Beamforming or AAS Technogies | Advanced Antenna Technologies in WiMAX

Beamforming or AAS Technogies

Beamforming technologies may be encountered behind several wordings: smart antenna, beamforming and Adaptive Antenna System (AAS). In the following beamforming will be used.

Beamforming Basics

The main objective of beamforming technology is to take benefit from the space/time nature of the propagation channel. Indeed, due to multiple reflections, diffraction and scattering on the transmitter to receiver path in a cellular environment, the energy reaching the BS comes from multiple directions, each direction being affected by a different attenuation and phase.

In a macrocellular environment (i.e. the antenna of the BS is above the rooftop) the signals reaching the BS are inside a cone. The angular spread of the signal depends on the environment. In a urban environment, the angular spread is of the order of 20 degrees. In a more open environment, like in a rural environment, the angular spread is a few degrees.

In the uplink, the beamforming technology principle is to coherently combine the signals received for N antenna elements of an antenna array. A generic beamforming diagram is shown in Figure 1. A block diagram of a beamforming receiver (respectively transmitter) with an N-element antenna array is shown in Figure 1 (respectively Figure 2). In the case of a block diagram of a beamforming receiver with an N-element antenna array, a signal processing unit analyses the same signal received from the N antenna elements and computes weights (w_i) that are applied on each path for combining.

Figure 1: Example of a block diagram of a beamforming receiver with an N-element antenna array

Figure 2: Example of a block diagram of a beamforming transmitter with an N-element antenna array

On the downlink, the processing is very similar to the uplink. Based on the information measured on the signal received in the uplink, it is possible to estimate the Direction of Arrival (DoA) from the uplink signal and to apply different weights, z_i (amplitude and phase), to the different transmit paths of the same signal, so that the resulting antenna pattern focuses towards the direction of the user.

Since the weights in the downlink depend on the uplink signals, this assumes certain channel reciprocity between the uplink and downlink signals since the BS do not know the downlink spatial channel response. Actually, the reciprocity can more realistically be assumed in the case of the TDD system since the uplink and downlink signals use the same frequency at different time intervals. On the FDD system, the reciprocity is more difficult to assess.

In fact, beamforming technology encompasses several techniques. First implementations of beamforming were based on simple antenna switching mechanisms: in that approach, the elements of the antenna array where simply switched on or off according to the received signals. This has the advantage of simplicity but the possibility for beamforming is limited. Today, beamforming uses an adaptive array: the amplitude and phase of each antenna element can be set independently. This has the advantage of having the possibility to achieve infinity of beams.

With adaptive beamforming, several optimising strategies may be used. The signal processing unit must maximise the received CINR. This can be achieved by having a resulting antenna pattern such that the antenna array creates a null in the direction of arrival of a strong interferer. However, the number of interferers that can be cancelled are limited by the number of elements constituting the array: with N antenna elements, it possible to have at most null N−1 interferers. In addition, this technique requires a good knowledge of the radio environment (which may imply additional overheads). This explains why in many implemented systems this method is mainly used in the uplink, where the BS can have maximum knowledge of the radio environment.

Finally, an advanced implementation of beamforming can enable SDMA (Spatial Division Multiple Access). Provided that two or more users are sufficiently separated in space, it is possible to send them at the same time, on the same physical resources, different information on different beams. Nevertheless, the use of SDMA is quite difficult in a mobile environment where MSs that may be well separated at a given moment may be in the same direction at the next moment. 

Radio Engineering Consideration for WiMAX Systems

WiMAX solutions include fixed WiMAX and mobile WiMAX. According to the type of terminal offered (outdoor CPE (Consumer Premise Equipment), self-install indoor CPE, mobile terminal–PCMCIA, laptop, smart phones), each solution has a different deployment method.

LOS/NLOS Propagation
Coverage from a BS is linked to several radio parameters. The first one is the propagation environment. Depending on the relative locations of the BS and the terminal, several models can be used to evaluate the losses due to propagation.

LOS and Near LOS Propagation
Line-of-Sight (LoS) and Near Line-of-Sight (NLoS) propagations may happen when the BS and the MS are deployed outdoor, above the average height of the environment. This may be the case of the deployment of a fixed WiMAX solution in a rural environment with the BS located on a high altitude point (or at the top of a mast) and the SS (then a CPE, for a fixed WiMAX) deployed on the rooftop of the customer's house.

LOS propagation requires that the first Fresnel zone is free from any obstacle. In that case, the propagation losses are proportional to the square of the distance between the BS and the SS. However, in a practical deployment, this happens very seldom. Usually, the Fresnel zone is obstructed and/or there are few obstacles on the BS to the CPE transmission path. In that case, near LOS models are used. However, in the case where there are few obstacles (a building, a water tower, a hill), the obstacles are modelled by knife-edge. An example of such a model can be found in Reference.

NLOS Propagation
NLOS (Non-Line-of-Sight) propagation occurs when the terminal is located indoor and/or at ground level. In this situation, there is in most cases no direct path between the BS and the terminal, there is a high number of obstacles on the BS to MS path (buildings, trees, cars, etc.) and the receiver may receive several copies of signal that experienced several reflections/diffractions on different obstacles. This type of propagation is typical of cellular deployments.

In the case of WiMAX, NLOS propagation corresponds to a deployment of a fixed or mobile WiMAX using self-install indoor terminals (wireless DSL deployment), or a deployment of a mobile WiMAX with mobile terminals (PCMCIA, laptop with integrated chipset, multimode mobile phones, etc.).

In NLOS, the losses versus the distance are much higher that in LOS. Usually, the distance decay exponent is between 3 and 4. An example of a propagation model that can be used to evaluate propagation losses in NLoS is the Erceg model.

Radio Parameters and System Gains
In order to evaluate the range, additional parameters are required. First, the value of some propagation parameters depends on additional radio parameters such as:

• Frequency band. WiMAX could be deployed at different frequencies (2.3 GHz, 2.5 GHz and 3.5 GHz); the higher the frequency, the higher the propagation losses.

• BS antenna height. Propagation losses decrease if the antenna height is increased.

• Terminal antenna height. The lower the terminal, the higher the losses (for mobile deployment, the height of the terminal is usually taken to be between 1.5 and 2m).

Then, depending on the deployment scenario, a margin modelling the fluctuations of the radio environment needs to be considered.

In the case of the LOS/near LOS scenario, the BS and the MS are fixed. However, the propagation loss fluctuates (according, for example, to geoclimatic parameters). Hence, to evaluate the range, a margin needs to be evaluated according to the signal availability required (e.g. 99%) and the geoclimatic parameters. A method to evaluate the margin is available in Reference.

In the case of the NLOS scenario, in order to reflect the distribution of the obstacles in the coverage area, a margin, called the shadowing margin, needs to be taken into account for the evaluation of the range. This shadowing effect is modelled by a lognormal distribution with a standard deviation, which depends on the environment (typically from 10dB in an urban environment to 5 dB in rural environments). The resulting margin also depends on the signal probability availability on the cell area (usually between 90 and 95%). In addition, other margins may be included in the propagation losses: indoor margin (in the case of indoor coverage requirements), interference margin (to reflect the interference level generated by other cells transmitting at the same frequency), body losses, etc.

Finally, to evaluate the range, the system gain may be evaluated. The system gain is the maximum signal level difference that can be accepted on the BS-to-terminal path so that the receiver may decode a signal with sufficient quality. The system gains depend on many specific parameters:

• Transmission power of the BS/terminal.

• Antenna gain of the BS/terminal. The gain at the terminal side depends on the type of terminal. Outdoor CPEs may have gains in excess of 14dBi while mobile terminals may have only a 0dBi antenna gain.

• Receiver sensitivity. The receiver sensitivity depends on the signal bandwidth, the modulation and the coding scheme. The IEEE 802.16 standards are providing minimum reference values but vendors may come with better performance solutions.

WiMAX Radio Features that Enhance the Range
For fixed WiMAX solutions deploying outdoor CPEs, the coverage may be of several km. However, to get range figures in line with an exiting cellular system for WiMAX cellular-like deployments, additional features are needed to compensate for the extra losses due to lower terminal antenna gains, indoor penetration and NLOS behaviour.

The radio enhancement feature applicable to fixed and mobile WiMAXs is subchannelisation. This is the possibility to concentrate the transmit power on a few subchannels and thus to play with trade-off between the coverage and the maximum data rate a terminal can get at the cell edge. Other enhancement features that are only applicable to the mobile WiMAX are:

• Convolutional Turbo Coding (CTC).

• Repetition Coding. For each retransmission, up to 3 dB gain can be obtained. This again allows a trade-off between the maximum range and data rate at the cell edge.

• Hybrid ARQ (HARQ). This is the capability of the receiver to combine several transmissions of the same MAC PDUs.

• Advanced antenna technologies: beamforming and MIMO.

Frequency Planning Guidelines
Again, according to the deployment scenario, the frequency planning guidelines are different.

• Fixed WiMAX

In the case of a fixed WiMAX with an outdoor CPE deployed in LOS/near LOS, similar frequency planning to that of Fixed Wireless Access (FWA) systems (e.g. PMP microwave transmission) can be used. At the radio site, several alternatives for sector configurations may be employed (1, 3, 4 and 6 sectors per site). In FWA-like deployment, the CPE has an external antenna with high gain and hence a reduced antenna beam width. Consequently, the antenna of the CPE provides significant interference protection; hence in order to achieve the CINR target on the service area, frequency planning with 3 and 4 frequencies may be used. For the fixed WiMAX with an indoor self-install CPE, which also have directive antennas, a similar frequency plan may be used.

For densification, more sectors (up to six) or more frequencies per sector can be deployed on the existing sides.

•  Mobile WiMAX

In the case of a mobile WiMAX deployment with mobile terminals, there is no interference reduction due to the MS antenna (omnidirectional antennas). However, because of the use of OFDMA and permutations (which gives some randomness in the use of subcarriers), and because of the better radio performance (more coding capabilities, beamforming and/or MIMO), it is possible to deploy an efficient system with a frequency planning reuse scheme of 3 and even 1.

In the case of MBS (see below), the frequency channel assigned for transmitting multicast/broadcast connections must be deployed in a Single Frequency Network (SFN) manner.

For densification, the same techniques used for existing cellular systems are employed: more frequencies per site, cell splitting, microcellular layer, indoor coverage from the indoor, etc.

Base Station Synchronisation
IEEE 802.16-2004 indicates that there are three options of BS synchronisation:

• Asynchronous configuration. Every BS uses its own permutation. The frame lengths and starting times are not synchronised among the base stations. This configuration can be used as an independent low-cost hot-spot deployment.

• Synchronous configuration. All the BSs use the same reference clock (e.g. by using GPS, or Global Positioning System). The frame durations and starting times are also synchronised among the BSs but each BS may use different permutations. Due to the time synchronisation in this scenario and the long symbol duration of the OFDMA symbol, fast handovers as well as soft handovers are possible. This configuration can be used as an independent BS deployment with a controlled interference level.

• Coordinated synchronous configuration. All the BSs work in the synchronous mode and use the same permutations. An upper layer is responsible for the handling of subchannel allocations within the sectors of the base station, making sure that better handling of the bandwidth is achieved and enabling the system to handle and balance loads between the sectors and within the system.

The standard indicates that, for TDD and FDD realisations, it is recommended (but not required) that all BSs should be time-synchronised to a common timing signal.

Defining Interference or "Think Receiver"

The Interference Protection Working Group of the FCC's Spectrum Policy Task Force recommends that the FCC should consider using the "interference temperature" metric to quantify and manage interference. "Interference temperature" is a measure of radio frequency (RF) power (power generated by other emitters and noise sources) available at a receiving

antenna to be delivered to a receiver. More specifically, it is the temperature equivalent of the RF power available at a receiving antenna per unit bandwidth, measured in units of degrees Kelvin. As conceptualized by the FCC, the terms "interference temperature" and "antenna temperature" are synonymous. The term "interference temperature" is more descriptive for interference management.

Interference temperature can be calculated as the power received by an antenna (watts) divided by the associated RF bandwidth (hertz) and a term known as Boltzman's Constant (equal to 1.3807 wattsec per ºKelvin). Alternatively, it can be calculated as the power flux density available at a receiving antenna (watts per meter squared), multiplied by the effective capture area of the antenna (meter squared), with this quantity divided by the associated RF bandwidth (hertz) and Boltzman's Constant. An "interference temperature density" can also be defined as the interference temperature per unit area, expressed in units of ºKelvin per meter squared and calculated as the interference temperature divided by the effective capture area of the receiving antenna (determined by the antenna gain and the received frequency). Interference temperature density can be measured for particular frequencies using a reference antenna with known gain. Thereafter, it can be treated as a signal propagation variable independent of receiving antenna characteristics.

As illustrated in Figure 1, interference temperature measurements can be taken at receiver locations throughout the service areas of protected communications systems, thus estimating the real-time conditions of the RF environment.

(Source: FCC)

Figure 1: Interference temperature illustrated

Forms of Interference

Interference can be classified into two broad categories: co-channel (CoCh) interference (internal) and out-of-channel interference (external). These forms of interference manifest themselves as shown in Figure 2.

(Source: IEEE)

Figure 2: Forms of interference

Figure 2 illustrates a simplified example of the power spectrum of the desired signal and CoCh interference. Note that the channel bandwidth of the CoCh interferer may be wider or narrower than the desired signal. In the case of a wider CoCh interferer (as shown), only a portion of its power will fall within the receiver filter bandwidth. In this case, the interference can be estimated by calculating the power arriving at the receive (Rx) antenna and then multiplying by a factor equal to the ratio of the filter's bandwidth to the interferer's bandwidth.

An out-of-channel interferer is also shown. Here, two sets of parameters determine the total level of interference. First, a portion of the interferer's spectral sidelobes or transmitter output noise floor falls CoCh to the desired signal, that is, within the receiver filter's passband. This can be treated as CoCh interference. It cannot be removed at the receiver; its level is determined at the interfering transmitter. By characterizing the power spectral density (psd) of sidelobes and output noise floor with respect to the main lobe of a signal, this form of interference can be approximately computed similarly to the CoCh interference calculation, with an additional attenuation factor due to the suppression of this spectral energy with respect to the main lobe of the interfering signal. Figure 3 details the relationship of these lobes to the transmitter.

Figure 3: Main lobe, side lobes, and back lobe

Second, the receiver filter of the victim receiver does not completely suppress the main lobe of the interferer. No filter is ideal, and residual power passing through the stopband of the filter can be treated as additive to the CoCh interference present. The performance of the victim receiver in rejecting out-of-channel signals, sometimes referred to as blocking performance, determines the level of this form of interference. This form of interference can be simply estimated in a manner similar to the CoCh interference calculation, with an additional attenuation factor due to the relative rejection of the filter's stopband at the frequency of the interfering signal.

Cofrequency/Adjacent-Area Case Operators are encouraged to arrive at mutually acceptable sharing agreements that would allow for the maximum provision of service by each licensee within its service area. Under the circumstances where a sharing agreement between operators does not exist or has not been concluded and where service areas are in close proximity, a coordination process should be employed.