotted and of users in outage increases, as shown in Fig. 3.

10 1

0.9 Proposed − 64k 0.8 FDMA/OFDMA Aloha − 64k TDMA/OFDMA Aloha − 64k Proposed − 128k 0.7 FDMA/OFDMA Aloha − 128k TDMA/OFDMA Aloha − 128k 0.6 Proposed − 256k FDMA/OFDMA Aloha − 256k 0.5 TDMA/OFDMA Aloha − 256k

0.4 Collision Probability 0.3

0.2

0.1

0 0 5 10 15 20 25 30 Number of Users Fig. 4. Collision probability of the different schemes for a cell radius of 500 m and different target rates.

TABLE I DISCRETE RATES AND SNR THRESHOLDS WITH 14 MODULATION AND 4

CODING SCHEMES [19]. Proposed − 250m 3.5 FDMA/OFDMA Aloha − 250m TDMA/OFDMA Aloha − 250m Proposed − 500m MCS 𝑟𝑙 (bits) 𝜂𝑙 (dB) 3 FDMA/OFDMA Aloha − 500m No Transmission 0 -∞ TDMA/OFDMA Aloha − 500m 2.5 Proposed − 1000m QPSK, R = 1/8 0.25 -5.5 FDMA/OFDMA Aloha − 1000m QPSK, R = 1/5 0.4 -3.5 TDMA/OFDMA Aloha − 1000m QPSK, R = 1/4 0.5 -2.2 2 QPSK, R = 1/3 0.6667 -1.0 QPSK, R = 1/2 1.0 1.3 Sum−Rate (Mbps) 1.5 QPSK, R = 2/3 1.333 3.4 QPSK, R = 4/5 1.6 5.2 16-QAM, R = 1/2 2.0 7.0 1 16-QAM, R = 2/3 2.6667 10.5 16-QAM, R = 4/5 3.2 11.5 0.5 64-QAM, R = 2/3 4.0 14.0 64-QAM, R = 3/4 4.5 16.0 0 64-QAM, R = 4/5 4.8 17.0 0 5 10 15 20 25 30 Number of Users 64-QAM, R = 1 (uncoded) 6.0 26.8

Fig. 5. Sum-rate of the different schemes for a target rate of 128 kbps and different values of the cell radius. C. Results with Different Cell Radii D. Results with Different Transmission Probabilities

In this section, we present simulation results for a target In this section, we consider a cell radius of 500 m and a rate of 128 kbps and three different values for the cell radius: target rate of 128 kbps. We study the impact of varying the 250 m, 500 m, and 1000 m. Fig. 5 shows the sum-rate results. transmission probability. The cases with 𝑝𝑇 =0.2, 𝑝𝑇 =0.5, Clearly, when the distance increases, the SNR received at and 𝑝𝑇 =0.7 are considered. Furthermore, we investigate a the AP or BS is significantly reduced. This decreases the dynamic scheme with an adaptive transmission probability. In achievable rate within a reserved time slot. However, the the dynamic scheme, the transmission probability over sub- proposed scheme shows significant superiority over the other channel 𝑖 is set to 𝑝𝑇 = 𝑝𝑇,0/rank(𝑖), with 𝑝𝑇,0 a constant schemes. For a relatively small number of users, the target and rank(𝑖) the rank of subchannel 𝑖 indicating its position rate can be achieved even when the distance increases. In fact, after sorting the subchannels in decreasing order of channel enough time slots are available with the proposed scheme to gain. Hence, when a subchannel has a high channel gain for a compensate the increase in the distance. When the number given user, the chances of transmitting over that subchannel of users increases, each user will need a higher number of increase, and the chances of transmitting over subchannels time slots to compensate the increased distance. This will with bad channel gains are reduced. We use 𝑝𝑇,0 =0.9 to lead to more collisions in the reservation phase. Hence, less increase the probability of selecting the subchannels having transmissions will occur in the transmission phase, which the best channel gain. Fig. 6 shows the collision probability reduces the achieved rate, as shown in Fig. 5, and leads to results. For fixed 𝑝𝑇 , reservations become more aggressive an increase in the number of users in outage. as 𝑝𝑇 increases, which leads to a slight increase in collision

11 the other schemes with capture effect. Furthermore, inves- 1 tigating certain techniques to ensure a better stability of 0.9 p = 0.2 T the proposed scheme is an important topic to study. Other p = 0.5 0.8 T extensions include the application of the proposed approach p = 0.7 0.7 T to various service types (e.g., video transmission), different Dynamic packet arrival rates (Poisson arrival for example, instead of 0.6 assuming there is always data to transmit), etc. 0.5

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