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2012 - Wireless Networking Symposium
Spatial Reuse Strategy In mmWave WPANs with Directional Antennas Qian Chent, Xiaoming Pengt, Juan Yang+ and Francois Chint t Institute for Infocomm Research, Singapore 138632 + National Mobile Communications Research Laboratory, Southeast University, China 210096 Email:{qchen.pengxm.chinfrancois}@
[email protected]
Ahstract- In this paper, we propose a spatial reuse strategy for millimeter-wave (mmWave) wireless personal area networks (WPANs) with directional antennas. By this strategy, two or more transmissions can be appropriately scheduled for spatial sharing based on the beamforming
(BF)
information. Moreover, we
analyze the performance of this strategy with consideration of the difference between idealistic and realistic directional antennas. Simulation results show that the strategy efficiency is related to antenna types, configurations and network topologies, and that the spatial multiplexing gain can be increased accordingly.
I. INTRODUCTION Numerous standards have been designed to provide very high throughput up to multi-gigabit speed level in a millimeter wave (mmWave) wireless personal area network (WPAN), e.g. IEEE S02.11ad draft, IEEE S02.15.3c and ECMA-3S7 [1] [3], which exploit the advantages of directional transmission techniques in the unlicensed 60 GHz frequency band. One of the most salient features of mmWave communication is that the signal in mmWave band between 30 GHz to 300 GHz degrades more significantly than that in the traditional 2.4 or 5 GHz band. In order to compensate this signal attenuation effect and provide robust communication link, a directional transmission technique is used in mmWave WPANs to exploit the high gain and less interference properties of directional antennas. Furthermore, the directional transmission also leads to a possibility of achieving spatial sharing among devices, thereby, increasing the network throughput accordingly. On the other hand, the difficulty in the implementation of spatial sharing is how to appropriately schedule the intended transmissions. Lin et al. [5] proposed a randomized exclusive region based scheduling scheme, namely, ERX, to decide a set of concurrent transmissions. In this scheme, the piconet controller (PNC) must collect the location information of all devices to make proper schedules. However, in mmWave WPANs, the accurate location information cannot be easily obtained at PNC by existing localization techniques. Moreover, another type of spatial sharing schemes with directional anten nas was proposed working in ad hoc or wireless sensor net works. In these schemes, two pairs of devices may potentially communicate simultaneously, depending on the directions of transmissions. If a device hears another device from a certain direction, then it defers only in that direction and intends towards other directions. For example, in [4], a directional network allocation vector (DNAV) table is established for keeping track of the directions towards which a device cannot
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initiate its transmission. However, this type of schemes suffers from the hidden terminal problem, which actually cannot eliminate the conflicts among concurrent transmissions. In this paper, we propose another spatial reuse strategy for mmWave WPANs with directional antennas. To the best of our knowledge, this is the first work towards spatial sharing in mmWave WPANs based on beamforming (BF) information. The main contributions of this paper are listed as below: • Propose a new spatial reuse strategy based on BF information which applies to mmWave WPANs. • Derive the spatial reuse condition and design the algo rithm of implementation. • Analyze the performance with consideration of the dif ference between idealistic and realistic antennas. • Evaluate the efficiency of this strategy and show the achieved spatial multiplexing gain. The rest of this paper is organized as follows: Section II introduces the system model. In Section III and IV, we show the details of the proposed spatial reuse strategy and analyze its performance. Then, the simulation results are illustrated in Section V. Finally, conclusions are drawn in Section VI. II.
SYSTEM MODEL
In this section, we will introduce the system model, ide alistic directional antenna model, BF mechanism and spatial sharing mechanism. A. System Model and Idealistic Directional Antenna Model
The system model considered in this paper is shown in Fig. 1: The mmWave WPAN consists of one designated device, which is called a personal basic service set (PBSS) control point (PCP) or access point (AP) in IEEE S02.11ad draft or a PNC in IEEE S02.15.3c standard, and N (1 :::; N :::; 254) non-PCP/non-AP stations (STAs) or devices. Here, PCP/AP provides the basic timing for the PBSS as well as the allocation of contention-based access period (CBAP) and service period (SP). In this mmWave WPAN, any pair of STAs including both the PCP/AP and the non-PCP/non-AP STAs can directly communicate with each other through the equipped directional antennas after completing the BF training. Moreover, we consider an idealistic directional antenna model which is widely used for spatial reuse study [6]. As illustrated in Fig. 1, the directional antenna of the PCP/AP consists of lvI beam sectors covering the entirety of 3600
2
measure during the existing reservation
measure during the candidate reservation
(SP,)
(SPo)
Measurement Duration
BTl
Fig. 2.
Fig. 1.
..
«
MeaslJement Duration ..
! SPc
(Candidate):
Time STAB OTT �-----BI-------_
I
SPe (Existing): STAA STA B
A mmWave WPAN and directional antenna model.
area, where lvI = 27T / ¢ and ¢ is the beamwidth expressed in radians. For each sector, we assume that the received signal outside the main lobe (the shadowing area) is thoroughly at tenuated and can be neglected accordingly. Similarly, all other non-PCP/non-AP STAs are also equipped with the directional antennas of this type. B. Beamforming and Spatial Sharing under IEEE 802.IIad
BF is a mechanism that is used by a pair of STAs to determine appropriate antenna system settings for both trans mission and reception. In IEEE 802.11ad, the BF training starts with a sector level sweep (SLS) and a beam refinement protocol (BRP) may follow if required. The purpose of the SLS is to select the best transceiver sectors between two participating STAs. Furthermore, the BRP phase is to enable iterative refinement of the antenna weight vector (AWV) of both transmitter and receiver at both participating STAs. After the successful completion of the BF training, BF is said to be established and the information of sector IDs and AWVs with the best quality can be obtained accordingly. On the other hand, the spatial sharing mechanism proposed in IEEE 802.11ad draft is to allow SPs belonging to different STAs in the same spatial vicinity to be scheduled concurrently. Fig. 2 shows an example of this process. First, the channel access time is divided into beacon inter vals (Bls), and each BI mainly consists of four parts: beacon transmission interval (BTl), association beamforming training (A-BFT), announcement time (AT), and data transfer time (DTT). SPs and CBAPs are allocated within each DTT. Second, we assume that SPe is an existing SP that has been allocated for STAs A and B, and SPc is a candidate SP that is allocated for STAs C and D. Here, candidate SP refers to a SP that is to be assessed for spatial sharing with other existing SPs. Moreover, we assume that the BF training has been performed between each pair. Then, for the sake of spatial sharing, the PCP/AP transmits a request to STAs C and D to measure over SPe's allocation and also transmits a request to STAs A and B to measure over SPc's allocation. After which, each STA performs the measurement and reports the result to the PCP/AP.
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!STAA
An example of spatial sharing measurement.
Lastly, the PCP/AP estimates the channel quality based on the feedback information. If the relevant interference level is lower than a predefined receive sensitivity, PCP/AP may schedule the candidate SPc overlapping with the existing SPe for the purpose of spatial sharing. In fact, this kind of spatial reuse mechanism is a blink selection process only based on measurement. No priori in formation can be provided for the PCPI AP to appropriately choose an exact existing SP for a candidate SP to assess. Thus, the efficiency of this kind of spatial sharing mechanism will severely degrade if there are many candidate SPs intending for spatial sharing with existing SPs. III. D ETAILS OF PROPOSED
SPATIAL REUSE
STRATEGY
In this section, we propose a spatial reuse strategy for mmWave WPANs which is based on the BF information. A. Beamforming Information Table
In mmWave WPANs, a pair of STAs must perform the BF training before communicate with each other. According to the current IEEE 802.1 I ad draft or IEEE 802.15.3c standard, the BF information is only kept at the participating STAs. In our proposed spatial reuse strategy, we assume that the BF information, such as selected sector, selected antenna, SNR, etc., will be eventually fed back to the PCP/AP after the BF training. This work can be easily fulfilled with no or minor modification of the current standard specifications. By this assumption, the PCP/AP is able to establish a BF information table (BIT) that records all the BF training results among STAs. Figure 3 shows a typical example of a mmWave WPAN which consists of one PCP/AP and six non-PCP/non-AP STAs A, B, C, D, E, and F. Suppose that each device has 12 sectors with each sector having 30° beamwidth. The sector numbers are arranged in a clockwise or a counter-clockwise from 1 to 12. Thus, a BIT of this network can be established at the PCP/AP, which is shown in Table I where the first two columns refer to the source STA a and destination STA b, respectively, and the third column S ( a, b) indicates the best sector ID used for directional transmission from STA a to STA b. B. Spatial Reuse Condition
With the assumption that the BIT has been established, we derive the spatial reuse condition for PCP/AP to assess whether or not two pairs of STAs can currently transmit without interference with each other.
3
Algorithm 1 Calculate the spatial reuse criteria �. Input:
Established BIT Existing SPe for pair (A,B); Candidate SPc for pair (C,D). Output: Calculate the value of � between SPe and SPc' 1: 2: 3:
o PCP/AP
4: 5: 6: 7: 8: 9: 10:
o Fig. 3. A mmWave WPAN: The shadowing area refers to the coverage area of each selected sector. TABLE I
BEAMFORMING INFORMATION TABLE AT PCP/AP (PARTIAL). DES b B
Sea, b)
SRC a
DES b
Sea, b)
A
1
D
A
C
3
D
B
6
A A
D
11
D
C
4
A
E
4
D
E
5
A
F
2
D
F
3
B B B B
A
7
E
A
C
5
E
B
11
D
9
E
C
4
E
7
E
D
1
F
5
E
F
3
C
A
8
F
A
F
B
10
SRC a
B
C
B
12
2
2
7
C
D
10
F
C
6
C
E
8
F
D
9
C
F
12
F
E
7
Firstly, we define a parameter Oa--+b,c = min(II S(a,b) S(a,c) II,M -II S(a,b) -S(a,c) II) to indicate the sector num ber difference between two best sectors S(a,b) and S(a,c) which are chosen by one source STA a to two different destination STAs band c, where the min function returns the minimum value of a set. Secondly, we assume that an existing SPe is allocated for a pair of STAs (A,B) and a candidate S Pc is allocated for another pair of STAs (C,D). Lastly, Algorithm 1 gives a way for PCP/AP to calculate the minimum value (denoted by �) of all o's among two intended concurrent transmission pairs (A,B) and (C,D). To better understand this algorithm, we use two transmission pairs (A,B) and (C,D) in Fig. 3 and the BIT in Table I as an example to show how Algorithm 1 works. If the source STA A is transmitting to its destination STA B with the best sector 1 after completing the BF training, then we consider the resulted interference from A to each of two STAs (C,D). From Table I, we can easily see that the Sector 3 and 11 would be the best sector for A transmitting to C and D, respectively, which are not the same as Sector
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� f-- 00. foreach X,Y E {A,B} and i,j E {C,D} do if S(x,y) -=I- S(x,i) & S(x,y) -=I- S(x,j)
& S(i,j) -=I- S(i,x) & S(i,j) -=I- S(i,y) then Ox--->y,j, Oi--->j,x, Oi--+j,y);
� = min(�,Ox--+y,i, else � f-- 0; return �; end if end return �.
1. Therefore, neither C nor D would be affected by A in consideration of idealistic direction antennas as mentioned in Section II-A. Furthermore, the values of OA--->B.C and OA--->B.D are calculated as {2,2}. In a similar way, if the source STA is considered as B, C, and D, respectively, we can follow the same procedure to check whether or not the current transmission would interfere with the other one. Moreover, it is easily obtained that the values of o's for the source STAs B, C, and D are given by {2,2}, {2,2}, {2,2}, respectively. Therefore, (A,B) and (C,D) would not interfere with each other for concurrent transmission, and the minimum value � of all the o's is equal to 2. Actually, Algorithm I can be intuitively understood as below: as long as any two pair of STAs does not fall in the coverage area of the selected sectors at both the source and destination of each other, then their assigned SPs can be scheduled overlapping with each other for purpose of spatial sharing. Here, the coverage range refers to the receive sensitivity range. Thus, the spatial reuse condition between any two transmission pairs is that � > O. Moreover, the value of � is also a factor to determine the accuracy of Algorithm 1: The greater the value of �, the more space to avoid interference among concurrent directional transmissions. Therefore, in the remaining part, we use � as the decision criteria of spatial reuse between a candidate SP and an existing SP. C. Spatial Reuse Strategy
Now, based on the established BIT and the spatial reuse criteria �, we propose a spatial reuse strategy for PCP/AP to choose the best existing SP for a considered candidate SP before send out the measurement requests. Assume that the PCP/AP has scheduled K number of existing SPs in each BI which are denoted by set {S Pk 11 :::; k :::; K}. If a candidate SPc comes, the PCP/AP can execute Algorithm 2 to choose the best existing SP (denoted by SP*) for the purpose of spatial sharing. Obviously, the best existing SP* has the maximum value of � (denoted by �*) with this candidate S Pc and must satisfy the condition that �* > O. Thus, SP* and SPc have a high probability to be scheduled overlapping with each other for spatial sharing.
4
Algorithm 2 Choose the best existing SP for candidate SP. Input:
Established BIT Existing SP set {SPkI1 :::; k :::; K}; Candidate SPc for pair ( a, b) . Output: Choose the best SP* for SPc' 1: �* +- 0, SP* +- 0. 2: foreach k = 1 to K do Calculate the value of � k between SPk and SPc 3: through Algorithm 1. 4: if � k > �* then 5: �* +- � k; 6: SP* +- SPk; 7: 8: 9:
end if end return S P* .
,, . -'-, _. _. _. _. _. _. _. _. _.-. B B-3dB
•
o
Fig. 4. Measurement model with directional antenna: STA A is transmitting through a selected sector pointing towards the direction of the arrow.
By this spatial reuse strategy, the efficiency of the SP sched ule can be dramatically improved, and the spatial multiplexing gain will be increased accordingly. IV. PERFORMANCE ANALYSTS In the previous section, we proposed a spatial reuse strategy under the assumption of idealistic antenna model. Now, we analyze its performance with consideration of the difference between idealistic and realistic antenna models.
where 8' = 811 or 812 is the angle between the line AB(or AC) and the center line of A's sector beam, j is the signal frequency in units of GHz, p is the path loss exponent, and d is the distance between A and B(or C) in units of meters. Moreover, PT is the transmit power of A, GT(8') can be calculated by (1) for 8 = 8', G R(8') is equal to 0 for the omnidirectional receive pattern, Lpath(j,p,d) is the path loss between A and B(or C), and A(p) expressed in units of dB is a constant value for a given p. C. Performance of Spatial Reuse Strategy
A. Realistic Directional Antenna Model
The most widely used realistic directional antenna model is considered as a main lobe of Gaussian form in linear scale and constant level of side lobes [7]. Using this model, the gain of a directional antenna expressed in units of decibel (dB), denoted by G(8), is given by: G(8) 8ml
=
=
{
Go
Cisz,
=
3.01·
(e�:dHr,
)2
2.6· 8-3dB
0
