Link-adaptation and transmit power control for unicast and multicast in

Link-adaptation and Transmit Power Control for Unicast and Multicast in IEEE 802.11a/h/e WLANs António Grilo1, Mário Nunes2 INESC/IST, R. Alves Redol, Nº9, 1000 Lisboa, Portugal 1 [email protected] 2 [email protected] Abstract This paper proposes a mechanism to combine linkadaptation and transmit power control (TPC) in IEEE 802.11a WLANs. The link-adaptation and TPC algorithms try to maximize the goodput of the WLAN, while minimizing transmit power. The algorithm runs at the sender, but it relies on feedback from the receiver. The latter notifies the sender about the Signal-toInterference-plus-Noise-Ratio experienced in previous transmissions, allowing a prediction of the channel status. An extension of the algorithm for multicast is also presented. The implementation of the proposed mechanism in IEEE 802.11a/h/e WLANs is discussed and its performance evaluated.

1. Introduction Communication in WLANs is subject to variable Signal-to-Interference-plus-Noise-Ratio (SINR) due to fading, path loss and interference phenomena. In WLANs that are able to use more than one coded modulation scheme, error resilience can be improved at the expense of bitrate reduction. This process is called link-adaptation and consists on changing bandwidth efficiency (bits/symbol) by adapting the modulation and code rate, taking into account that transmission is performed at a fixed symbol rate. In particular, the IEEE802.11a [1] physical (PHY) layer specification for 5 GHz WLANs defines 8 different bitrates achieved with different combinations of modulation and convolutional code rate. Link-adaptation can greatly improve the goodput of the WLAN by selecting the most efficient coded modulation scheme in each frame transmission, if the SINR experienced at the receiver is taken into account [2]. Transmit power control (TPC) is an important mechanism to minimize co-channel interference, to avoid unfairness due to the near-far effect and to save energy in battery-powered wireless stations (WSTAs). For IEEE 802.11a WLANs in particular, a special task group (IEEE 802.11h) was created in order to define the transmit power control protocol and associated dynamic frequency selection (DFS) mechanisms.

When combined with link-adaptation, TPC can decrease the transmit power level to the minimum required for transmission with the chosen transmit mode, while guaranteeing an error probability low enough to achieve the target goodput. This is particularly suitable for use in combination with polling based medium access mechanisms like the IEEE 802.11 Point Coordination Function (PCF) [3] and the polling based access of the upcoming IEEE 802.11e Hybrid Coordination Function (HCF). In contention based access, the use of different power levels by different stations may cause inefficiency due to the hidden terminal problem [4], though this problem can be overcome in IEEE 802.11 by transmitting the RTS/CTS frames at maximum power. This paper proposes a new SINR-based Mode-Power Adaptation (SMPA) mechanism. An innovative aspect of the proposed scheme is that it also supports mode-power adaptation in multicast data transmission. The performance of the proposed scheme in IEEE 802.11a WLANs with IEEE 802.11e QoS extensions is evaluated and compared with other link-adaptation schemes. The rest of this paper is organised as follows. Section 2 presents the related work that preceded this study. Section 3 presents an overview of the IEEE 802.11a standard. The SMPA algorithm for unicast is presented in section 4, followed by the multicast version in section 5. Some guidelines for the implementation of SMPA are presented in section 6. Section 7 presents an analysis of the performance of SMPA, which is followed by the simulation results in section 8. Finally, section 9 presents the main conclusions of this study.

2. Related work The Receiver-Based AutoRate (RBAR) protocol is proposed in [5]. RBAR is a SINR based link-adaptation protocol for IEEE 802.11 DCF networks and includes no TPC mechanism. RBAR is compared with Automatic Rate Fallback (ARF) [6], which is an ARQ based linkadaptation mechanism used in WaveLan products from Lucent. RBAR presents better performance than ARF, as it is able to react more quickly to changes in channel conditions due to fast fading effects. Nevertheless it is totally controlled by the receiver. The latter informs the

Proceedings of the 28th Annual IEEE International Conference on Local Computer Networks (LCN’03) 0742-1303/03 $ 17.00 © 2003 IEEE

estimates are more efficient than the simpler ARF algorithm, this paper compares SMPA and ARF assuming an error margin in CSI estimates.

3. The IEEE 802.11a PHY The IEEE 802.11a PHY layer is defined for operation in the 5 GHz U-NII band and is based on OFDM. It specifies 8 different transmit modes obtained with different combinations of modulation and convolutional code rate. Each transmit mode corresponds to a different bitrate. The IEEE 802.11a transmit modes are listed in Table 1, together with the respective number of bytes transmitted in one OFDM symbol (Bytes-per-Symbol – BpS). Table 1. IEEE 802.11a PHY modes. Mode

Modulation

1 2 3

BPSK BPSK QPSK QPSK 16-QAM 16-QAM 64-QAM 64-QAM

4 5 6 7 8

Code Rate 1/2 3/4 1/2

Bitrate

BpS

6 Mbps 9 Mbps 12 Mbps

3 4.5 6

3/4 1/2 3/4 2/3 3/4

18 Mbps 24 Mbps 36 Mbps 48 Mbps 54 Mbps

9 12 18 24 27

In general, error resilience decreases as the bitrate increases. The goodput that can be achieved with each of these transmit modes varies with the size of the packet and the status of the channel given by the SINR figure. Figure 1 depicts Goodput as a function of SINR for 2304octet packets according to the model presented in [3].

Goodput (bps)

sender about the required transmit mode during the RTS/CTS phase of frame transmission, which prevents the sender from combining rate adaptation and transmit power control in a flexible way. Besides, the protocol is not suitable for multicast link-adaptation, where the RTS/CTS mechanism is not used. A Joint Link-Adaptation and transmit Power control (JLAP) algorithm for HIPERLAN/2 is proposed in [7]. JLAP tries to maximize goodput while transmitting at the minimum possible transmit power. JLAP selects the mode-power pair based on a packet error rate estimate, the length of the unused part of the MAC frame and buffer occupancy. If the unused part is smaller than the lower threshold and buffers grow, this means that the capacity is too low and throughput must be increased, which is done by increasing transmit power. If the unused part is greater than the higher threshold, JLAP decreases transmit power. JLAP always chooses the PHY mode that achieves higher goodput at the chosen transmit power. A problem arises when the capacity of the cell is exceeded. In this case JLAP always chooses maximum transmit power, which increases interference with other cells, forcing these to increase their own transmit power levels. This problem is not solved in the referred paper and is relegated to future work. Moreover, reliance on the packet error rate presents additional disadvantages. A significant number of transmission attempts in order to infer a good packet error rate estimate (e.g., a minimum of 100 transmission attempts is needed to detect a packet error rate of 1u10-2). This can be too slow to cope with fast fading effects. The introduction of the issue of energy-efficient IEEE 802.11 PCF operation in IEEE 802.11a WLANs was presented in [3], which provides the basis for the present work. This paper also presents an analytical method to evaluate goodput and energy performance in PCF. While some interesting implementation guidelines are provided, no protocol is specified in detail and thus the impact of the feedback overhead is not evaluated. Moreover, multicast is not addressed. The present study aims to overcome the main handicaps of the previous studies in the following ways. Unlike RBAR, it places the control of the algorithm at the sender, while defining an efficient feedback protocol that allows the receiver to notify the sender with channel status information (CSI). While mode-power selection is based on the general guidelines provided in [3], it is defined in more detail in the present study, which also considers the impact of CSI feedback protocol on the performance. Moreover, the algorithm is extended to deal with multicast issues, which were not addressed in the previous studies. With the exception of JLAP, the previous studies assume perfect received power and SINR estimates, which is usually not the case. While the simulation results in [5] show that solutions based on CSI

4.50E+07 4.00E+07 3.50E+07 3.00E+07 2.50E+07 2.00E+07 1.50E+07 1.00E+07 5.00E+06 0.00E+00 0

5

10

15

20

25

C/(N+I) (dB)

BPSK 1/2 (6 Mbps) QPSK 3/4 (18 Mbps) 64-QAM 2/3 (48 Mbps)

BPSK 3/4 (9 Mbps) 16-QAM 1/2 (24 Mbps) 64-QAM 3/4 (54 Mbps)

QPSK 1/2 (12 Mbps) 16-QAM 3/4 (36 Mbps)

Figure 1. Goodput as a function of SINR for 2304-octet packets in IEEE 802.11a PCF.

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30

4. Link-adaptation and TPC algorithm for unicast This section describes the proposed SMPA scheme. All calculations are assumed to be in dB units. The algorithm runs at the sender and chooses the mode-power pair that maximizes goodput for each QoS-DATA frame, where transmitted power (Pt) is constrained to the interval [Pt_min, Pt_max]. In order to perform its selection, the sender must have an estimate of the channel quality experienced at the receiver in previous transmissions. The channel status information (CSI) has the following three components: x SINRold: Instantaneous SINR that the receiver has experienced when the sender transmitted a frame with power Ptold. x LMold: Link Margin, or ratio of the received signal power to the minimum required for the reception of a frame without errors. The estimate is based on the received signal power experienced when the sender transmitted a frame with power Ptold. x Ptold: Transmit power of the packet that resulted on the SINRold and LMold estimates performed at the receiver. The SINRold can be estimated at the receiver based on the received signal power (Prold) and the co-channelinterference-plus-noise (CCI+N) power. A method to obtain these estimates in IEEE 802.16 networks is provided in [8]. Another method is presented in [9] in the context of cellular mobile radio networks. The LMold estimate must take into account both the minimum received power (or receiver sensitivity – RS) and minimum SINR (SINR_min) required for the reception of a frame without errors. LMold can thus be computed at the receiver as: LM old

MIN (Pr old  RS , SINR old  SINR _ min)

(1)

In order to transmit a data frame of length L, the sender starts by choosing the PHY transmit mode based on a threshold scheme similar to those presented in [9] and [5]. The objective is to maximize the physical bitrate, minimizing the frame transmission time and thus reducing the transmit energy. Taking into account the goodput vs SINR relationship depicted in Figure 1, a SINR threshold can be defined for each transmit mode k (SMTk), which is the SINR value above which transmit mode k achieves higher goodput than the previous mode. For instance, for mode 8 of IEEE 802.11a, 64-QAM ¾, SMT8 |23.5 dB. Note that SMTk may be a function of packet size. The sender tries to transmit at the maximum possible bitrate and thus assumes at first that transmission is done at Pt_max. Based on SINRold, he calculates SINR_maxold,

which is the SINR value that would be experienced by the receiver if Ptold=Pt_max. SINR_maxold can be calculated as follows: SINR _ max old

SINR old  ( Pt _ max  Pt old )

(2)

The transmit mode is then selected. Let M1,…,MN represent the set of transmit modes in decreasing order of maximum goodput, as follows: M1 Mi MN

if SINR_maxold t SMT1 if SMTi+1> SINR_maxold t SMTi otherwise

After selecting transmit mode m that maximizes goodput, the sender estimates the minimum transmit power (Pt) required for the reception of the frame without errors. Pt must result in at least SPTm at the receiver and is subject to the Link Margin constraint relative to Ptold: Pt

MAX ( Pt _ max (SINR _ maxold  SPTm ),

Pt old  LM old )

(3)

SPTm is the SINR threshold used to calculate transmit power for mode m. It indicates the SINR value above which the goodput for mode m approaches its maximum. For instance, for mode 8 of IEEE 802.11a, 64-QAM ¾, SPT8 |24 dB. As SINRold and LMold are calculated at the receiver, a feedback mechanism must exist in the form of a tuple. A problem arises if the frame reception fails due to an abrupt change in channel status, as the receiver is unable to recalculate and/or identify the sender. The recovery is achieved by setting transmit power at Pt_max and lowering the bitrate upon detection of a failed transmission (e.g. detection of a missing ACK frame). The bitrate decrease continues until a frame is successfully received. In order to speed up the process, after a failed transmission at transmit mode Mm, the algorithm selects the new value for m as m ªm / 2º . After a transmission succeeds, the receiver estimates the new tuple and provides it to the sender, allowing again the selection of the most suitable mode-power pair for the current channel conditions. In order to keep the tuple updated at the sender while there is no data being transmitted, special SINR_REPORT and SINR_REPORT_ACK frames can be used. These should be limited to a minimum in order to minimize overhead. In this way, a receiver should only send a SINR_REPORT frame after detection of a significant change in channel quality. This detection is performed by

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calculating the optimal transmit mode for the received packet taking into account its SINR estimate, and comparing it with the optimal transmit mode taking into account the SINR estimate in the previous packet reception. If these are different, the receiver updates the tuple at the sender by means of a SINR_REPORT frame. In order to maximize its probability of reception, the SINR_REPORT is transmitted at the lowest bitrate and maximum transmit power. It is acknowledged by a SINR_REPORT_ACK frame carrying the tuple relative to communication in the opposite direction. The number of SINR_REPORT retransmissions should be limited in order to minimize overhead and power consumption.

5. Link-adaptation and TPC for multicast hierarchical data A special case of SMPA occurs in multicast scenarios where the sender is transmitting packets to several receivers in simultaneous. The latter may experience different channel quality depending on the location and/or speed, which introduces the problem of choosing the mode-power pair that better satisfies the group of users. A possible solution is to select the mode-power pair that satisfies the worst receiver, assuring good reception by all receivers. Nevertheless this can be very wasteful of resources if the common bitrate is too low, causing long packet transmission times. Any other solution will necessarily cause packets not to be received by all users. A compromise solution can be achieved if the data has a hierarchical structure. A good example is an MPEG4encoded video stream with space or temporal scalability. Space scalability allows the decoder to treat a subset of streams produced by the coder to rebuild and display textures, images and objects with a reduced space resolution. For textures, a maximum of 11 levels of space scalability is supported, while a maximum of 3 scalability levels is supported for the video sequence [10]. In [11] an MPEG4 video stream composed by a base layer substream and two enhanced layer sub-streams is transmitted over IP Differentiated Services using an Assured Forwarding service class [12], characterised by 3 different drop precedence levels. Within a service class, the drop precedence indicates which packets should be dropped first in case of congestion, queue overflow, etc. Drop precedence is thus related to the relative importance of the packet, which is higher for low drop precedence. Packets that belong to the base layer are assigned low drop precedence, as they are essential to decode the overall MPEG4 stream. Enhancement layers are assigned higher drop precedence. In the proposed scheme, besides its traditional IP-layer role, drop precedence is also used for mode-power selection in multicast. Packets with low

drop precedence (DP1) should be received by all stations and thus their transmit mode is selected as for unicast, taking into account the worst receiver of the multicast stream, i.e. the receiver that presents the lowest SINR_maxold. On the other hand, for packets with the highest drop precedence (DP3), the transmit mode is selected taking into account the highest SINR_maxold found among all the receivers of the multicast stream. A decision must be taken regarding the middle drop precedence (DP2). The proposed scheme selects the DP2 transmit mode by maximizing the mean packet rate m ( MPR ), in packets-per-second, given by the expression:

MPR m

(1  Pem ( L)) / T m ( L)

(4)

where Pem (L) is the average packet error rate at mode m taking into account all receivers of the multicast stream, and Tm(L) is the transmit time of a packet of length L with transmit mode m. After selecting the transmit mode m, the chosen multicast transmission power is equal to the maximum of the unicast transmission powers for mode m, taking into account the potential receivers for that mode. This is expressed in the following expression: j  Re ceivers : SINR _ max old t SMTm , j m Pt mcast

MAX ( Pt mj )

(5)

In order to prevent ACK frame explosions and the requirement to retransmit different packets to different users, multicast data is usually transmitted in sequence with no acknowledgements and ARQ. Consequently, the tuple must be sent by the receivers in special SINR_REPORT frames, which should be limited to a minimum in order to minimize overhead. In this way, a receiver should only send a SINR_REPORT frame after detection of a significant change in channel quality. This detection is performed by calculating the optimal transmit mode for the received packet taking into account its SINR estimate, and comparing it with the optimal transmit mode taking into account the SINR estimate in the previous packet reception. If these are different, the receiver updates the tuple at the sender by means of a SINR_REPORT frame. In order to maximize its probability of reception, the SINR_REPORT is transmitted at the lowest bitrate and maximum transmit power. It is acknowledged by a SINR_REPORT_ACK frame carrying the tuple relative to communication in the opposite direction. The number of SINR_REPORT retransmissions

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should be limited in order to minimize overhead and power consumption. In case the channel quality of a receiver drops below the minimum to allow reception of DP1 packets, the sender is unable to detect this because, as already mentioned, there is no ARQ mechanism in multicast. In this situation, receivers must rely on Beacon frames in order to detect the channel quality drop. Of course this may cause a higher latency in mode-power adaptation. As a final note, SINR_REPORT and SINR_REPORT_ACK frames can also be used in the unicast scenario in order to keep the channel status information updated at the sender while there is no data being transmitted.

6. Implementation in IEEE 802.11a/h/e SINRold and LMold are calculated at the receiver. Consequently, the transmit power level must be carried in all frames. A proposal for the inclusion of the transmit power level in the SERVICE field of the Physical Layer Convergence Protocol (PLCP) header was already presented to the IEEE802.11h task group [13] (see Figure 2). According to this proposal, the transmit power level (TXPWR_LEVEL) is coded in 4 bits, which allows 15 power levels spanning from -19 dBm to 23 dBm, in 3 dBm steps. As 9 SERVICE field bits are reserved for future use in the current IEEE 802.11a specification, this solution does not introduce additional overhead. Smaller steps can provide better efficiency (see below), though requiring more bits to encode the transmit power level. Scrambler Initialization (7 bits)

TXPWR_LEVEL (4 bits)

Reserved (4 bits)

Figure 2. Revised SERVICE field as proposed in [13]. In order to allow efficient mode-power adaptation, the tuple should be readily available at the sender, which can be accomplished by including it in the header of all frames, namely the CTS, ACK and poll frames. This allows mode-power optimisation in a frameby-frame basis during long bursts (e.g. IEEE 802.11e Transmission Opportunities - TXOPs), but requires changes to the IEEE 802.11 MAC specification in order to include the tuple in the frame headers. The proposed information element is a SINR Report Element encoded in 3 octets. The transmit power and Link Margin are encoded each in one octet, following the encoding of these fields in the IEEE 802.11h TPC Report Element [14]. The SINR occupies one octet and takes values in the interval [-63, 63] in steps of 0.5 dB (Figure 3).

SINR (8 bits)

TXPWR_LEVEL (8 bits)

LM (8 bits)

Figure 3: SINR Report element format. This information element shall also be included in SINR_REPORT and SINR_REPORT_ACK frames. The latter shall be implemented as Spectrum Management Action frames as defined in the IEEE 802.11h specification (Figure 4). Category (8 bits)

Action (8 bits)

Dialog Token (8 bits)

SINR Report Element (24 bits)

Figure 4: SINR_REPORT and SINR_REPORT_ACK frame body format. The Category field shall be set to 0 in order to identify a Spectrum Management Action. The Action field identifies the action (SINR_REPORT or SINR_REPORT_ACK) and shall be encoded with values in the range [5, 255], which are reserved for future use: SINR_REPORT=5 and SINR_REPORT_ACK=6. The Dialog Token field is a non-zero value chosen by the SINR_REPORT sender, which identifies the transaction. Finally, the frames include the 24-bit SINR Report Element. The performance of mode-power adaptation can be further improved if SINR_REPORT frames are granted high priority, which is not possible to achieve with the original IEEE 802.11 MAC specification, except in PCF mode. The future IEEE 802.11e will specify a set of QoS enhancements to the MAC [15]. Among other enhancements, IEEE 802.11e introduces prioritization in both contention-based and contention-free access. Prioritization is based on the IEEE 802.1d [16] specification of 8 priority levels ranging between 0 (lowest) and 7 (highest). The latter corresponds to the Network Control (NC) traffic class. In IEEE 802.11a/h networks with IEEE 802.11e MAC, SINR_REPORT frames should be considered as belonging to the NC traffic class.

7. SMPA performance in IEEE 802.11a WLANs The IEEE 802.11a/e performance analysis framework used in this paper is an adaptation of the IEEE 802.11 PCF performance analysis framework proposed in [3]. As the main concern of this paper is overall transmit power (i.e. the power emitted by the antennas) as opposed to overall power consumption, we take a different approach from the original framework by taking into account the transmit power for QoS CF-Poll and CF-Ack frames besides QoS Data frames. On the other hand the extra

Proceedings of the 28th Annual IEEE International Conference on Local Computer Networks (LCN’03) 0742-1303/03 $ 17.00 © 2003 IEEE

Pt (dBm)

power consumed by the several hardware blocks of the WLAN card is discarded. The objective of this analysis is to evaluate the advantages of the proposed mode-power adaptation scheme relative to a system that only performs linkadaptation while transmitting at the maximum power level. The considered SMT and SPT values are listed in Table 2.

24 23 22 21 20 19 18 17 16 15 0

5

10

15

20

25

30

C/(N+I) (dB)

Table 2. SINR thresholds used to select the mode-power pair. Tx Mode SMT (dB) SPT (dB)

w/ TPC 3.0 dBm step

1

2

3

4

5

6

7

8

4 5

N/A N/A

7.5 8.0

10.5 11.0

14.0 14.5

17.0 17.5

22.0 22.5

23.5 24.0

Transmit mode 2 is not used because it always presents lower goodput than mode 3. Other relevant parameters are listed in Table 3. Table 3. Performance evaluation parameters.

SINR_min (dB) RS (dBm) Noise Floor (dBm) L (octets)

As can be seen, Pt = Pt_max when SMTm d SINR d SPTm. After that, Pt tends to decrease while SPTm d SINR d SMTm+1. This is not always possible, due to the granularity of transmit power. With 1.5 dBm steps, more opportunities are found to decrease Pt, while for 3 dBm steps Pt never decreases for SINR values below 20.5 dB. The impact of TPC on goodput is minimal (see Figure 7), as it is almost the same that results from transmission at the maximum transmit power (23 dBm).

-19 23 4 -86 -93 230 4

The results consider three different situations: linkadaptation+TPC with a transmit power resolution of 3 dBm, link-adaptation+TPC with a transmit power resolution of 1.5 dBm and link-adaptation with no TPC fixing Pt=23 dBm. Figure 5 depicts the selected transmit mode as a function of SINR, which is independent of TPC. Figure 6 shows the transmit power, measured in dBm units.

Pt=23 dBm

Figure 6. Transmit power (Pt).

Goodput (bps)

Pt_min (dBm) Pt_max (dBm)

w/ TPC 1.5 dBm step

4.50E+07 4.00E+07 3.50E+07 3.00E+07 2.50E+07 2.00E+07 1.50E+07 1.00E+07 5.00E+06 0.00E+00 0

5

10

15

20

25

30

C/(N+I) (dB) w/ TPC 3.0/1.5 dBm step

Pt=23 dBm

Figure 7. Goodput as a function of SINR. The impact on average transmit energy-per-bit is depicted in Figure 8.

8

Energy-per-bit (J-6/bit)

7

Tx Mode

6 5 4 3 2 1 0 0

5

10

15

20

25

30

C/(N+I) (dB)

5.00E-02 4.50E-02 4.00E-02 3.50E-02 3.00E-02 2.50E-02 2.00E-02 1.50E-02 1.00E-02 5.00E-03 0.00E+00 0

5

10

15

20

25

30

C/(N+I) (dB)

Figure 5. Transmit mode as a function of SINR.

w/ TPC 3.0 dBm step

w/ TPC 1.5 dBm step

Pt=23 dBm

Figure 8. Average transmit energy-per-bit (J-6/bit).

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With or without TPC, the transmit energy-per-bit tends to decrease as the SINR increases. This is due to the increase in bitrate, as shown by the staircase form of the Pt = 23 dBm curve. For the same transmit power, a higher bitrate corresponds to a shorter transmit time, decreasing transmit energy. For SINR values below 5 dB, the transmit energy increases abruptly due to the increasing number of retransmissions per packet at the lowest bitrate. TPC energy reduction is depicted in Figure 9. This is a measure of the fraction of average transmit energy-per-bit for Pt = 23 dBm that is reduced by using TPC.

is approximately 33%. The maximum duration of a contention-free burst is configured as 8 ms. The CAP Timer update interval is configured as 5120 Ps. This follows the recommendation expressed in [18] that the CAP Timer should be updated at uniform intervals that are multiples of 64 Ps, and no less than 1024 Ps. In this model, packet losses happen when the maximum allowed retransmission attempts (aShortRetryLimit and aLongRetryLimit) are exceeded or the maximum queuing delay of packets exceed the configured limit (dot11MSDULifetime). Table 4. MAC parameters.

TPC Energy Reduction

80.0% 70.0%

IEEE 802.11e MAC

60.0% 50.0% 40.0%

aSlotTime Beacon interval aFragmentationThreshold

30.0%

aRTSThreshold

20.0% 10.0% 0.0% 0

5

10

15

20

25

SIFS PIFS DIFS dot11MSDULifetime aShortRetryLimit aLongRetryLimit dot11DefaultCPTXOPLimit dot11CAPRate dot11CAPMax CAP Timer update time

30

C/(N+I) (dB) w/ TPC 3.0 dBm step

w/ TPC 1.5 dBm step

Figure 9. TPC energy reduction. TPC energy reduction for 1.5 dBm power steps is always higher or equal than energy reduction for 3 dBm power steps. While the former is able to reduce transmit energy in transmit modes 3, 4, 5, 6 and 8, the latter is only able to reduce transmit energy in modes 6 and 8. It should also be noted that in either case, transmit energy reduction is easier to achieve in transmit mode 8, i.e. for high SINR values, which are typically obtained when the user stations are located near to the Access Point (AP).

8. Simulation results The performance of SMPA in IEEE 802.11a/h/e WLANs was evaluated through computer simulation. The simulation model is an extension of the model already used in [17]. The parameters presented in Table 3 are kept. User data is transmitted with HCF contention-free access while SINR_REPORT frames are transmitted with the IEEE 802.11e prioritized contention-based medium access. Table 4 lists the most relevant MAC parameters used in the simulations. A dot11CAPRate value of 21Ps means that, on average, only one third of each unit of 64Ps can be used in contention-free bursts. This means that the maximum fraction of time occupied with polling-based transmission

20 Ps 100 ms 1024 octets 500 octets 20 Ps 40 Ps 60 Ps 60 ms 7 3000 Ps 21Ps 8000 Ps 5120 Ps

The considered path loss (Pl) model is given by the following expression: Pl

Pl (d 0)  n u 10 log 10 (d / d 0)  10 u ¬(d / 5) ¼

(6)

This indoor model combines the usual log-distance model plus a wall-attenuation factor given by the last term ( 10 u ¬(d / 5) ¼ ). This corresponds to a building floor in which walls are spaced by 5 m and each wall causes an attenuation of 10 dB. The latter is a typical attenuation value for concrete walls [19]. The considered path loss exponent is n=3, and the reference distance is d0=1m. Pl(d0), or path loss experienced at the reference distance is calculated with the Friis free space propagation model [19]: Pr(d 0)

Pt u Gt u Gr u O (4S ) 2 u d 0 2 u l

2

(7)

where Pr is received power, Gt and Gr are the transmit and receive antenna gains, O is the carrier wavelength (in

Proceedings of the 28th Annual IEEE International Conference on Local Computer Networks (LCN’03) 0742-1303/03 $ 17.00 © 2003 IEEE

All simulation results correspond to an average over 10 trials. Worst-case confidence intervals are provided. The confidence interval for each point is calculated assuming that the variable has standard normal distribution with average P and standard deviation V estimated from the samples. The 95% confidence interval is calculated as follows: § V · ¸¸ © n¹

P r 1.96 ˜ ¨¨

where n is the number of samples. In the first simulation, a user terminal moves in a straight line with a speed of 1 m/s between two positions located respectively at points (-10 m, 12 m) and (10 m, 12 m) considering the AP’s location at the origin (0, 0). The user is receiving a downstream MPEG4 video stream from the AP. SMPA was evaluated in two configurations, with and without the use of SINR_REPORT management frames. As already explained in section 4, when SINR_REPORT frames are used, they are only transmitted if the optimal transmit mode for the last received packet is different from the optimal transmit mode for the previous packet, and there are no DATA frames awaiting transmission that can carry the CSI information. In this way SINR_REPORT frames are only sent when absolutely necessary, minimising the overhead. Figure 10, Figure 11, Figure 12 and Figure 13 depict the experienced packet error probability, average transmission delay, transmit time occupancy (or the amount of consumed network resources) and transmit energy-per-bit as a function of the Pr error magnitude. The worst-case confidence intervals were respectively r0.23%, r0.15 ms, r0.1% and r4.74E-04 J-6 around the depicted values. MPEG4 Traffic 13.00%

Packet Error Probability

meters) and l is the system loss factor. These results consider unitary antenna gains and l=1. The simulated channel is also subject to fast multipath fading. The latter is simulated based on the Rayleigh fading model provided in [5]. The simulations consider two types of traffic sources. The first model is the Source Type 1 defined for performance evaluation of 802.14 [20]. It consists of a Poisson arrival distribution, which has the following message sizes and respective probabilities: (64, 0.6), (128, 0.06), (256, 0.04), (512, 0.02), (1024, 0.25) and (1518, 0.03). Each busty data source generates a data rate of 630 Kbps. This traffic is designated assorted data traffic in this paper. For the MPEG4 video source model we have used a trace of a real video stream of an e-learning session (Lecture Room-Cam video stream [21]. Due to the difficulty of finding a multi-layered stream with spatial scalability, we have decided to use a single-layered stream and assign different drop precedence to ‘I’, ‘P’ and ‘B’ frames. In this way, DP1 is assigned to the ‘I’ frames, which form the base layer of the stream. DP2 and DP3 are assigned to ‘P’ and ‘B’ frames, respectively. MPEG4 frames are fragmented in RTP packets with a size of 1024 octets. The data rate of the video stream is approximately 630 Kbps, which allows direct comparison with assorted data transmission. The performance of the SMPA scheme is strongly dependent on the accuracy of the Pr and SINR estimates at the receiver. It is assumed that each Pr estimate has an associated error given by a uniform probability distribution in the range >Pr  E ( Pr ), Pr  E ( Pr )@ , where E ( Pr ) is the magnitude of thr Pr error. In the first simulation scenario, performance of SMPA in unicast is compared with ideal mode-power adaptation and ARF, taking into account the magnitude of the error in the Pr estimate. It is assumed that SMPA knows Pr error magnitude and subtracts it to each Pr estimate. Ideal mode-power adaptation assumes that the Pr estimate is accurate and the sender knows the SINR experienced by the receiver in any frame transmission in advance without the need of feedback. The ARF algorithm corresponds to the adaptation presented in [5], which works as follows. When no acknowledgement is received after two succeeding data frame transmissions, the sender decreases the bitrate. Each additional missing acknowledgement causes the bitrate to decrease. If 10 consecutive data transmissions are successful, the sender tries to increase the bitrate. This can also happen if a timeout (60 ms) expires during which no failed data transmissions occur.

12.50% 12.00% 11.50% 11.00% 10.50% 0

1

2

3

4

5

Pr error (dB) Ideal

ARF

SMPA w/o SINR_REPORT

SMPA w/ SINR_REPORT

Figure 10. Packet error probability as a function of Pr estimate error, for the MPEG4 video stream.

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Average Delay (ms)

MPEG4 Traffic 9.00 8.00 7.00 6.00 5.00 0

1

2

3

4

5

6

Pr error (dB) Ideal

ARF

SMPA w/o SINR_REPORT

SMPA w/ SINR_REPORT

Figure 11. Average transmission delay as a function of Pr estimate error, for the MPEG4 video stream.

Time Occupancy (%)

MPEG-4 Traffic 18.00% 16.00% 14.00% 12.00% 10.00% 8.00% 6.00% 0

1

2

3

4

5

6

per-bit in SMPA (|5u10-2 J-6/bit) is higher than in ideal mode-power adaptation because the latter is able to detect when transmission is not possible (SINR < SMT1), preventing failed transmission attempts. ARF presents the worst performance with packet error probability above 12%, average transmission delay slightly above 8 ms, transmit time occupancy around 15 % and transmitted energy-per-bit slightly below 6u10-2 J-6/bit. As the Pr error increases, the performance of SMPA is degraded. SMPA presents worse results than ARF for a Pr error greater than 5.0 dB, 5.0 dB and 4.5 dB and 4.5 dB respectively in terms of packet error probability, transmission delay, transmit time occupancy and transmit energy-per-bit. Notice that the use of SINR_REPORT frames in SMPA is irrelevant in unicast. The acknowledgement frames are enough to notify the sender about the estimates. The second simulation is equal to the first except that the traffic source used is now assorted data. Figure 14, Figure 15, Figure 16 and Figure 17 depict the experienced packet error probability, average transmission delay, transmit time occupancy and transmit energy-per-bit as a function of Pr error magnitude. The worst-case confidence intervals were respectively r0.43%, r1.38 ms, r0.2% and r8.12E-04 J-6 around the depicted values.

Pr error (dB)

Assorted Data Traffic

Ideal

ARF

SMPA w/o SINR_REPORT

SMPA w/ SINR_REPORT

Packet Error Probability

16.50%

Figure 12. Time occupancy as a function of Pr estimate error, for the MPEG4 video stream. MPEG-4 Traffic

15.50% 14.50% 13.50% 12.50% 0

Energy-per-bit (J-6/bit)

7.00E-02

1

2

3

4

5

Pr error (dB)

6.00E-02 5.00E-02

Ideal

ARF

4.00E-02

SMPA w/o SINR_REPORT

SMPA w/ SINR_REPORT

3.00E-02 2.00E-02 0

1

2

3

4

5

6

Pr error (dB) Ideal

ARF

SMPA w/o SINR_REPORT

SMPA w/ SINR_REPORT

Figure 14. Packet error probability as a function of Pr estimate error, for assorted data traffic.

Figure 13. Transmit energy-per-bit as a function of Pr estimate error, for the MPEG4 video stream. When the Pr estimate is accurate (Pr error = 0), SMPA presents a packet error probability and transmition delay performance closer to the ideal mode-power adaptation. SMPA packet error probability is around 11.25% and average transmission delay is around 6 ms. Transmit time occupancy is between 12% and 13%. Transmit energy-

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6

invariant. It can be approximated by the following expression [19]:

Average Delay (ms)

Assorted Data Traffic 17.00 15.00 13.00

Tc (t ) |

11.00 9.00 7.00 5.00

where v(t ) 0

1

2

3

4

5

6

Pr error (dB) Ideal

ARF

SMPA w/o SINR_REPORT

SMPA w/ SINR_REPORT

Figure 15. Average transmission delay as a function of Pr estimate error, for assorted data traffic.

Time Occupancy (%)

Assorted Data Traffic 20.00% 18.00% 16.00% 14.00% 12.00% 10.00% 0

1

2

3

4

5

6

Pr error (dB) Ideal

ARF

SMPA w/o SINR_REPORT

SMPA w/ SINR_REPORT

Figure 16. Time occupancy as a function of Pr estimate error, for assorted data traffic. Bursty Data Traffic 8.00E-02

Energy-per-bit (J-6/bit)

9˜O 16 ˜ S ˜ v(t )

7.00E-02 6.00E-02 5.00E-02 4.00E-02 3.00E-02 0

1

2

3

4

5

6

Pr error (dB) Ideal

ARF

SMPA w/o SINR_REPORT

SMPA w/ SINR_REPORT

Figure 17. Transmit energy-per-bit as a function of Pr estimate error, for assorted data traffic. As can be seen, the packet error probability of SMPA is now slightly higher than that of ARF. This is easily explained by the relationship between the coherence time of the channel and burst size. The coherence time of a channel is a statistical measure of the time duration over which the channel impulse response is essentially

(8)

is the speed along the line-of-sight

between the sender and receiver at time t, and O is the wavelength of the carrier frequency. The MPEG4 video stream is characterized by long packet bursts spaced by 40 ms. As such, the majority of the packets of each burst are transmitted back-to-back within a time interval shorter than the coherence time of the channel (approximately 10 ms), which means that the estimate for a packet is usually valid for the next packet. On the contrary, assorted data sources generate small bursts (usually one packet long) with packet Poisson-distributed inter-arrival times that are usually greater than the coherence time. This means that the estimate for a packet is usually invalid by the time the next packet is generated. As SMPA increases the bitrate faster than ARF based on the CSI estimate for the previous packet, drops in channel quality between succeeding packets increase the number of transmission attempts required to lower the bitrate until the transmission is successful, which results in a greater probability of packet loss due to exceeding the number of transmission attempts (aShortRetryLimit, aLongRetryLimit). On the other hand, ARF increases the bitrate very slowly, which means that it usually stays on lower bitrates, assuring an higher probability of successful transmission with less transmission attempts. The fact that, on average, ARF keeps the bitrate lower than SMPA also explains why transmission delay, transmit time occupancy and transmit energy-per-bit are still higher in ARF for a Pr error lower than 5.0 dB, 6.0 dB and 5.0 dB respectively. While this result seems odd at first sight, one should remember that in this case packet losses are due to exceeding the maximum number of transmission attempts rather than exceeding the maximum queuing delay (dot11MSDULifetime). While SMPA loses packets slightly more often than ARF, its successful transmissions are accomplished at higher bitrates, which reduces the average transmission delay. The use of SINR_REPORT frames is again practically irrelevant in terms of performance. In order to illustrate the SMPA operation in multicast, a scenario is presented in which 5 users receive the same MPEG4 video stream. The users are stopped and located respectively at the following distances (d) from the AP: 4, 6, 11, 11.5 and 14 meters. Multipath fading is not considered at this time in order to allow a better understanding of the SMPA mechanism in multicast.

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Table 5. Multicast scenario settings. User d Pr_max (dBm)

SINR_max (dB) LM (dB) Unicas t Tx mode Pt for Unicast (dBm)

1

2

3

4

5

4

6

11

11.5

14

42.0 51.0

57.3 35.7

75.2 17.8

75.8 17.2

78.3 14.7

44 8

28.7 8

10.8 6

10.2 6

7.7 5

-4

11.3

22.7

23

22.8

The corresponding multicast transmit modes and Pt values for the three drop precedence classes are listed in Table 6, where a Pt resolution of 3 dB is now considered.

Mean Packet Rate (Packet/s)

The scenario settings are listed in Table 5, where the Pr, SINR_max and LM values for each user are also included. The LM and Pt encoding limitations are not considered in Table 5.

Tx mode Pt (dBm)

DP2

DP1

8 14

6 23

5 23

The transmission modes selected for DP3 and DP1 correspond to the optimal modes for the best and worst receiver respectively. The transmission mode for DP2 corresponds to the maximization of the Mean Packet Rate given by expression (4) and depicted in Figure 18. Between mode 1 and 5, the Mean Packet Rate increases due to the gradual decrease Tm(L), while Pem (L) stays insignificant. In mode 6, user 5 is unable to receive the data frames, which means that Pem (L) suffers a decrease. Nevertheless this is compensated by the decrease of Tm(L) and mode 6 presents the maximum Mean Packet Rate. With mode 7, users 3 and 4 are unable to receive data frames, which means that Pem (L) drops heavily, causing the decrease of Mean Packet Rate. The latter rises again in mode 8 because Pem (L) is kept approximately constant while Tm(L) decreases.

3000.00 2500.00 2000.00 1500.00 1000.00 500.00 0.00 1

2

3

4

5

6

7

Tx Mode

Figure 18. Mean Packet Rate for each transmission mode, considering 1024-octet packets. Table 7 shows the fraction of packets received per drop precedence per terminal, demonstrating that SMPA works as intended. DP1 packets are received by all terminals with small losses, while DP3 packets are received only by the best receivers (users 1 and 2). DP2 packets are received by users 1, 2, 3 and 4 with small losses while user 5 is only able to receive 4.8 %.

Table 7. Packets received in multicast.

Table 6. Transmit mode and transmit power in multicast. DP3

3500.00

User

1 2 3 4 5

Received packets (%) DP3 DP2 DP1 100.0 100.0 100.0 100.0 100.0 100.0 99.1 100.0 0.0 0.0 0.0

93.7 4.8

100.0 99.0

9. Conclusions This paper has presented a SINR-based Mode-Power Adaptation (SMPA) algorithm that can be used in WLANs with polling-based medium access. SMPA tries to maximize the goodput of the WLAN, while using the minimum transmit power that assures correct packet delivery. The algorithm is based on SINR estimates provided by the receiver by means of a suitable protocol. The implementation of this protocol in IEEE 802.11a/h WLANs was discussed. An extension of the algorithm for multicast scenarios was also presented. The performance of SMPA was analysed. It was shown that the maximization of goodput reduces the transmit energy, due to the minimization of transmit time. The use of Transmit Power Control with link-adaptation

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can reduce even more the average transmit energy-per-bit for some ranges of SINR. The performance of SMPA was also evaluated by means of computer simulation and compared with ideal link-adaptation and ARF link-adaptation with fixed transmit power in unicast scenarios. This comparison took into account the magnitude of the error in the received power estimates required by SMPA. It was shown that for good received power estimates, SMPA performs better than ARF when the traffic is more correlated and features long bursts (e.g. MPEG4 video traffic). On the other hand, ARF presents a lower packet loss when bursts are small and packets are spaced more uniformly. The ARF algorithm does not support multicast as it relies on acknowledgement frames. SMPA is independent of the ARQ mechanisms and its use in multicast was demonstrated through computer simulation.

10. References [1] IEEE 802.11a, “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY Specifications), Standard, IEEE, August 1999. [2] D. Qijao, S. Choi, “Goodput enhancement of IEEE 802.11a wireless LAN via link adaptation”, IEEE Proceedings ICC’2001, Helsinki, Finland, June 2001. [3] D. Qijao et al, “Energy-Efficient PCF Operation of IEEE 802.11a Wireless LAN”, IEEE INFOCOM’02, New York, June 2002. [4] S. Gray, V. Vadde, “Throughput and Loss Packet Performance of DCF with Variable Transmit Power”, IEEE 802.11-01/227, May 2001. [5] G. Holland, “A Rate-Adaptive MAC Protocol for Wireless LANS”, Mobicom'1, Rome, Italy, 2001. [6] Kamerman, L. Monteban, “WaveLAN-II: A highperformance wireless LAN for the unlicensed band”, Bell Labs Technical Journal, pages 118-133, Summer 1997. [7] M. Radimirsch, “An Algorithm to Combine Link Adaptation and Transmit Power Control in HIPERLAN Type 2 ”, in Proceedings of Personal Indoor and Mobile Radio Conference 2002 (PIMRC’2002), September 2002.

[9] K. Balachandran et al, “Channel Quality Estimation and Rate Adaptation for Cellular Mobile Radio”, IEEE Journal on Selected Areas in Communications, Vol. 17, Nº. 7 July 1999. [10] ISO/IEC 14496-2:2001, “Information Technology – Coding of Audio-Visual Objects – Part 2: Visual”, July 2001. [11] T. Ahmed et al, “Encapsulation and Marking of MPEG-4 Video over IP Differentiated Services”, in proceedings of the Sixth IEEE Symposium on Computers and Communications ISCC'2001, July 2001. [12] J. Heinanen et al, “RFC 2597: Assured Forwarding PHB Group”, IETF, June 1999. [13] S. Choi et al, “Transmitter Power Control (TPC) and Dynamic Frequency Selection (DFS) Joint Proposal for 802.11h WLAN”, IEEE 802.11-01/169r2, May 2001. [14] A. Myles, “IEEE 802.11h Draft Normative Proposal”, IEEE 802.11-02/402r0, June 2002. [15] S.Kandala et al, “Normative Text for TGe Consensus Proposal”, IEEE 802.11-02/612r0, Sep. 2002. [16] IEEE Std. 802.1d (ISO 15802-3), “Media Access Control (MAC) Bridges”, 1998. [17] Grilo, M. Nunes, “Performance Evaluation of IEEE802.11e”, Proc. 13th IEEE International Symposium on Personal Indoor and Mobile Radio Communications (PIMRC’2002), vol.1, Lisboa, Portugal, Sep. 2002, pp. 511-517. [18] D. Kitchin, “HCF Channel Access Rules”, IEEE 802.11-02/015r1, Jan. 2002. [19] T. Rappaport, Wireless Communications: Principles and Practice, 2nd edition, Prentice Hall, 2002. [20] J. Limb et al, “Performance Evaluation Process for MAC Protocols”, IEEE 802.14/96-083R3, Mar. 1996. [21] Video traces http://www.eas.asu.edu/trace.

[8] K. Olszewski, “Channel Quality Measurement Method for 802.16ab PHY Layers”, IEEE 802.16abc01/45, October 2001.

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at