MULTIHOP RELAY CHANNEL MEASUREMENTS

Recently, several channel models have been developed for various environments and system topologies such as the COST 231, Stanford University Interim (SUI), and WINNER models. Unfortunately, statistical models of this type make a number of general assumptions that are not always met in practical WiMAX scenarios. For example, the SUI models are suitable for FWA (rather than MWA) and they are based only on cellular-type measurements in the 1.9 GHz band. The COST 231 models do not formally apply beyond 2 GHz. The WINNER models do reach to frequencies at 5 GHz, but does not include frequency correction factors to enable their use in the 3.5 GHz band envisaged for WiMAX. Furthermore, there is little by way of empirical data collected in specifically multihop scenarios in the 3.5 GHz making an evaluation of these models in their intended deployments difficult. This part presents the results of a measurement campaign conducted in outdoor city environment to subject these models to empirical scrutiny.

1 MEASUREMENT CAMPAIGN

Measurements were carried out along seven routes in Bristol city centre, United Kingdom, comprised of five RS locations as shown in Figure 1. These routes were identified from a ray-tracing analysis as being likely to exhibit deep shadowing from the BS (which was mounted on the roof of a building approximately 30 m above the top of a hill). The system under consideration is a 2-hop DL, as shown schematically in Figure 2. A commercial 3.5 GHz panel antenna was used with a beamwidth of 90° in azimuth and 10° in elevation with 17 dBi gain. Since the BS antenna was not omnidirectional, its orientation and downtilt are important. Prior to commencing the main campaign, power measurements were taken at each location for downtilts between 0° and 10° following which during measurement the antenna was set to the downtilt which maximized the received power. The antenna was oriented approximately southwesterly for routes 1–5 and northeasterly for routes 6 and 7.

 

Figure 1: Map of Bristol, United Kingdom, showing the BS, the 7 MS routes, and the 5 RS locations.

 

Figure 2: Schematic of multihop measurement system. f₁ = 2.58 GHz, f₂ = 3.467 GHz.

The RS was on a portable pump-up mast that was left fixed in place once on location. Two dipoles were mounted on top of the mast at either end of a beam that could be rotated in a plane. One end was fixed to act as the RS–MS transmitter, and the other end rotated around it to act as the BS–RS receiver; this rotation was to permit measurements of the local variation of the BS–RS signal. The BS transmitted 20 W before antenna gain and after cable loss, etc., on 3.59 GHz and the RS 3.67 W on 3.467 GHz.

The MS was constructed on a trolley that was pulled or pushed along the selected routes. A laptop on the trolley provided “command and control.” The two wheels on one axle of the trolley were equipped with electronic pulse counters that incremented the counts on the laptop approximately every 4 mm. Also connected to the laptop was a spectrum analyzer which received the signal from the antenna. The antenna (mounted at 1.65 m above ground) was the same type of dipole as used at the RS. The mean length of a route was about 90 m, and each route was repeated with the RS at a selection of heights of {2,3,4,5} m.

Path lengths were determined by using survey grade GPS equipment to obtain positional fixes for the start and end of each route and the trolley’s distance pulses to estimate the route in between. Elevation was assumed to change linearly along a route. Path loss at a particular point was determined as the difference between the transmitted and received power after taking account of the system gains and losses in cables, amplifiers, etc. The effect of antenna patterns was approximated as being the gain in the LoS direction between the transmitter and receiver.

HIGHLY EFFICIENT MULTIHOP RELAY TOPOLOGIES

The big challenge for broadband wireless system design comes up with the right balance between capacity and coverage that offers good quality and reliability at a reasonable cost. It is important to look at system spectral efficiency more broadly to include the notion of coverage area. Results presented in previous sections have demonstrated the high potential benefits for relay deployment with radio resource sharing, in terms of interference, MIMO combination, and multiuser transmission. To implement the radio resource reuse and achieve highly efficient relay deployments, appropriate frequency reuse and multiuser access strategies are required. Relay systems must be based on a topology that fully exploits effective resource assignment based on the spatial separation of nodes. In this section, we propose directional distributed relay for highly efficient multiuser transmission with reduced demands on radio resource.

Figure 1 depicts the directional distributed relaying architecture. This is based on a paired radio resource transmission scheme, and it is possible to achieve one radio resource to one user (or one group of users) in average, even with multihop relay. The radio resource can be defined as either frequency (e.g., subcarriers in an OFDMA symbol) or time (e.g., OFDMA time slots). Transmissions in the BS coverage are the same as the IEEE 802.16e standard. For relay links, paired transmissions are applied, where the BS forms two directional beams, or uses two sector antennas to communicate with RS1 and RS2 simultaneously. A paired radio resources are required: f₁ and f₂. The first radio resource (f₁) is applied to the RS–BS1 link and also to the RS2–MS links (in the RS2 coverage); while the second resource (f₂) is applied to the BS–RS2 link and also to the RS1–MS links (in the RS1 coverage). Radio resources are shared between the RSs and MSs. Each end-user employs a single pair of radio resources, on average.

 

Figure 1: Directional distributed relaying with paired radio resource.

Using the sharing scheme outlined above the interference can be controlled at the BS and RS nodes. In this relay configuration there are only two sets of interference, as also illustrated in Figure 1. The interference between the BS and MS groups (I₁ and I₂) can be detected and controlled by the BS. First, the BS could employ an adaptive array to exploit the spatial separation of the groups. Second, since the received power by each MS in each MS group is known to the BS, the BS can apply interference avoidance  between the two groups based on measured signal to interference plus noise ratio (SINR) and power control, where the transmit power of the two RSs are controlled for balancing the SINR according to the service requirement. Furthermore, in this scenario the expected level of interference is small because the BS connects to the MSs through a relay, which means the relay SNR-gain will be much higher than the SNR_access level. Interference between RSs (I₃ and I₄) can be reduced by array processing (including the use of sector antennas) at the RSs. Interference measurement for the efficient resource assignment can be achieved during the neighborhood discovery procedure. To achieve high levels of SINR (e.g., 10–25 dB), array processing, including the use of sector antennas at the RS, is desirable.

This proposed topology is fully compatible with the existing 802.16e standard and no modifications are required at MSs. Alternative deployments topologies are also possible based on the same concept, such as a single RS to cover a coverage hole. In such cases, the radio resource sharing is performed between the RS and its BS. It could be complicated for statistical studies as the performance is fully dependent on the deployment scenario. However, it is much more feasible in a realistic application environment by employing real channel measurements and ray tracers.

CURRENT DEVELOPMENTS IN WiMAX TECHNOLOGY

WiMAX technology encompasses broadband wireless equipment which is designed in compliance with the IEEE 802.16 standard and certified by the WiMAX Forum. The IEEE 802.16e standard leverages several differences and enhancements over the 802.16-2004 standard to support mobile subscribers.

Scalable orthogonal frequency division multiple access (SOFDMA): Introduced in the 802.16e amendment over fixed WiMAX’s OFDM, SOFDMA supports scalable channel bandwidths from 1.25 to 20 MHz, using quadrature amplitude modulation (QAM; 16QAM or 64QAM) or quaternary phase shift keying (QPSK) modulation. SOFDMA enables additional resource allocation flexibility and adaptively optimized multiuser performance.

Advanced antenna technologies: MIMO PHY layer techniques have the potential to significantly increase bandwidth efficiency based on the premise that operation occurs in a rich scattering multipath environment. 802.16e defines optional support for such advanced antenna technologies. Major advantages of MIMO include diversity gains, multiplexing gains, interference suppression, and array gains. The inclusion of MIMO techniques alongside flexible subchannelization and adaptive modulation and coding (AMC) enables mobile WiMAX technology to improve system coverage and capacity.

In addition, 802.16e presents many advanced features for performance enhancements, such as handover support, quality-of-service (QoS) support, and energy savings mechanisms for handheld support, etc.

Multihop relay: Another milestone in the development of WiMAX was the introduction of multihop relay running as the 802.16j multihop relay project, which targets on OFDMA PHY layer and Medium Access Control (MAC) layer enhancements for licensed bands to enable the operation of RSs. The objectives of 802.16j are to enhance coverage, throughput, and system capacity by specifying 802.16 multihop relay capabilities and functionalities of interoperable RSs and BSs. Several technical topics were focused on, mainly including relay concepts, frame structure, network entry, bandwidth request, security, mobility management, routing, path management, interference control and radio resource management, etc.

The concept of multihop relaying is already well developed in the fixed telecommunications world. Microwave radio relays have been widely used to transmit digital and analog signals over long distances, with examples including telephony and broadcast television. With the evolution of mobile networks, wireless relays have been further developed for cellular transmission. Analog repeaters are sometimes used in cellular systems to extend coverage into regions that are uncovered by the standard network. Digital relaying for cellular applications was initially investigated to enhance coverage for delay-insensitive traffic. More recently, Streaming21 of the United States announced the availability of a 3G relay server in mid-2006, a carrier-grade mobile streaming solution that allows mobile operators and content providers to deliver multimedia contents to mobile phone subscribers over GPRS and 3G networks. Now, the relay concept is being further developed in 802.16j to supporting both digital repeater and decode-forward (DF) relaying with various techniques, such as cooperative relaying, intelligent radio resource management (RRM) for radio resource reuse, smart antenna on RSs for direction-controlled transmission, relay grouping, etc.

While the IEEE and the WiMAX Forum strive to address the technological challenges of high mobile, NLOS WiMAX services, large throughput and coverage, etc., commercial service providers face additional operational challenges including spectrum limitations, security vulnerabilities, and QoS implementations. Many researchers have been working toward developing mechanisms that provide highly efficient mobile WiMAX, which is the core topic in this chapter. Further development is expected in a newly approved project from the IEEE, namely 802.16m—Advanced Air Interface to meet IMT-advance requirements. It is targeting data rates of 100 Mbps for mobile applications and 1 Gbps for fixed applications, cellular, macro- and microcell coverage, with currently no restrictions on the RF bandwidth.