Tan Zhenhui,Qiao Xiaoyu

(Beijing Jiaotong University， Beijing 100044， P. R. China)

Abstract:The short-distance low-power technologies for wireless access communications,including Bluetooth,Ultra-Wideband(UWB)and Wireless Fidelity(Wi-Fi),focus on the physical layer and Medium Access Control(MAC)layer.Bluetooth is a wireless protocol for exchanging data over short distances.It is an alternative to RS232 data cables.UWB is a radio technology that can be used at very low energy levels for short range.It provides flexible high-bandwidth wireless access.Wi-Fi enables the wireless connection of fast-access devices and mobile devices in Wireless Local Access Network(WLAN)and it is suitable for long distances.The standardizations of the above technologies have been developed and published soon after the emergence of these technologies.Among all the standards,Bluetooth and Wi-Fi are the most important ones.

I n a short-distance low-power wireless access network,the communication range is less than I 00 m and the transmitted power is between I-I00 mw.This kind of access network gives a solution for last-mile access and hot-spot communication coverage.Compared with the wired Local Area Network(LAN),it allows mobile users either access or quit from the network easily.It makes the terminal devices portable and exploration of wireless network resources easy.However,it has some disadvantages,such as low reliability,low data transmission rate and high power consumption.The data encryption and user security are also the problems need to be solved.

The short-distance low-power wireless access technologies,such as Bluetooth(IEEE 802.I5),Ultra-Wideband(UWB)(IEEE 802.I5.3a)and Wireless Fidelity(Wi-Fi)(IEEE 802.II),focus on the Physical Layer and Medium Access Control(MAC)Layer.Bluetooth is an alternative technology to RS232 data cables.UWB connects various electronic devices,providing flexibility and portability of wideband wireless access.Wi-Fiis a seamless extension to the short-distance wireless environment as an alternative to wired LAN.

1 Bluetooth

In I994,Ericsson began to study low-power communication system to replace cable communication system mobile in short distance..In I998,Ericsson,Nokia,IBM,Toshiba and Intel organized the Bluetooth Special Interest Group(SIG).In I999,the Bluetooth SIG released the first version of Bluetooth protocol.In 2000,3COM,Agere,Microsoft and Motorola joined in the Bluetooth SIG.In 2002,the IEEE 802.I 5 group ratified the Bluetooth protocol as IEEE 802.I5.I［I］.

As a Wireless Personal Area Network(WPAN)access technology,Bluetooth is conceived as an alternative standard to short-distance,low-cost cable transmission.It is also used either as a bridge between other networks or a node in the Ad hoc network.An active Bluetooth device listens and responds to the callings of the master device.Once the master device knows the address of the user device,it starts to connect with the user device.The user device responds to the queries of the master device,establishes frequency-hopping sequence synchronization for authentication and communication.The devices which are not used for authentication or transmission are in the bandwidth and power-saving mode.

Bluetooth replaces short cables and transmits an Equivalent Isotropic Radiation Power(EIRP)of 30-I00 mVor I5-20 d Bm.Bluetooth operates in the license-free band at 2.4-2.4835 GHz.To avoid interfering with other protocols that use the same bandwidth,such as IEEE 802.IIg,Bluetooth protocol divides the band into 79 channels(each I MHz wide)and changes channels up to I,600 times per second by Gaussian Frequency Shift Keying(GFSK)modulation and Frequency-Hopping Spread Spectrum(FHSS)features.The Automatic Repeat Request(ARQ)error correction mechanism is used on the MAC layer to protect against strong narrow-band interference.The earlier version Bluetooth protocols support I Mbit/s data rate,however,the data rate are adaptable to change according to different wireless application environments.

In terms of service quality,the connectionless-oriented Asynchronous Connectionless Link(ACL)of Bluetooth uses complex channel admission control and planning policy to ensure Quality of Service(QoS).Asynchronous service QoSparameters include long-period data rate,peak data rate,latency and jitter.Bluetooth also provides synchronous services at a constant bit rate on Synchronous Connection-Oriented(SCO)links.The definition of Bluetooth space capacity is the ratio of the total data transmission rate over the covered area.Generally,Bluetooth covers 20 picocells with a diameter of I 0 m.The maximum data transmission rate of a picocell is 400 kbit/s.The Bluetooth space capacity is 25 kbit/s per square meter.The traffic in all the picocells is controlled by the Bluetooth host point.The picocell host controls the subordinate access channels.The LAN access protocol is used to connect external networks.

Allthe Bluetooth scatternets are kind of Ad hoc networks with different types of topologies.The scatternet has a picocell sub-architecture,in which a global connection function is operated between mobile stations by the IPlayer using a globaladdressing and routing mechanism.

Bluetooth initializes the communication link between devices by a query process.The host starts the query program periodically to discover the MAC address list of the devices in the covered area.The master device queries through paging using the MAC addresses and clocks of the user devices.Bluetooth defines the wireless interface and the communication stack for allowing mutual device authentication and service provisioning.Figure I shows a Bluetooth communication stack.

The link management layer in Figure I is responsible to the type of link structure,authentication,encryption,QoS,power consumption and transmission format.The control layer provides a command interface for the link management layer and baseband,and provides relevant interfaces to hardware devices.The Logical Link Control and Adapatation Protocol(L2CAP)provides connection-oriented and connectionless data services to the upper layers(for example,the segmentation and reassembly from the upper-layer protocol data unit,and the support for QoS).The RFCOMM Point-to-Point Protocol(PPP)can be used directly on L2CAP.

Bluetooth devices work in the master and slave mode［2］.In the basic Bluetooth network,one master device and seven slave devices form a picocell There would be up to 255 slave mobile stations in the waiting mode,when mobile stations do not exchange data.Picocells are able to be connected to make up a scatternet.Ascatternet is a multi-hop wireless network in terms of topology,in which there are no direct path between two nodes,so they must be relayed through other nodes.In addition,two picocells can communicate through their common node,which would be the master node or a slave node.

Bluetooth supports three types of encryptions:No encryption(Type I),encryption at the service layer after a channel is established(Type 2)and the encryption at the link layer before a channelis established(Type 3).There are two levels of encryptions:Trusted and untrusted.There are three levels of encrypted services:Open service,authenticated service and authenticated and authorized service.The authentication and encryption at the link layer depend on four basic parameters:Bluetooth-corresponding 48-bit address identifier,special authentication key,special encryption key and I28-bit random frequency variable.

2 UWB

Different from the conventional continuous wave radio technology,UWB encodes mass information on the short-pulse sequence and allows the transmission on UWB spectrum with the extension of very low power spectrum.

The UWBtechnology can be used for the wireless Ad hoc network,Free Space Optical(FSO),Zig Bee,Radio Frequency Identification(RFID),Radio over Fiber(RoF),Bluetooth and Home Radio Frequency(HomeRF)systems.The short-distance low-power wireless access system using UWB has the following advantages:

(I)High capacity

Signaling energy spreads on the UWBspectrum with white noise,which increases channel capacity.

(2)Low possibility of multipath fading and high interference immunity

The resolution of short-pulse signals in multipath channels is higher than I ns,which can greatly reduce the effect of multipath fading.The interference immunity is proportional to the signal bandwidth.The greater the signal bandwidth is,the higher the interference immunity is.

(3)Time diversity and frequency diversity

UWB pulses last a very short time,leaving blank spaces between the previous and current pulses.It is easy for the rake receiver to combine UWB pulse signals from different paths to increase the density of detected signals.As the preferred technology for the Personal Area Network(PAN),the IEEE 802.I 5.3a UWBhas many advantages,including high data rate from I00 Mbit/s to I Gbit/s,low power consumption and low cost.However,it uses a very high bandwidth,which may cause interference and compatibility problems when it shares a channel with other communication systems［3］.

Figure 2 illustrates the protocol architecture of a UWB system.The IEEE I394 convergence layer is similar to the IEEEI 394 link layer.It is responsible for the mapping between the transaction layer and the UWB lower layers.The convergence layer contains an IEEE I394 Specific Service Convergence Sublayer(SSCS)and a Common Part Convergence Sublayer(CPCS).

▲Figure 1. Bluetooth communication stack.

▲Figure 2. Protocol stack architecture.

▲Figure 3. UWB centralized control network topology.

In the Physical Layer and Data Link Layer of the UWB system,the phases of the short pulse sequence are modulated by information symbols.UWBpulses can be modulated using biphasic Gaussian pulses or pulse amplitude,or modulated strictly using pulse intervals according to the position of the very-narrow Gaussian pulses.The typical pulse bandwidth is 0.2-2.0 ns,and the pulse interval is between I 0-I00 ns.Pulse positions can be changed,at random or pseudo-random intervals.

Figure 3 illustrates a UWBcentralized control network topology.This architecture adopts P2Pcommunication and Ad hoc centralized control in the Ad hoc network.It consists of several picocells.Each picocell selects a master station and controls other slave stations automatically.If UWBis used for the network topology in a distributed control mode,it is difficult to control the output power and interference of each node.

To further expand the coverage of the UWBsystem,data flow control and network layer management connect the UWBwireless bus and the IEEEI394 backbone network through two types of bridges,as shown in Figure 4.The IEEE I394 bridge works as an interface to the wired and wireless buses.It provides the following functions for the data flow between the IEEEI394 backbone network and the UWB wireless bus:

·Establishing synchronous channels;

·Logicalmapping between channels and UWB data link channels;

·Asynchronous packets and control data flow routing between the wired and wireless;

·Clock synchronization between IEEEI394 and UWB;

·Synchronization between frames and groups;

·Adding or removing IEEEI394 nodes and UWB mobile terminals;

·Configuring relevant frequency resources.

3 Wi-Fi

In I 997,IEEEpublished IEEE 802.II standards,which is the basis of Wi-Fi.Wi-Fioperates at 2.4 GHz(2.4-2.4835 GHz in North America,2.47I-2.497 GHz in Japan),and supports I Mbit/s or 2 Mbit/s data rate.Wi-Fi provides wireless connections for fast access devices and mobile devices within WLAN.

The Wi-Fistandard specifies the MAC programs accessing the physicalmedia,MAC layer processing portability and the cross-cell hand-over between adjacent cells.In I997,IEEE released IEEE 802.IIa［4］that uses the rates of 6,9,I2,24,36,48 and 54 Mbit/s,with the

bandwidth of 5 GHz.It cannot use the IEEE 802.IIh dynamic frequency selection and auto-sensing power control technologies.After the verifications of Lucent and Harris,IEEE approved IEEE 802.IIb［5］that uses the rate of 5.5 or II Mbit/s.In 2003,IEEE released IEEE 802.IIg［6］,which has similar performance to IEEE 802.IIa,and is compatible with IEEE 802.IIb.

Besides using the 5 GHz frequency-bands,Wi-Fiuses the Direct Sequence Spread Spectrum(DSSS)with a bandwidth of I 6 MHz,Complementary Code Keying(CCK)and Orthogonal Frequency-Division Multiplexing(OFDM)technologies.DSSScan reduce broadband noise.Wi-Fisends and receives signals at the same frequency.

The gain for retransmitting packets comes from time diversity.Wi-Fiadapts to the rate at the Physical Layer and is transparent to the higher layers.It uses different modulation depending on different rates.Wi-Fisupports the rates between I-54 Mbit/s.Wi-Fiuses ARQ error correction,coding and multiplex technologies at the MAC layer to protect against the effect of noise.IEEE 802.IIa and the cordless phone share the 5 GHz frequency band.Wi-Firesists interference by using a transmission power controltechnology.

▲Figure 4. A bridge connecting a UWB wireless bus and an IEEE 1394 backbone network.

Once a Wi-Fimobile station is powered on,it scans all the wireless channels and searches for the primary network that transmits beacons.It selects the Ad hoc network that contains the Wi-Fimobile station,and authorizes,authenticates and connects the Wi-Fi mobile station through the Access Point(AP),and provides QoSfrom best-effort to priority assurance.When Wi-Fimobile stations become components of the Ad hoc network,they have the capability to discover a new network or disconnect from the old network and form a new network.Wi-Fimobile stations can roam among communication networks,share public distribution system resources and allow seamless mobile transmission.

When Wi-Fimobile stations become the infrastructure of the fixed communication,the access point will release authentication and convergence and enter a sleep status.

Wi-Fihas defined two types of access:Distributed Coordination Function(DCF)and Point Coordination Function(PCF).The simplest network architecture is an Independent Basic Service Set(IBSS).It contains the Ad hoc topology of at least two base stations.A Basic Service Set(BSS)is the expanded network of an Extended Service Set(ESS).In the fixed communication infrastructure,an extended service set is a set of BSSes connected through a distribution system.Figure 5 illustrates the typical components of a Wi-Fi network.Services provided by base stations include base-station service and distribution system service.Distribution system services allow data to be transmitted between the base stations in different BSSes.

To ensure reliable access,Wi-Fi authenticates user devices at the link layer.User devices are authenticated in two ways.One is Open System Authentication(OSA)and the other is Shared Key Authentication(SKA).Communication devices in an IEEE 802.IX/EAPframework can provide algorithm options at different security levels.

The Wi-Fi WLAN architecture is based on the BSS.A BSSis a set of mobile or fixed Wi-Fimobile stations,which access network transmission media with certain a control mode.Wi-Fi allows four interference-free BSSes.The BSSdata transmission rate in a coverage with a diameter of I00 m is 900 kbit/s,or 3I.4 Mbit/s in a coverage with a diameter of I0 m.IEEE 802.IIg supports a space capacity of 0.I kbit/s per square meter at the lowest rate,and 400 kbit/s per square meter at the highest rate.

The multiplexing technologies that Wi-Fiuses include DSSSand OFDM.DSSSuses the II-bit Barker series.The II-chip sequence modulates one information bit.IMbit/s and 2 Mbit/s modulations use Binary Phase Shift Keying(BPSK)and Orthogonal Phase Shift Keying(QPSK),respectively.CCK is the I 6-bit sequence number 4 or 8 information bits.

In the fixed network and the Ad hoc network,Wi-Fimobile stations run the MACprotocolusing DCF,and use Carrier Sense Multiple Access/Collision Avoidance(CSMA/CA)as the channel access technology.There are many types of basic DCFaccess.Mobile stations can optimize and select the Request To Send/Clear To Send(RTS/CTS)mechanism to reduce the collision caused by hidden terminals.Network Allocation Vector(NAV)calculates the termination time of the data transmission as a mobile station counter.IEEE 802.IIe defines the Enhanced Distribution Channel Access(EDCA)and Hybrid Coordination Function Controlled Channel Access(HCCA)as the new coordination modes to provide the Hybrid Coordination Function(HCF)function.EDCA provides 8 data priority levels.Each mobile station assigns a different Inter-Frame Space(IFS)value to the channels at different priority levels.A queue with a higher level uses a shorter IFSto get a higher priority of channel access and a higher priority of back-off time.In HCCA,a mobile station is responsible for providing the HCF function with centralized control.HCCA ensures the QoSof service rate,time delay and jitter.

Wireless packets are often lost in the short-distance low-power wireless access system,so it is very difficult to maintain the synchronization between encryption and decryption at any time interval.The Wi-Fisecurity framework uses the Wireless Equivalent Protocol(WEP)that utilizes the RC4 stream cipher.In 2002,it defined the Wireless Protection Access(WPA)as an improvement of WEP,and is used by IEEE 802.IIi.WPA uses 802.I X/EAPas the framework,uses the Temporary Key Integrity Protocol(TKIP)for cipher encryption and the Extensible Authentication Protocol(EAP)for authentication.In 2004,the IEEE 802.IIi Task Group announced WPA2 as an integrated authentication framework based on IEEE 802.IXand EAP.Different authentication and keys for different application environments generate different EAP.Two cipher suites are defined,i.e.,the upgrade software TKIP for the existing devices and Advanced Encryption Standard(AES)-based Counter Mode with Cipher Block Chaining Message Authentication Code Protocol(CCMP).

4 Power Consumption Control

The short-distance low-power wireless access system is mainly used for the communication access of pocket-size terminals which are mobile in a short distance and have a limited power supply.Bluetooth and UWB require very low power consumption.Wi-Fiis used for long-distance connections and supports the access of mobile terminals with a certain power.

?Figure 6.Wi-Fi power management statuses in a fixed-architecture network.

4.1 Bluetooth Power Management

Bluetooth power can be managed using the following methods:Backup mode without data switching but clock running only;and data switching mode in which terminal devices are connected to the picocell host.There are 4 modes of connections:Activated mode,in which devices in the picocell are in active status;listening mode,devices are in listening status with low power consumption;hold mode,in which the device ACLtraffic is blocked in a period of time;waiting mode,in which devices are no long members of the picocell but are still synchronous with the picocell host,with the lowest power consumption.

Cambridge Silicon Radio(CSR)has developed a Complementary Metal-Oxide Semiconductor(CMOS)single-chip Bluetooth device.Single-chip Bluetooth includes baseband BlueCore and wireless BlueCore.Baseband BlueCore contains the firmware that provides link controller,link management and host controller interface layer functions.Wireless BlueCore provides flash storage and power-saving functions.BlueCore supports two modes of low power consumption:Doze mode,which lowers the processor clock rate;and deep sleep mode,in which most of circuits are in closed status.

4.2 Wi-Fi Power Management in Fixed Infrastructure Network

Wi-Fidevices have two statuses:Wake-up and sleepy status in which devices neither transmit nor receive signals.The devices have two power management modes:Activated mode and power-saving mode.Figure 6 illustrates the Wi-Fipower management statuses in a fixed-architecture network.To enter power-saving mode from activated mode,mobile stations must send the power management bits in the packet header to the access point.The access point stores the traffic of all the mobile stations that are in power-saving mode.With certain algorithm,the mobile stations in power-saving mode are switched over to the activated mode to receive beacons.The mobile stations without access traffic returns back to power-saving mode.

4.3 Wi-Fi Power Management in Ad Hoc Network

There is no access point in the Ad hoc network,so the mobile stations in power-saving mode transmit Ad hoc Traffic Indication Message(ATIM)frames to switch over to the wake-up status during the ATIM window period.The activated stations store the traffic of all the mobile stations in power-saving mode.Mobile stations receiving the ATIM frames keep in the wake-up status to receive message traffic until the next ATIM window,and then go back to power-saving mode.

Compared with the power management in the fixed infrastructure network,because no access point can be used as a reference station,the wake-up or sleepy status of mobile stations is evaluated by other mobile stations in the Ad hoc network.During the ATIM window period,the transmission and reception of ATIM frames obey the DCFand Carrier Sense Multiple Access/Collision Avoidance(CSMA/CA)access algorithms.

5 Latest Development of Short-Distance Low-Power WACS System

The short-distance low-power wireless access system is standardized at a fast speed.There are many standards,but Bluetooth and Wi-Fiare the most useful ones.

So far there have been five core versions of technologies since Bluetooth was published.In 2007,the Bluetooth Special Interest Group released the latest version of Bluetooth technology,2.I+Enhanced Data Rate(EDR).The transmission rate of Bluetooth has improved greatly since then.With new EDRfunction,Bluetooth can support a rate up to 2 or 3 Mbit/s,while previous Bluetooth generally provides a rate of I Mbit/s.In 2009,the Bluetooth Special Interest Group［7］launched one high rate solution and one low power-consumption solution.

In 2006,the Bluetooth Special Interest Group announced to use the WiMedia Alliance's UWB technology as the high-speed solution for Bluetooth.The Bluetooth Special Interest Group also utilizes Wi-Fiby adding 802.IIa,b and g into the Bluetooth 802.II AMP specification to realize device interoperation and high-speed data transmission.In 2008,the Bluetooth Special Interest Group announced a high-speed development policy formally.It developed a new wireless Radio Frequency(RF)solution as a replacement,which supports the temporary use of the backup wireless connection preset in the consumer Bluetooth devices to realize a higher transmission rate besides the previous support for the Bluetooth protocol,functions,security and pairing.

The Bluetooth low-power consumption specification will meet the consumer electronic product industry's requirement for the interactive remote control in communications.The low power consumption technology is designed for the long life of the Bluetooth devices.

To use the UWB technology,it is necessary to solve the problem of coexistence with other wireless systems.For the coexistence of UWB and 4G,Detection And Avoidance(DAA)has been paid great attention as an anti-interference technology for UWB in Japan and Europe.Current researches include the MB-OFDM-UWBsolution integrating OFDM,spread spectrum and interleave and frequency hopping mechanisms;the MIMO-OFDM-UWB assumption that uses the Multi-Input Multi-Output(MIMO)technology in the UWB system;applications integrating UWBand Mesh,and the convergence of UWB and smart antennas.

The Wi-Fi Alliance launched the Wi-Fi Protected Setup(WPS)technology［8］in the later 2006.This technology integrates the safest encryption WPA/WAP2.Furthermore,the simple configuration makes it easier for the WLAN products to be accepted by ordinary customers Wi-Fiis planning to use the MIMO technology in the next standard version.Airgo has developed the MIMO chip at a data rate of I 08 Mbit/s,which is compatible with the existing Wi-Fi.The IEEE 802.IIn Task Group has defined the MIMO physical layer specifications.