Sunday, November 1, 2009

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:

6.5 Burst Profile

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
Open table as spreadsheet

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
Open table as spreadsheet

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
Open table as spreadsheet

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.)
Open table as spreadsheet

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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