Ad Hoc Networks
Technologies enabling ad hoc

The most widespread technologies for wireless ad hoc networks are WiFi based on IEEE 802.11 and Bluetooth based on IEEE 802.15.

## WiFi

WiFi is wireless network technology used especially for LAN (Local Area Networks) with small range (at most hundreds of meters) and provides high speed internet access to users inside households, offices, shopping centres, etc. The bit rates achieved by WiFi equipment depends on the supported standard. The most common bit rates are up to 54 Mbps per one access points (IEEE 802.11a/ IEEE 802.11g) and newly up to 300 Mbps (IEEE 802.11n).

The advantage of WiFi is especially in its low cost and that it can be used in licence-free ISM (Industry, Scientific and Medical) frequency bands. To be more specific, 2.4 GHz and 5GHz bands are usually dedicated for WiFi.

- 2,4 GHz – For most of the Europe countries is dedicated 13 channels, with bandwidth equal to 22 MHz at frequencies between 2.4 GHz to 2.484 GHz, are assigned. Note that for America, only 11 channels is assigned. The adjacent channels are partially overlapping. Consequently, only three channels could be found that do not interfere with each other
- 5 GHz – The band is further divided into three sub-bands:
  - 1. sub-band (5150 – 5250 MHz) – used inside buildings
  - 2. sub-band (5250 – 5350 MHz) – used inside buildings (the UEs must be able to control their power and to switch to another band if interference to radar occurs)
  - 3. sub-band (5350 – 5450 MHz) – the same rules as for the second sub-band are applied

Since WiFi is wide spread technology, allocated radio channels become often overloaded. Consequently, lots of modifications in optimal sharing of radio channels and the access to it have to be specified. That is why many versions of WiFi have been approved so far in order to avoid collisions, to improve its throughput, and capacity.

All equipments based on IEEE 802.11 are tested by WECA (Wireless Ethernet Compatibility Alliance) organization, which was renamed in 2002 on WiFi alliance. The WiFi alliance tests whether the equipments fulfil all necessary requirements or not.

The IEEE 802.11 standards define MAC layer that is together with LLC (Logical Link Control) a part of second layer of RM-OSI (Reference Model - Open System Interconnection) model. The purpose of MAC layer is to handle an access to the radio channel, fragmentation, and defragmentation of data packets, specifications of control frames, etc. In addition, WiFi standards specify several types of physical layers differing especially in various modulation techniques (frequency hopping, direct sequence spread spectrum or OFDM). As already mentioned, WiFi can use infrastructure or ad hoc topology (optionally also mesh, as it will be described later).

## Bluetooth

Bluetooth specifies short range wireless communication. It is characterized by low transmitting power and low price. The purpose of the Bluetooth is to replace metallic interconnection of various electronic devices such as mobile phones, headsets, laptops, etc.

The basic transmission rate of the Bluetooth is 1 Mbps, but it can be extended up to 3 Mbps for version 2.0 with EDR (Enhanced Data Rates). In addition, high speed transmission up to 24 Mbps can be reached by Bluetooth v3.0 + HS (High Speed). This version enables to exploit WiFi bands for Bluetooth communication. Last version, Bluetooth v4.0 is focused on lowering power consumption. Thus, this version is also denoted as LE (Low Energy).

In all version of Bluetooth, FHSS (Frequency Hopping Spread Spectrum) is utilized to cope with the problem of interference and fading. In FHSS, all devices use a frequency hopping pattern of pseudo-randomly selected a carrier frequency for transmission of a packet. The carrier frequency is hopped 1600 times per second.

Physical layer transmission of Bluetooth is performed in license-exempt frequency bands of 2.4 GHz with bandwidth of 83.5 MHz. Specifically, frequencies between 2400 MHz and 2483.5 MHz are occupied. In basic transmission rate and EDR, the bandwidth is split into 79 transmission channels and two guard bands. Lower and upper guard bands are of 2 and 3.5 MHz bandwidth respectively. Both basic rate and EDR use full duplex and TDD transmission scheme.

The data are modulated with a binary GFSK (Gaussian Frequency Shift Keying) for basic 1 Mbps transmission in Bluetooth v1.0. For higher bit rates in later versions, PSK (Phase Shift Keying) based modulations are used. The modulation π/4-DQPSK (pi/4 Rotated Differential Quadrature Phase Shift Keying) and 8 DPSK (8 phase Differential Phase Shift Keying) used in EDR mode enable to reach 2 and 3 Mbps respectively. To reach 24 Mbps, so called AMP (Alternate MAC/PHYs) operation must be enabled. After establishing EDR radio channel, AMP finds alternative wider band and shift the transmission to this band.

A different physical channel and access to the channel are defined to enable low power consumption. Contrary to basic rate and EDR, only GFSK modulation is considered by Bluetooth LE. The same bandwidth but with different subchannelization pattern is used. The band is split into 40 channels separated by 2 MHz with 2 and 1.5 MHz lower and upper guard bands. For access the channels, either TDMA or FDMA can be used. Three channels are used for "Advertisement" and 37 channels for data communication. In advertisement channels, devices can indicate what they intend to perform (i.e., setting a connection or preparing for data transmission)

Bluetooth supports direct point-to-point as well as point-to-multipoint communication. All communicating devices sharing the same channel is denoted as a piconet. In each piconet, a device is master to all others and the other devices are slaves to the master one. The device initiating the communication is labelled as the master. All slave devices must be synchronized to the master’s clock and must follow the same hopping pattern as the master. The communication is able only between master and slave, but not between two slave devices. Up to seven slaves can be active in each piconet for basic data rate and EDR. Beside the seven active devices, additional up to 255 non-communicating devices (in parked state) can be also included in the piconet. In Bluetooth LE, the number of active devices is limited by amount of master's radio resources.

A device can be included in more than one piconet. In this case, the piconets mutually overlap each other and the topology is denoted as scatternet. The scatternet is composed of several piconets while even in scatternet, each piconet still contains only one master device. Each device can be the master only in one piconet in frame of the scatternet. In all other piconets, the device must take over the role of the slave. Therefore, the role of master and slave can be switched among devices in scatternet to avoid the situation when a device is the master in two piconets. If two piconets partly overlapping and having different masters, different pattern of frequency-hopping is used in each piconet. Therefore, a device participating in more than one piconet must apply time multiplexing and access individual piconets one-by-one.

Topology for Bluetooth