9.3 Transmission of Downlink and Uplink Subframes

9.3 Transmission of Downlink and Uplink Subframes

Downlink and uplink transmissions coexist according to one of the two duplexing modes: TDD or FDD. They are sent through the downlink and uplink subframes. More specific information is now given about each of these two subframes for OFDM PHY. The structure of downlink and uplink subframes is the same for TDD and FDD.

9.3.1 OFDM PHY Downlink Subframe

An OFDM PHY downlink subframe consists of only one downlink PHY PDU, this PDU being possibly shared by more than one SS. A downlink PHY PDU starts with a long preamble, which allows PHY synchronisation for listening SSs. A listening SS synchronises to the downlink using the preamble (see Section 9.3.5 and Chapter 11). The preamble is followed by a Frame Control Header (FCH) burst. The FCH contains the Downlink Frame Prefix (DLFP) which specifies the burst profile and length of at least one downlink burst immediately following the FCH. Several downlink burst profile and lengths, up to four after the FCH, may be indicated in the DLFP. An HCS field occupies the last byte of the DLFP.

For OFDM PHY, the standard indicates that the DLFP is one OFDM symbol with the most robust modulation and coding scheme. The modulation and coding scheme can be considered to be BPSK with a coding rate of 1/2. In the DLFP, the following are specified:

  • The location and profile of the first downlink burst (immediately following the FCH).

  • The location and profile of the maximum possible number of subsequent bursts. The locaation and profile of other bursts are specified in the DL-MAP MAC management message (see Section 9.4). The profile(s) is specified either by a 4-bit Rate_ID (for the bursts indicated by the DLFP) or by DIUC (in DL-MAPs).

Each downlink burst may be sent to one (unicast) or more SSs (multicast or broadcast). A DL-MAP message (indicator of the downlink frame use, see below), if transmitted in the current frame (a case where no DL-MAP is needed: the DLFP indicates all the burst profiles of the downlink subframe), must be the first MAC PDU in the burst following the FCH. A UL-MAP message (indicator of the uplink frame use, see below) immediately follows either the DL-MAP message (if there is one) or the FCH. If UCD and DCD messages are transmitted in the frame, they immediately follow the DL-MAP and UL-MAP messages. The FCH is followed by one or many downlink bursts. The same burst profile can be used more than one time (this is a 16e update of 802.16-2004 which required each burst being transmitted with a different burst profile). These downlink bursts are transmitted in order of decreasing robustness of their burst profiles. The general format of a downlink subframe is shown in Figure 9.4. Burst profile indicators are DIUC and UIUC, described in the sequel.

Image from book
Figure 9.4: Details of the OFDM PHY downlink subframe. Each downlink burst may be sent to one (unicast) or more SSs (multicast or broadcast).

9.3.2 OFDM PHY Uplink Subframe

Figure 9.5 represents the structure of an uplink subframe. An OFDM PHY uplink subframe consists of three global parts in this order:

  • Contention slots allowing initial ranging. Via the Initial Ranging IE, the BS specifies an interval in which new stations may join the network (see Chapter 11 for the initial ranging procedure). Packets transmitted in this interval use the RNG-REQ (Ranging Request) MAC management message and are transmitted using a contention procedure as collision(s) may occur with other incoming SSs (see Chapter 10 for the contention procedures).

  • Contention slots allowing bandwidth requests. Via the Request IE, the BS specifies an uplink interval in which requests may be made for a bandwidth for uplink data transmission (see Chapter 10 for bandwidth request).

  • One or many uplink PHY PDUs, each transmitted on a burst. Each of these PDUs is an uplink subframe transmitted from a different SS. A PDU may transmit an SS MAC messages.

Image from book
Figure 9.5: Details of the OFDM PHY uplink subframe.

9.3.3 OFDMA PHY Frame

For the OFDMA PHY Layer, the frame format is evidently different, taking into account that data mapping is made on two dimensions: time and subcarriers. Figure 9.6 shows an example of an OFDMA frame in the TDD mode. This figure includes nonmandatory OFDMA frame elements.

Image from book
Figure 9.6: Example of an OFDMA frame in the TDD mode. (Based on References [2] and [10].)

The transitions between modulations and coding take place on slot boundaries in the time domain (except in the AAS zone) and on subchannels within an OFDMA symbol in the frequency domain. The FCH is transmitted using the QPSK rate 1/2 with four repetitions using the mandatory coding scheme. Then, the FCH information is sent on four adjacent subchannels with successive logical subchannel numbers in a PUSC zone. The FCH contains the DLFP which specifies the length of the DL-MAP message that immediately follows the DLFP and the repetition coding used for the DL-MAP message.

The OFDMA frame may include multiple zones (such as PUSC, FUSC, PUSC with all subchannels, optional FUSC, AMC, TUSC1 and TUSC2). The transition between zones is indicated in the DL-MAP by the STC_DL_Zone or AAS_DL_IE. Both of these DIUCs are extended DIUC (=15) specific assignments. DL-MAP and UL-MAP allocations cannot span over multiple zones. Figure 9.7 shows an OFDMA frame with multiple zones. In the first PUSC zone of the downlink (first zone), the default renumbering sequence is used for cluster logical numbering.

Image from book
Figure 9.7: Illustration of the OFDMA frame with multiple zones. (Based on Reference [2].

The frame structure used for the uplink includes:

  • Allocation for ranging. The uplink ranging subchannel is allocated for SSs for ranging (initial/periodic/handover ranging) and bandwidth requests.

  • The fast feedback slot includes four bits of payload data, whose encoding may contain CINR measurements, handover operation messages, extended rtPS bandwidth request, etc. The BS may allocate a CQICH (Channel Quality Information CHannel) (also called a fast-feedback channel) using a CQICH_IE (CQICH_allocation_IE or CQICH_Control_IE) for periodic CINR reports. This uplink channel state information feedback is used for some handover and MIMO operations. The CQICH also exists for the downlink.

  • Other optional signalling data allocations are handover-related subchannels, MIIO-related subchannels, HARQ UL subchannel, HARQ ACK subchannel, Power_control_IE, AAS_UL_IE, etc.

  • Allocation for data transmission.

9.3.4 Frame Duration

Frame duration possible values are dependent on the PHYsical Layer. The frame duration values for the OFDM (WiMAX) PHY Layer are shown in Table 9.1 with the corresponding frame duration codes. For the OFDMA PHY Layer, a value is added to this list: 2 ms. For mobile WiMAX (OFDMA) system profiles only a 5 ms duration is mandatory.

Table 9.1: Frame duration possible values for OFDM (WiMAX) PHY Interface (based on [1].)
Open table as spreadsheet

Frame duration code

Frame duration (ms)

















The frame duration is decided by the BS. This value is transmitted in the DCD message on the frame duration code (on 8 bits), as seen in Section 9.5.2 and Annex B. For the two duplexing systems, the rule is the following:

  • In an FDD system, the uplink and downlink channels are located on separate frequencies. A fixed duration frame is used for both uplink and downlink transmissions.

  • In the case of TDD, the uplink and downlink transmissions occur at different (complementary) times while sharing the same frequency. A TDD frame contains one downlink and one uplink subframe. The general format of an OFDM PHY TDD frame is shown in Figure 9.3. For OFDMA PHY, the format is evidently different, taking into account the two dimensions: time and subcarriers (see Figure 9.6).

    We now describe the preambles used in 802.16.

9.3.5 Preambles

A 802.16 preamble is a standard-defined sequence of symbols known by the receiver. The preamble is used by the PHYsical Layer for synchronisation and equalisation. The preamble must be taken into account for precise computation of a useful data rate.

For the OFDM PHY Layer, all preambles are structured as either one (short preamble) or two (long preamble) OFDM symbols. The OFDM symbols are defined by the values of the composing subcarriers. The Cyclic Prefix (CP) of those OFDM symbols has the same length as the CP of data OFDM symbols.

The long preamble is used in the following cases:

  • the first preamble in the downlink PHY PDU:

  • the initial ranging preamble;

  • the AAS preamble.

The short preamble is used in the following cases:

  • the first preamble in the uplink PHY PDU, when no subchannelisation is applied:

  • in the downlink bursts that fall within the STC-encoded region, the preamble transmitted from both transmit antennas simultaneously;

  • a burst preamble on the downlink bursts when indicated in the DL-MAP_IE.

In the case where the uplink allocation contains midambles, the midambles consist of one OFDM symbol and are identical to the preamble used with the allocation.

For the OFDMA PHY Layer, the preamble is a number of subcarriers.

[2]IEEE 802.16e, IEEE Standard for Local and Metropolitan Area Networks, Air Interface for Fixed Broadband Wireless Access Systems, Amendment 2: Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands and Corrigendum 1, February 2006 (Approved: 7 December 2005).

[10]WiMAX Forum White Paper, Mobile WiMAX - Part I: a technical overview and performance evaluation, March 2006.