SYSTEM PARAMETERS AND Three control FRAME STRUCTURE channels 22 packet data channels The frame structure described in this section supports both control information, which is needed to perform the dpa procedure, as well as the bearer traffic. a frame is 20 ms. The con trol part uses a staggered schedule, in which only one base station at a time, from a group of four adjacent base stations, transmits informa tion for DPA.3 The bearer traffic. on the other EE5 hand, is transmitted on the assigned radio resources 4(“ channels”) without a staggered schedule. To implement a staggered schedule four frames(80 ms) are grouped as a"super frame. Effectively this achieves a reuse factor 24 OFDM blocks I 104 OFDM blocks in 8 slots of 4 for control information while allowing a reuse factor of I for bearer traffic by using DPA Figure 2. Division of radio resources in time and frequency domains to allow i.e. all traffic channels can be used ever DPA for high-peak-rate data services; small radio resources, needed for low- where). A reuse factor of 4 and three sectorized delay services, occupy only one slot, which is further divided into four mini lots to allow coding across different frequency clusters error protection for the control channels, where as interference avoidance based on dpa with admission control provides good quality for the achieve this frequency diversity for small resources traffic channels a slot is divided into mini-slots at the cost of The total bandwidth is divided into 8-kHz sub- reduced efficiency due to TDMA overhead channels ("tones"). In the time domain, this can be constructed by grouping OFDM blocks(blocks HIGH-PEAK-RATE DATA SERVICES: of OFDM subchannels) with a 125 us signaling LARGE RADIO RESOURCES interval and a 31.25 us guard time to accommo date significant delay spread in macrocells. In the 528 subchannels(4.224 MHz) are organized into following discussion, the duration of an OFDI 2 clusters of 24 subchannels(192 kHz)each block (or simply"block"), 156.25 us, is used as the and 8 time slots of 13 OFDM blocks each within basic time unit in the discussion of the frame a 20-ms frame of 128 blocks, figure 2 shows this structure. A frame of 20 ms is equivalent to 128 resource allocation scheme. The control channel blocks. This corresponds to a 6. 4 kbaud block rate. functions are defined in[3]. This allows flexibili- Also a total of 528 subchannels are considered in channel resulting in a 4. 224 MHz bandwidth. The discus- blocks of control overheard to perform the dPA sion below focuses on the case of oPSK modula- tion and 1/2-rate coding(1 b/s/Hz), resulting in a This arrangement of tone clusters is simila total rate of 3.3792 Mb/s without considering to the arrangements in the band-division multi- other overheads Coding and modulation schemes ple access(BDMA) proposal by Sony. Figure 3 with higher efficiency could provide higher rates, depicts this operation. Each tone cluster would specially for microcellular environment contain 22 individual modulation tones plus 2 Considerations for organization of resources guard tones, and an OFDM block would have a are the resolution of resource size, the overhead time duration of 156.25 us with 31.25 us for required to allocate individual resources, and the guard time and ramp time to minimize the 3 The grouping can be expected size of objects to be transmitted over a effects of delay spread up to about a 20-us span. configured similar to con- resource. Minimization of overhead can be Of the 13 OFDM blocks in each traffic slot, two ventional frequency plan into large resources, but if many objects are rela- leading block for synchronization(phase/fre- using a regular and repet- tively small in size or higher-layer protocols gener- quency/timing acquisition and channel estima- tive pattern, for example, ate small objects that the lower layers must carry, on)and a trailing block as guard time for with timing groups I and there will be a need to allocate small resources to separating consecutive time slots. A single radio 2 altemating in the odd equire resources that are small locally in time pattern, by which the packets are transmitted in there. d groups 3 and 4 achieve good efficiency. Also, streaming data may resource is associated with a frequency-hoppi rows an avoid the need for buffe ource bits before using eight different tone clusters in each of the transmission, which causes delay. A 2 Mb/s system eight traffic slots. Coding across eight traffic 4 The word"resource"is with 20-25 resources would support about 80-100 ts for user data, as shown in Fig. 3, exploits used to emphasize that th kb/s rates locally in time. This rate would be suit- frequency diversity which gives sufficient coding assignment of radio chan- able for high-bit-rate data services. If supporting gain for performance enhancement in the fading nels for traffic bearers can about 10 kb/s locally in time were desirable(e. g, channel. This arrangement supports 22 resources be a general combination voice or audio services of 8 kb/s with additional in frequency that can be assigned by DPA. Tak- of time slots, frequen coding for error correction in wireless channels), ing into account overhead for OFDM block sub-carriers and user this would be equivalent to about 200 resources. In guard time, synchronization, slot separation, and codes. The user code con- the following, small resource assignment is consid- DPA control, a peak data rate of 2. 1296(3.3792 trol the sequence by which ered using only one of the 8 slots in a 20-ms frame. x 22/24 x 11/13 x 104/128)Mb/s is available for a given user access differ- Frequency hopping over different slots is employed packet date services using all 22 radio resources, ent frequency sub-carmier to gain frequency diversity for large resources. To each 96.8 kb at different time slots. Magazine·JuIEEE Communications Magazine • July 2000 83 SYSTEM PARAMETERS AND FRAME STRUCTURE The frame structure described in this section supports both control information, which is needed to perform the DPA procedure, as well as the bearer traffic. A frame is 20 ms. The con￾trol part uses a staggered schedule, in which only one base station at a time, from a group of four adjacent base stations, transmits informa￾tion for DPA.3 The bearer traffic, on the other hand, is transmitted on the assigned radio resources4 (“channels”) without a staggered schedule. To implement a staggered schedule, four frames (80 ms) are grouped as a “super￾frame.” Effectively, this achieves a reuse factor of 4 for control information while allowing a reuse factor of 1 for bearer traffic by using DPA (i.e., all traffic channels can be used every￾where). A reuse factor of 4 and three sectorized antennas in each base station provide extra error protection for the control channels, where￾as interference avoidance based on DPA with admission control provides good quality for the traffic channels. The total bandwidth is divided into 8-kHz sub￾channels (“tones”). In the time domain, this can be constructed by grouping OFDM blocks (blocks of OFDM subchannels) with a 125 ms signaling interval and a 31.25 ms guard time to accommo￾date significant delay spread in macrocells. In the following discussion, the duration of an OFDM block (or simply “block”), 156.25 ms, is used as the basic time unit in the discussion of the frame structure. A frame of 20 ms is equivalent to 128 blocks. This corresponds to a 6.4 kbaud block rate. Also, a total of 528 subchannels are considered, resulting in a 4.224 MHz bandwidth. The discus￾sion below focuses on the case of QPSK modula￾tion and 1/2-rate coding (1 b/s/Hz), resulting in a total rate of 3.3792 Mb/s without considering other overheads. Coding and modulation schemes with higher efficiency could provide higher rates, especially for microcellular environments. Considerations for organization of resources are the resolution of resource size, the overhead required to allocate individual resources, and the expected size of objects to be transmitted over a resource. Minimization of overhead can be achieved by organizing the available bandwidth into large resources, but if many objects are rela￾tively small in size or higher-layer protocols gener￾ate small objects that the lower layers must carry, there will be a need to allocate small resources to achieve good efficiency. Also, streaming data may require resources that are small locally in time to avoid the need for buffering source bits before transmission, which causes delay. A 2 Mb/s system with 20–25 resources would support about 80–100 kb/s rates locally in time. This rate would be suit￾able for high-bit-rate data services. If supporting about 10 kb/s locally in time were desirable (e.g., voice or audio services of 8 kb/s with additional coding for error correction in wireless channels), this would be equivalent to about 200 resources. In the following, small resource assignment is consid￾ered using only one of the 8 slots in a 20-ms frame. Frequency hopping over different slots is employed to gain frequency diversity for large resources. To achieve this frequency diversity for small resources a slot is divided into mini-slots, at the cost of reduced efficiency due to TDMA overhead. HIGH-PEAK-RATE DATA SERVICES: LARGE RADIO RESOURCES 528 subchannels (4.224 MHz) are organized into 22 clusters of 24 subchannels (192 kHz) each and 8 time slots of 13 OFDM blocks each within a 20-ms frame of 128 blocks. Figure 2 shows this resource allocation scheme. The control channel functions are defined in [3]. This allows flexibili￾ty in channel assignment while providing 24 blocks of control overheard to perform the DPA procedures. This arrangement of tone clusters is similar to the arrangements in the band-division multi￾ple access (BDMA) proposal by Sony. Figure 3 depicts this operation. Each tone cluster would contain 22 individual modulation tones plus 2 guard tones, and an OFDM block would have a time duration of 156.25 ms with 31.25 ms for guard time and ramp time to minimize the effects of delay spread up to about a 20-ms span. Of the 13 OFDM blocks in each traffic slot, two blocks are used as overhead, which includes a leading block for synchronization (phase/fre￾quency/timing acquisition and channel estima￾tion) and a trailing block as guard time for separating consecutive time slots. A single radio resource is associated with a frequency-hopping pattern, by which the packets are transmitted using eight different tone clusters in each of the eight traffic slots. Coding across eight traffic slots for user data, as shown in Fig. 3, exploits frequency diversity which gives sufficient coding gain for performance enhancement in the fading channel. This arrangement supports 22 resources in frequency that can be assigned by DPA. Tak￾ing into account overhead for OFDM block guard time, synchronization, slot separation, and DPA control, a peak data rate of 2.1296 (3.3792 x 22/24 x 11/13 x 104/128) Mb/s is available for packet date services using all 22 radio resources, each 96.8 kb/s. 3 The grouping can be configured similar to con￾ventional frequency plan￾ning for reuse factor 4 using a regular and repeti￾tive pattern, for example, with timing groups 1 and 2 alternating in the odd rows and groups 3 and 4 in the even rows. 4 The word “resource” is used to emphasize that the assignment of radio chan￾nels for traffic bearers can be a general combination of time slots, frequency sub-carriers and user codes. The user code con￾trol the sequence by which a given user access differ￾ent frequency sub-carriers at different time slots. ■ Figure 2. Division of radio resources in time and frequency domains to allow DPA for high-peak-rate data services; small radio resources, needed for low￾delay services, occupy only one slot, which is further divided into four mini￾slots to allow coding across different frequency clusters. x x x 528 tones divided into 22 24-tone clusters Frequency Assignment channel Paging channel Pilot channel 24 OFDM blocks 104 OFDM blocks in 8 slots Three control channels 22 packet data channels