e call capacity. And as described in Sec. 28.2.5, data terminal movement affects the data ca- pacity as well. Since the waveform is affected by delay spread and Doppler distortion, the data capacity goes down. There is another capacity tradeoff available to the CDMA data service provider. It is reasonable for the end-to-end data service to retransmit bad packets using automatic repeat request (ARQ). In this case, a higher frame error rate (FER) simply means more channel time sending packets again rather than a higher error rate to the data service subscriber. Thus the capacity a, with retries taken into account, is given in Eq. (29.4): (29.4) Since a frame is a fixed number of bits, the FER depends on the BER. For a given data rate, there is an optimal point where the tradeoff between minimum power (low Eb/No) and high FER reaches an overall least energy per bit. Lowering Eb/No will create many more errors without saving much power, and raising Eb/No will use more power without fixing many frame errors. Managing this tradeoff is an im- portant way for the wireless data service provider to maximize data capacity. The op- timal data performance depends on the speed of motion of the user terminal. Another option to increase capacity is to use a longer interleaving length SO that bursty channel errors are still corrected. The price of longer interleaving is a longer de- lay in packet transmission. Also, a longer interleaving length means that each service has larger chunks of data. If the packet is smaller than the interleaving length, then the system has to pad the rest of the interval. 2The 1/a estimate is not quite exact because it ignores the fact that a CDMA channel does not interfere with itself. While this can be significant for large-capacity users, 1/a is a good estimate of the share of the CDMA carrier a data service uses. Personal computer owners may be aware of the issue of hard-disk cluster size, where each file uses a whole number of clusters. The smallest quantum of disk space is typically something li