Dealing with Interference with WiMAX

Interference—Some Assumptions

The primary objection to wireless systems is the concern that there are or will soon be too many operators on the same frequency, which will cause so much interference that the technology will become unusable. This issue is not that simple.

Such an assumption relies largely on the use of unlicensed spectrum, where, according to Larry Lessig's "tragedy of the commons" scenario, multiple operators broadcast on the same unlicensed (read "free") spectrum, ultimately rendering it useless. Although this scenario may already be evident in the case of Wi-Fi variants (largely limited to the 2.4 GHz range), WiMAX is considerably different. WiMAX currently has no problems, only solutions.

Since 1927, interference protection has always been at the core of federal regulators' spectrum mission. The Radio Act of 1927 empowered the Federal Radio Commission to address interference concerns. This act primarily focused on three parameters: location, frequency, and power. The technology of the time did not permit consideration of a fourth element: time. In the modern sense, one might consider that a spectrum used by cell phones in a metropolitan area (dense population with millions of users) would command a very high price at a spectrum auction. At the other end of the "spectrum," a frequency band, say 2.5 GHz, in an exurban or rural market may go for very little money at an auction or at resell by a spectrum broker. It is entirely possible that the wireless service provider may find a very low cost licensed spectrum and enjoy a protected spectrum, which will largely negate the concern over interference from other broadcasters (the purpose of the Radio Act of 1927 in the first place).

What OFDM Means to WiMAX

To the telecommunications industry, an WiMAX OFDM-based system can squeeze a 72 Mbps uncoded data rate (~100 Mbps coded) out of 20 MHz of channel spectrum. This translates into a spectrum efficiency of 3.6 bps per Hz. If five of these 20 MHz channels are contained within the 5.725 to 5.825 GHz band, giving a total band capacity of 360 Mbps (all channels added together with 1 ×frequency reuse). With channel reuse and through sectorization, the total capacity from one BS site could potentially exceed 1 Gbps.

OFDM has manifold advantages in WiMAX, but among the more notable advantages is greater spectral efficiency. This is especially important in licensed spectrum use, where bandwidth and spectrum can be expensive. Here, OFDM delivers more data per spectrum dollar. In unlicensed spectrum applications, OFDM mitigates interference from other broadcasters due to its tighter beam width (less than 28 Mhz) and guardbands, as well as its dispersal of the data across different frequencies so that if one flow is "stepped on" by an interfering signal, the rest of the data is delivered on other frequencies.

QoS: Error Correction and Interleaving

Error correcting coding builds redundancy into the transmitted data stream. This redundancy allows bits that are in error or even missing to be corrected. The simplest example would be to simply repeat the information bits. This is known as a repetition code. Although the repetition code is simple in structure, more sophisticated forms of redundancy are typically used because they can achieve a higher level of error correction. For OFDM, error correction coding means that a portion of each information bit is carried on a number of subcarriers; thus, if any of these subcarriers has been weakened, the information bit can still arrive intact.

Interleaving is the other mechanism used in OFDM systems to combat the increased error rate on the weakened subcarriers. Interleaving is a deterministic process that changes the order of transmitted bits. For OFDM systems, this means that bits that were adjacent in time are transmitted on subcarriers that are spaced out in frequency. Thus errors generated on weakened subcarriers are spread out in time; that is, a few long bursts of errors are converted into many short bursts. Error correcting codes then correct the resulting short bursts of errors.

QPSK Versus QAM

Rather than attempting to be all things to all subscribers, WiMAX delivers a gradation of QoS dependent on distance of the SS from the BS: The greater the distance, the lower the guarantee of QoS. WiMAX utilizes three mechanisms for QoS; from highest to lowest, these mechanisms are 64-QAM, 16-QAM, and QPSK. Figure 1 illustrates modulation schemes.

Figure 1: Modulation schemes focus the signal over distance.

By using a robust modulation scheme, WiMAX delivers high throughput at long ranges with a high level of spectral efficiency that is also tolerant of signal reflections. Dynamic adaptive modulation allows the BS to trade throughput for range. For example, if the BS cannot establish a robust link to a distant subscriber using the highest order modulation scheme, 64-QAM, the modulation order is reduced to 16-QAM or QPSK, which reduces throughput and increases effective range. Figure 2 demonstrates how modulation schemes ensure throughput over distance.

Figure 2: Modulation schemes ensure a quality signal is delivered over distance by decreasing throughput.

QPSK and QAM are the two leading modulation schemes for WiMAX. In general the greater the number of bits transmitted per symbol, the higher the data rate is for a given bandwidth. Thus, when very high data rates are required for a given bandwidth, higher-order QAM systems, such as 16-QAM and 64-QAM, are used. 64-QAM can support up to 28 Mbps peak data transfer rates over a single 6 MHz channel. However, the higher the number of bits per symbol, the more susceptible the scheme is to intersymbol interference (ISI) and noise. Generally the signal-to-noise ratio (SNR) requirements of an environment determine the modulation method to be used in the environment. QPSK is more tolerant of interference than either 16-QAM or 64-QAM. For this reason, where signals are expected to be resistant to noise and other impairments over long transmission distances, QPSK is the normal choice.

Multiplexing in OFDM

As shown in Figure 3, an efficient OFDM implementation converts a serial symbol stream of QPSK or QAM data into a size M parallel stream. These M streams are then modulated onto M subcarriers via the use of size N (N ≤ M) inverse FFT. The N outputs of the inverse FFT are then serialized to form a data stream that can then be modulated by a single carrier. Note that the N-point inverse FFT could modulate up to N subcarriers. When M is less than N, the remaining N−M subcarriers are not in the output stream. Essentially, these have been modulated with amplitude of zero.

Figure 3: Block diagram of a simple OFDM transmitter

Although it would seem that combining the inverse FFT outputs at the transmitter would create interference between subcarriers, the orthogonal spacing allows the receiver to perfectly separate out each subcarrier. Figure 4 illustrates the process at the receiver. The received data is split into N parallel streams that are processed with a size N FFT. The size N FFT efficiently implements a bank of filters, each matched to N possible subcarriers. The FFT output is then serialized into a single stream of data for decoding. Note that when M is less than N, in other words fewer than N subcarriers are used at the transmitter, the receiver only serializes the M subcarriers with data.

Figure 4: Block diagram of a simple OFDM receiver

Legacy QoS Mechanisms

The following paragraphs describe legacy mechanisms.

FDD/TDD/OFDM

WiMAX incorporates a number of time-proven mechanisms to ensure good QoS. Most notable are TDD, FDD, FEC, FFT, and OFDM. The WiMAX standard provides flexibility in spectrum usage by supporting both FDD and TDD. Thus, it can operate in both FDD/OFDM and TDD/OFDM modes. It supports two types of FDD: continuous FDD and burst FDD.

In continuous FDD, the upstream and downstream channels are located on separate frequencies, and all CPE stations can transmit and receive simultaneously. The downstream channel is always on, and all stations are always listening to it. Traffic is sent on this channel in a broadcast manner using TDM. The upstream channel is shared using TDMA, and the BS is responsible for allocating bandwidth to the stations.

In burst FDD, the upstream and downstream channels are located on separate frequencies. In contrast to continuous FDD, not all stations can transmit and receive simultaneously. Those that can transmit and receive simultaneously are referred to as full-duplex capable stations while those that cannot are referred to as half-duplex capable stations.

A TDD frame has a fixed duration and contains one downstream subframe and one upstream subframe. The two subframes are separated by a guard time called transition gap (TG), and the bandwidth that is allocated to each subframe is adaptive. The TDD subframe is illustrated in Figure 1.

(Source: IEEE)

Figure 1: TDD subframe

Within a TDD downlink subframe, transmissions coming from the BS are organized into different modulation and FEC groups. The subframe header, called the FCH, consists of a preamble field, a PHY control field, and a MAC control field. The PHY control field is used for physical information, such as the slot boundaries, destined for all stations. It contains a map that defines where the physical slots for the different modulation/FEC groups begin.

The groups are listed in ascending modulation order, with QPSK first, followed by 16-QAM and then 64-QAM. Each CPE station receives the entire DL frame, decodes the subframe, and looks for MAC headers indicating data for the station. The DL data is always FEC coded. Payload data is encrypted, but message headers are unencrypted. The MAC control is used for MAC messages destined for multiple stations.

This variation uses burst single-carrier modulation with adaptive burst profiling in which transmission parameters, including the modulation and coding schemes, may be adjusted individually to each SS on a frame-by-frame basis. Channel bandwidths of 20 or 25 MHz (typical United States allocation) or 28 MHz (typical European allocation) are specified. Randomization is performed for spectral shaping and to ensure bit transitions for clock recovery.

Forward Error Correction (FEC)

WiMAX utilizes FEC, a technique that doesn't require the transmitter to retransmit any information that a receiver uses for correcting errors incurred in transmission over a communication channel. The transmitter usually uses a common algorithm and embeds sufficient redundant information in the data block to allow the receiver to correct. Without FEC, error correction would require the retransmission of whole blocks or frames of data, resulting in added latency and a subsequent decline in QoS.

Need QoS? Throw more bandwidth at it! Throughput and latency are two essentials for network performance. Taken together, these elements define the "speed" of a network. Whereas throughput is the quantity of data that can pass from source to destination in a specific time, round-trip latency is the time it takes for a single data transaction to occur (the time between requesting data and receiving it). Latency can also be thought of as the time it takes from data send-off on one end to data retrieval on the other (from one user to the other). Therefore, the better throughput (bandwidth) management, the better the QoS

Service Flow - WiMAX

Service Flow

Minimizing customer intervention and truck roll is very important for WiMAX deployments. The Provisioned Service Flow Table, Service Class Table, and Classifier Rule Table are configured to support self-installation and auto-configuration. When customers subscribe to the service, they tell the service provider the service flow information including the number of UL/DL connections with the data rates and QoS parameters, along with the types of applications (for example, Internet, voice, or video) the customer intends to run. The service provider preprovisions the services by entering the service flow information into the service flow database. When the SS enters the BS by completing the network entry and authentication procedure, the BS downloads the service flow information from the service flow database. Figure 1 provides an example of how the service flow information is populated. It indicate that two SSs, identified by MAC address 0x123ab54 and 0x45fead1, have been preprovisioned. Each SS has two service flows, identified by sfIndex, with the associated QoS parameters that are identified by qosIndex 1 and 2, respectively. qosIndex points to a QoS entry in the wmanIfBsServiceClassTable that contains three levels of QoS: Gold, Silver, and Bronze. sfIndex points to the entry in the wmanBsClassifierRuleTable and indicates which rules shall be used to classify packets on the given service flow.

(Source: Intel)

Figure 1: Service flow provisioning

When the SS with MAC address 0x123ab54 registers into the BS, the BS creates an entry in the wmanIfBaseRegisteredTable. Based on the MAC address, the BS will be able to find the service flow information that has been preprovisioned. The BS will use a dynamic service activate (DSA) message to create service flows for sfIndex 100001 and 100002, with the preprovisioned service flow information. This can be seen in Figure 1. It creates two entries in wmanIfCmnCpsServiceFlowTable. The service flows will then be available for the customer to send data traffic

How WiMAX Works

Like most data communications, WiMAX relies on a process consisting of a session setup and authentication. The RLC manages and monitors the quality of the service flow. With WiMAX, this process is a series of exchanges (DLs and ULs) between the BS and SS. A complex process determines what FDD and TDD settings will be used for the service flow, FEC, sets encryption, bandwidth requests, burst profiles, and so on. The process starts with channel acquisition by the newly installed SS.

Channel Acquisition

The MAC protocol includes an initialization procedure designed to eliminate the need for manual configuration. In other words, the subscriber takes the SS out of the box, plugs in power and Ethernet, and connects almost immediately to the network. The following paragraphs describe how that is possible without laborious user setup or service provider truck roll.

Upon installation, the SS begins scanning its frequency list to find an operating channel. It may be preconfigured by the service provider to register with a specified BS. This feature is useful in dense deployments where the SS might hear a secondary BS due to spurious signals or when the SS picks up a sidelobe of a nearby BS antenna. Moreover, this feature will help service providers avoid expensive installations and subsequent truck rolls.

After selecting a channel or channel pair, the SS synchronizes to the DL transmission from the BS by detecting the periodic frame preambles. Once the PHY is synchronized, the SS will look for the periodically broadcasted DCD and UCD messages that enable the SS to determine the modulation and FEC schemes used on the BS's carrier.

Initial Ranging and Negotiation of SS Capabilities

Once the parameters for initial ranging transmissions are established, the SS will scan the UL-MAP messages present in every frame for ranging information. The SS uses a backoff algorithm to determine which initial ranging slot it will use to send a ranging request (RNG-REQ) message. The SS will then send its burst using the minimum power setting and will repeat with increasingly higher transmission power until it receives a ranging response.

Based on the arrival time of the initial RNG-REQ and the measured power of the signal, the BS adjusts the timing advance and power to the SS with the ranging response (RNG-RSP). The response provides the SS with the basic and primary management CIDs. Once the timing advance of the SS transmissions has been correctly determined, the ranging procedure for fine-tuning the power is done via a series of invited transmissions.

WiMAX transmissions are made using the most robust burst profile. To save bandwidth, the SS next reports its PHY capabilities, including which modulation and coding schemes it supports and whether, in an FDD system, it is half-duplex or full-duplex. The BS, in its response, can deny the use of any capability reported by the SS. See Figure 1 for an illustration of this process.

Figure 1: Channel acquisition process between an SS and BS

It should be noted here how complex this setup procedure is. The purpose thus far is to ensure a high quality connection between the SS and the BS.

SS Authentication and Registration

Wi-Fi has been dogged with a reputation for lax security. Perhaps the best "horror story" deals with a computer retailer who installed a wireless LAN. A customer purchased a Wi-Fi equipped laptop and, anxious to enjoy it, powered it up in the parking lot of the retailer. The new laptop owner was immediately able to tap into the retailer's Wi-Fi network and was able to capture some customer credit card information. Fortunately, the new laptop owner was a journalist, not a con artist. The story, much to the chagrin of the national retailer and the Wi-Fi industry, made the national news. The Wi-Fi industry has had to work hard to shake the reputation of having loose security measures. A similar story will not easily, if ever, occur with WiMAX.

Each SS contains both a manufacturer-issued factory-installed X.509 digital certificate and the certificate of the manufacturer. The SS in the Authorization Request and Authentication Information messages sends these certificates, which set up the link between the 48-bit MAC address of the SS and its public RSA key, to the BS. The network is able to verify the identity of the SS by checking the certificates and can subsequently check the level of authorization of the SS. If the SS is authorized to join the network, the BS will respond to its request with an authorization reply containing an authorization key (AK) encrypted with the SS's public key and used to secure further transactions.

Upon successful authorization, the SS will register with the network. This will establish the secondary management connection of the SS and determine capabilities related to connection setup and MAC operation. The version of IP used on the secondary management connection is also determined during registration.

IP Connectivity

After registration, the SS attains an IP address via DHCP and establishes the time of day via the Internet Time Protocol. The DHCP server also provides the address of the TFTP server from which the SS can request a configuration file. This file provides a standard interface for providing vendor-specific configuration information. Se Figure 2 for an illustration of this process.

Figure 2: SS authentication and registration

Connection Setup

Now comes the connection setup, where data (the content) actually flows. WiMAX uses the concept of service flows to define one-way transport of packets on either the DL or the UL. Service flows are characterized by a set of QoS parameters, such as those for latency and jitter. To most efficiently utilize network resources, such as bandwidth and memory, WiMAX adopts a two-phase activation model in which resources assigned to a particular admitted service flow may not be actually committed until the service flow is activated. Each admitted or active service flow is mapped to a MAC connection with a unique CID. In general, service flows in WiMAX are preprovisioned, and the BS initiates the setup of the service flows during SS initialization.

In addition, the BS or the SS can dynamically establish service flows. The SS typically initiates service flows only if there is a dynamically signaled connection, such as a switched virtual connection (SVC) from an ATM network. The establishment of service flows is performed via a three-way handshaking protocol in which the request for service flow establishment is responded to and the response acknowledged.

In addition to supporting dynamic service establishment, WiMAX supports dynamic service changes in which service flow parameters are renegotiated. These service flow changes follow a three-way handshaking protocol similar to the one dynamic service flow establishment uses.

The MAC and WiMAX Architecture

The WiMAX DL from the BS to the user operates on a point-tomultipoint basis as illustrated in Figure 1. The WiMAX wireless link operates with a central BS with a sectorized antenna that is capable of handling multiple independent sectors simultaneously. Within a given frequency channel and antenna sector, all stations receive the same transmission. The BS is the only transmitter operating in this direction, so it transmits without having to coordinate with other stations except the overall TDD that may divide time into UL and DL transmission periods. The DL is generally broadcast. In cases where the DL-MAP does not explicitly indicate that a portion of the DL subframe is not a specific SS, all SSs capable of listening to that portion of the DL subframe will listen.

Figure 1: Typical WiMAX architecture for point-to-multipoint distribution

The MAC is connection-oriented. Connections are referenced with 16-bit connection identifiers (CIDs) and may require continuously granted bandwidth or bandwidth on demand. As described previously, both bandwidths are accommodated. A CID is used to distinguish between multiple UL channels that are associated with the same DL channel. The SSs check the CIDs in the received PDUs and retain only those PDUs addressed to them.

The MAC PDU is the data unit exchanged between the MAC layers of the BS and its SSs. It is the data unit generated on the downward direction for the next lower layer and the data unit received on the upward direction from the previous lower layer.

Each SS has a standard 48-bit MAC address, which serves as an equipment identifier because the primary addresses used during operation are the CIDs. Upon entering the network, the SS is assigned three management connections in each direction. These three connections reflect the three different QoS requirements used by different management levels:

• Basic connection—transfers short, time-critical MAC and radio link control (RLC) messages.

• Primary management connection—transfers longer, more delay-tolerant messages, such as those used for authentication and connection setup. The secondary management connection transfers standards-based management messages such as Dynamic Host Configuration Protocol (DHCP), Trivial File Transfer Protocol (TFTP), and Simple Network Management Protocol (SNMP). In addition to these management connections, SSs are allocated transport connections for the contracted services.

• Transport connections—are unidirectional to facilitate different UL and DL QoS and traffic parameters; they are typically assigned to services in pairs.

SSs share the UL to the BS on a demand basis. Depending on the class of service utilized, the SS may be issued continuing rights to transmit, or the BS may grant the right to transmit after receiving a request from the user.

Service Classes and QoS

Within each sector, users adhere to a transmission protocol that controls contention between users and enables the service to be tailored to the delay and bandwidth requirements of each user application. This is accomplished through four different types of UL scheduling mechanisms. These mechanisms are implemented using unsolicited bandwidth grants, polling, and contention procedures. The WiMAX MAC provides QoS differentiation for different types of applications that might operate over WiMAX networks:

• Unsolicited Grant Services (UGS)—UGS is designed to support constant bit rate (CBR) services, such as T1/E1 emulation and VoIP without silence suppression.

• Real-Time Polling Services (rtPS)—rtPS is designed to support real-time services that generate variable size data packets, such as MPEG video or VoIP with silence suppression, on a periodic basis.

• Non-Real-Time Polling Services (nrtPS)—nrtPS is designed to support non-real-time services that require variable size data grant burst types on a regular basis.

• Best Effort (BE) Services—BE services are typically provided by the Internet today for web surfing.

The use of polling simplifies the access operation and guarantees that applications receive service on a deterministic basis if required. In general, data applications are delay tolerant, but real-time applications, like voice and video, require service on a more uniform basis and sometimes on a very tightly controlled schedule.

For the purposes of mapping to services on SSs and associating varying levels of QoS, all data communications are in the context of a connection. Service flows may be provisioned when an SS is installed in the system. Shortly after SS registration, connections are associated with these service flows (one connection per service flow) to provide a reference against which to request bandwidth. Additionally, new connections may be established when a customer's service needs change. A connection defines both a service flow and the mapping between peer convergence processes that utilize the MAC. The service flow defines the QoS parameters for the PDUs that are exchanged once the connection has been established.

Service flows are the mechanism for UL and DL for QoS management. In particular, they facilitate the bandwidth allocation process. An SS requests UL bandwidth on a per connection basis (implicitly identifying the service flow). The BS grants the bandwidth to an SS as an aggregate of grants in response to per connection requests from the SS.^[1]

The modulation and coding schemes are specified in a burst profile that may be adjusted adaptively for each burst to each SS. The MAC can make use of bandwidth-efficient burst profiles under favorable link conditions then shift to more reliable, although less efficient alternatives, as required to support the planned 99.999 percent link availability (QPSK to 16-QAM to 64-QAM).

The request-grant mechanism is designed to be scalable, efficient, and self-correcting. The WiMAX access system does not lose efficiency when presented with multiple connections per terminal, multiple QoS levels per terminal, and a large number of statistically multiplexed users.

Along with the fundamental task of allocating bandwidth and transporting data, the MAC includes a privacy sublayer that provides authentication of network access and connection establishment to avoid theft of service, and it provides key exchange and encryption for data privacy.

OFDM Variants 2–11 GHz

The need for NLOS operation drives the design of the 2—11 GHz PHY. Because residential applications are expected, rooftops may be too low (possibly due to obstruction by trees or other buildings) for a clear sight line to a BS antenna. Therefore, significant multipath propagation must be expected. Furthermore, outdoor-mounted antennas are expensive, due to both hardware and installation costs. The four 2—11 GHz air interface specifications are described in the following paragraphs.

WirelessMAN-OFDM This air interface uses OFDM with a 256-point transform (see OFDM description later in this chapter). Access is by TDMA. This air interface is mandatory for license-exempt bands.

The WirelessMAN-OFDM PHY is based on OFDM modulation. It is intended mainly for fixed access deployments where SSs are residential gateways deployed within homes and businesses. The OFDM PHY supports subchannelization in the UL. There are 16 subchannels in the UL. The OFDM PHY supports TDD and FDD operations, with support for both FDD and H-FDD SSs. The standard supports multiple modulation levels including Binary Phase Shift Keying (BPSK), QPSK, 16-QAM, and 64-QAM. Finally, the PHY supports (as options) transmit diversity in the DL using Space Time Coding (STC) and AAS with Spatial Division Multiple Access (SDMA).

The transmit diversity scheme uses two antennas at the BS to transmit an STC-encoded signal to provide the gains that result from second-order diversity. Each of two antennas transmits a different symbol (two different symbols) in the first symbol time. The two antennas then transmit the complex conjugate of the same two symbols in the second symbol time. The resulting data rate is the same as without transmit diversity.

Figure 1 illustrates the frame structure for a TDD system. The frame is divided into DL and UL subframes. The DL subframe is made up of a preamble, Frame Control Header (FCH), and a number of data bursts. The FCH specifies the burst profile and the length of one or more DL bursts that immediately follow the FCH. The downlink map (DL-MAP), uplink map (UL-MAP), DL Channel Descriptor (DCD), UL Channel Descriptor (UCD), and other broadcast mes-sages that describe the content of the frame are sent at the beginning of these first bursts. The remainder of the DL subframe is made up of data bursts to individual SSs.

Figure 1: Frame structure for a TDD system (Source: IEEE)

Each data burst consists of an integer number of OFDM symbols and is assigned a burst profile that specifies the code algorithm, code rate, and modulation level that are used for those data transmitted within the burst. The UL subframe contains a contention interval for initial ranging and bandwidth allocation purposes and UL PHY protocol data units (PDUs) from different SSs. The DL-MAP and UL-MAP completely describe the contents of the DL and UL subframes. They specify the SSs that are receiving and/or transmitting in each burst, the subchannels on which each SS is transmitting (in the UL), and the coding and modulation used in each burst and in each sub-channel.

If transmit diversity is used, a portion of the DL frame (called a zone) can be designated to be a transmit diversity zone. All data bursts within the transmit diversity zone are transmitted using STC coding. Finally, if AAS is used, a portion of the DL subframe can be designated as the AAS zone. Within this part of the subframe, AAS is used to communicate to AAS-capable SSs. AAS is also supported in the UL.

WirelessMAN-OFDMA This variant uses orthogonal frequency division multiple access (OFDMA) with a 2048-point transform. In this system, addressing a subset of the multiple carriers to individual receivers provides multiple access. Because of the propagation requirements, the use of AASs is supported.

The WirelessMAN-OFDMA PHY is based on OFDM modulation. It supports subchannelization in both the UL and DL. The standard supports five different subchannelization schemes. The OFDMA PHY supports both TDD and FDD operations. The same modulation levels are also supported. STC and AAS with SDMA are supported, as is multiple input, multiple output (MIMO). MIMO encompasses a number of techniques for utilizing multiple antennas at the BS and SS in order to increase the capacity and range of the channel.

The frame structure in the OFDMA PHY is similar to the structure of the OFDM PHY. The notable exceptions are that subchannelization is defined in the DL as well as in the UL, so broadcast messages are sometimes transmitted at the same time (on different subchannels) as data. Also, because a number of different subchan-nelization schemes are defined, the frame is divided into a number of zones that each use a different subchannelization scheme. The MAC layer is responsible for dividing the frame into zones and communicating this structure to the SSs in the DL-MAP and UL-MAP. As in the OFDM PHY, there are optional transmit diversity and AAS zones, as well as a MIMO zone.

Wireless High Speed Unlicensed Metro Area Network (WirelessHUMAN) WirelessHUMAN is similar to the aforementioned OFDM-based schemes and is focused on Unlicensed National Information Infrastructure (UNII) devices and other unlicensed bands.

The Function of the PHY - WiMAX

As the name might imply, the purpose of the PHY is the physical transport of data. The following paragraphs will describe different methods to ensure the most efficient delivery in terms of bandwidth (volume and time in Mbps) and frequency spectrum (MHz/GHz). A number of legacy technologies are used to get the maximum performance out of the PHY. These technologies, including OFDM, TDD, FDD, QAM, and Adaptive Antenna System (AAS), will be described in the following pages or chapters.

OFDM: The "Big So What?!" of WiMAX

OFDM is what puts the max in WiMAX. OFDM is not new. Bell Labs originally patented it in 1970, and it became incorporated in various digital subscriber line (DSL) technologies as well as in 802.11a. OFDM is based on a mathematical process called Fast Fourier Transform (FFT), which enables 52 channels to overlap without losing their individual characteristics (orthogonality). This is a more efficient use of the spectrum and enables the channels to be processed at the receiver more efficiently. OFDM is especially popular in wireless applications because of its resistance to forms of interference and degradation. In short, OFDM delivers a wireless signal much farther with less interference than competing technologies. Figure 1 provides an illustration of how OFDM works.

Figure 1: The significance of OFDM: A focused beam delivering maximum bandwidth over maximum distance with minimum interference

TDD and FDD

WiMAX supports both time division duplex (TDD) and frequency division duplex (FDD) operation. TDD is a technique in which the system transmits and receives within the same frequency channel, assigning time slices for transmit and receive modes. FDD requires two separate frequencies generally separated by 50 to 100 MHz within the operating band. TDD provides an advantage where a regulator allocates the spectrum in an adjacent block. With TDD, band separation is not needed, as is shown in Figure 2. Thus, the entire spectrum allocation is used efficiently both upstream and downstream and where traffic patterns are variable or asymmetrical.

Figure 2: A TDD subframe

In FDD systems, the downlink (DL) and uplink (UL) frame structures are similar except that the DL and UL are transmitted on separate channels. When half-duplex FDD (H-FDD) subscriber stations (SSs) are present, the base station (BS) must ensure that it does not schedule an H-FDD SS to transmit and receive at the same time.

Figure 3 illustrates this relationship.

Figure 3: ULs and DLs between BSs and SSs

Adaptive Antenna System (AAS)

AAS is used in the WiMAX specification to describe beam-forming techniques where an array of antennas is used at the BS to increase gain to the intended SS while nulling out interference to and from other SSs and interference sources. AAS techniques can be used to enable Spatial Division Multiple Access (SDMA), so multiple SSs that are separated in space can receive and transmit on the same subchannel at the same time. By using beam forming, the BS is able to direct the desired signal to the different SSs and can distinguish between the signals of different SSs, even though they are operating on the same subchannel(s), as shown in Figure 4.

Figure 4: AAS uses beam forming to increase gain (energy) to the intended SS.

Certification Process | WiMAX Certifications

The WiMAX certification process is the steps, processes and tests which are performed to ensure that products will perform as desired and will reliably interoperate with other devices that are certified.

Testing for WiMAX products can be performed using testing conformance standards. The WiMAX test standards include a protocol implementation conformance statement (PICS), a test suite structure (TSS) and a radio conformance testing (RCT).

A protocol implementation conformance statement proforma is a document that is provided by a company or testing facility that states that the product or system provides and supports a specific set of commands and protocols. The 802.16 system has many capabilities and WiMAX devices typically are designed to support only a limited set of the protocols and capabilities.

A test suite structure (TSS) is a set of testing equipment configurations and procedures that are used to evaluate the operation and performance of products or systems. For the WiMAX system, the TSS performs operational testing of key functions of WiMAX devices including connecting radio links, authenticating, setting up, and changing services.

Radio conformance tests are a set of procedures that are used to evaluate the operation and performance of the radio part of wireless products or systems.

Service Flows and Classes

WiMAX services are the providing of information transfer to or between users. Information transfer services can have a variety of characteristics that can be selected and varied by the operator. The WiMAX system uses services flows to identify specific transmission characteristics for specific user services and a single user may have multiple service flows. Common sets of service characteristics may be defined by service classes.

Service Flows

Service flows are communication channels (e.g. a stream of packets) that have particular service characteristics associated with the transfer of data. For example, a communication link might have several service flows associated with it; a real time service flow for voice communication, a high-integrity service flow (low error rate) for data file transfer and a best effort service flow for Internet web browsing.

Each service provided on a WiMAX system is associated with a service flow. Service flows are requested, established and ended. When subscriber stations request services, the system can evaluate and negotiate the requested characteristics at any time.

A single WiMAX subscriber station may have multiple service flows for each connection (service flows can be different in different directions). Service flows can be dynamically added, changed and ended.

Service flows are uniquely identified by a service flow identifier (SFID). A SFID is associated with a specific connection identifier to determine the service characteristics a specific user will receive on that particular device

Unsolicited Grant Service (UGS)

Unsolicited grant service is a service flow in which the transmission system automatically and periodically provides a defined number of timeslots and fixed packet size that is used by a particular receiver. UGS is commonly used to provide services that require a constant bit rate (CBR) such as audio streaming or leased line (e.g. T1 or E1) circuit emulation.

UGS provides a constant bit rate for a single connection. A subscriber device may need additional bandwidth for an additional service that is added to a connection or to temporarily provide more bandwidth on the UGS connection. To request more bandwidth on a UGS connection, a poll me bit or slip indicator bit may be used.

A poll me bit is a signaling message in a data field within the header of a data packet that indicates that the device would like to be polled. The poll me bit indicates to the base station that the subscriber device needs to be polled for a service other than for the current UGS service.

For transmission to synchronous connections, timing inaccuracies may result in the need to transfer additional bits if the clock of one connection is slightly faster than the other connection. When the buffer of the faster connection indicates the number of bits to be transmitted may soon run out, a slip indicator bit may be used. The slip indicator is a signaling message within the header of a data packet that indicates that the data transmission queue of that device is changing (slipping) and that the device needs more bandwidth to keep up with the transmission queue. This allows the base station to temporarily assign additional bandwidth until the transmission buffer has caught up.

Figure 1 shows how WiMAX unsolicited grant service (UGS) operates. Subscriber stations are assigned to receive and transmit during assigned time intervals. The subscriber station may use the poll me bit in the header to indicate it wants to be polled so it can send data for another service. When the base station receives the poll me bit, it sends a polling message which allows the subscriber station to send a packet of data that is independent of the UGS packets.

Figure 1: Wireless Unsolicited Grant Service (UGS)

Scheduling Services

Overview

Scheduling services are the medium access control functions (data flow control) that define how and when devices will receive and transmit on a communication system. The types of services that WiMAX can provide range from guaranteed bandwidth with low delay unsolicited grant service (UGS) to random access best effort (BE) service. WiMAX systems use a grant management system to coordinate the request for new services and changes to existing services (such as requesting more bandwidth). The WiMAX system uses a combination of time division multiple access, polling and contention based flow control to provide specific types of services to users.

Time division multiple access (TDMA) is a process of sharing a single radio channel by dividing the channel into time slots that are shared between simultaneous users of the radio channel. When a subscriber communicates on a WiMAX system using TDMA, he/she is assigned a specific time position on the radio channel. By allowing several users to use different time positions (time slots) on a single radio channel, TDMA systems can guarantee a constant data rate with a minimal amount of flow control overhead.

Polling is the process of sending a request message (usually periodically) for the purpose of collecting events or information from a network device. The receipt of a polling message by a device starts an information transfer operation for a specific time period. Polling may be performed with specific units (unicast), to groups of units (multicast) or to all units (broadcast).

Unicast polls are requests for data transmission or responses to commands that are only sent between a sender (polling device) and receiver (polled device). When a subscriber station is responding to a unicast polling message, no other devices are allowed to transmit.

Multicast polls are requests for data transmission or responses to commands that are sent from a polling device to several receiving devices which are part of a multicast group. When a device receives a multicast polling message for its group, it will respond if it has data to send. When a subscriber station is responding to a multicast polling message, others may also have information to transmit. For multicast poll messages, subscriber stations must use contention based access (on the contention slot) to send their data.

Broadcast polls are requests for data transmission or responses to commands that are sent from a polling device to all devices that are able to receive its broadcasted polling message. When a device receives a broadcast polling message, it will respond if it has data to send. For broadcast poll messages, subscriber stations must use contention based access (on the contention slot) to send their data.

The amount of time between polling messages is called the polling cycle. The time between polling cycles is a balance between delay (more polling messages is less delay) and overhead (more polling messages increases the percentage of data that is used for control messages).

Figure 1 illustrates the different types of polling that are used in the WiMAX system. A device that is part of a multicast group, has received a multicast polling message, must compete for access to send its data. Finally, for a broadcast polling message, any device that has data will compete for access to send its data.

Figure 1: WiMax Polling Types

Contention based access control is the independent operation (distributed access control) of communication devices (stations). In a contention-based system, communication devices randomly request service from channels within a communication system. Because communication requests occur randomly, two or more communication devices may request service simultaneously. The access control portion of a contention based session usually involves requiring the communication device to sense for activity before transmitting and listening for message collisions after sending its service request. If the requesting device does not hear a response to its request, it will wait a random amount of time before repeating the access attempt. The amount of time waited between retransmission requests increases each time a collision occurs.

The WiMAX system defines time periods that subscriber stations can use for contention based access. When subscriber units desire to initiate requests to the system that are not scheduled from a polling message, they must access the process during the contention time slots period. The contention time slot period is periodically broadcast on the downlink channel along with other channel access control information.

Figure 2 shows how contention based access control can be performed on a WiMAX system. Channel descriptors are periodically broadcasted on the downlink radio channel that provides the time intervals for the contention slots. Subscriber devices that use contention based access must compete during these time periods. The WiMAX subscriber station will initially attempt to access the system at a relatively low power level. If the subscriber station does not hear a response to its request, it will wait a random amount of time, increase its transmitted power level and attempt access again. The subscriber station will continue to wait increasing amounts of time each time and increases its transmitted power level each time an access attempt fails until it receives a response from the system.

Figure 2: WiMax Contention Based Access Control

The WiMAX system uses a grant management process for the requesting and allocation (granting) of resources (such as transmission time or bandwidth). Subscriber stations can request changes to the type of services they require (e.g. increases or decreases in bandwidth) by transmitting a bandwidth request header and the system can decide to grant, adjust or not authorize the grant request.

The WiMAX system can grant resources based on a connection or based on a specific subscriber station. A grant per subscriber station is the allocation of transmission bandwidth that affects the transmission for all the connections associated with a subscriber station. A grant per connection is the assignment of bandwidth which only affects the transmission for a specific connection on a subscriber device.

Bandwidth requests can be in aggregate or incremental form. An aggregate request is a message that defines the amount of a resource (such as transmission bandwidth) that is requested to provide for a combined group of applications or services. An incremental request is a message that defines the additional amount of a resource (such as transmission bandwidth) that is requested to provide for an application or service. Bandwidth request messages may be sent as stand alone messages or they may be piggybacked in the payload of another packet of data.

Error Rate | QoS

Error rate is a ratio between an amount of information that is received in error as compared to the total amount of information that is received over a period of time. Error rate may be expressed in the number of bits that are received in error on the number of blocks of data (packets) that are lost over a period of time. WiMAX error rates can be affected by a number of factors including signal quality and system configuration. Some of the common error rate measures for WiMAX include bit error rate (BER) and packet loss rate (PLR).

Bit Error Rate (BER)

BER is calculated by dividing the number of bits received in error by the total number of bits transmitted. It is generally used to denote the quality of a digital transmission channel. Bit errors can occur randomly over time (random errors) or in group (burst errors).

Random errors are bits in a received digital signal that are received in error that occur in such a way that each error can be considered statistically independent from any other error. Burst errors are the distortion or failure of a digital receiver to correctly decode groups of digital bits. Burst errors typically have a high bit error ratio (BER) compared to the overall BER of a communication link or channel.

Packet Loss Rate (PLR)

Packet loss rate is a ratio of the number of data packets that have been lost in transmission compared to the total number of packets that have been transmitted. Some applications (such as digital television) are more sensitive to packet loss rate than bit error rates.

Quality of Service (QoS)

QoS is one or more measurements of desired performance and priorities of a communications system. The WiMAX system is designed with the ability to apply different QoS levels to downlink and uplink connections as well as provide multiple service types on a single connection to each user. QoS measures for WiMAX systems may include service availability, data throughput, delay, jitter, and error rate.

Service Availability

Service availability is the ratio of the amount of time an authorized user is able to access the services compared to the total time service is supposed to be available. Service availability can be affected by a variety of factors including admission control and oversubscription.

Admission control is the process of reviewing the service authorization level associated with users and determining the extent to which network resources will be allocated if they are available. Admission control is used to adjust, limit or assign the use of limited network resources to specific types or individual users. Admission control may allow for the assignment of higher access level priority for specific types of users such as public safety users.

Oversubscription is a situation that occurs when a service provider sells more capacity to end customers than a communications network can provide at a specific time period. This provides a benefit of reduced network equipment and operational cost.

Oversubscription is a common practice in communications networks as customers do not continuously use the maximum capacity assigned to them and they access the networks at different time periods. Unfortunately, over-subscription in telecommunications can cause problems when customers do attempt to access the network at the same time. For example, when customers open their presents at a holiday event (e.g. Christmas) and attempt to access the Internet at the same time.

Radio Link Control (RLC) | WiMAX Operation

Radio link control protocol is a layer 2 (link layer) that is used to coordinate the overall flow of data packets across the radio link. RLC uses error detection and data retransmission to increase the reliability of the radio link while reducing the error rate. WiMAX radio link control functions include power level control, periodic ranging, burst profile changes and bandwidth requests.

Power control is the process of adjusting the power level in a wireless system where the base station receiver monitors the received signal strength of mobile radios. Control messages are transmitted from the base station to the mobile telephone commanding it to raise and lower its transmitter power level as necessary to maintain a good radio communications link.

Ranging may need to be performed after the subscriber station has been inactive for a while. A timer (the T4 timer) that is continuously reset as the subscriber station communicates with the system helps determine this. If the subscriber station (SS) has not communicated with the system in awhile, the timer will not be reset and it will expire. If the timer expires, the SS must again perform ranging with the system.

The base station is responsible for assigning burst profiles. However, the subscriber station may request changes to the burst profile. This may occur as a result of an increase in the bit error rate of the received signal due to fading or interference. The subscriber station may request a change in burst profile that is more robust or offers a higher data transmission rate. The base station may grant the request, negotiate parameters or reject the request.

During a WiMAX communication session, changes in bandwidth may be requested. The subscriber station may send bandwidth request messages to the base station to increase or decrease its allocated bandwidth. Bandwidth request messages may be sent as independent messages or they may be piggybacked with other messages.

Medium Access Control | WiMAX Operation

Medium access control is the process used by communication devices to gain access to a shared communications medium or channel. The methods for controlling access to WiMAX systems may be assigned (“non-contention based”) or random (“contention based”).

When the WiMAX system uses contention free access control the subscriber station must wait for polling messages before responding. If contention based access control is used (e.g. best effort service), the subscriber device must compete for access to send its packets. The WiMAX system can mix contention free and contention based access on the same radio channel.

Contention free access is provided by defining time periods that specific devices will use when communicating with the system. Because all the devices listening to the WiMAX radio channel can hear these messages, devices will not transmit during the assigned time periods.

Contention based access is provided through the use of contention slots and the collision sense multiple access (CSMA) process. The WiMAX channel descriptors define specific time periods (“contention slots”) that contention based WiMAX devices must use when accessing the WiMAX system. Contention slots are dedicated time intervals (time slots) on a communication channel that can be used to allow devices to randomly request service from a system.

When contention based WiMAX subscriber stations access the system, they first obtain the contention time slot interval and the system access parameters (e.g. initial access transmit power level). After the contention slot time period has started, the subscriber station begins to transmit an access message at a low RF power level. If the subscriber station hears a positive response to its access request message, it can transmit its package. If the subscriber device does not hear a response (e.g. another device has transmitted at the same time), it must stop transmitting and wait a random amount of time before attempting to access the system again. Each time the device attempts to access the system and fails, it must wait a longer amount of time before attempting to access the system again. This prevents the possibility of many collisions between devices that are attempting to access the system at approximately the same time.

Figure 1 illustrates how the WiMAX system can mix contention free and contention based access control on a WiMAX radio channel. This diagram shows that the downlink channel contains downlink and uplink descriptor messages that define when subscriber stations are allowed to transmit. For unicast polled devices (contention free), they are assigned specific time periods to transmit from a polling message. For multicast polled, broadcast polled or best effort devices (contention based), they compete during the contention time slot periods.

Figure 1: WiMax Access Control

Initial Ranging | WiMAX Operation

Initial ranging is the process of estimating the distance or propagation time between a transmitter and receiver. Ranging information may be used to assist in the establishment of operating parameters for the transmitter and receiver. The transmitter power level and packet transmission delay time ensure packets do not overlap with transmission from other devices.

During the initial ranging process, the base station is assigned the basic CID that will be used to control the radio operations of the subscriber device. After the basic CID is assigned, a primary management CID may be assigned to allow for authentication and the establishment of other CID channels. A secondary CID may be assigned to allow the downloading of configuration files and the assignment of an IP address using dynamic host configuration protocol (DHCP).

Figure 1 depicts the basic channel acquisition processes that may be used in the WiMAX system. The subscriber station begins by scanning a set of potential WiMAX frequencies. If it finds a WiMAX radio channel, it synchronizes with the RF channel and acquires the downlink channel descriptor (DCD) and uplink channel descriptor (UCD) messages to determine how to access the system. The subscriber station then sends initial ranging request messages to get the attention of the system and to receive timing adjustment information. This process starts by transmitting at a lower RF power level and gradually increasing until the system responds with an assignment of basic and primary control identifiers (CID). The subscriber station then sends its transmission capabilities to the base station and the WiMAX system responds with an authorization or denial of service for these transmission capabilities.

Figure 1: 802.16 Channel Acquisition and Initial Ranging