Possible applications are: medical monitoring, office automation, sensor networks ... operating with a fractional bandwidth greater than 0.25 (i.e. a. -3dB bandwidth which is at ... within a network by using contention-free techniques (e.g. polling) ...
MEDIUM ACCESS TECHNIQUES IN ULTRA-WIDEBAND AD HOC NETWORKS Hiroyuki Yomo∗, Petar Popovski, Carl Wijting*, István Z. Kovács*, Nico Deblauwe, Angel F. Baena, and Ramjee Prasad Department of Communication Technology, Aalborg University, Denmark {yomo, petarp, carl, istvan, ndeblauw, quillo, prasad}@kom.auc.dk
Abstract--Short-range wireless systems have recently gained a lot of attention to provide seamless, multimedia communications around a user-centric concept in so called wireless personal area networks (WPAN). Ultrawideband (UWB) technology presents itself as a good candidate for the physical layer (PHY) of WPAN, both for high and low data rate applications. Although the PHY issues for UWB in WPAN environments have been extensively studied, the problems related to the medium access techniques remain largely unexplored, especially for targeted WPAN scenarios. In this paper, we consider time-hopping spread spectrum (TH-SS) UWB communication system and discuss how to design medium access techniques in order to achieve efficient resource sharing in WPAN environments. We evaluate throughput performance by computer simulation in multi-network scenario, and discuss the impact of TH parameters on the throughput performance. Furthermore, we propose a link adaptation mechanism which can adapt TH parameters according to interference conditions among collocated WPANs. This mechanism aims to maximize the throughput achieved by WPANs while keeping the robustness with respect to the basic timing mechanisms used in the TH-SS communications. Index terms—Wireless Personal Area Network (WPAN), ad-hoc networking, Ultra-wideband (UWB), Medium Access Control
1.
INTRODUCTION
Short-range wireless systems have recently gained a lot of attention to provide seamless, multimedia communications ∗
around a user-centric concept in so called Wireless Personal Area Networks (WPANs). Salient requirements of these systems include low-power operation, use of license-free wireless links, low cost, scalability and the use of ad hoc networking techniques. Ultra-wideband (UWB) technology presents itself as a good candidate for the physical layer (PHY) of such systems, both for high and low data rate applications. Possible applications are: medical monitoring, office automation, sensor networks, information services, and banking / financial applications [1]. UWB radio communications are an extreme form of spread spectrum communication systems, generally defined as operating with a fractional bandwidth greater than 0.25 (i.e. a -3dB bandwidth which is at least 25% of the center frequency used). Alternatively, Federal Communications Commission (FCC) in USA defines a UWB device as any radio with a -10dB fractional bandwidth greater than 0.20 or occupying at least 500 MHz of the spectrum [2]. In this paper we will consider UWB systems based on Impulse Radio (IR-UWB), which use very short pulses, and have been known for a long time. IR-UWB meets the requirements for short-range wireless systems and additionally, possesses the following desirable features: mitigated multi-path fading effects, possibility of high bit rates and a unique location ability. Indeed, the IEEE 802.15 Task Group2, which deals with high rate PHY for WPAN, has formed a study group (SG3a) to consider UWB as a candidate for this PHY [3]. Recently, the PHY issues for UWB in WPAN environments have been extensively studied [4] [5] [6]. However, the problems related to the medium access techniques remain largely unexplored, especially regarding the targeted ad hoc networking scenarios. In ad hoc networking, the devices are interconnected via spontaneously created, disposable connections, without relying on a pre-existing infrastructure. These scenarios pose seriously challenging research tasks,
This paper describes work undertaken partly in the context of the IST-2001-34157 Power aware Communications for Wireless OptiMised personal Area Network (PACWOMAN). The IST program is partially funded by the EC.
since the same medium should be used by many mutually interfering WPANs under the stringent synchronization conditions imposed by the IR-UWB. The two basic modulation schemes used in IR-UWB are Pulse Position Modulation (PPM) and Bi-Phase Modulation (BPM), while the two common channelization techniques are TimeHopping Spread Spectrum (TH-SS) and Direct Sequence Spread Spectrum (DS-SS). This work analyzes only IR-UWB communication systems using TH-SS and PPM. Since WPAN technology uses license-free wireless links, the radio resources should be shared among the collocated and uncoordinated WPANs. Then, devices belonging to a single IR-UWB WPAN should actually share the channel not only among themselves but also with the devices belonging to the neighboring, physically collocated networks. In such a situation, there are two different levels of multiplexing we need to achieve. First, the medium access control (MAC) protocols should coordinate the transmissions of all devices within a network by using contention-free techniques (e.g. polling), random-access (e.g. carrier-sensing), or a combination of both. At the second level, the multiple collocated networks must share the resources in a sense to minimize the mutual interference, which can be achieved through adapting the parameters of TH used within the networks. In this paper, we consider TH-SS UWB communication system and we show how to design TH parameters in order to achieve efficient resource sharing at the second level of multiplexing, that is, resource sharing among multiple networks. We evaluate throughput performance by computer simulation in multi-network scenario, and discuss the impact of TH parameters on the throughput performance. Furthermore, we propose a link adaptation mechanism that can adapt TH parameters according to interference conditions. This mechanism aims to maximize the throughput achieved by WPANs while keeping the robustness with respect to the basic timing mechanisms used in the TH-SS communications. 2.
ULTRA-WIDEBAND PHYSICAL LAYER (UWB PHY)
2.1. UWB radios UWB radio communications are an extreme form of spread spectrum communications. The FCC has defined an UWB device as any device with a -10dB fractional bandwidth, FBW , greater than 0.20 or occupying at least 500MHz of the spectrum [2]: FBW
f − fL =2 H ≥ 0.20 fH + fL
(1)
where the upper, f H , and lower, f L frequencies correspond to the -10dB bandwidth. The FCC also regulated the spectral shape and maximum power spectral density ( ≈ -41dBm/MHz) of the UWB
radiation in order to limit the interference with other communication systems. The ETSI regulations in EU are expected to follow the FCC but with a more restrictive spectral shape, motivated by a different management of the available spectrum [7]. The UWB signals generation methods can be grouped in two major categories:
• Single-Band (SB) based: employing one single transmission frequency band, and • Multi-Band (MB) based, employing two or more frequency bands, each with at least 500 MHz bandwidth. In the SB solution, the UWB signal is generated using very short, low duty-cycle, baseband electrical pulses with appropriate shape and duration. Due to the carrier-less characteristics (no sinusoidal carrier to raise the signal to a certain frequency band) these UWB systems are also referred to as carrier-free or IR-UWB communication systems [6]. The MB UWB systems can be implemented carrier less (different pulse shapes/lengths are used according to the frequency band) [8] or carrier based (multi-carrier like) [9]. In the IR-UWB solutions, typically the radiated pulse signals are generated without the use of local oscillators or mixers, thus potentially a simpler and cheaper construction of the transmitter (TX) and receiver (RX) is possible, as compared to the conventional narrow-band systems. The characteristics of the pulse used (shape, duration), determine the bandwidth and spectral shape of the UWB signals. The most common pulse shapes used in IR-UWB are: Gaussian monocycle (and its derivatives) and Hermitian pulses. The low transmit power levels together with the ultra-fine time resolution of the system can increase considerably the synchronization acquisition time and the complexity of the receiver. The large transmission bandwidth in UWB, in the order of n × 100MHz (with n = 5K 70 ), has as result a higher immunity to interference effects and improved multipath fading robustness. The multipath can be resolved down to differential delays of nanosecond or less (i.e. 30cm or less spatial resolution). In order to convey the information symbols in UWB communications several approaches for the modulation techniques exist, mostly based on the classical base-band modulation types. Modulating the UWB pulse characteristics such as amplitude (PAM), time position (PPM), phase (PM), shape (PSM), or any combination of these, can be used. Another direct consequence of the large communication bandwidth is the possibility to accommodate many users, even in multipath environments. The two most common channelization and multiple access (MA) techniques in IR-UWB are: • Direct Sequence Spread Spectrum (DS-SS), similar to code division multiple access (CDMA) communication systems but using an UWB radio pulse as chip pulse shape and, • Time Hopping Spread Spectrum (TH-SS), which uses UWB pulses pseudo-randomly shifted in time domain.
For simplicity, we restrict our study to TH-SS (SB) IR-UWB communication systems using binary PPM (BPPM). Furthermore, we make abstraction of the pulse shape and of the corresponding optimal detection techniques that can be used.
users/devices, and the multiple-access pulse collisions are the only sources of link level errors that are considered and analyzed. 3.
MEDIUM ACCESS ISSUES FOR IR-UWB WPAN
2.2. TH-SS UWB description For a multi-user (device) scenario, the format of the transmitted TH-SS IR-UWB signal, stx(k ) , corresponding to the k − th user (device) is given by: s tx( k ) (t ( k ) ) =
∞
∑w
p (t
(k )
− jT f − c (jk )Tc − d (jk )δ )
(2)
j = −∞
where t ( k ) is the k − th transmitter's clock time; w p (t ) is the used UWB pulse; T f is the time frame allocated for each UWB pulse; Tc is the time shift step used in channelization / MA together with the c (jk ) TH code (sequence) allocated to each communication channel/user (device); δ is the additional time shift used for M-PPM of the information symbols,
d (jk ) .
To
increase
the
reliability
of
the
communication link, the same symbol can be repeated a certain number of times N p , increasing the processing gain of the system, i.e. d (jk ) = const. for j = [ p K p + N p ] and ) any integer p . A TH sequence c (k is typically a pseudoj
random sequence with period N p , where each element of the sequence is an integer in the range [0 K N h − 1] . The time frame T f is divided into equally spaced TH shifts, Tc , so that T f ≥ N h Tc . The TH codes can be chosen based on their auto- and cross-correlation properties, number of estimated users, etc. similarly to the CDMA systems. The M-PPM is achieved by using the additional time shift δ and Tc ≥ Mδ . In the remaining of this paper we consider only the binary PPM (BPPM) case, thus M = 2 and d (jk ) = 0 or d (jk ) = 1 .
In a multi-user scenario (ad-hoc networks, etc.) several transmitted signals, as given by (2), propagate through the radio channel, interfere with each other and add up at the input of the receivers of all the users. Additionally, due to the multipath fading, delay, attenuation and other noise sources in the radio channel, the received signals are further distorted. The choice for the systems parameters T f , Tc and δ is dependent on the desired system performances (throughput, reliability, etc.) and the radio propagation channel characteristics (multipath fading, dispersion, etc.). In our work we abstract the radio channel propagation mechanism in terms of signal fading, spread and additional noise. The absolute signal attenuation and delays, introduced by the different relative geographical locations of the
3.1. Medium Access Techniques The role of medium access techniques in wireless networks is to coordinate transmission access to common radio resources so that the interference among different transmissions is avoided or decreased and capacity (number of communication links with satisfied quality of service (QoS)) is maximized in the network. In the WPAN scenario, we have multiple collocated WPANs, each of which is composed of personal devices of a user such as PDAs, laptop PCs, etc. Since WPAN technology uses license-free wireless links, the radio resources should be shared among the collocated and uncoordinated WPANs. Thus, devices belonging to a single WPAN should actually share the channel not only among themselves but also with the devices belonging to the neighboring, physically collocated networks. In the design of medium access techniques in such a WPAN context, we need to take account of two different control levels of interference. The first level of interference is Intra-WPAN Interference which is the interference among links located in single WPAN. This level of interference can be controlled by employing traditional MAC protocols. For instance, in Bluetooth based WPAN, polling scheme with Master/Slave operation, which is a contention-free MAC technique, has been adopted for this coordination [10]. Also, random-access techniques such as carrier sense multiple access (CSMA), or a combination of contention-free and random access techniques can be employed. The second level of interference is Inter-WPAN Interference, which is the interference among links located in different WPANs. Since it is difficult to coordinate the transmissions and completely avoid interference among devices in these "uncoordinated" WPANs, the multiple collocated networks must share the resources in a sense to minimize the mutual interference, which can be achieved by adopting the transmission technologies that have the inherent immunity to the interference like SS technologies. In this paper, we focus on IR-UWB WPAN employing THSS transmission as shown in the previous section. Each communication channel is defined with single TH sequence, and one or several TH sequence(s) can be allocated to each WPAN. In each WPAN, the use of the TH sequence(s) is coordinated either by Master/Slave operation or it is regulated in distributed manner. Assuming that the Intra-WPAN interference is perfectly controlled by some MAC protocol in each WPAN, in this paper, we consider how to control second level of interference, that is, Inter-WPAN interference. The only way to control the Inter-WPAN interference is to change TH parameters such as N p , N h , Tc according to different situations, e.g., number of interfering WPANs. In order to discuss adaptive control of the Inter-WPAN
12
10
To investigate the interactions between the UWB physical layer and the MAC layer a custom-made simulation tool has been developed. The simulator investigates the interactions between collocated WPANs. One WPAN is taken as the WPAN under investigation and the other WPANs are considered as interfering networks.
Now the performance of the system with a varying number of collocated WPANs is considered. All WPANs are using the same parameters. The number of TH shifts or bins within the TH sequence of the WPAN is varied from 2 to 8. Figure 1 depicts the throughput for the case of a pulse repetition of 10. The highest throughput is obtained for the case where only 2 channels are assumed to be present and N h = 2 . With an increasing number of other networks (channels) the performance of the N h = 2 case degrades. As can be seen from Figure 1 after 10 networks became active the performance for the case of N h = 4 starts to outperform the case of N h = 2 . The better performance of networks applying a larger N h can be explained from the fact that each WPAN generates less traffic and thus it reduces the total amount of generated interference. Since the system is in an interference limited domain the performance is improved, in other words the additionally generated traffic in the case of
8
6
4
2
0
A WPAN is modeled as a cluster of nodes between which the communication channel is defined by a unique TH sequence. Since here we are interested in the inter-network interference the internals of a WPAN are not modeled. For the sake of simplicity, we assume that only one TH sequence is allocated to a WPAN. The WPAN is only represented by the TH sequence, which is used to transmit a constant flow of information. We assume perfect coordination of this TH sequence within a WPAN, which means that there is always only one transmission at a time in each WPAN.
2
4
6
8
10
12
14
16
18
20
Number of channels Number of channels
Fig. 1 Throughput for N p = 10 and N h = 2, 4, 8 PuRe = 10 Np=10
0
10
base=2 Nh=2 base=4 Nh=4 base=8 Nh=8 -1
10
-2
10
BER BER
In accordance with the regulations of the FCC the frequency band was chosen as f L = 3.1 GHz and f H = 10 GHz with a center frequency of 6.85 GHz, resulting in an effective pulse duration of 162.64 ps. The selected modulation method is Disjoint BPPM. The propagation aspects of the wireless channel were modeled using the free space model. The area used in the simulations is a square of 15m. Assuming that the delay spread of the channel is less than 20 ns, the size of one bin, containing two pulse positions, was set to be 2 × 20 = 40 ns. The transmitted power in the WPANs is, P = −2.5 dBm. The simulation results are averaged over 5 runs of simulations of more than 12.000 transmitted bits.
x 10
base=2 Nh=2 base=4 Nh=4 base=8 Nh=8
Throughput
3.2. Simulation Model and Results
PuRe = 10 Np=10
5
14
Throughput
interference, we have to investigate how these parameters affect the interference among different links with different TH sequences, how these parameters interact with each other, and how the throughput and bit error rate (BER) performance are influenced by changing these parameters. Therefore, in the following section, we investigate the relationship between network performance and these parameters by simulation.
-3
10
-4
10
-5
10
-6
10
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20
Number of channels Number of channels
Fig. 2 BER for N p = 10 and N h = 2, 4, 8 N h = 2 is lost in collisions. This gives an intuition for possibilities to optimize the UWB system for the number of collocated networks.
From Figure 2 it can be seen that the BER of the system increases as the number of collocated WPANs increases. The BER can be used as a measure to indicate the number of collocated WPANs. Based upon Figure 2 a heuristic for adjusting the value of N h for a fixed target BER can be developed, which can be used to create a reliable packet flow after coding. For example if the target is set to 10 −2 then the system can start with applying N h = 2 , with 4 or more collocated WPANs the number of bins is increased to 4, and with 12 collocated WPANs the number is further increased to 8. Maintaining a target BER is investigated further in Figure 3, which depicts the number of WPANs that can co-exist while
• Soft link adaptation - The values of N h and N p are
20
Nh = 2 Nh = 4 Nh = 8
changed, but N h ⋅ N p = const., such that the transmission rate is constant, i.e. Tb = const.
No. channels for BER < 0.1
15
The hard link adaptation requires explicit exchange of information about the spreading sequence among all members of a WPAN, since they should have the same representation of the current nominal bit rate within the WPAN. Hence, the hard link adaptation necessarily introduces a communication overhead. The hard link adaptation determines the value of:
10
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0 2
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Np
Fig. 3 Number of maximum channels for a fixed BER for varying N p (BER @ 10 −1 and N h 2-20).
maintaining a target BER, here the target was set to be 10 −1 . The number of WPANs (channels) is studied as a function of the processing gain, parameterized by the number of bins in a frame. The higher the processing gain N p , the less sensitive the system is to interference, as before also the number of bins N h can be increased, reducing the number of transmissions and thus the collision (probability).
α=
Tb Tc
(4)
On the other hand, the soft link adaptation can be applied autonomously, by each device in the WPAN, during its transmission and it does not introduce a communication overhead. Let the minimal value of the hopping base, used in ) in (2) are the system, be fixed at min N h = h0 , such that c (k j chosen pseudorandomly and uniformly from [0 K h0 − 1] . For given α , the maximal value of the processing gain is N αp = hα . For soft link, the transmitter uses chip puncturing 0
by transmitting only N p pulses per bit, where N p < N αp and the value q =
4.
SYSTEM DESIGN FOR INTERFERENCE MITIGATION
4.1. Hard/Soft Link Adaptation The discussion in the previous section sets a basis to consider the link adaptation in the WPANs based on IR-UWB. In this case, the link adaptation is a mechanism to adapt the TH parameters in order to maximize the throughput achieved by the WPAN. Our proposal for a link adaptation procedures considers that the system has a fixed chip duration Tc . This choice will keep the pulses intact, while all the changes for the actual transmission at the physical layer will be induced by change of parameters at the MAC/link layer. To start with, we assume that the time frame is fully used for chip transmission, i.e. T f = N hTc . Then, the bit duration can be represented as a function of N h and N p as follows: Tb ( N h , N p ) = N h N pTc
and the nominal bit transmission rate is Rb =
(3) 1 Tb
.
Np N αp
≤1.
At the conceptual level, the puncturing can be done pseudorandomly: If a pulse should be transmitted in certain ) slot, as determined by c (k j , the WPAN device transmits the pulse with probability q and with probability 1 − q the device stays silent in that chip interval. Note that this decision does not need to be coordinated with the receiver, who will perceive the lack of pulse as an erasure in the channel. The average number of pulses transmitted per bit is q ⋅ N αp . To support soft link adaptation, we assume that the devices within a WPAN share a TH sequence defined with a TH base of size h0 . By using the puncturing rate q < 1 , the actual TH base N h is increased, N h =
α qN p
> h0 . Alternatively, during
the hard link adaptation, all WPAN devices can adopt the same value of q , which is autonomously changed afterwards by each WPAN member via soft link adaptation. The actual puncturing may be performed according to a pattern different from the described pseudorandom one.
The WPAN communication is adapted to the channel conditions by varying N h and N p . This variation should be
To see the applicability of the hard and soft link adaptation, we must consider the interference model for a WPAN.
robust with respect to the basic timing mechanisms used in the TH-SS communication. We define two basic techniques that can be used in the link adaptation:
4.2. Interference Mitigation
• Hard link adaptation - The nominal bit transmission rate i.e. Tb is changed;
Since a WPAN uses open access in the unlicensed spectrum, we consider that the errors are induced from the interference with other devices with unlicensed operation, such that the communication is interference-limited. For the purpose of
link adaptation, WPAN differentiates between 2 interferer types: 1.
Interference from a collocated non-WPAN entity;
2.
Interference from identical, collocated WPAN.
The essential difference between the two is that the interference from a non-WPAN interferer is regarded as statistically invariant. That is, the WPAN considers that the interference that the WPAN itself causes to the non-WPAN entity will not initiate change in the transmission pattern of the non-WPAN entity. The reliability of the bit transmission with respect to a non-WPAN interferer can be increased only by increasing the number of transmitted pulses per bit, Np. If the satisfactory value of Np is determined, then there should be no puncturing and a hard link adaptation set the value of Tb in the WPAN to be: Tb = N p h0Tc
(5)
The situation is different when the interference is induced by collocated, identical WPANs. The interference that a WPAN is causing to the collocated WPANs is determined by the number of pulses transmitted per second, which, for fixed Tb and given q can be expressed as
qN αp Tb
. Each WPAN is
1
considered as a selfish agent that applies a strategy to maximize its throughput but it is aware that the other WPANs are also applying such strategies. If all collocated WPANs increase the pulse repetition rate, the transmission reliability is not improved. In order to host more collocated WPANs, as the results in previous section show, the TH base of each WPAN should be increased. This can be achieved by the described chip puncturing. We have described the mechanisms to apply link adaptation. However, one key issue remains - how will the WPAN know whether the interference is caused by WPAN or non-WPAN device? We outline the basic procedure: A WPAN sets the value of Tb to be low and sets initial value of q < 1 . If the BER is not satisfactory, each device tries autonomously to vary q in order to reach higher throughput. If there is unsatisfactory BER for some predefined timeout, a device in the WPAN (e.g. the master) may initiate hard link adaptation to increase Tb . The investigation of the optimal interplay between the hard/soft link adaptation to maximize the throughput is out of the scope of this paper. 5.
CONCLUSION
In this paper, we have discussed medium access techniques in IR-UWB WPAN using TH-SS and PPM. We have evaluated BER and throughput performance in multiple WPAN scenario by computer simulation and shown that the choice of TH parameters largely influences the BER and throughput performance. Based on these simulation results, we have proposed a link adaptation mechanism which can adapt TH 1
This terminology is borrowed from the game theory
parameters according to interference conditions. This mechanism maximizes the throughput achieved by WPANs while keeping the robustness with respect to the basic timing mechanisms used in the TH-SS communications. Our further investigations include the implementation and performance evaluation of the proposed link adaptation mechanism in dynamic WPAN scenario. Also, the future work includes the design of key functionalities for medium access such as control channel, beacon channel, synchronization in IR-UWB context. 6.
REFERENCES
[1] PACWOMAN Consortium, “State-of-the-Art of the WPAN networking paradigm”, Deliverable 5.1, PACWOMAN IST-2001-34157, 2002. Available: http://www.imec.be/pacwoman. [2] Federal Communications Commission “Revision of Part 15 of the Commission's Rules Regarding Ultra-Wideband Transmission Systems”, First Report and Order, ET Docket 98-153, FCC 02-48, April 2002. [3] IEEE Working Group for WPAN. Available: http://www.ieee802.org/15 [4] M. Z. Win and R. A. Scholtz, “Ultra-Wide Bandwidth Time-Hopping Spread-Spectrum Impulse Radio for Wireless Multiple Access Communications”, IEEE Trans. Commun., Vol. 48, No. 4, pp. 679-691, April 2000. [5] J. Foerster, E. Green, S. Somayazulu, and D. Leeper, “Ultra-wideband technology for short- and mediumrange wireless communications,” Intel Technical Journal, May 2001, http://developer.intel.com/technology/itj [6] M. Z. Win, R.A. Scholtz, "Impulse Radio: How it works", IEEE Comm. Letters, Vol .2, No.1, pp. 36-38, January 1998. [7] Domenico Porcino, “UWB Regulations \& Coexistence: From the FCC First Report and Order to the Path to Approval in Europe”, Tutorial at International Workshop on Ultra Wideband Systems, Oulu, Finland, June 2003. [8] N. Askar, "Overview of General Atomics PHY proposal to the IEEE 802.15.3a", IEEE P802.15, March 2003. [9] J. Joe, "Cellonics UWB Pulse Generators", International Workshop on Ultra Wideband Systems, Oulu, Finland, June 2000 [10] Bluetooth SIG, Bluetooth Core Specification, ver. 1.1, February 2001, http://www.bluetooth.com
