WiMAX Quality of Services (QoS)

The IEEE 802.16 standard supports up to five QoS classes. The level of
quality of service differentiation is per service flow. Each of the service flow
is having one of the scheduling types; best effort (BE), non-real time polling
service (nrtPS), real-time polling service (rt-PS), extended real-time polling
service (ert-PS) or unsolicited grant service (UGS).
WiMAX provides the five QoS classes through an architecture that is able to
process requests, perform access control and allocate the required radio
resources that are able to meet the requests that are accepted. The five QoS
classes are described as follows.

• UGS: this is designed to support real-time data streams that consist of fixed sized packets issued at periodic intervals, such as back haul and voice over IP (VoIP) without silence suppression.
  

• Ert-PS: this is designed for the extended real-time services of variable rates such as VoIP with silence suppression, interactive gaming, and video telephony.
  

• Rt-PS: this is designed to support real-time data streams of variable rates that are issued at periodic intervals, such as MPEG video, audio and video streaming, and interactive gaming.
  

• Nrt-PS: this is designed to support delay-tolerant data streams consisting of variable-sized data packets such as file transfer protocol (FTP), browsing, video download, and video on demand.
  

• BE: this is designed to support data streams for which there is no minimum service requirements, and no guarantee of timely delivery of packets such as E-mail and Internet browsing.
  

WiMAX differentiates the service flows at the IP layer through the DiffServ
code points (DSCP). DSCP is a field in the header of IP packets used for
classifying packets entering the network in order to provide QoS guarantees.
From an IP transport perspective, the WiMAX network is divided into
multiple DSCP domains. One domain is between the base station and the
ASN gateway (ASN-GW) in every ASN termed as ASN DiffServ domain.
The second domain, CSN DiffServ domain, is between the ASN-GWs and
the HAs. The third domain is between the HAs and Internet or operator
service network.

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Physical Layer or The Physical Layer of WiMAX

The 802.16 Physical Transmission Chains

The modulation and OFDM transmission are the major building blocks of the WiMAX PHYsical Layer. The transmission chains of WiMAX are described for both OFDM and OFDMA PHYs.

1 The Global Chains

The PHY transmission chains of OFDM and OFDMA are illustrated in Figures 1 and 2. The blocks are the same with the small difference that OFDMA PHY includes a repetition block. The modulated symbols are then transmitted on the OFDM orthogonal subcarriers. In the following, WiMAX channel coding building blocks are described.

Figure 6.1: OFDM PHY transmission chain

Figure 6.2: OFDMA PHY transmission chain

2 Channel Coding

The radio link is a quickly varying link, often suffering from great interference. Channel coding, whose main tasks are to prevent and to correct the transmission errors of wireless systems, must have a very good performance in order to maintain high data rates. The 802.16 channel coding chain is composed of three steps: randomiser, Forward Error Correction (FEC) and interleaving. They are applied in this order at transmission. The corresponding operations at the receiver are applied in reverse order.

3 Turbo Coding

Turbo codes are one of the few FEC codes to come close to the Shannon limit, the theoretical limit of the maximum information transfer rate over a noisy channel. The turbo codes were proposed by Berrou and Glavieux (from ENST Bretagne, France) in 1993. The main feature of turbo codes that make them different from the traditional FEC codes are the use of two error-correcting codes and an interleaver. Decoding is then made iteratively taking advantage of the two sources of information

4 Transmission Convergence Sublayer (TCS)

The Transmission Convergence Sublayer (TCS) is defined in the OFDM PHY Layer and the Non-WiMAX SC PHY Layer. The TCS is located between the MAC and PHY Layers. If the TCS is enabled, the TCS converts MAC PDUs of variable size into proper-length FEC blocks, called TC PDU.

The TCS is an optional mechanism for the OFDM PHY. It can be enabled on a preburst basis for both the uplink and downlink through the burst profile definitions in the uplink and downlink channel descriptor (UCD and DCD) messages respectively. The TCS_ENABLE parameter is coded as a TLV tuple in the DCD and UCD burst profile encodings. At SS initialisation, the TCS capability is negotiated between the BS and SS through SBC-REQ/SBC-RSP MAC messages as an OFDM PHY specific parameter. The TCS is not included in the OFDMA PHY Layer.

Finally, the burst profiles of OFDM and OFDMA PHY, an important building block of IEEE 802.16 MAC layer, are described:

5 Burst Profile

The burst profile is a basic tool in the 802.16 standard MAC Layer. The burst profile allocation, which changes dynamically and possibly very fast, is about physical transmission. Here the parameters of the burst profiles of WiMAX are summarised. The burst profiles are used for the link adaptation procedure.

5.1 Downlink Burst Profile Parameters

The burst profile parameters of a downlink transmission for OFDM and OFDMA PHYsical layers are proposed in Table 1. The parameter called FEC code is in fact the Modulation and Coding Scheme (MCS). For OFDM PHY, there are 20 MCS combinations of modulation (BPSK, QPSK, 16-QAM or 64-QAM), coding (CC, RS-CC, CTC or BTC) and coding rate (1/2, 2/3, 3/4 and 5/6). The most frequency-use efficient (and then less robust) MCS is 64-QAM (BTC) 5/6. For OFDMA PHY, there are 34 MCS combinations of modulation (BPSK, QPSK, 16-QAM or 64-QAM), coding (CC, ZT CC, CTC, BTC, CC with optional interleaver) and coding rate (1/2, 2/3, 3/4 and 5/6).

Table 1: Downlink burst profile parameters for OFDM and OFDMA PHYsical layers
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Burst profile parameter	Description
Frequency (in kHz)	Downlink frequency
FEC code type	Modulation and Coding Scheme (MCS); there are 20 MCSs in OFDM PHY and 34 MCSs in OFDMA PHY (as updated in 802.16e)
DIUC mandatory exit threshold	The CINR at or below where this burst profile can no longer be used and where a change to a more robust (but also less frequency-use efficient) burst profile is required. Expressed in 0.25 dB units.
DIUC minimum entry threshold	The minimum CINR required to start using this burst profile when changing from a more robust burst profile. Expressed in 0.25 dB units
TCS_enable (OFDM PHY only)	Enables or disables TCS

5.2 Uplink Burst Profile Parameters

The burst profile parameters of an uplink transmission for an OFDM PHY and an OFDMA PHY are proposed in Tables 2 and 3 respectively.

Table 2: Uplink burst profile parameters for the OFDMA PHYsical Layer
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Burst profile parameter	Description
FEC type and modulation type	There are 20 MCSs in OFDM PHY
Focused contention power boost	The power boost in dB of focused contention carriers
TCS_enable	Enables or disables TCS

Table 3: Uplink burst profile parameters for the OFDMA PHYsical Layer
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Burst profile parameter	Description
FEC type and modulation type	There are 52 MCSs in OFDMA PHY
Ranging data ratio	Reducing factor, in units of 1 dB, between the power used for this burst and the power used for CDMA ranging encoded as a signed integer

5.3 MCS Link Adaptation

The choice between different burst profiles or, equivalently, between different MCSs is a powerful tool. Specifically, choosing the MCS most suitable for the state of the radio channel, at each instant, leads to an optimal (highest) average data rate. This is the so-called link adaptation procedure. In the following chapters the MAC procedures that can be used for the implementation of link adaptation are described. The link adaptation algorithm in itself is not indicated in the 802.16 standard. It is left to the vendor or operator.

The order of magnitudes of SNR thresholds can be obtained from Table 4, proposed in the standard for some test conditions. These SNR thresholds are for a BER, Bit-Error Rate, measured after the FEC, that is smaller than 10⁻⁶.

Table 4: Received SNR threshold assumptions , Table 266. (From IEEE Std 802.16-2004 . Copyright IEEE 2004, IEEE. All rights reserved.)
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Modulation	Coding rate	Receiver SNR threshold (dB)
BPSK	1/2	6.4
QPSK	1/2	9.4
QPSK	3/4	11.2
QAM-16	1/2	16.4
QAM-16	3/4	18.2
QAM-64	1/2	22.7
QAM-64	3/4	24.4

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Different types of Digital Modulation

1.Digital Modulations

As for all recent communication systems, WiMAX/802.16 uses digital modulation. The now well-known principle of a digital modulation is to modulate an analogue signal with a digital sequence in order to transport this digital sequence over a given medium: fibre, radio link, etc. (see Figure 1). This has great advantages with regard to classical analogue modulation: better resistance to noise, use of high-performance digital communication and coding algorithms, etc







Figure 1: Digital modulation principle

2.Binary Phase Shift Keying (BPSK)

The BPSK is a binary digital modulation; i.e. one modulation symbol is one bit. This gives high immunity against noise and interference and a very robust modulation. A digital phase modulation, which is the case for BPSK modulation, uses phase variation to encode bits: each modulation symbol is equivalent to one phase. The phase of the BPSK modulated signal is π or -⁻π according to the value of the data bit. An often used illustration for digital modulation is the constellation. Figure 5.2 shows the BPSK constellation; the values that the signal phase can take are 0 or π.

Figure 2: The BPSK constellation
3. Quadrature Phase Shift Keying (QPSK)

When a higher spectral efficiency modulation is needed, i.e. more b/s/Hz, greater modulation symbols can be used. For example, QPSK considers two-bit modulation symbols.

Table 1 shows the possible phase values as a function of the modulation symbol. Many variants of QPSK can be used but QPSK always has a four-point constellation (see Figure 3). The decision at the receiver, e.g. between symbol ‘00’ and symbol ‘01’, is less easy than a decision between ‘0’ and ‘1’. The QPSK modulation is therefore less noiseresistant than BPSK as it has a smaller immunity against interference. A well-known digital communication principle must be kept in mind: ‘A greater data symbol modulation is more spectrum efficient but also less robust.’

Figure 3: Example of a QPSK constellation

Table 1: Possible phase values for QPSK modulation

Open table as spreadsheet

Even bits	Odd bits	Modulation symbol	ϕk
0	0	00	π/4
1	0	01	3π/4
1	1	11	5π/4
0	1	10	7π/4

4. Quadrature Amplitude Modulation (QAM): 16-QAM and 64-QAM

The QAM changes the amplitudes of two sinusoidal carriers depending on the digital sequence that must be transmitted; the two carriers being out of phase of +π/2, this amplitude modulation is called quadrature. It should be mentioned that according to digital communication theory, QAM-4 and QPSK are the same modulation (considering complex data symbols). Both 16-QAM (4 bits/modulation symbol) and 64-QAM (6 bits/modulation symbol) modulations are included in the IEEE 802.16 standard. The 64-QAM is the most efficient modulation of 802.16 (see Figure 4). Indeed, 6 bits are transmitted with each modulation symbol.

Figure 4: A 64-QAM constellation

The 64-QAM modulation is optional in some cases:

• license-exempt bands, when the OFDM PHYsical Layer is used

• for OFDMA PHY, yet the Mobile WiMAX profiles indicates that 64-QAM is mandatory in the downlink.

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Benefits of Wireless Communications with MVPN (Mobile Virtual Private Network)

For wireless operators deploying latest-generation cellular systems based on packet-switched data such as GPRS and CDMA2000, and especially those targeting business customers for significant portion of their revenue stream, the importance of services based on MVPN technologies is hard to underestimate. For operators, MVPN is not only one of the required technologies for business customers' private network access but also a foundation for other services requiring interaction with private networks such as m-commerce, virtual presence and gaming applications, and multimedia applications (which includes Voice over IP-based services).

The benefits of deploying Mobile VPNs for businesses and institutions include:

• Uninterrupted, media and location-independent connectivity to private networks

• Seamless private network access mobility

• Connectivity to a particular Internet service provider (ISP) or application service provider (ASP)

• Mobile remote access outsourcing possibilities

• Secure m-commerce enabler

• Constant remote-workers reachability

• Higher cost-effectiveness

As a result, businesses, which already had a positive experience with wireline VPN services, are now looking to wireless operators for extending these services into wireless environments. In our view, during the next few years as the latest generations of cellular systems and other wireless technologies take off, an enormous market opportunity awaits wireless carriers who can meet demands for services requiring private network access.

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